557 IL6of 1996-1 Erosion and Accretion Trends Along the Laice n/lichigan Shore at North Point Marina and Illinois Beach State Park Year-1 (1995) Report of a Four-Year Study of Coastal Geology and Coastal Geologic Processes By: Michael J. Chrzastowski Anthony M. Foyle and 0. Brian Trask Illinois State Geological Survey Coastal and Wetlands Geology Section 615 E.PeatwDdy Drive Champaign, Illinois 61820-6964 Submitted to: Illinois Department of Natural Resources Office of Capital Development / Engineering 524 South Second Street Lincoln Tower Plaza Springfield, Illinois 62701-1787 April 1996 DNR Contract No. 951 5E Illinois state Geological Survey / Open File Series 1996-1 /-•V? W Erosion and Accretion Trends Along the Lake l\/!icliigan Shore at North Point Marina and Illinois Beach State Park Year-1 (1995) Report of a Four-Year Study of Coastal Geology and Coastal Geologic Processes By: Michael J. Chrzastowski Anthony M. Foyie and C. Brian Trask Illinois State Geological Survey Coastal and Wetlands Geology Section 615 E. Peat)ody Drive Champaign. Illinois 61820-6964 Submitted to: Illinois Department of Natural Resources Office of Capital Development / Engineering 524 South Second Street Lincoln Tower Plaza Springfield, Illinois 62701-1787 April 1996 DNR Contract No. 951 5E Illinois State Geological Survey / Open File Series 1996-1 EXECUTIVE SUMMARY The Illinois State Geological Survey (ISGS) began a four-year study in 1995 to examine erosion and accretion trends along the Lake Michigan shore at North Point Marina (NPM) and the North and South Units of Illinois Beach State Parte (IBSP). This study is funded by the Illinois Department of Natural Resources (DNR) which is responsible for coastal management at these facilities. The goal of the study is to develop a sediment budget for the coastal system to provide a basis for planning and implementing long-term coastal-management strategies. An immediate objective is to provide information on erosion and accretion trends relevant to ongoing coastal- management concems. In the marina vicinity, and in the northem part of the IBSP North Unit, survey data collected in 1995 were compared with data from 1987 through 1992. In the three-year interval 1992-1995, a minimum of 14,200 cu yds/yr of littoral sediment moved south across the WI-IL state line. Most of this sediment accumulated in the nearshore between the state line and the NPM north breakwater. Some sediment was transported southward around the north breakwater and accumulated lakeward of the marina entrance and inside the entrance. The marina entrance has been a sediment trap since the breakwaters were constmcted in 1988-89. The 1992-95 data comparison strongly suggests that littoral sand crossing the state line has bypassed the north breakwater and has contributed to shoaling in the marina entrance. Net erosion dominated across the lake bottom from the marina south breakwater to the Camp Logan headland. Locally, this erosion lowered the lake bottom to elevations below those that existed in 1987 prior to marina construction. For the entire shore along the marina property, the most severe erosion continues lakeward of the south parking area, undermining the existing line of shore defense. Without additional engineering measures to protect this area, additional subsidence of the existing shore defense is certain. As this shore defense subsides, erosion will advance landward toward the parking access roads. The shore in the northem part of the IBSP North Unit, between the marina/state parte boundary and the Camp Logan headland, has undergone extreme coastal change. Shoreline and nearshore changes between 1987 and 1995 were evaluated to assess overall trends. Between 1987 and 1989 the reach gained 13 acres of beach from the southward dispersion of sediment dredged from the marina basin. Since 1989, shoreline recession has occun-ed. As of 1995 only 5 acres remains of this previous 13-acre gain. The 1989-1995 rate of beach area loss has been 1.3 acres/yr. During this same time Interval, a total of 202,000 cu yds of beach nourishment was supplied to this shore (avg. 33,700 cu yds/yr). The nourishment slowed the rate of shoreline recession, but the nourishment volumes have been insufficient to counteract an annual net loss of sediment from the beach and nearshore. A preliminary analysis of the nearshore sediment budget suggests that a nourishment rate of at least 68,400 cu yds/yr would be required annually downdrift of the marina to maintain a balanced sediment budget and to halt net erosion. In July 1995, beach nourishment consisting of fine to medium sand was supplied to the north ends of both the North Unit (20.000 cu yds) and the South Unit (24,000 cu yds). Monitoring of the nourishment documented that the sand was neariy all dispersed into the shallow nearshore by November 1995. At both sites, the most rapid dispersion occurred during a single stomi on September 7-8. Digitized by the Internet Archive in 2012 with funding from University of Illinois Urbana-Champaign http://archive.org/details/erosionaccretion19961chrz CONTENTS EXECUTIVE SUMMARY i PART 1: STUDY DESCRIPTION 1 INTRODUCTION 1 Background 1 Purpose and Scope 4 Project Funding 4 Units of Measure 5 Tenninology 5 SETTING 7 Geologic Framework 7 Waves and Littoral Transport 8 METHODS 10 Field Studies 10 Data Processing 11 OVERVIEW OF 1995 DATA COLLECTION 13 General Statement 13 North Point Marina (NPM) 13 Illinois Beach State Park (IBSP) / North Unit 14 Illinois Beach State Park (IBSP) / South Unit 14 Regional Data Collection 14 PREVIOUS COASTAL MONITORING AT NORTH POINT MARINA 14 General Statement 15 Marina Construction History (1987-1989) 15 Coastal Monitoring (1987-1992) 17 Lake-Bottom Volumetric Changes (1987-1992) 18 PART 2: 1995 STUDY FINDINGS 23 NORTH POINT MARINA 23 General Statement 23 North Beach 25 North Beach / North Breakwater Nearshore 31 North Beach Bar 37 Marina Entrance 41 South Breakwater Nearshore 48 South Partying Area Nearshore 53 Submerged Riprap 58 South Partying Area Proposed Shore Protection 64 Volumetric Analysis of Sand Beneath the South Partying Area 64 Breakwater Conditions 68 ILLINOIS BEACH STATE PARK / NORTH UNIT 75 General Statement 75 Dispersion of 1994 Beach Nourishment 75 Monitoring of 1995 Beach Nourishment 76 Spring Bluff Nearshore 80 Spring Bluff Shoreline Changes Since Marina Construction 85 Model for Shoreline Recession at Spring Bluff Beach 91 ILLINOIS BEACH STATE PARK/ SOUTH UNIT 94 General Statement 94 Monitoring of 1995 Beach Nourishment 94 REGIONAL COASTAL MONITORING 101 General 101 Beach and Nearshore Profile Scheme 101 Compilation of Dredge Records 102 Regional Littoral Sediment Budget 104 PART 3: STUDY APPLICATIONS AND SUMMARY 111 RECOMMENDATIONS 111 SUMMARY OF YEAR-1 KEY FINDINGS 113 North Point Marina 113 Illinois Beach State Pari^ / North Unit 115 Illinois Beach State Parte / South Unit 116 Regional Coastal Monitoring 116 ACKNOWLEDGMENTS 117 REFERENCES 117 APPENDIX A Annual Bathymetric Contour Maps of North Point Marina Vicinity 1987 - 1992 120 APPENDIX B Annual Lake-Bottom Change Maps for North Point Marina Vicinity 1987 - 1992 127 APPENDIX C Profiles across the submerged riprap 1990 - 1995 134 APPENDIX D Maps related to proposed shore defense at the NPM south parking area 137 APPENDIX E Diver report from 1990 survey along the NPM north breakwater 140 APPENDIX F Profiles across the 1995 nourishment stockpile at IBSP North Unit 141 APPENDIX G Profiles across the 1995 nourishment stockpile at IBSP South Unit 145 APPENDIX H Regional scheme for nearshore profiling 147 III FIGURES 1-1 Map of the Illinois coast of Lake Michigan showing location of the study area 2 1-2 Map of the study area showing major place names and geographic features 3 1-3 Historical positions of the Low Water Datum (LWD) zero-depth contour along the nearshore near the present site of North Point Marina 9 1-4 Oblique aerial view of the North Point Marina construction site 16 1-5 Record of 1989 erosion at the fan delta 20 1-6 Record of the 1990 erosion at the fan delta 21 2-1 Index map of place names in the North Point Marina vicinity 24 2-2 Anticipated shoreline accretion history at North Beach 26 2-3 1995 shoreline position at North Beach and annual shorelines for the six years 1987 through 1992 29 2-4 1995 position of the of the LWD zero-depth contour at North Beach compared to the annual position of this contour for 1987 through 1992 30 2-5 1995 bathymetry of the North Beach/north breakwater nearshore 34 2-6 Isopach map of 1992-1995 lake-bottom changes in the North Beach/north breakwater nearshore 35 2-7 Isopach map of 1987-1992 lake-bottom changes in the North Beach/north breakwater nearshore 36 2-8 Schematic illustration of the location of North Beach bar 39 2-9 1995 bathymetry in the North Point Marina entrance area 44 2-10 Isopach map of 1992-1995 lake-bottom changes in the North Point Marina entrance area 45 2-11A Profile comparisons across the NPM marina entrance from 1989 through 1995. ... 46 2-1 1 B Profile comparisons across the inner part of the marina entrance from 1989 through 1995 47 2-12 1995 bathymetry of the south breakwater nearshore 50 2-13 Isopach map of 1992-1995 lake-bottom changes in the south breakwater nearshore 51 IV 2-14 1995 bathymetry of the south parking area nearshore 55 2-15 Isopach map of 1992-1995 lake-bottom changes in the south parking area nearshore 56 2-16 View of the riprap looking south from the south breakwater in October 1989 60 2-17 Same view of the riprap six months later in April 1990 60 2-18 Comparison of the position of the 10-fl LWD contour lakeward of the south parking area between 1989 and 1995 63 2-19 Isopach map of the NPM south parking area showing thickness of sand beneath the parking area that was derived from dredging the marina basin 67 2-20 Cross section showing the relationship between the marina north breakwater and the adjacent lake-bottom depression 71 2-21 A View looking north toward the north breakwater "sag" from the north end of the south breakwater 72 2-21 B Closer view of north breakwater "sag" looking north from the south breakwater ... 72 2-22 View showing localized subsidence of capstones along the middle section of the south breakwater 73 2-23 Map summary of changes to the North Unit nourishment stockpile between June and November 1995 79 2-24 1995 bathymetry of the Spring Bluff nearshore 82 2-25 Isopach map of 1992-1995 lake-bottom changes in the Spring Bluff nearshore ... 83 2-26 Maps showing 1987-1995 changes in position of the shoreline (A) and the Low Water Datum (LWD) zero-depth contour (B) between the North Point Marina south breakwater and the Camp Logan headland 88 2-27 View showing the influence of submerged nearshore ruins on shoreline configuration just south of the marina/state park boundary 90 2-28 Potential logarithmic-spiral shorelines between the NPM south breakwater and the Camp Logan headland 93 2-29 Location of the 1995 beach nourishment site in the Illinois Beach State Pari^ South Unit 97 2-30A Map summary of changes to the northem half of the South Unit nourishment stockpile between June and November 1995 98 2-30B Map summary of changes to the southem half of the South Unit nourishment stockpile between June and November 1995 99 2-31 View looking southward along the South Unit nourishment site three days before completion of the June/July 1995 nourishment 100 2-32 Same view as Figure 2-31 showing what remained of the South Unit nourishment stockpile in November 1995 100 2-33 Preliminary graphical littoral sediment budget between the WI-IL state line and Waukegan HartDor 110 TABLES 1-1 Factors for converting from U.S. customary units to metric units 5 1-2 Conversion factors for adjustment between different lake-level and topographic datums used in this study 12 1-3 1987 through 1992 annual accretion and erosion volumes and net volumetric change for the nearshore area between the Wisconsin-Illinois state line and the north steel sheetpile at Camp Logan 22 2-1 Summary of erosion and accretion volumes In the North Beach nearshore 32 2-2 Summary characteristics of the North Beach bar 40 2-3 Summary of erosion and accretion volumes in the south breakwater nearshore ... 52 2-4 Summary of erosion and accretion volumes in the south parking area nearshore . . 57 2-5 Summary characteristics of the lake-bottom depression adjacent to the north breakwater (1988-1995) 69 2-6 Summary of erosion and accretion volumes in the Spring Bluff nearshore 84 2-7 Dredge volumes for Commonwealth Edison Waukegan Generating Station 102 2-8 Dredge volumes for Waukegan Harbor 104 VI PART1: STUDY DESCRIPTION INTRODUCTION Background The historical record of coastal change along the Illinois shore of Lake Michigan docunfients that the state's most dynamic coastal area is the northernmost 9.7 miles between the Wisconsin- Illinois (WI-IL) state line and Waukegan Harbor (Fig. 1-1). Nowhere else on the Illinois shore has erosion been as severe or has accretion been as pronounced. The temporal scales for erosion and accretion have been short-term resulting in some of the most rapid rates of coastal change documented along the shore of southern Lake Michigan. Of the total 9.7 miles of lakeshore between the state line and Waukegan Harbor, 6.5 miles (67 percent) is state-ov^med (Fig. 1-2). This consists of the coastal zone at North Point Marina (NPM) near the state line, and the North and South Units of Illinois Beach State Parte (IBSP). Both the marina and state parte are managed by the Illinois Department of Natural Resources (Illinois DNR) and are among the most heavily used DNR recreation and conservation areas in the state. NPM represents a state investment of neariy 42 million dollars that Incorporates state-of-the-art marina design for 1500 boat slips, making this the largest marina in the Great Lakes Region. IBSP not only provides lakeshore recreation, but also preserves Illinois' last remaining stretch of natural shoreline along Lake Michigan, and has the state's last remaining concentration of coastal wetlands and dunes. The effident management of both the marina and state park is dependent on an understanding of the coastal geology and ongoing processes of coastal change. Prior to the mid-1980s, several studies of coastal geology and coastal processes were completed along this segment of the Illinois shore (e.g., Tetra Tech, 1980). Within the last ten years, however, the locations and rates of coastal change have been influenced by record high lake levels (1986) and unprecedented beach and nearshore changes resulting from the 1987-1989 construction of NPM. Near the marina and along much of the state parte shore to the south, the beaches, nearshore, and littoral sediment transport system are still adjusting to the addition of this major coastal facility. Kenosha Wl IL Winthrop Harbor .4 'NorthPoint Marina Zion r'-j \ . Illinois Beach I STUDY State Park I AREA Waukegan ,_ s -'--v-i. \ North Chicago Lake Bluff v_ Lake Forest Waukegan Harbor Great Lakes Harbor (U.S. Navy) Highland Park Lake Michigan Glencoe Winnetka ' Kenilworth.rij.. Wilmette -'- I 10 20 Km ;--■-., Q b Figure 1-1 Map of the Illinois coast of Lake Michigan showing location of the study area. Prairie Harbor Yacht Club WINTHROP HARBOR North Point Marina Wl IL Illinois Beach State Park North Unit Hosah Park (City of Zion) ^j Commonweatth Edison " Zion Nuclear Power Plant Lake Michigan Illinois Beach State Park South Unit WAUKEGAN Commonwealth Edison Waukegan Generating Station (coal Tired) Grand Avenue 1 Mile 2 Kitometers Lakeshore managed by Illinois Department of Natural Resources (DNR) Waultegan Harbor Landward limit of beach-ridge plain Figure 1-2 Map of the study area showing major place names and geographic features. Purpose and Scope In 1995, the Illinois State Geological Survey (ISGS) began a four-year study of coastal geology and coastal geologic processes along the northernmost segment of the Illinois shore of Lake Michigan. This study focuses on NPM and IBSP, but also examines erosion and accretion trends on a regional basis between the WI-IL state line and Waukegan Harbor. The ultimate goal is the development of a sediment budget for the coastal zone identifying sediment sources and sinks, sediment transport pathways, and average annual rates of lake-bottom change. This information is needed for ongoing and future management of coastal sand resources. This report is the first in a series of four annual reports that summarizes yeariy findings during the course of this four-year study and provides information relevant to ongoing coastal management of NPM and IBSP. Two aspects of this Year-1 report are important as a perspective to the report content and focus. 1) Because the collection, processing, and interpretation of field data are continuing as this report is submitted, some of the findings presented here are interim in nature. As additional data are collected and evaluated, these interim findings will be expanded in subsequent annual reports. 2) The four-year study will have equal emphasis on coastal erosion and accretion trends at both the marina and the entire state partt. However, this Year-1 report focuses primarily on the marina vicinity and on the northem segment of IBSP North Unit, located immediately south of the marina. The reason for this focus is that this part of the study area is the area of most rapid coastal change and has some of the most immediate coastal-management concems. Project Funding The primary funding for the Year-1 study was a contract with the Illinois Department of Conservation (IDOC). A reorganization of state agencies in July 1995 resulted in IDOC merging into a newly fonned Illinois Department of Natural Resources (DNR). Additional first-year funding on a cost-share basis was provided by ISGS from general revenue funds for ISGS study of Lake Michigan coastal geology. This Year-1 report is a contract deliverable for DNR (IDOC) Contract No. 9515E. Units of Measure All measurements in this report are given In U.S. customary units {i.e., feet, miles, acres, cubic yards). This is to facilitate comparison of present-year data with previous coastal monitoring data and with past and present engineering projects at the marina and state park. Table 1-1 provides factors for converting to metric units. Table 1-1 Factors for converting from U.S. customary units to metric units. | U.S. customary Conversion factor Metric Length foot 0.3048 meter mile 1.609 kilometer i .■..:. : . :::Area .::::.::::::::.:_ square foot 0.0929 square meter square yard 0.8361 square meter square mile 2.59 square kilometer acre 0.4047 hectare Volume cubic yard 0.7646 cubic meter To convert from U.S. customary units to metric units, multiply by the conversion factor in the central column. Terminology Terms used in this report which are common to Lake Michigan coastal monitoring are defined as follows: bathymetry erosion / accretion This refers to the measurement of water depths. The compilation of water-depth data along a survey line is the basis for constmcting a bathymetric profile of the lake bottom; compilation of such data across an area is the basis for producing a bathymetric map. The loss (erosion) or gain (accretion) of sediment. Erosion and accretion can have a vertical component as well as a lateral component. Unless otherwise stated, the erosion and accretion discussed in this report refer to vertical change. fan delta isobath isopach littoral transport Low Water Datum (LWD) nearshore net erosion / net accretion updrift / downdrift A geologic term used to describe a high-relief and lobate sediment deposit commonly formed where intemnontane streams flow out onto a lowland area. In this report, tan delta" is used to describe the high-relief, lobate sand and gravel reservoir that was fomied on the south side of North Point Marina as a result of the hydraulic dredging of the marina basin. The term '^an delta" is used to refer to the entire sand body, both above and below water level. A line on a bathymetric map connecting points of equal water depth or equal lake-bottom elevation (i.e.. equal bathymetry). In this report, the terms "isobath" and "bathymetric contour" are used interchangeably. A line on a map connecting points of equal thickness of a specific material. In this report, isopach maps are presented for the thickness of sediment gained or lost between given time intervals. These isopach maps indicate the vertical amount of lake-bottom erosion and accretion that are determined from a temporal comparison of bathymetric data. The movement of sediment along the beaches and nearshore by waves and wave-induced currents. The sediment involved in the transport is referred to as "littoral drift." This is the reference plane, or datum, for measuring lake levels and lake-bottom depths (bathymetry). The datum facilitates comparison of lake-bottom elevations from different months or years independent of changes in lake level. All lake-bottom elevations in this report are referenced to LWD. The nearshore is here defined as the zone between the shoreline and water depths of about 20 to 25 ft. This is the zone of major littoral sediment transport along the lake bottom. The nearshore does not include the beach or any other areas above mean lake level. When all erosion and accretion volumes for an area are summed, the Del change is determined. The predominant (i.e., strongest) waves along a coast produce a net transport direction for littoral sediment. Updrift refers to the direction from which net transport originates; downdrift refers to the direction towards which net transport occurs. For the segment of Illinois coast discussed in this report, net littoral transport is southward, driven by northeriy waves. Therefore, updrift implies "to the north," and downdrift implies "to the south." 6 SETTING Geologic Framework The study area is located along the coast of the Zion beach-ridge plain, a low-lying coastal sand and gravel plain that extends from Kenosha, Wl to North Chicago, IL (Fig. 1-1). The western border of the plain is marked by north-south trending bluff line which is approximated by a north- south right-of-way of the Chicago and Northwestem Railroad (Fig. 1-2). The Zion beach-ridge plain is a coastal feature that was fomied by the accretion of sediment that was transported along the coast by wave action. Although much of the plain appears flat, it consists of hundreds of curvilinear sand and gravel ridges that are relict beach ridges and dunes. These ridges approximate the position of fomier shorelines and formed as the sand plain accreted southward and lakeward by the addition of sand and gravel. Radiocarbon dating indicates that this plain first migrated southward across the WI-IL state line about 3,700 years ago (3,700 yrs B.P.; Larsen, 1985). Prior to the late 1800s, the leading edge of accretion was along the shore at Waukegan and North Chicago. Beginning in 1883 with the start of constmction of Waukegan Harbor, and to a greater degree after expansion of the hartjor stmctures in 1906, the southward migration of the sand plain ceased because the hartDor jetties formed a partial to near-total banner to littoral transport (Chrzastowski and Trask, 1995). The origin of the sand and gravel that comprise the beach-ridge plain is uncertain. Possibly this was originally a large glaciofluvial deposit located along the coast near Kenosha. Waves would have eroded this material and moved it southward where it was redeposited along the shore to form the beach-ridge plain. The original source area ceased to exist in the last several hundred to thousand years, and since that time the beach-ridge plain has been recycling its sand and gravel supply as it continues to migrate southward. Wave-induced erosion has been occurring along the northern part of the plain; the eroded sediment then moves southward by wave action to accrete along the southern part of the plain. These processes of northem erosion and southern accretion have resulted in a long-temn southward migration of the beach-ridge plain that is analogous to the movement of a tank tread. The coast at NPM and IBSP North Unit are within the erosional zone. The transition from net erosion to a stable or net accretional coast occurs in IBSP South Unit along the shore between the state pari^ lodge and the mouth of Dead River (Fig. 1-2). The long-tenn historical shoreline recession along the Illinois shore just south of the state line has been the most severe of any documented along the Illinois shore (Jennings, 1990; Illinois State Geological Survey, 1988). Near the present site of NPM, the annual shoreline and nearshore recession has averaged 10 ft/yr between 1872 and 1987 (Fig. 1-3). In 1872, the beach was located at what is now the most lakeward tip of the NPM north breakwater. Persistent erosion and shoreline recession along this reach was one reason for the late 1970s abandonment of the residential community (Spring Bluff) that previously occupied this lakeshore. One of the impacts of constructing North Point Marina is that the facility now creates a coastal hard point that limits the maximum possible shoreline recession in the area. Waves and Littoral Transport The wave climate between the WI-IL state line and Waukegan Harbor is such that 90 percent of the waves in the nearshore are less than 3 ft in height (Booth, 1994). The highest waves are typically assodated with storms occurring between autumn and spring. During severe storms, nearshore wave heights may reach 8 or 9 ft. This coast is exposed to waves approaching from the either the northeast or southeast quadrants. Northeasteriy waves tend to be highest since winds from the northeast quadrant have the longest fetch (i.e., distance over water). The predominance of northeasteriy waves is responsible for net southeriy transport of sediment along the beaches and nearshore. Southeasteriy waves intermittently cause reversals in the littoral transport direction by moving sediment northward, but oel littoral transport is to the south. 8 1 ■ — — Wl ,\{ \V\ « IL K I \A\ [ \ r \^^^^ 11 » Lake \ \ J) Michigan V A^i \ * \\> ^^ -y\\ ^ i NorthPoint \ 4^ Marina \ \ \\H \ \ 1 1 ' 3v" ' Low Water Datum (LWD) Zero contours V V, '■■ 10/^ if \ > _ 1910 ^\ \ 1937 \ 1946 1992 « 4 \ tl 1 1 ^w^ 1995 \ u 1 ■ www 1 *\ \ Pre-constructon shoreline \ May 1987 '• 1000 Feet • 1 1 1 \\\t 1 300 Meters \ w ; Figure 1-3 Historical positions of the Low Water Datum (LWD) zero-depth contour along the nearshore near the present site of North Point Marina. The LWD reference approximates the shoreline, but permits a comparison that is independent of differences in lake level for the different years mapped. The 1987 shoreline position approximates the shore configuration just prior to marina construction. 9 METHODS' Field Studies Establishing horizontal and vertical control All horizontal and vertical control for the field surveys relied on existing bench marks or other survey marks in the study area. Where necessary, new control points were established by oinning traverses from existing control. All survey work for establishing and verifying horizontal and vertical control used a LIETZ/SOKKISHA Set 4-A Total Station. A digital record of all survey data was made with a LIETZ SDR20 Electronic Note Book used in conjunction with the Total Station. Data were downloaded at the end of each field day onto a laptop computer and later processed using MICROSOFT Excel spreadsheet software. All horizontal control (X-Y data) was done in Illinois state plane coordinates. Vertical control (Z data) was referenced to Mean Sea Level and subsequently corrected to Low Water Datum for presentation in map format (see Data Processing). Surveys of beach profiles and monitoring of beach nourishment A LIETZ Total Station and one or more prism poles were used for collecting all beach profile data. Beach profile data were typically collected between the backshore vegetated-ridge line and the shallow nearshore (to depths of approximately 5.5 ft below lake level at the time of survey). The Total Station and back-sight prism were set at reference points of known X (Northing) and Y (Easting) in state plane coordinates, and known elevation (Z). The prism pole was then positioned on successive measurement points along predetemnined profile lines, generally at a given Northing, and the respective X, Y, and Z data were obtained for each point. To monitor nourishment stockpiles, a similar procedure was employed to determine stockpile topography (profile survey) and areal extent (circumnavigation survey). To determine rates of stockpile erosion and magnitudes of volume change, the location of the crest along the stockpile's erosional scarp was mapped by positioning the prism pole at 45-ft increments along this line. Elevations along the scarp crest were typically 1 to 12 ft above lake level at the times of survey. Collection of bathymetric data Bathymetric data were collected using one of two methods. ^Use of specific product names in this report is for informational purposes only and does not constitute endorsement by the Illinois State Geological Survey. 10 1) Fathometer Method Bathymetric data beyond wading depths were collected with a ROSS Model 803 Portable Survey Fathometer mounted onboard a 12.5-ft ZODIAC-type inflatable boat. The fathometer measured depth in feet. The calibration was checked at the beginning of each day of data collection. During profiling, the survey boat was maintained on the desired profile line by a person onshore using a transit fixed on the azimuth of the line (typically N90E). Radio or visual signals to the boat operator were used to keep the boat within one boat width (5.6 ft) of the line. Offshore distance to the boat was measured by a MOTOROLA Mini-Ranger III system (accurate to +/- 10 ft) which uses a microwave signal to determine distance between a transceiver mounted on the boat and an onshore transponder. The Mini-Ranger III system includes a control console onboard the survey boat that provides an LED display of distance in meters from the onshore transponder. The fathometer operator monitored the console display to make fix marks and annotations on the fathometer record to provide a location reference for the depth (Z) data. 2) Wading Method Shallow-water bathymetric data were collected to wading depths (approximately 5.5 ft ) to provide coverage for shallow-water areas where the boat-mounted fathometer could not always provide a good record. This procedure, which was an extension of the beach- profiling transects, involved a person wading into the water along the designated profile line holding a prism pole on successive points. An onshore Total Station operator then shot these points to obtain X-Y-Z data. Data-point spacing was such that all significant lakebed elevation changes (> 0.5 ft) were recorded. A wet suit aided prolonged stay in the water. Data Processing Datums Three different datums are used for data compiled in this report. These are: • Lakes Michigan-Huron Low Water Datum (LWD) • Intemational Great Lakes Datum (IGLD) 1955 or 1985 • National Geodetic Vertical Datum (NGVD) 1929 (also called Mean Sea Level (MSL)) Table 1-2 shows conversion factors for adjusting elevations between these different datums. Datums for bathymetry All bathymetric profiles and maps in this report are referenced to LWD. All bathymetric measurements collected by boat-mounted fathometer required a correction to LWD based on lake level relative to LWD at the time of each bathymetric profile. For these corrections, houriy lake-level data were compiled from lake-level gauges at Milwaukee, Wisconsin and Calumet Harbor, Illinois. These gauges are operated by the National Ocean 11 Table 1-2 Conversion factors for adjustment between different lake-level and topographic datums used in this study. Given Datum (in feet) To Convert to Datum (in feet) | LWD' IGLD^ (1955) IGLD2(1985) NGVD^(1929) LWD + 576.80 + 577.50 + 578.10 IGLD(1955) - 576.80 + 0.70 + 1.30 IGLD(1985) - 577.50 -0.70 + 2.00 NGVD(1929) -578.10 -1.30 + 2.00 Acronyms: LWD = Low Water Datum (also called Chart Datum) IGLD = International Great Lakes Datum NGVD = National Geodetic Vertical Datum 'LWD is the datum used for all lake-bottom depths reported by the ISGS. This is also the datum used for all depths on Lake Michigan nautical charts published by the National Ocean Service (NOS). and is commonly the datum used for profile data reported by the U.S. Army Corps of Engineers (COE). *IGLD is the intemational datum for reporting Great Lakes water levels. The datum adjustment from 1955 to 1985 was necessary to compensate for regional crustal uplift due to post-glacial rebound. Ail lake levels reported by NOS and COE since 1992 are referenced to IGLD 1985. *NGVD (1929) is the datum used for all topographic information on U.S. Geological Survey topographs maps of the Illinois coast of Lake Michigan. This datum is also refen-ed to as Mean Sea Level (MSL). Service (NOS) of the National Oceanic and Atmospheric Administration (NOAA). Because the study area is located approximately midv^ay between the gauges at Milwaukee and Calumet Harbor, the hourly lake levels for the two gauges were averaged to compute hourly lake levels during the times of bathymetric survey. The difference between these hourly lake levels and LWD was an hourly con-ection used to reference fathometer-derived lakebed elevations to the LWD reference plane. This correction ranged from 1.8 to 2.2 ft for the bathymetric data collected during summer 1995. Datums for topography LWD was also the datum used for elevations above lake level such as beach elevations and elevations on the beach nourishment stockpiles. LWD was used because it pemnitted direct comparison of profile and map data above and below water. Any compiled elevation data that were referenced to IGLD, NGVD, or MSL were adjusted to LWD using the correction factors given in Table 1-2. Constmcting and Measuring Isopach Maps Isopach maps were constructed by superimposing bathymetric (and topographic) contour maps to obtain contour intersection points. Elevation changes at these intersection points were used 12 to create an isopach contour map depicting areas and magnitudes of elevation change. The isopach maps were then used to compute erosion and accretion volumes. To compute volumes, an electronic, digital planimeter (LIETZ Planix-7) was used to measure the area within each isopach contour interval. Each of these areas was multiplied by the mid-contour value to give volume {e.g., the area between the 2 and 3 ft contours was multiplied by the mid-contour value, 2.5 ft). All volumes between contours were then summed to give total erosion and accretion volumes and net volume change. For all calculations, annual lake-bottom changes of less than one foot were not included in the calculations. This cutoff allowed for possible elevation en-ors in the collection and processing of the fathometer data. OVERVIEW OF 1995 DATA COLLECTION General Statement Data collection during 1995 had two major objectives: 1) To establish baseline data to be used for annual comparisons during the four- year study and for comparison with older data sets; 2) To provide data applicable to the ongoing management of coastal resources In the study area. The primary areas of data collection were along the beaches and nearshore at NPM and IBSP. Because this four-year study is concerned with the entire coastal setting from the WI-IL state line to Waukegan HartDor, some data collection and data compilation was regional in scope covering the entire reach. The following is an overview of the 1995 data collection and compilation as of the end of November 1995. Results derived from the 1995 data are discussed in the sections following this overview. North Point Marina (NPM) • Bathymetric surveys A total of 49 bathymetric profile lines were run between the WI-IL state line and the marina/state pari< boundary. An additional 9 lines were run in the marina entrance. These data were collected to document 1995 bathymetry, and were used for comparison with data collected annually between 1987 and 1992. • Sand volume calculations at NPM south parking area Pre- and post-construction topographic and bathymetric data were compiled to compute the volume of sand derived from the 1987 marina basin dredging operation that now resides beneath the south partying area. 13 Illinois Beach State Park f IBSP) / North Unit • Bathymetric surveys A total of 17 bathymetric profile lines were run between the marina/state park boundary and the Camp Logan headland to document 1995 bathymetry and for comparison with data collected annually between 1987 and 1992. • Beach nourishment monitoring Beach nourishment totaling 20,000 cu yds was placed at the northem end of the North Unit in July 1995. A baseline topographic and wading survey was conducted prior to the placement of the nourishment. Subsequent monitoring was done monthly to document dispersal rates. • Transport rate of coarse-grained beach nourishment Beach nourishment totaling 32,000 cu yds of small gravel (pea gravel) was placed at the north end of the North Unit in September 1994. The extent of downdrift dispersion of this gravel was mapped to determine short-term (1994-1995) littoral transport rates. Illinois Beach State Park fIBSP) / South Unit • Beach nourishment monitoring Beach nourishment totaling 24,000 cu yds was placed at the north end of the South Unit in June and July 1995. A baseline survey was done in the eariy stages of this nourishment to detemriine pre-fill beach topography and adjacent nearshore bathymetry. Subsequent monitoring was done monthly during and following the nourishment emplacement to document erosion and dispersal rates. Regional Data Collection • Uttoral sediment transport near the south end of IBSP Dredge records and records of grain-size analyses were compiled for sediment dredged at the Commonwealth Edison Waukegan Generating Station. These data, combined with similar data for dredging at Waukegan Harbor, were used to estimate minimum littoral sediment transport rates at the downdrift end of the state pari^. • Analysis of historical bathymetric records from 1872 through 1974 Bathymetric data sets from 1872, 1910, and 1974 were compiled to assist in evaluation of long-temi shoreline position changes, historical lake-bottom changes, and distribution of long-term erosional and accretional zones between the state line and Waukegan Hartjor. These analyses will be discussed in the report of 1996 studies. • Establishing control for regional nearshore profiling X-Y coordinates were detenmined for a series of 22 profile lines between the WI-IL state line and Waukegan HartDor. This effort was a precursor for a land survey to establish monuments for these lines and the 1996 collection of bathymetric data. 14 PREVIOUS COASTAL MONITORING AT NORTH POINT MARINA General Statement In order to understand beach and lake-bottom changes that are presently occurring in the vicinity of North Point Marina (NPM), It is valuable to review the construction history of the marina and the changes that occurred along the beaches and nearshore in the eariy post-construction phase. The following review spans the interval from 1987, which is the year construction began in the nearshore, to 1992, which was the end of a five-year, post-construction monitoring program conducted by ISGS. Marina Construction History (1987-1989) NPM is the largest coastal engineering project built along the Illinois coast of Lake Michigan in the last several decades, and it is one of the largest Lake Michigan coastal engineering projects of this century. The marina basin covers approximately 72 acres. The basin straddles the pre- construction shoreline and was built by a combination of dredging a basin into the beach and backshore area, and building shore-attached breakwaters to enclose the lakeward side of the basin. During construction, sediment hydraulically dredged from the basin was discharged along the south side of the south breakwater. Although some uncertainty remains about the exact volume of material dredged and discharged, the best available estimate is 1.5 million cu yds (Moffatt and Nichol Engineers, 1986). The sediment was primarily sand but consisted of a mix of silt, sand, and gravel. The grain-size distribution was similar to that of the existing beach and nearshore. Ground-breaking ceremonies for the marina occurred on July 18, 1986, but it was In spring 1987 that construction began to significantly alter the beach and nearshore. By summer 1988, dredging had progressed such that the discharge of sand and gravel was raising the beach elevation on the south side of the marina project and extending new beach area into Lake Michigan. This elevated and lobate depositional feature on the south side of the marina has been refenred to as a "fan delta" (see Terminology). In tenms of coastal processes at NPM and the IBSP North Unit, the fomiation of this fan delta was the single most significant event associated with marina construction. The fan delta added a major volume of sediment to the littoral system; it was then the site of the most rapid coastal changes in the first few years following marina construction. Figure 1-4 shows the fan delta when it was nearing its maximum lakeward extent in summer 1988. 15 Figure 1-4 Oblique aerial view of the North Point Marina construction site on September 14, 1988 looking toward the south. Sediment dredged from the marina basin is being discharged by slurry pipe to create the fan delta. The fan delta reached its maximum lakeward extent within three months after this photograph. 16 Ail breakwater construction and dredging was completed by late 1988. By the end of 1988 the surface of the fan delta was graded to an elevation ranging from 14 to 18 ft LWD. This grading was in preparation for construction of marina parking facilities and access roads, now referred to as the "south parking area." Construction began on the access roads and south parking lots in late summer 1989. To defend this south parking area from shoreline erosion, four major phases of constmcting shore defenses occurred: 1) In late fall 1988, riprap was placed from the NPM south breakwater southward for a distance of 600 ft along the shoreline of the fan delta. This was apparently in anticipation of the parking area construction which would begin the following year. 2) In November 1989, a revetment consisting of concrete "cubes" (3x3x4 ft), toe stone, and splash stone was constructed lakeward of the southem two-thirds of the parking area. 3) In December 1990, the revetment was extended northward an additional 350 ft to intercept the south breakwater. By this time the riprap placed in 1988 had been undermined sufficiently to become a submerged feature, which made the revetment extension necessary. 4) First in December 1989, and then on several subsequent occasions through at least 1992, additional riprap was placed along the shoreline. These efforts were a remedial action to compensate for lakeward shifting and undermining of the concrete cubes. Construction of the south parking area atop the fan delta was a major change in land use from the initial project plan to use the fan delta as a "feeder beach" (Moffatt & Nichol Engineers, 1986). The feeder beach was to have served as a sand stockpile that would erode by wave action and supply beach nourishment to the downdrift beach and nearshore areas. The re- assignment of the feeder beach area to serve as a parking area, and the resulting construction of the paridng area shore defense, was the impetus for importing additional sand to the beach area south of the paridng area to create a substitute feeder beach. Nourishment stockpiles were created here in 1990 (150,000 cu yds), 1994 (32,000 cu yds), and twice in 1995 (20,000 cu yds, July; 33,000 cu yds, December 1995 - January 1996). Coastal Monitoring (1987-1992) The bathymetric surveys conducted by the ISGS in the NPM vicinity from 1987 through 1992 extended from the state line southward 7700 ft (1.46 miles) to the Camp Logan headland. Bathymetric data were collected with a fathometer along a series of east-west lines having a 300-foot spacing in 1987, and a 100-foot spacing from 1988 through 1992. In 1987, position 17 control for the survey boat was a visual range-azimuth procedure using an onshore transit and alidade. From 1988 through 1992, position control was an electronic range-azimuth procedure using an onshore transit and a microwave distance-measurement system (MiniRanger III) as discussed in the METHODS section. Profile lines were identified by number from the state line (profile 1) to Camp Logan (profile 78). Appendix A contains annual bathymetric maps resulting from the six years of data collection between 1987 and 1992. Appendix B contains isopach maps showing areas and vertical magnitudes of annual lake-bottom change during this interval. These lake-bottom change maps were produced by comparing the bathymetry for successive years. Appendix B also contains a table summarizing the major lake-bottom changes that occurred. A brief summary is given below. Between 1987 and 1992, there was Initially widespread accretion to the south of the marina as basin dredging built the fan delta. Once the discharge of dredged sediment ceased, erosion dominated. On the north side of the marina, widespread accretion occurred updrift of the north breakwater and a bar developed extending from the state line to the north breakwater. The accretion and bar development resulted from littoral sediment that came southward across the state line. The extreme beach and lake-bottom changes were concentrated in the area of the fan delta. The addition of sediment to this area rapidly created an extensive and thick sand body. Once the sediment supply ceased, rapid erosion began, causing landward recession of both the undenA^ater slope and the scarp along the lakeward face of the above-water area. In 1989 and 1990, ISGS monitoring of shoreline and scarp recession lakeward of the south parking area documented the most rapid recession ever recorded along the Illinois lakeshore. A line of riprap along the northem shoreline of the fan delta temporarily halted shoreline recession there, but fomned a headland around vk^ich wave refraction eroded a hook-shaped embayment to the south opposite the south parking area (Fig. 1-5). The maximum shoreline recession documented between January and October 1989 was 200 ft (Terpstra and Chrzastowski, 1992). By January and February 1990, the riprap had subsided sufficiently to allow erosion to occur landward of it (Fig. 1-6). As of 1995, this submerged riprap remains as prominent lake-bottom feature. Lake-Bottom Volumetric Changes (1987-1992) Table 1-3 summarizes the annual lake-bottom accretion, erosion, and net change for the survey area from the WI-IL state line south to Camp Logan, and from the ft LWD contour lakeward to 18 the limit of bathymetric change (about the 20 ft LWD contour). Thus, this is a composite of lake- bottom changes in the marina vicinity that includes both the updrift area north of the north breakwater (North Beach) which was influenced by sediment entering from Wisconsin, and the downdrift area (south of the south partying area) which was Influenced by sediment added and eroded at the dredge-discharge and beach nourishment sites. The volumes in Table 1-3 provide a quantitative perspective for the maps In Appendix B. Net accretion occun-ed in the two-year interval 1987-1989. This accretion was largely the product of dredging the marina basin and discharging this sediment on the south side of the marina. Within that two-year Interval, greater net accretion occurred during 1987-1988, approximately twice that of 1988-1989. The 1987-1988 accretion corresponds with the time of greatest annual discharge of dredge sediment. 19 riprap 1989 scarp positions Concrete-cube revetment (constructed Nov 1989) N 100 1 i 200 Feet 1 1 50 Meters Figure 1-5 Record of 1989 erosion at the fan delta that occurred south (downdrift) of the riprap. These recession lines are along the crest of a 6- to 12-ft high scarp that was parallel to the shoreline. The southern tip of the riprap acted as a headland around which northerly waves refracted and eroded the hook-shaped embayment. Construction of the cube revetment in November 1989 halted the landward recession. 20 1990 scarp positions 200 Feet 50 Meters Figure 1-6 Record of the 1990 erosion at the fan delta that occun-ed landward of the line of riprap. These recession lines are along the crest of a 6- to 10-ft scarp that was parallel to the shoreline. In December 1990, construction of a northward extension of the original cube revetment halted the landward recession. The riprap is now a submerged feature. 21 Table 1-3 1987 through 1992 annual accretion and erosion volumes and net volumetric change for the nearshore area between the Wisconsin-Illinois state line and the north steel sheetpile at Camp Logan. ^ Year Interval Accretion Erosion Net Change (+ accretion; - erosion) cuyds cu yds per shoreline cuyds cu yds per shoreline ft^ cuyds cu yds per shoreline ft^ 1987-88 309,000 40 52,000 7 +257,000 +33 1988-89 235,000 31 107,000 14 +128,000 +17 1989-90 112,000 15 241,000 31 -129,000 -17 1990-91 105,000 14 126,000 16 -21,000 -3 1991-92 65,000 8 165.000 21 -100,000 -13 Summation 1987-1992 826,000 108 691,000 89 +135.000 +17 ^ Al volume calculations are for lake-bottom changes greater than 1 foot and are recorded to the nearest thousand cu yds. Nearshore area is defined as that area between the shoreline and approximately the 20 ft LWD Isobath. The upper boundary to the volume calculations is ft LWD. Annual comparisons are for data collected in the late spring or summer of each year. ^ Numbers are based on a shoreline distance of 7700 ft which is the north-south distance between the WI-IL state line and the limit of mapping at the Camp Logan headland. From 1989 through 1992, the annual net change was erosion. The severe erosion In 1989-1990 is interpreted to be due to rapid removal of sediment from over-steepened nearshore slopes and other unstable areas at, and adjacent to. the fan delta. This period of rapid erosion also corresponds to the time when shore protection was first placed lakeward of the south parking area. Though the net change during 1990-1991 was erosion, the magnitude of this erosion (21,000 cu yds) was significantly lower than that of the preceding and following one-year interval. This is in part due to the addition of approximately 150,000 cu yds of sand and gravel to the shore south of the marina in autumn 1990. This sediment was obtained from dredging Prairie Harbor Yacht Club. Erosion of this stockpile was a major source of sand to the nearshore. The 1987 to 1992 summation of nearshore volume changes results in net accretion. Even if the volumes are corrected for 54,200 cu yds of nearshore sediment "locked up" beneath the south parting area (see Volumetric Analysis of Sand Beneath the South Partying Area), the 1987-1992 net change is accretion totaling 80,800 cu yds. Net erosion dominated from 1989 through 1992, but the total loss of sediment was less than that gained between 1987 and 1989 from marina dredging. 1995 data indicate that it was at some time between 1992 and 1995 that total erosion in the nearshore exceeded gains resulting from dredging the marina basin. 22 PART 2: 1995 STUDY FINDINGS NORTH POINT MARINA General Statement The primary maintenance operation at North Point Marina (NPM) during 1995 that impacted either the beach or nearshore involved hydraulic dredging within the marina along the approach to the recreational basin. The dredging occurred between June and August. The area dredged had experienced shoaling over several years, and dredging was necessary to maintain project depths. A total of approximately 7,000 cu yds was dredged (J. LaBelle, North Point Marina General Manager, pers. com.), transported by dredge pipe along the crest of the south breakwater, and discharged on the lakeward side of the south breakwater at the junction with the riprap-defended south paridng area. The sediment input to this area resulted in a short-lived beach and shoal area at the junction between the south breakwater and the riprap shore defense. By September, the accretion in the dredge-discharge area had eroded such that no above-water area remained. Along the shore at the south parking area, no additional riprap or sand was added prior to December 1995. No regrading occurred across the sand fill landward of the line of riprap and other stone. Large, arcuate erosional scars were present on the top surface of this fill resulting from stonm events in spring 1995 and the preceding winter. During 1995, planning continued for building a submerged breakwater (or "reef) and a shoreline revetment to together protect the shore of the south parking area from further erosion. No construction occunred in 1995. The following discussion summarizes 1995 data and observations for specific locations in the NPM vicinity. Figure 2-1 is an index map of place names in the NPM vicinity referred to in this report. 1995 studies at NPM are discussed under the following headings: I North Beach II North Beach / North Breakwater Nearshore III North Beach Bar IV Marina Entrance V South Breakwater Nearshore VI South Parking Area Nearshore VII Submerged Riprap VIII South Partying Area Proposed Shore Protection IX Volumetric Analysis of Sand Beneath the South Partying Area X Breakwater Conditions 23 Lake Michigan Kenosha Co. North breakwater South breakwater Nearshore zones North Beach / north beakwater nearshore south breakwater nearshore . Submerged riprap south parking area nearshore Concrete-cube and riprap revetment Figure 2-1 Index map of place names in the North Point Marina vicinity. 24 I North Beach North Beach is the name given to the recreational beach adjacent to the north side of North Point Marina. This beach extends for approximately 1,000 ft between the steel sheetpile jetty at the WI-IL state line and the NPM north breakwater (Fig. 2-1). Prior to NPM construction, Moffatt & Nichol Engineers (1986) reported on potential impacts of the marina construction on North Beach and the adjacent nearshore. It was anticipated that North Beach would accrete as the north breakwater acted as a banier to net southward transport of littoral sediment. During the seven years since breakwater construction (1988-1995), North Beach has been accretlonal, with the 1995 shoreline positioned as much as 60 ft lakeward of the 1987 shoreline. However, when compared with changes predicted prior to the time of construction, the shoreline changes at North Beach have been minimal. Anticipated accretion history Figure 2-2 shows one of several models of anticipated beach accretion at North Beach presented by Moffatt & Nichol Engineers in 1986. This model assumes that North Beach and the adjacent nearshore receives a littoral sediment supply of 60,000 cu yds/yr. This was considered one of the best estimates of littoral transport at the time of making the model. A rapid shoreline advance was predicted to occur in the first year following breakwater constnjction. The annual rate of shoreline advance was predicted to gradually decrease as the beach and nearshore built into deeper water. About two to three years after breakwater construction, beach accretion was predicted to result in a shoreline that was tangential to the curved, northeast face of the north breakwater. When this shoreline configuration occurred. North Beach would be at or near its accretion capacity, and natural bypass of the north breakwater would begin. The potential adverse impact of this natural bypass would be the deposition of sand as a submerged spit extending southward from the south end of the north breakwater, or as a shoal area in the marina entrance. Either scenario would result in undesirable shoaling at the entrance area. In order to mitigate this potential for marina entrance shoaling, Moffatt & Nichol Engineers (1986) recommended that the anticipated accretion at North Beach be monitored. It was also recommended that the predicted accretional area at North Beach be dredged and the material artificially bypassed to the nearshore south of the marina. 25 Shoreline of Nov 1985 used in accretion model (lake level - + 3.8 ft LWD) Shoreline of May 1987 (lake level +3.67 ft LWD) , -^ Year number following 4 *l breakv\/ater construction Wl IL Predicted natural bypass of north breakwater by Year 2 or 3 1 00 Meters 400 Feet Figure 2-2 Anticipated shoreline accretion history at North Beach for the first five years following completion of the NPM north breakwater (modified from Moffatt and Nichol Engineers, 1986). Model is based on a littoral-sediment input of 60,000 cu yds/yr. 26 Actual accretion history Figures 2-3 and 2-4 show the actual accretion history at North Beach by comparing the locations of annual shorelines (Fig. 2-3) and zero-ft LWD contours (Fig. 2-4) for 1987 (pre-construction) through 1992, and for 1995. Comparison of the shorelines shows differences in the extent of the emergent beach from year to year. The advantage of comparing the locations of the 0-ft LWD is that annual changes in the location of this contour are independent of annual fluctuations in lake level. In the first year following breakwater construction, accretion did occur in the nearshore of the southern half of the beach. This was the maximum single-year accretion. It is illustrated north of the north breakwater by the 1988 0-ft LWD contour lying up to 140 ft lakeward of the 1987 0-fl LWD contour (Fig. 2-4). During the 1987-1988 interval, erosion occurred along the middle and northem sections of North Beach as the beach and nearshore was being reconfigured into an arcuate shape. Between 1988 and 1992, the position of the zero contour remained fairiy stable. The major change to occur was vertical accretion up to 2 ft in the nearshore along the southem end of the beach between 1991 and 1992. This accretion caused the 1992 0-ft LWD contour to migrate up to 80 ft lakeward of its 1991 location. By 1995, the 0-ft LWD contour occupied a position similar to that held during 1991. In the southem part of North Beach, the 0-fl LWD contour moved landward approximately 100 ft, while along the central part of the beach, the 1995 0-ft LWD contour had moved approximately 100 ft lakeward of its 1992 location. The 1995 shoreline was, on average, approximately 35 ft landward of its 1992 location indicating a less steep inner nearshore in 1995. The 1995 shoreline closely approximated the 1989 shoreline. Rather than the crescent-shaped beach that was anticipated. North Beach has maintained a rather uniform shoreline configuration since 1988. Substantial accretion has occun-ed across the shallow neartiore such that out to 200 ft from the 1995 shoreline, lake-bottom elevations are shallower than 2 ft LWD. This shoaling has been beneficial to the recreational use of this beach by providing a broad area for shallow wading, and eliminating the steep, lake-bottom slope common along much of the neighboring recreational shore at IBSP. Reasons for variance from anticipated shoreline accretion at North Beach The lack of major beach accretion at North Beach, contrary to the 1986 model prediction by Moffatt & Nichol Engineers, can be attributed to two factors: 27 1) The volume of littoral sediment crossing the WI-IL state line annually has been less than the 60,000 cu yds/yr assumed in the model; 2) Changes have occurred to the shoreline, shore structures, and sand management practices on the Wisconsin side of the state line since the model was made. The volume estimate of 60,000 cu yds/yr used in the model prepared by Moffatt & Nichol Engineers was in fact an over-estimate. As discussed in the subsequent section on the North Beach/north breakwater nearshore, accretion history in the area has averaged 10,800 cu yds/yr. As discussed in a later section on preliminary sediment budget (page 105), the minimum estimate of transport across the state line is 14,200 cu yds/yr. or 4.2 times less than the estimated 60,000 cu yds/yr. An equally important factor in restricting major changes in the location of the North Beach shoreline may be related to changes on the neighboring Wisconsin shore. Since completion of NPM, significant changes have been made at Prairie Harbor Yacht Club (previously called Prairie Harbor and Prairie Cove Marina). In 1989, the marina basin was dredged and enlarged, and new jetties were constructed at the marina entrance. Annual and semi-annual maintenance dredging of the Prairie Hart)or entrance channel captures sand that might othen^^ise add to the volume crossing the state line. The dredged sand is stockpiled inland or backpassed to the updrift shore. For the littoral sediment that does cross the state line, the transport pathway is influenced by the Prairie Harbor north jetty which is a rubble-mound structure extending 150 ft. lakeward of the shoreline (Fig. 2-1). Since it was constructed, the structure has apparently influenced southward littoral transport pathways by partially deflecting sediment transport away from North Beach. The offshore-deflected sediment stream is then better aligned to bypass the NPM north breakwater. This influence is best demonstrated by the location and orientation of the North Beach bar (Fig. 2-1). If past trends continue, North Beach can be expected to be a low-maintenance beach with no adverse trends in shoreline accretion or erosion. However, persistent accretion is occuning across much of the shallow nearshore, and dredging may be necessary to prevent this area from becoming akin to a tidal flat. 28 Prairie Harbor rubble-mound jetty dredged channel Lake Michigan Wl IL Average daily mean lake level (ft; LWD) Calumet Harbor, IL 1987 (May) +3.67 1988 (Jun) +2.34 1989 (Jun) +2.04 1990 (Jun) +1.52 1991 (Jun) +2.36 1992 (Sep) +1.64 1995 (July) +2.03 Figure 2-3 1995 shoreline position at North Beach and annual shorelines for the six years 1987 through 1992. The 1987 shoreline approximates conditions prior to constmction of the north breakwater. The differences in shoreline positions are in part influenced by differences in lake level. 29 f Prairie Harbor rubble-mound jetty dredged channel Lake Michigan Wl IL Low Water Datum (LWO) zero contours 1987 1988 1989 1990 1991 1992 o 9 1995 Figure 2-4 1995 position of the of the LWD zero-depth contour at North Beach compared to the annual position of this contour for 1987 through 1992. The 1987 zero contour approximates conditions prior to construction of the north breakwater. 30 II North Beach / North Breakwater Nearshore The North Beach/north breakwater nearshore is here defined as the lake bottom between the WI-IL state line and the southern tip of the north breakwater (Fig. 2-1). Bathymetric data from this nearshore area provide a means of mapping lake-bottom changes and computing volumetric changes at the updrift end of the study area. These data are particularly valuable for calculating the volume of sediment coming across the WI-IL state line. FigurB 2-5 shows the 1995 nearshore bathymetry. For an analysis of nearshore changes, two lake-bottom comparisons were made. The first map comparison, for 1992 to 1995 (Fig. 2-6), compares the most recent three years of change. The second map of lake-bottom change, for 1987 to 1992 (Fig. 2-7), compares bathymetry prior to construction (1987) with bathymetry five years following construction. Table 2-1 presents the volume-change data in tabular fonri. 1995 bathymetry (Figure 2-5) The North Beach/north breakwater nearshore generally has a smooth lake bottom from the shoreline out to depths of 22 ft LWD. At greater depths an irregular lake bottom suggests a bottom of exposed clay (i.e., glacial till). Prominent 1995 bathymetric features in the 1995 bathymetry are the North Beach bar centered about 650 ft lakeward of the North Beach shoreline, and a lake-bottom depression along the northeast-facing section of the north breakwater (Fig. 2-5). 1992-1995 lake-bottom changes (Figure 2-6) For the three-year interval 1992-1995, the North Beach/north breakwater nearshore was net accretional. The accretion volume (+44,100 cu yds) exceeded the erosion volume (-13,100 cu yds) by a factor of 3.4. The net accretion totaled 31,000 cu yds (Table 2-1). Accretion All accretion in excess of 1 ft occun-ed In a NNW-SSE trending band that closely con-esponded in position to where a large offshore trough was located in 1992 (Appendix A; Map A-6). This elongate accretionary zone, defined by the +1-ft isopach contour, can be traced southeastward along the lakeward side of the north breakwater to just offshore of the breakwater's southern tip (Fig. 2-6). This accretion lobe wraps around the south tip of the north breakwater and extends into the marina entrance (see Marina Entrance). Along the southem third of this accretionary zone, accretion between 5 to 6 ft is associated with partial infilling of a 16-ft deep elongate trough that was present on the 1992 bathymetric map (Appendix A; Map A- 6). This partial infilling caused the axis of the trough to shallow (to 1 1 ft LWD) and move closer to the toe of the north breakwater (Fig. 2-5). 31 Table 2-1 Summary of erosion and accretion volumes in the North Beach nearshore.^ 1987-1992 1992-19952 1987-1995 Accretion (+) (cu yds) 75,000 44,100 119,100 Erosion (-) (cu yds) 19,800 13,100 32.900 Net change (cu yds)^ +55,200 +31,000 +86,200 Annual net change (cu yds/yr)^ +11,000 +10,300 +10,800 Normalized annual net change (cu vds/vr/shoreline ft)'' +7 +6 +7 ^ All volumes are computed for lake-bottom elevation changes in excess of 1 ft and occurring belov*^ Low Water Datum (LWD). Volumes are rounded to the nearest 100 cu yds. ^ Three-year comparison; erosion and accretion volumes are for the three-year summation and annual net change is a three-year average. ' Net accretion is indicated by a positive number and net erosion is indicated by a negative number. * Shoreline distance (1600 ft) is based on measurement along a north-south line bounded to the north and south by the defined limits of the nearshore reach. Numbers are rounded to the nearest v^rhole number. Net accretion is indicated by a positive number and net erosion is indicated by a negative numt>er. Erosion A large erosional area, bounded by the -1 ft contour and the north breakwater. Is present landward of the 1995 North Beach bar along the southern part of the North Beach nearshore (Fig. 2-6). The maximum erosion is 3 ft within a nan-ow band lying along the north- facing section of the north breakwater. Between 1987 and 1992 this area had been accretional (Fig. 2-7). 1987-1992 lake-bottom changes (Figure 2-7) Between 1987 and 1992, the North Beach/north breakwater nearshore was also net accretional. The accretion volume (+75,000 cu yds) exceeded the erosion volume (-19,800 cu yds) by a factor of 3.8. The net accretion totaled 55,200 cu yds (Table 2-1). Most accretion (between 4 and 5 ft) occured in a NNW-SSE trending band approximately 1000 ft offshore. This accretional band is associated with the 1992 location of the North Beach bar (see North Beach Bar) which extended southward from the WI-IL state line towards the NPM north breakwater. The second major accretional area in the 1987-1992 comparison occun-ed in a WNW-ESE trending band (300 to 900 ft in N-S width) in the southem part of the North Beach area. This accretion area extended from the 1987 0-ft LWD contour east-southeast toward the North Beach bar. This localized accretion resulted from entrapment of littoral drift by the NPM north breakwater, as well as probable sediment input related to construction of the north breakwater. Minimal accretion occun-ed lakeward of the 1992 location of North Beach bar. 32 Two major erosional areas are evident in the 1987-1992 comparison (Fig. 2-7). One is a broad area located landward of the 1992 North Beach bar and approximately 300 to 800 ft lakeward of the northem half of North Beach. Here, maximum erosion was between 3 and 4 ft. This erosion is assumed to be related to an adjustment of the lake bottom as a result of development of North Beach bar. The other major erosion area was a NW-SE trending nan-ow erosional zone along the lakeward side of the north breakwater. Here, erosion between 2 to 3 ft occun-ed approximately 70 ft northeast of the north breakwater. 33 64: 6« Figui Nod hem mapping 100 200 300 Fed 50 1^0 Metere RubUe-mound breakwiter* and revetments Cultural raatufM(1B92) lllinots Stale Plane coofdinatet Figure 2-5 1995 bathymetry of the North Beach/north breakwater nearshore. 34 IS ? z ■ g j a '■" a M Ul ; f o e C "5 (0 9> c o c !£ o to m ■c o Z (A 0) o> c (0 u E o x: o ja 0) to o> • CM a> a. ta E o (0 o. o (0 <9 CM e 35 o m 9 a o c o (D 0) o 2 « x: c E o o I 0) w CM o> a> I GO a> o a. ID E o m o. o I CM £ 3 36 Summary of 1987-1995 lake-bottom changes During the eight-year interval 1987-1995, the loci of erosion and deposition in the North Beach nearshore varied (Appendix B, Maps B-1 to B-5; Figs. 2-6 and 2-7). The major changes were related to repositioning of the North Beach bar. This is discussed further in the next section. What has been consistent through the eight years has been a trend of net accretion. Between 1987 and 1995, accretion was 86,200 cu yds (Table 2-1), which gives an average net accretion over these eight years of 10,800 cu yds/yr. The significance of this accretion volume is that it documents that the lake bottom updrift of the north breakwater has been a net accretional area since the marina was constructed. The north breakwater has acted as a partial barrier to littoral transport. The accretion volume provides data important in calculating a minimum estimate of the average annual supply of littoral sediment that crosses the WI-IL state line. This is done in a subsequent section discussing the sediment budget (see Regional Coastal Monitoring). One value of comparing two time intervals Is to identify any possible trends of increasing or decreasing rates of lake-bottom change. For these two comparisons (1987-1992 and 1992-1995), the trend has consistently been net accretion across this nearshore at a relatively constant rate of 10,800 cu yds/yr. Hi North Beach Bar Background and significance The North Beach bar is a submerged bar that extends from the WI-iL state line southward toward the NPM north breakwater (Figs. 2-1; 2-5; 2-8). The bar is a prominent depositional feature that was first documented In the ISGS bathymetric survey of 1990 (Appendix A; Map A-4) and has persisted to 1995. Several characteristics pertaining to the bar are: • The bar as a prominent lake-bottom feature was not present between 1987 and 1989, although in both 1987 and 1989 accretional areas just south of the state line may have been precursors of the bar (Appendix A; Map A-1 , Map A-3). • Since 1990, the bar has been defined as a shoal area above the 6-, 7-, or 9-ft LWD contours. • The bar has consistently been located at least 550 ft lakeward of its contemporaneous shoreline. • Between 1990 and 1995, the maximum crest elevation of the bar occurred in 1995 between 4 and 5 ft LWD. 37 The significance of the North Beach bar\Anthin the context of coastal management at the marina and state park is that: 1) of the littoral sediment that crossed the state line between 1990 and 1995 and accreted updrift of the NPM north breakwater, the greatest accretion thicknesses were associated with the bar; 2) the bar identifies part of a primary pathway of sediment transport, extending from the state line toward the NPM north breakwater and further southward toward the marina entrance; 3) the bar is a localized sand reservoir that could be mined for downdrift beach nourishment. Bathymetric changes (Figure 2-8) The North Beach bar has been a dynamic lake-bottom feature since it first developed in 1990. Figure 2-8 is a schematic diagram showing locations of the bar axis during 1990, 1991, 1992 and 1995. Table 2-2 summarizes morphologic characteristics of the bar. From 1990 to 1992, the axis of the bar moved progressively lakeward which appears to have facilitated eventual sediment bypassing around the NPM north breakwater between 1992 and 1995. By 1995, a significant change had occurred with the bar axis moving landward and developing a north-south orientation. Bathymetric data over the next few years will be needed to determine whether this change in location and orientation is a short-lived phenomenon, or whether it signifies major change in littoral sediment supply and/or nearshore dynamics. 38 -|- 2122000 N North Beach Bar North Beach Bar 100 200 300 Feet 100 Meters Bathymetnc contours in feet below Low Water Datum (LWD) -h Northern limit of bathymetnc survey ^^^ Shoreline swe Geological Rubble-mound breakwater survey Illinois State Plane coordinates Figure 2-8 Schematic illustration of the location of North Beach bar for the years 1 990, 1 991 , 1992, and 1995. Shown is the deepest lake-bottom closed contour that defines the outline of the bar and encloses the bar axis. 39 Table 2-2 Summary characteristics of the North Beach bar. 1990 1991 1992 1995 Distance offshore of North Beach (ft)^ 600 800 750 550 Proximity of southem tip to NPM north breakwater (ft) 100 150 200 250 Shallowest closed contour (ft)^ 6 8 8 5 Deepest closed contour (ft)^ 7 8-9' 9 6 Length (ft)* 1000 1000 1100 750 Area (sq yds) 10,700 17,100 15,200 14,300 Volume (cu yds)* 34.000 59,000 51,000 34,000 ^ Distance offshore is the average distance measured due east of the shoreline. ^ Contours are referenced to Low Water Datum (LWD). ' During 1991, most of the bar, with the excepton of the northern part, had an atypical plateau-like morphology. * Bar length is measured along the bar axis. ' Volumes are rounded to the nearest thousand cubic yards. II Volumetric analysis Annual sediment volumes were calculated for the North Beach bar by subtracting an inferred non-banred bathymetric profile from the barred bathymetric profile for each of the years of survey (1990, 1991, 1992, and 1995). Four east-west profiles were used (ISGS Profiles 2, 4, 6, and 8) to obtain representative bar cross-sectional areas along the length of the bar. Areas were measured using a digital planimeter. The cross-sectional areas were then Integrated along the north-south length of the bar to provide the bar volume for each year. Detemriining the bar volume was dependent on the boundaries chosen to define this bathymetric feature. The following boundaries were used: • North boundary: • West boundary: • South boundary: • East boundary: The WI-IL state line (note: the bar apparently continues north of the state line, thus these volume calculations apply only to the Illinois segment). A line along the axis of the trough on the landward side of the bar- trough pair (as seen on bathymetric profiles). Defined by the southem edge of the bar, as interpreted from bathymetric maps. Defined as the break in slope between the lakeward edge of the bar and the smooth lake bottom lying lakeward of the bar (as seen on bathymetric profiles). Of the four boundaries, this was the most subjective. 40 During the four years of measurement, the bar volume ranged between 34,000 and 59,000 cu yds. Peak volumes were attained in 1991 and 1992. By 1995, the bar volume had returned to that of 1990. These variations in bar volume are likely a natural response to changes in wave dimate, sediment supply, and lake level. During these four years of measurement, the average bar volume was 44,500 cu yds. IV Marina Entrance Background The marina entrance developed a well-defined accretional area soon after breakwater construction (Appendix B; Map B-1). When entrance shoaling was first recognized in 1988, some debate ensued conceming the source of the sediment. Bathymetric data collected by ISGS suggested that some or all of this sediment may have been derived from erosion of the fan delta on the south side of the marina. At its maximum extent in 1988, the emergent and submerged parts of the fan delta extended further lakeward than the north-south segment of the south breakwater. Northward transport of sediment from this discharge area by waves from the south and southeast could readily move sand into the sheltered marina entrance area. Comparison of ISGS bathymetric data collected along the south breakwater in 1988 and 1989 (Appendix B; Map B-2) show northward progression of an accretionary wedge along the south breakwater consistent with this premise of northward sediment transport. An altemate mechanism for the entrance shoaling was presented by Moffatt & Nichol Engineers (1990). They suggested that the sediment was derived from the considerable volumes of sand that were distributed as roadbed along the north and south breakwaters during construction. Wave action during and following construction would have allowed some of this roadbed material to leak out of the breakwaters and cause lake-bottom accretion at the marina entrance. Data coliection The ISGS began collecting limited profile data in the marina entrance area in 1989. This profiling consisted of a series of nine lines aligned between landmarics such as the ends of breakwaters or breakwater daymart^ers. The data collection focused on the area between the tips of the north and south breakwaters and the entrance to the commercial basin. However, one line extended from the north breakwater to the entrance to the recreational basin. Data from these surveys are incorporated in bathymetric maps A-3 through A-6 (Appendix A). Maps B-3 through B-5 (Appendix B) document 1989-1992 annual lake-bottom changes in the marina entrance. All nine marina-entrance survey lines were resurveyed in July 1995 to allow profile comparisons 41 with data from 1989 through 1992. During July and August 1995, maintenance dredging was undenA^ay in the approach channel to the recreational basin. Dredging was planned for the marina entrance area, but none occurred here in 1995. 1995 bathymetry (Figure 2-9) The 1995 bathymetry In the marina entrance (Fig. 2-9) documents the lake-bottom asymmetry between the ends of the north and south breakwaters. The 10 ft LWD contour defines a trough area extending from the marina entrance toward the commercial basin. This trough is likely a primary pathway for currents flowing into and out of the marina. Such currents develop when differences in water level occur between the open lake and the marina basin. 1992-1995 lake-bottom changes (Figure 2-10) Figure 2-10 is an isopach map identifying lake-bottom changes In the marina entrance between 1992 and 1995. Two accretional tends are significant. First, the area of maximum accretion (up to 6 ft) occurs about 200 ft northwest of the north end of the south breakwater. Second, an accretionary lobe extends around from the lakeward side of the north breakwater and turns Into the marina entrance. This latter feature indicates that, between 1992 and 1995, there was sediment bypass of the north breakwater, at least as far south as its southern tip. This accretional lobe is part of a narrow accretional zone that extends southward from the WI-IL state line (see Fig. 2-6). Thus, some of the sediment that accumulated within the marina entrance area between 1992 and 1995 was likely derived from updrift of the north breakwater and from north of the WI-IL state line. The only other time such an accretional lobe was documented from the state line to the marina entrance was between 1989 and 1990 (Appendix B; Map B-3). Detailed sediment transport studies are needed to conclusively verify that sediment from updrift of the marina has accumulated within the marina entrance. However, the overall configuration of the 1992-1995 isopach contour pattem is consistent with such a transport pathway. 1989-1995 lake-bottom changes (Figures 2-11A,B) The principal area of lake-bottom change in the marina entrance area between 1989 and 1995 occun-ed around the north end of the south breakwater. Figure 2-11 A shows profile data across the marina entrance between the ends of the north and south breakwaters. In 1987, prior to breakwater construction, the lake bottom sloped gently from about 1 1 ft LWD at what is now the tip of the south breakwater, to about 14.5 ft LWD at what is now the tip of the north breakwater. Subsequent shoaling and erosion resulted in development of a cross-sectional asymmetry with a broad shoal area on the western tvy«>thirds of the entrance and a nan-ow trough on the eastem third adjacent to the north breakwater. Maximum accretion in the entrance area occun-ed 42 between 1989 and 1990, then slowed between 1990 and 1991. The overall profile configuration has remained rather unifonm since 1991. As of 1995, the marina entrance trough had a maximum depth of about 15 ft LWD. This is about 0.5 ft below the base of the nearby north breakwater. It is unknown if the stability of the breakwater toe has been affected by this depression, but no shifting of breakwater stone can be seen above water. The trough axis has apparently assumed a stable position, but continued monitoring of this feature is wanranted. If the trough is caused by the deflection and channeling of currents along the west (marina basin) side of the north breakwater, then the depression would occur even if the shoal area adjacent to the south breakwater was absent. Altematively, it is possible that if the shoal area adjacent to the northem tip of the south breakwater is dredged, the increased cross-sectional area of the entrance would result in currents no longer being channeled along the northeastem side of the entrance. Deposition would then occur and reduce the depth of the trough adjacent to the north breakwater. In either case, continued monitoring is important since the lake bottom in this area is at or below the base elevation of the adjacent breakwater and could lead to undemriining of the breakwater. Figure 2-1 1 B shows profile comparisons running approximately north-south between the north end of the south breakwater and the middle daymarker of the north breakwater. Lateral accretion on the northward nose of the south breakwater shoal has persisted since 1989. Between 1992 and 1995, lateral accretion advanced the shoal northward by about 60 ft and was accompanied by up to 6 ft of vertical accretion. There has also been persistent lake-bottom accretion on the lake bottom in the northem half of this profile near the approach to the commercial basin. Between 1989 and 1995, accretion reduced depths from 13 ft LWD to about 11 ft LWD. Just under 1 ft of accretion occunred here between 1992 and 1995. Nearby, at the entrance to the commercial basin. 3 to 4 ft of accretion occurred between 1992 and 1995 (Fig. 2-10). The isopach map shown in Figure 2-10 was used to compute the volume of net accretion occuning in the marina entrance between 1992 and 1995. The limits for the volume computation are shown on the index map in Figure 2-11 A. For this three-year period, the net accretion within this area totaled 9,200 cu yds. This is an average annual net accretion of 3,100 cu yds/yr. 43 ■2120400 N 643300 E 644500 E 100 200 300 Feet 50 100 Meters + Rubble-mound breakwaters and revetments Illinois State Plane coordinates Bathymetric contours in feet below Low Water Datum (LWD) Contour interval 1 foot Illinois State Geological Survey Figure 2-9 1995 bathymetry in the North Point Marina entrance area. 44 North breakwater 2121500 N 644500 E -2120400 N 643300 E 644500 E 100 200 300 Feet 50 Contours in feet Contour interval 1 foot 100 Meters _ Rubble-mound brealcwaters and revetments -|- Illinois State Plane coordinates Fl-:-!-!-:-:':-] Lake-bottom erosion greater than 1 ft Illinois State Geological Survey Figure 2-10 Isopach map of 1992-1995 lake-bottom changes in the North Point Marina entrance area. The accretional lobe along the south and east side of the north breakwater is the southem part of a 1992-1995 accretional lobe that is continuous from the WI-IL state line (see Figure 2-6). 45 o E 3 «^ Q § o Q West (south breakwater) East (north breakwater) 2 - 6 - 8 - 5 o « 10 12 14 - 16 1987 (pre-construction) profile V.E. =20x 4CX3 350 300 250 200 150 1C» 50 Distance in feet from North Breakwater outer daymarker Dashed line shows limits for calculating 1992-1995 net accretion volume daymarker North breakwater South breakwater Figure 2-11 A Profile comparisons across the NPM marina entrance from 1989 through 1995. Profile line ains approximately west to east between the ends of the south and north breakwaters. Index map shows profile locations and limits of 1992-1995 volume calculation. 46 North (north breakwater) South (south breakwater) 100 200 300 400 500 Distance in feet from North Breakwater middle daymarker 1987 (pre-construction) profile 600 Figure 2-1 1B Profile comparisons across the inner part of the marina entrance from 1989 through 1995. Profile line mns approximately north to south, from the middle daymari^er of the north breakwater to the north end of the south breakwater. See inset map on Figure 2-1 1A for location. 47 Implications of accretion and erosion trends The bathymetric data for the marina entrance from 1989 to 1995 indicate the following: 1) Since the constmction of the brealo/»/aters, the marina entrance has been a sediment trap. Some lake-bottom erosion has occurred fomriing a trough along the inner side of the north brealcwater, but the net change has been accretion. The major area of accretion is located adjacent to the north end of the south breakwater. 2) Between 1991 and 1995, the lake-bottom profile between the ends of the two breakwaters did not change significantly. Thus along this profile an equilibrium configuration was achieved by 1991 which has persisted. Subsequent to 1991, sediment coming Into the marina entrance was deposited further inside the entrance area. 3) Comparison of the 1992 and 1995 bathymetric data indicates an accretional lobe extending down the lakeward side of the north breakwater and wrapping around the tip of the breakwater. This feature indicates that sand from updrift of the marina has bypassed the north breakwater and has a pathway into the marina entrance. If sediment transport persists along this pathway, shoaling in the marina entrance will persist. 4) Over the past three years (1992-1995) the average annual accretion solely in the marina entrance area has been just over 3,000 cu yds/yr. The need for future maintenance dredging in this area should be anticipated. This dredging will likely need to extend into the commercial basin. V South Breakwater Nearshore The south breakwater nearshore is here defined as the area lying between the southem tip of the north breakwater and the approximate mid-point in the north-south segment of the south breakwater (Fig. 2-1). The northem and southem boundaries are coincident with ISGS profile lines 17 and 32, respectively. 1995 bathymetry (Figure 2-12) The 1995 bathymetry of the south breakwater nearshore is shown in Figure 2-12. No nearshore bar is present. A prominent feature is a trough that extends from the marina entrance lakeward to at least the 16 ft LWD isobath. A similar feature most recently occun-ed in 1989 and 1990 (Appendix A; Maps A-3 and A-4). Depths within 100 ft of the south breakwater range from 6 to 10 ft LWD; the shallower depths occur adjacent to the north end of the breakwater at the marina entrance. A regular contour pattem lakeward to depths of about 30 ft LWD suggests a sandy lake bottom. However, an in-egular contour pattem occurs between the 21 and 27 ft LWD 48 isobaths, approximately 1000 ft lakeward of the marina entrance. Diver inspections in 1992 confirmed that an outcrop of compacted clay {i.e., glacial till) dominates this area (Ellen Marsden, Illinois Natural History Survey, pers. com). 1992-1995 lake-bottom changes (Figure 2-13) The south breakwater nearshore v^ras net erosional during the three-year interval 1992-1995 (Table 2-3). With the exception of one large accretional area, most of the 1995 lake bottom lies approximately 1 ft lower in elevation than the 1992 lake bottom. Development of the trough that extends lakeward from the marina entrance locally resulted In 4 ft of lake-bottom erosion. AccfBtion The most significant accretion on the 1992-1995 isopach map (Fig. 2-13) is a lobate accretional area having an axis running NNE-SSW. The orientation and configuration of this feature indicates an influx of littoral sediment from the north. Up to 350 ft In width, and with vertical accretion of up to 3 ft, this feature extends south-southwestward from a position approximately 500 ft lakeward of the south tip of the north breakwater to a position approximately 200 ft lakewanj of the southern part of the south breakwater. The feature is not apparent on the 1995 bathymetric map (Fig. 2-12) because most of the deposition occurred within a depression on the 1992 lake bottom. This accretion is separate from the previously discussed accretional lobe that extends along the north breakwater. The accretion possibly represents a deeper-water sediment transport pathway extending southward from the North Beach / north breakwater nearshore. Erosion The greatest amounts of erosion occur In patches along the lakeward side of the south breakwater, and in a zone extending east-southeastward from the marina entrance (see the Marina Entrance section). The lake bottom is primarily erosional landward and lakeward of the lobate accretional feature discussed above, with 4 ft maximum erosion occurring along the trough leading lakeward from the marina entrance. 49 £ o CO k. CO c "5 (0 o o 10 a> CM & 3 50 ■2119200N 644000E UmmSauGtUafalSunf UmK o( data eompatiaon Rubbl^-nKMMKl braakwatsr -\- Hlinoit Stat* Ptana coonttnataa •;•!•:■!•:•:■: 1 Lake-bottom arosion greater than 1 ft -2121000 N 645500 8 100 200 300 Feet SO 100 Meter* Contour* In feet Positiva numbere mdicata accretion Negative numbers indicate erosion Contour interval 1 foot Figure 2-13 Isopach map of 1992-1995 lake-bottom changes in the south breakwater nearshore. 51 1987-1992 lake-bottom changes The south breakwater nearshore had net accretion between 1987 and 1992 (Table 2-3). Accretion occurred primarily along the length of the south breakwater and adjacent to the southern tip of the north breakwater. Most accretion occunred between 1988 and 1989 along the south breakwater (Appendix B; Map B-2) . This accretion is interpreted as the result of southerly waves and wave-induced currents moving sediment northward from the fan delta. This accretion along the south breakwater became subject to major erosion between 1989 and 1990 (Appendix B; Map B-3). Table 2-3 Summary of erosion and accretion volumes in the south breakwater nearshore.^ 1987-88 1988-89 1989-90 1990-91 1991-92 1992-95^ Accretion (+) (cu yds) 10.300 54,800 15,200 22,000 14,600 9,100 Erosion (-) (cuyds) 29,400 16.900 39,600 10,000 11,700 28,900 1 Annual net change (cu yds/yr)^ -19.100 +37.900 -24,400 +12,000 +2,900 -6,600 Normalized annual net change (cu vds/yr/shoreline ft)^ -13 +25 -16 +8 +2 -4 ^ All volumes are computed for lake-bottom elevation changes in excess of 1 ft and occurring below Low Water Datum (LWD). Volumes are rounded to the nearest 100 cu yds. ^Three-year comparison; erosion and accretion volumes are for the three-year summation and annual net change is a three-year average. Total net change from 1992-1995 equals -19,800 cu yds. ' Net accretion is indicated by a positive number and net erosion is indicated by a negative number. * Shoreline distance (1500 ft) is based on measurement along a north-south line bounded to the north and south by the defined limits of the nearshore reach. Numbers are rounded to the nearest whole number. Net accretion is indicated by a positive number and net erosion is indicated by a negative number. Summary of 1987-1995 nearshore bathymetric change The net. eight-year lake-bottom change lakeward of the south breakwater between 1987 and 1995 amounts to net erosion of 10,500 cu yds (Table 2-3). Between 1987 and 1992, the annual net change varied from year to year because of gains and losses associated with the supply and transport of sediment from the fan delta. Between 1992 and 1995 this nearshore became net erosional. The development of the 7,200 cu yd accretional lobe between 1992 and 1995 was not of sufficient volume to outweigh the total erosion. Most of the construction-related accretion between 1987 and 1989 occun-ed between the 52 breakwater and about 400 ft lakeward of the breakwater (Appendix B; Maps B-1, B-2). Within this band, which generally lies landward of the 1995 12-ft LWD contour, the 1995 bathymetry indicates that the bottom elevations are now 1 to 4 ft shallower than the 1987 pre-construction lake bottom. Thus some of the construction-related accretion still resides along this zone. In contrast, lakeward of the 1995 12-ft contour, much of the 1995 lake bottom generally lies 1 ft lower than the 1987 lake bottom. VI South Parki ng Area Nearshore The south parking area nearshore is here defined as the area lying between the approximate mid-point of the north-south segment of the NPM south breakwater, and a line projected east of the east-west trending section of Dead Dog Creek located about 2000 ft to the south (Fig. 2-1). These northern and southem limits con-espond to ISGS profiles 32 and 52 respectively. These limits were chosen in consideration of a proposal to constmct a submerged offshore breakwater (or reef) to protect the south parking area (Appendix D). The southem part of this nearshore reach crosses south of the marina/state park boundary. Figure 2-14 shows the 1995 bathymetry in the south parking area nearshore. Figure 2-15 is an Isopach map showing lake-bottom changes based on a comparison of 1992 and 1995 bathymetry. 1995 bathymetry (Figure 2-14) The two prominent features on the lake bottom off the south parking area are the submerged riprap that extends for about 600 ft southward from the south breakwater, and an enclosed trough that occurs between the submerged riprap and the shoreline (Fig. 2-14). Minimum depths across the submerged riprap in 1995 ranged from 5 to 3 ft LWD; a 9 ft LWD contour defines the enclosed trough. Characteristics of the submerged riprap are discussed in a subsequent section. Other than the area of the submerged riprap, the lake bottom opposite the south partying area is mostly featureless to at least 20 ft LWD, and contours are generally parallel to the shoreline. An irregular contour pattern lakeward of the 22 ft LWD contour suggests exposed clay {i.e., glacial till). The steepest lake-bottom slopes occur adjacent to the shoreline and along the margins of the submerged riprap. 1992-1995 lake-bottom changes (Figure 2-15) The south parking area nearshore was net erosional between 1992 and 1995 (Fig. 2-15). Between the shoreline and the 12-ft LWD contour, which generally lies within 400 ft of the shoreline, erosion lowered the lake bottom at an average rate of approximately 0.25 ft/yr. Approximately 80 percent of the 1 995 lake bottom lies at an elevation below that of the 1 992 lake bottom. The maximum erosion, up to 5 ft, occun-ed within 200 ft of the shoreline. 53 Accretion Accretion in excess of 1 ft was confined to a nan'ow strip of nearshore within 100 ft of the shoreline of the IBSP-North Unit beach nourishment site, located at the south end of the south paricing area. Accretion was the direct result of beach nourishment during September 1994 and July 1995. Most of the accretion is attributed to placement of the 1995 nourishment, since the area was mapped during and soon after placing the nourishment and before the material could be redistributed by waves. Minor accretion occun'ed in the small embayment where the south breakwater joins the riprap shore defense along the south parting area. This accretion during summer 1995 resulted from accumulation of material that was being dredged from the NPM entrance and being dumped, via slurry pipe, at the south end of the south breakwater. By September 1995, all subaerial expression of this feature had been removed by wave erosion. Erosion Erosion was pervasive along this 2000 ft stretch of nearshore. The most severe erosion occunred in a nan^>w swath landward of the 10 ft LWD isobath and within approximately 300 ft of the 1995 shoreline. Most lake-bottom erosion was on the order of 1 to 2 ft (Fig. 2-15) with a localized maximum of 5 ft. This erosion maximum occurred along the shallow nearshore adjacent to the concrete cube and riprap revetment that protects the south parking area. Erosion also occurred farther offshore along the line of submerged riprap (see Submerged Riprap). 1987-1992 lake-bottom changes The south paridng area nearshore between 1987 and 1992 had net accretion (Table 2-4). Most of the nearshore accretion occurred between 1987 and 1989 (Appendix B, Maps B-1 and B-2) and was the direct result of building the fan delta (244,500 cu yds). However, some of this accretion (54,200 cu yds; Table 2-4) is now located landward of the south parking area's concrete cube and riprap revetment that was installed between October 1989 and December 1990. This 54,200 cu yds has thus been "locked up" and unavailable to the nearshore system since 1989-1990. Thus, while 229,600 cu yds of net nearshore accretion occurred between 1987 and 1989, only 175,400 cu yds of this material remained accessible to any subsequent erosion. In comparing rates of change in the nearshore, the latter volume is a more accurate representation of change during the 1987-1989 period. Using the adjusted volume, the net accretion between 1987 and 1992 amounted to 6,800 cu yds. Erosion volumes were greatest between 1989 and 1990 when the supply of dredge material had ceased and the oversteepened and unstable beach and nearshore started to erode rapidly. Erosion has continued since 1990, but at decreasing annual rates (Table 2-4). 54 North Point Marina recreational basin 644000 E Joins Figure 2-24 646000 E _i_ 100 200 300 Feet 2117300 N 21 17300 N- 50 100 Meters Wmw stale Geologicai Survey Bathymetric contours in feet below Low Water Datum (LWD) Contour interval 1 foot Shoreline (July 1995) Rubble-mound breakwater Concrete cube and riprap revetment Cultural features (1992) Illinois State Plane coordinates Figure 2-14 1995 bathymetry of the south parking area nearshore. 55 North Point Marina recreational basin 2119200 N- 64S500E — — •— ~-— 1992 shoreline 1992 0nLWDisot>ath —————— 199S shoreline — 1995 OnLWD Isobath — Limit of data comparison Rubble-mound breakwater Concrete cube and riprap revetment Cultural features (1992) Illinois State Plane coordinates 1 Lake-bottom erosion J greater than 1 ft 100 200 300 Feet 100 Meters Contours in feet Positive numtwrs indicate accretion Negative numbers indicate erosion Contour interval 1 foot 644000 E Joins Figure 2-25 2117300 N Figure 2-15 Isopach map of 1992-1995 lake-bottom changes in the south parking area nearshore. 56 Table 2-4 Summary of erosion and accretion volumes in the south parking area nearshore.^ 1987-88 1988-89 1989-90 1990-91 1991-92 1992-952 Accretion (+) (cu yds) 69,200= 122,100 121,100= 122,400 400 1,300 600 1,500 Erosion (-) (cu yds) 12,000 2,900 105,200 33,900 31,800 33,300 Annual net Change (cu yds/yr)^ +57,200= +110,100 +118,200= +119,500 -104.800 -32,600 -31,200 -10,600 Normalized annual net change (cu yds/yr/ shoreline ft)^ +29= +55 +59= +60 -52 -16 -16 -5 ^ All volumes are computed for lake-bottom elevation changes in excess of 1 ft and occurring belovi/ Lov»^ Water Datum (LWD). Volumes are rounded to the nearest 100 cu yds. ^ Three-year comparison; erosion and accretion volumes are for the three-year summation and annual net change is a three-year average. Total net change for 1992-1995 equals -31,800. ' Net accretion is indicated by a positive number and net erosion is indicated by a negative number. * Shoreline distance (2000 ft) is based on measurement along a north-south line bounded to the north and south by the defined limits of the nearshore reach. Numbers are rounded to the nearest v^fhoie number. Net accretion is indicated by a positive numt)er and net erosion is indicated by a negative number. * Volume excludes 54,200 cu yds of sediment now lying beneath the south parking area landward of the cube revetment and therefore unavailable to the nearshore. Summary of 1987-1995 nearshore bathymetric change The net, eight-year, lake-bottom change between 1987 and 1995 amounted to -25,000 cu yds. indicating an erosional nearshore. This number excludes the 54.200 cu yds of material now trapped behind the south parking area revetment and located below ft LWD, as discussed above. Net accretion occun-ed between 1987 and 1989, but since 1989, net erosion has dominated. The transition from an accretional to an erosional nearshore resulted from termination of dredge disposal operations at the fan delta In 1988. Addition of beach nourishment at IBSP-North Unit since 1990 has not been sufficient to counteract nearshore erosional losses. An important observation lakeward of the south paricing area is that the 1995 lake bottom in this area between the 5 and 12 ft LWD contours (100-400 ft offshore) lies 2 to 6 ft deeper than it did in 1987. The average deepening has been 2 to 3 ft. Thus, erosion has not only removed all 57 sediment derived from the fan delta, but it has also lowered the lake bottom below the elevation that existed prior to marina construction. Lakeward of the 12 ft LWD contour, the 1995 lake bottom lies about 1 ft deeper than it was in 1987. The lake-bottom lakeward of the south partying area has had the most severe lake-bottom erosion observed anywhere in the nearshore adjacent to the marina. However, the erosion to the south, in the North Unit nearshore, is even more severe (see IBSP North Unit, Spring Bluff nearshore). VII Submerged Riprap Background The name "submerged riprap" refers to approximately 600 ft of submerged rock that was originally placed as above-water shore protection along the lakeward edge of the fan delta between late autumn 1988 and earty winter 1989. This riprap subsequently was undermined and eventually became submerged. The undemnining was caused by wave-induced erosion of sand lakeward of and beneath the riprap. The location of the submerged riprap relative to the NPM facility is shown in Figure 2-1. The 1995 bathymetry at the submerged riprap is shown in Figure 2-14. In general, the submerged riprap forms a broad arc extending southward from the elbow in the south breakwater. The submerged riprap Is important because it has significantly influenced the lake-bottom morphology lakeward of the northern half of the south parking area. In addition, the submerged riprap will lie beneath a segment of a proposed submerged reef which is part of planned shore protection for the south paricing area (Appendix D; Patrick Engineering, 1993, Exhibit V-5; 1995, Sheet C-1). The source of the riprap material was excess breakwater and revetment stone left over from marina constmction, demolition debris from the construction site, and other miscellaneous stone, brick, and debris from the site. This material was dumped along the lakeward flank of the fan delta from the wateriine up to the top surface of the sand pile which was then about four to six feet above lake level. Originally, no stone was placed below the water line. The southern limit of the riprap was determined by the amount of available stone and debris. Placement of the riprap was intended to halt the rapid recession of the shoreline that was occuning in fall 1988 along the northem edge of the fan delta. Although the riprap temporarily halted shoreline recession, the riprap rapidly shifted and subsided as erosion continued to remove underiying sand across the submerged part of the fan delta. The riprap was entirely submerged within two years of its placement. Comparison of the photographs in Figures 2-16 and 2-17 shows the amount of shifting and settling that occun-ed over one winter (winter of 1989-90). In the discussion of coastal monitoring at NPM between 1987 and 1992 in Part 1, Figure 1-5 illustrates the shore erosion pattem that occun-ed south (downdrift) of the southern end of the 58 illustrates the shore erosion pattern that occun-ed south (downdrift) of the southern end of the riprap while the stone was still emergent during 1989. Figure 1-6 shows the erosion that occun-ed landward of the riprap during 1990 once the stone subsided sufficiently for waves to overtop it. Bathymetric conditions 1990 - 1992 By summer 1990, some sections of the riprap had subsided sufficiently such that a bathymetric survey could be conducted with a shallow-draft boat crossing over the submerged stone along Its southern part (Appendix A; Map A-4). Subsequent surveys in 1991 and 1992 could cross along all five lines of the ISGS survey net that intercept the riprap area (Profiles 37 through 41). These surveys documented continued lowering of the riprap and the erosion of the adjacent lake- bottom. As of 1992, minimum depth over the submerged riprap was to 2 ft LWD adjacent to the south breakwater. The submerged flank of the south breakwater supported the riprap and limited the degree of riprap subsidence in this area. Toward the south, away from breakwater support, the minimum depth contour was 3 ft LWD (Appendix A; Map A-6). An enclosed depression with a maximum depth contour of 7 ft LWD was located between the submerged riprap and the shoreline. On the east (lakeward) side of the submerged riprap, the 1992 survey documented a rather steep profile descending from 5 ft LWD to 10 ft LWD in a minimum distance of 65 ft (1:13 slope). Bathymetric conditions 1995 As of summer 1995, the lake-bottom depression on the landward side of the submerged riprap had increased to a maximum depth between 9 and 10 ft LWD (Fig. 2-14). The depression thus deepened by at least 2 ft between 1992 and 1995. On the east side of the riprap, the 1995 lake bottom descended from 5 ft LWD to 10 ft LWD at an average slope of about 1:16. This reduction in slope since 1992 is related to a decrease in the elevation of the submerged riprap. Adjacent to the south breakwater, the minimum depth recorded across the submerged riprap remained at to 2 ft LWD, the same as recorded in 1992. Here the riprap has apparently subsided as far as possible over the submerged flank of the south breakwater. Away from the Influence of the south breakwater, the minimum-depth contour Is generally 5 ft LWD, although a contour of 3 ft LWD persists along the southern part of the submerged stone. The area enclosed by the 5 ft LWD contour has had 2 ft of subsidence since 1992. The area of 3-ft minimum depth had a similar depth in 1992 and thus represents a localized area of stability. Other than the two stable areas, the 1992-1995 comparison generally documents continued subsidence over this three- year Interval. 59 4>^:ji^ J*-' Figure 2-16 View of the riprap looking south from the south breakwater in October 1989. Note the variety of stone sizes and shapes. For comparison with the photograph in Figure 2-17, note the position and height above water of the concrete block supporting an upright steel pole, at left center with black arrow (Photo date October 4, 1989). ■^"j-. Figure 2-17 Same view of the riprap six months later in April 1990 showing the degree of riprap subsidence and landward erosion (Photo date April 13, 1990). 60 Magnitudes and rates of riprap settling Comparison of the bathymetric profile data across the submerged riprap for 1991, 1992. and 1995 documents the magnitudes and rates of settling of the riprap and erosion of the adjacent lake bottom. As previously noted, data exist for five profile lines across the riprap (ISGS profiles 37 through 41; Appendix C). There is some uncertainty in the measurements of riprap settling made from profile comparisons because slight differences in position of the survey boat in different years can cause measurements to be made to different rocks or different parts of the same rock. These profiles also include the bathymetric data for 1990 which was collected prior to the riprap subsiding below water level. Along all profile lines, during the four years 1991-1995, the crest elevation of the riprap settled between 1 and 3 ft. This gives an average annual rate of settling of 0.5 ft/yr. Along two of the five lines (profiles 37 and 39), the annual rate of settling from 1992 to 1995 slowed compared to 1991-1992. Along two of the lines (profiles 38 and 41), the rate of change from 1992 to 1995 increased compared to 1991-1992. Thus, varied conditions appear to occur with different parts of the riprap showing either decreasing or Increasing rates of settling. Whether these trends are real or an artifact of survey-boat positioning and slight inaccuracies in the data collection is uncertain. The important observation is that most of the riprap continues to subside. As of 1995, this riprap had yet to reach an equilibrium lake-bottom elevation. The settling of the submerged riprap does not involve the stone sinking into the lake-bottom sediment. The settling is a response to a widespread lowering of the lake bottom caused by the net erosion of lake-bottom sand (see South ParWng Area Nearshore; Fig. 2-15). As of 1995, the base of the submerged riprap is at about 8 to 9 ft LWD. Considering past changes across the nearshore at the fan delta, erosion has been active to at least 15 ft LWD. Thus, if the erosion process continues, the submerged riprap has the potential to subside several more feet. Influence of the submerged riprap on lake-bottom changes Comparison of bathymetric data from 1989 through 1995 shows that the nearshore adjacent to the south breakwater and south partying area continues to deepen. Continued lakebed erosion is maribta-mound breakwateis and matmants Cultural features (1992) Concrete cutw and riprap revetment Illinois State Plarie coordinates mnii SMa Gwlogial Suney 643500 E — [— 2117800 N +- Figure 2-19 Isopach map of the NPM south parking area showing thickness of sand (and gravel) beneath the parking area that was derived from dredging the marina basin in 1987-1988. 67 X Breakwater Conditions Engineering background The north and south outer breakwaters are classified as "rubble-mound" breakwaters consisting of alternate layers of different sized quarry stone. The surficial stone of the breakwaters consists of 2- to 10-ton dolomite quany blocks. Although the north and south breakwaters superficially appear to be of uniform design, a total of ten different cross-sectional designs (A through H, J and K) occur along twelve different segments of the breakwaters. These different designs consist of variations in stone size, crest elevation, and toe characteristics. Variations were engineered for the anticipated maximum wave energies that could impact different segments of the breakwaters. The layer of base stones for the north and south breakwaters rests on either the beach or lake bottom that existed at the time of constmction in 1 987 and 1 988. For the north breakwater, the landward end is built on former beach which had a maximum elevation of about + 7 ft LWD. The base elevation progressively decreases eastward to a maximum depth of about 14.5 ft LWD at the lakeward end of the breakwater near the marina entrance. The landward end of the south breakwater is also built on the 1987 beach which had a maximum elevation of about + 7 ft LWD. The base elevation decreases eastward to a maximum of about 11 to 12 ft LWD along the north- south segment of the south breakwater just south of the marina entrance. For most segments of both breakwaters, the design crest elevation is +11.9 ft LWD (590 ft MSL 1929). The exception is the middle section of the south breakwater which has a design elevation of +13.9 ft LWD (592 ft MSL 1929). Constmction of the outer breakwaters began in spring 1987 and was completed by autumn 1988. Thus, as of summer 1995, these stmctures are 7 to 8 years old. In a time frame of one or more decades, shifting and settling of stone can be expected for such massive stmctures. However, the observations discussed below suggest that some shifting and settling has occun-ed in excess of that expected for stmctures of this age. North breakwater The possibility of lake-bottom erosion along the base of the north breakwater was noted even before the breakwaters were constmcted, and is discussed by Moffatt & Nichol, Engineers (1986, p. 86-87) in a report on potential coastal impacts near the marina. The reason for concem is that the north breakwater is subject to the direct impact of northeasteriy waves which are the highest- energy waves along this coast. Two primary factors are potentially capable of causing scour along the toe of the north breakwater. These are: 68 1) Incident and breakwater-reflected waves could increase shear stresses on the lakebed and thereby induce erosion. 2) Increased cun^nt velocities could result when northeasterly waves induce set-up in the North Beach nearshore area, and the resultant southward flow of water around the breakwater occurs at velocities capable of eroding the lake bottom. Significant lake-bottom erosion adjacent to the north breakwater may undermine the breakwater and cause differential settling of the breakwater stone. Documented lake-bottom erosion adjacent to the north breakwater Bathymetric data indicate the development of an elongate lake-bottom depression generally trending NW-SE along the northeast-facing lakeward side of the north breakwater. First documented in 1988, this feature can be seen in the bathymetric maps included in Appendix A (Maps A-2, A-4, A-5, and A-6) and in the 1995 bathymetry along the north breakwater (Fig. 2-5). Table 2-5 summarizes the lake-bottom changes recorded from 1988 through 1995. Table 2-5 Summary characteristics of the lake-bottom depression adjacent to the north breakwater (1988-1995). 1 1988 1989 1990 1991 1992 1995 Distance from north breakwater (ft) ^ 150 200 70 100 90 40 Maximum depth of depression (ft LWD) ^ 16 15 13 16 16 11 Minimum closed contour depth (ft LWD) ^ 15 n/a^ 13 16 15^ n/a« Depression relief (ft) ^ 2 1 2 3 6 2 Depression length (ft) ^ 280 300 600 650 650 850 Reference Appendix map (or Figure) A-2 A-3 A-4 A-5 A-6 (2-5) ^ Distance is measured orthogonal to the lakeward face of the breakwater from the waterline. ^ Depth is based on the deepest contour that defines the depression. The north breakwater base elevation adjacent to the depression lies between 1 and 1 3.5 ft LWD. ' Depth is that of the shallowest closed contour that completely defines the depression. * Difference in elevation between the base of the depression and the lake-bottom immediately to the north and south. * Length of the depression is measured parallel to the lakeward face of the north breakwater. ® Depression is a southeastward-opening trough without a contained closed contour. 1 ^ Depression is part of a larger, southeastward-opening trough. 1 The lake-bottom depression adjacent to the north breakwater is a dynamic feature. The general NW-SE orientation of the feature has been fairty constant, but the depression has varied in width, length, maximum depth, and distance lakeward of the breakwater. Between 1988 and 1995, 69 the depression grew in length from 280 ft to 850 ft (Table 2-5) and moved closer to the north breakwater. The base elevation of the north breakwater in the vicinity of the depression is 10 to 13.5 ft LWD. When the depression reached its maximum recorded depth of about 16.5 ft LWD in 1988, 1991, and 1992, It was up to 3.5 ft below the base elevation of the nearby segment of breakwater. Figure 2-20 shows a cross section of the north breakwater and the adjacent depression for the six years that the depression appeared on bathymetric data. This particular line of section illustrates the worst-case conditions in 1991 and 1992 when the base of the depression was 3.5 ft below the base of the breakwater and within 60 ft of the breakwater. 1990 diver Inspection In a cooperative effort with the ISGS in August 1990, U.S. Geological Survey (USGS) divers made a reconnaissance SCUBA inspection of the lakeward edge of the extended toe of the north breakwater to detemiine if erosion was causing stone instability (Circe and Blackwood, 1991). Near the inner daymarker, they observed evidence of voids beneath some of the breakwater stone suggesting a winnowing of sand. Appendix E contains text and an illustration from this diver survey. f 995 observations No above-water evidence of stone shifting or settling along the north breakwater was apparent at any time during ISGS field studies at NPM through 1992. During field work in summer 1995, however, an apparent sag In the breakwater crest was observed along the northeast-facing section between the inner and outer daymarkers. The location of the sag corresponds with the segment of the breakwater that is adjacent to the lake-bottom depression previously discussed. Figures 2-21 A and 2-21 B are views from the south breakwater looking northward to the north breakwater. Figure 2-21 A provides an overview; Figure 2-21 B is a closer view centered on the area of maximum sag. In this closer view, the photo has been composed to use the distant lake- level horizon as a reference. This horizon is visible in the area of sag, but is below the crest of the breakwater on either side of the sag. An additional, less extensive area of possible differential settling occurs to the right (eastward) of these two views. The amount of apparent vertical settling on the north breakwater Is unknown at this time. Data were not collected as of this report to determine differences between existing crest elevations and those at the completion of breakwater construction. Also, no eariy post-construction photographs have been recovered to make photographic comparisons with present conditions. A visual estimate is that the maximum displacement in the central sag Is 1.5 to 2 ft. To accurately detemiine the degree and extent of settling, a detailed survey is recommended to compare existing breakwater elevations with eariy post-construction data. 70 O IT) O up c o w w Q. 0) ■o E o I (D 8 (0 (0 0) ■D C (0 k. fl) "5 (0 £ ja jC r o c (0 c •c (0 E 0) CM S 3 il 71 Figure 2-21 A View looking north toward tlie north breakwater "sag" from the north end of the south breakwater. The extent of a sag in the crest can be seen by comparing the breakwater crest against the horizon. The sag is located between the inner and outer daymarkers (not visible in the photo) (Photo date July 18, 1995). Figure 2-21 B Closer view of north breakwater "sag" looking north from the south breakwater. Photo is centered on the area of sag. The extent of sag is indicated by the horizon that can be seen in the distance where the crest elevation has decreased. Note two adult persons at left for scale (Photo date July 18, 1995). 72 South breakwater The middle section of the south breakwater was constructed with a crest elevation 2 ft higher than both the more northern and southem sections {i.e., 592 ft MSL compared to 590 ft MSL). Thus, even if the breakwater has not settled, there is a perception of differential settling on either side of this higher segment. However, some settling of stone has been suspected since at least 1992 along a segment at the southem end of the breakwater (landward of the inner daymarker; C. Price, NPM Harbomnaster, pers. comm.). f 995 observations Figure 2-22 shows an area of discontinuity in crest elevation at a location about 360 ft north of the inner daymarker. Pipe for the ongoing hydraulic dredging of the marina is visible in the photo and provides a useful indicator of in-egularities in the breakwater crest elevation. This particular site of stone displacement is the most severe observed anywhere on the south breakwater. Estimated settlement is 1 to 1.5 ft. Figure 2-22 View showing localized subsidence of capstones along the middle section of the south breakwater. Discharge pipe for hydraulic dredging lies along the breakwater crest. This sag is located about 360 ft north of the inner daymarker which is visible in the distance. Note colored 1-ft increment bands on the prism pole for scale (Photo date July 18, 1995). 73 Possible causes of breakwater differential settling Additional observations and survey data are needed before the magnitude and causes of any differential settling can be determined. Where apparent settling has occurred, the stone along both the lakeward and landward sides of both the north and south breakwaters shows no apparent outward displacement. Thus, the settling is unifomi across the width of the breakwater. The possible factors contributing to the settlement are: 1) compaction of sediment beneath the breakwaters; 2) shifting, settling, and/or winnowing of quarry-run stone at the base of the breakwaters; or 3) winnowing of lakebed sand from beneath the breakwaters. A cause and effect relationship may exist between the apparent sag in the north breakwater and the nearby lake-bottom depression. Since the depression has at times had a maximum depth below the base of the breakwater, the depression may have been associated with processes that removed sand from beneath the breakwater. 74 ILLINOIS BEACH STATE PARK/ NORTH UNIT General Statement Studies in 1995 within the limits of the North Unit of Illinois Beach State Park (IBSP) focused on its northern part. The area studied consisted of approximately 3000 ft of shore between the NPM south partying area and the headland formed by the steel-sheetpile shore defense at the north end of Camp Logan. This segment of the North Unit was also studied by ISGS between 1987 and 1992 as part of annual bathymetric mapping in the vicinity of NPM. Three geographic names are used in this discussion of the IBSP North Unit. These names were adapted by ISGS to assist In identifying geographic areas and are defined as follows: North Unit nourishment stockpile: This refers to the beach area immediately south of the NPM south parWng area where sand has been placed for beach nourishment since 1990. The nourishment stockpile lies between the south parking area and the mouth of Dead Dog Creek (Fig. 2-1). Spring Bluff beach: This refers to the beach area extending from the mouth of Dead Dog Creek to the north side of the Camp Logan headland. The name is derived from the former community of Spring Bluff that existed along this lakefront area prior to the 1980s. Camp Logan headland: This refers to the sheetpile-protected shoreline protrusion at Camp Logan. The name does not refer to a specific place along this shoreline promontory, but is a general name for the area. 1995 studies at IBSP North Unit are discussed under the following headings: I Dispersion of 1994 Beach Nourishment II Monitoring of 1995 Beach Nourishment III Spring Bluff Nearshore IV Spring Bluff Shoreline Changes Since Marina Construction V Model for Spring Bluff Shoreline Recession I Dispersion of 1994 Beach Nourishment In September 1994, 32,000 cu yds of new sediment was added to the North Unit nourishment stockpile. This 1994 nourishment was a moderately sorted, angular, fine gravel ("pea gravel") from an inland quany. Based on visual comparison with grain-size charts, the grain size ranged from 2 to 15 mm. The median grain size was approximately 6 mm. This nourishment was 75 coarser than the medium to coarse sand previously supplied to the nourishment stockpile. Most of that pre-existing sand (and gravel) came from the 1987-1988 dredging of the marina basin, but some v^as also supplied in 1990 from dredging at Prairie Hartjor Yacht Club on the Wisconsin side of the WI-IL state line. The distinctive size and angularity of the 1994 nourishment gravel allows it to be readily identified along the downdrift beaches. In July 1995, the downdrift leading edge of this material could be identified 2800 ft to the south at the Camp Logan headland. Here the pea gravel dominated much of the beach. The steel-sheetpile on the north side of the headland had acted as a partial barrier to southward transport of this material. Some pea gravel was present as a progressively decreasing fraction of the beach sand population at sites to the south of the Camp Logan headland, but the bulk of the pea gravel was on the updrift side of the structure. Because the headland did not totally block southward transport of the gravel, and because the dispersion of the gravel in the nearshore was not monitored, the downdrift leading edge of the gravel on the beach represents a minimum dispersion distance. The downdrift dispersion distance results in a minimum transport rate of 2800 flin 10 months or 3350 ft/yr (0.6 mi/yr) for the pea gravel. This is a useful reference number for ongoing nourishment projects as a means of estimating expected rates of dispersion. This minimum rate of 0.6 mi/yr applies to moderately sorted pea gravel moving along the beach by swash and backwash; transport rates may be higher in the shallow nearshore in the zone of plunging waves. Finer-grained nourishment, such as that supplied to the North and South Units in July 1995, would be expected to move downdrift at a rate exceeding 0.6 mi/yr. II. Monitoring of 1995 Beach Nourishment Nourishment characteristics and emplacement Between July 15 and July 27, 1995, new nourishment was added to the North Unit stockpile. This sediment was a moderately to pooriy sorted fine to medium sand. Granules and pebbles comprised no more than 10 percent of the sediment. Some clay-silt clasts were present as a minor component. The total volume added was 20,000 cu yds. The nourishment was placed such that the contribution was about 40 cu yds per shoreline foot of the nourishment stockpile. Based on a comparison of ISGS profile surveys before and after the nourishment was supplied, the 1995 nourishment increased the stockpile area from 8200 sq yds (June 1995) to 11.400 sq yds (July 1995). The nourishment material was obtained from storage stockpiles at the Commonwealth Edison Waukegan Generating Station (Fig. 1-2). The sediment was originally derived from maintenance 76 dredging of that facility's intake and discharge channels and cooling basin prior to spring 1995. The material was tmcked to the North Unit nourishment site and dumped along the lakeward flanks of the existing stockpile. The final stage of distributing the new material involved grading the top surface to an average slope of approximately 1:14 so that the lakeward edge of the stockpile ranged from 1 to 4 ft above lake level. This grading was done in part for public safety to minimize high scarps along the lakeward edge of the stockpile. Monitoring scheme In June 1995. ISGS established a monitoring scheme consisting of a series of four east-west profiles at 100-ft spadngs across the existing stockpile v^ere the new material was to be added. These lines were extended into the shallow nearshore. Survey points were also collected around the perimeter of the stockpile and along the scarp crest for comparison with future scarp positions. The June 1995 survey provided documentation of the topography and geometry prior to the 1995 nourishment. Surveys were repeated in late July 1995 to document conditions immediately after the nourishment operations. Subsequent surveys were done on a monthly basis through November 1995. Monitoring observations (July - November 1995) As the nourishment was supplied and distributed, some regrading of the pre-existing nourishment stockpile occurred. Lakeward displacement of the local shoreline thus resulted from both the addition of nourishment and the "skimming or of some pre-existing sediment that was pushed lakeward. As a result of the regrading. the southern part of stockpile was lowered by 0.5 to 2 ft compared to its elevation prior to the addition of the July 1995 nourishment. Erosion along the lakeward margin of the stockpile began immediately following completion of the nourishment operations. The resulting scarp and shoreline recession were minimal, however, compared to the changes that occun-ed during a storm of September 7 and 8 which was the first major storm event to impact this new stockpile. Nearshore waves reached heights of 6 to 8 ft. The two-day duration of high waves exacertaated the shore erosion. Through the remainder of 1995, no stomi occurred with comparable wave energy. Data collected during the September 7-8 storm event document the erosional impact of this single event. Appendix F shows data for the four profiles across the stockpile between late June and eariy November 1995. Along all profiles, most of the landward shift of the nourishment profile occun-ed during the storm of September 7-8. The September 8 scarp position was located 40 to 70 ft landward of its location on July 28. Figure 2-23 shows map-view positions of the scarp crest prior to and following the addition of the 1 995 nourishment. By September 8, much of the 77 scarp crest had retreated landward to its approximate location prior to the addition of the 1995 nourishment. Just north of Profile 48, severe erosion brought the scarp crest landward of the pre-nourishment position. The profiles in Appendix F indicate that as the stockpile eroded, most of the sand accumulated on the lake bottom at depths less than 5 ft LWD. Thus little, If any, nourishment sand was lost to deep water. The nourishment was being dispersed downdrift In a nan-ow band within 250 ft of the shoreline. By November 1995. all of the July 1995 nourishment (20,000 cu yds) had been dispersed from the stockpile. Essentially, it was the single September storm that was responsible for this dispersion. A principal factor influencing erosion patterns and rates along the North Unit stockpile is an elongate distribution or "apron" of boulders and cobbles on the beach along the northern part of the nourishment stockpile. The boulders and cobbles extend southward as far as Profile 49. This rock apron is the product of southward dispersion of different generations of shore-defense materials from the NPM south paridng area since 1989. The rock armors the shore and prevents the northem part of the stockpile from eroding as rapidly as would occur otherwise. The influence of this rock amnoring can be seen in Figure 2-23 v^ere the nourishment stockpile has rts maximum width between the concrete cube revetment and Profile 48. The stockpile has its minimum width along the first segment of unarmored shore between Profiles 49 and 50. Nourishment in December 1995 Additional sand totaling 33,000 cu yds was placed at the North Unit nourishment site starting in mid-December 1995 and continuing until eariy January 1996. The nourishment sand was obtained from the stockpile at the Waukegan Generating Station as was done in July 1995. Unlike the wori< of July, this December/January nourishment included placement of sand into numerous embayed erosional areas along the landward side of the revetment protecting the south paridng area. These erosional features on the landward side of the revetment had been present since at least spring 1995. Data conceming the dispersion of this December/January nourishment will be included in the report for 1996 (Year-2) study findings. 78 2118100 2118000 - 2117400 -6/30/95 -7/28/95 -9/8/95 -10/13/95 -11/9/95 ISGS Profile 48 ISGS Profile 49 ISGS Profile 50 ISGS Profile 51 Easting (feet, Illinois State Plane Coordinates) Figure 2-23 Map summary of changes to the North Unit nourishment stockpile between June and November 1995. The June 30 scarp position was surveyed prior to nourishment; the July 28 data show maximum lakeward extent of the stockpile nourishment. 79 Ill Spring Bluff Nearshore The Spring Bluff nearshore is here defined as the area lying between a line projected east of the east-west trending section of Dead Dog Creek and the Camp Logan headland, located about 2600 ft to the south. These limits con-espond to ISGS profiles 52 and 78, respectively. The northern limit for this reach was chosen in consideration of a proposal to construct a submerged breakwater (or reef) to defend the NPM south parking area (Appendix D). For comparison of 1995 bathymetric data and future data, it is advantageous to establish this limit south of the proposed construction area. Figure 2-24 shows 1995 bathymetry for the Spring Bluff nearshore, and Fig. 2-25 shows lake- bottom elevation changes based on comparison of 1992 and 1995 bathymetry. Table 2-6 summarizes volumetric changes. 1995 Bathymetry (Figure 2-24) The major feature in the Spring Bluff nearshore is a bar-trough pair that occurs within 250 ft of the shoreline (Fig. 2-24). Minimum depths along the bar are approximately 3.5 ft LWD. The relief across this bar-trough pair increases from approximately 1 ft in the north to 4 ft adjacent to the Camp Logan headland. Submerged ruins of fornier concrete bulkheads and other shore structures within 100 ft of the shoreline in the northem part of the nearshore cause localized bathymetric in^gularities. Elsewhere, a regular pattern of bathymetric contours suggests a sand- covered lake bottom. The steepest lake-bottom slopes occur adjacent to the shoreline, as shov^ on Figure 2-24 by the closely spaced contours. Depths 100 ft lakeward of the shoreline range from 3 ft LWD in the north to 7.5 ft LWD in the south. 1992-1995 lake-bottom changes (Figure 2-25) The Spring Bluff nearshore was net erosional between 1992 and 1995 (Fig 2-25; Table 2-6). The net volumetric change of -61 ,800 cu yds yielded a three-year average lake-bottom erosion rate of 20,600 cu yds/yr (Table 2-6). Accretion Maximum accretion occurred within 300 ft of the 1995 shoreline and was associated with the nearshore bar. A bar was present along this shore prior to the marina construction as documented in the 1987 bathymetry (Appendix A; Map A-1). Beginning with the southward dispersion of dredged sediment in 1988, the bar became a more prominent feature, and it has persisted as a prominent feature in all subsequent surveys (Appendix A; Maps A-2 to A-6). The 1992-1995 bar accretion relates to continued southward dispersion of sediment from the North Unit nourishment stockpile, as well as to a landward shift of the bar-trough pair which caused 80 infilling of the trough present in 1992 (Appendix A; Map A-6). Figure 2-25 documents the continued dynamic behavior of the Spring Bluff nearshore since construction of the marina. As shown by the lake-bottom change maps in Appendix B, the Spring Bluff nearshore had a dynamic history of accretion and erosion between 1988 and 1992. This resulted from continued adjustment to the input of sediment from the North Unit nourishment stockpile combined with a persistent trend of shoreline recession. The 1992-1995 changes are the most recent changes in this continuing trend. Localized accretional areas occun-ed in water depths of 14 ft and 18 ft LWD. These were associated with infilling of northeast-trending bathymetric depressions that were present on the 1992 lake bottom. Erosion Maximum erosion occurred in a nanx)w linear band lying approximately 100 ft lakeward of the 1995 shoreline and landward of the bar. In this band, average lake-bottom erosion was about 2 ft, v^^th a localized maximum of more than 6 ft (Fig 2-25). The erosion is directly related to landward migration of the 0-ft LWD isobath between 1992 and 1995. This landward migration averaged about 50 ft. Lakeward of the bar, lake-bottom erosion and accretion were generally on the order of 1 ft or less. However, along the southem half of the nearshore reach, a narrow erosional band, marked by erosion from 3 to more than 6 ft, relates to landward migration of the 1992 bar-trough pair. Considering the 1992-1995 nearshore erosion per shoreline foot, the Spring Bluff nearshore (-8 cu yds/yr/shoreline ft) was two times greater than that at the NPM south breakwater (-4 cu yds/yr/shoreline ft; Table 2-3), and 1.6 times greater than that at the south parking area (-5 cu yds/yr/shoreline ft; Table 2-4). Between 1992 and 1995, the Spring Bluff nearshore was the area of most extensive and rapid erosion between the WI-IL state line and the Camp Logan headland. 81 Ne; 644000 Figure 2-2 — — — — Shoreline (July 1995) Cullural features (1992) —I— lllinots Slate Plane coordir 100 200 300 Feet 100 Maters ow Water Datum (LWD) Soulhem limit o' 1995 mapping ■ SbM GKHogKd Sir««y Figure 2-24 1995 bathymetry of the Spring Bluff nearshore, Bathymetric data were being collected during the early stage of the July 1995 nourishment and no major dispersion had yet occurred into this map area. 82 Joins Figure 2-15 2117000 r 645500E 1992 OnLWD Isobath ■^•— — ••- — 1995 shoreline 1 995 OnUWD Isobath — Limit of data comparison Cultural features (1992) — j — Illinois State Plana coordinataa t:':':::-:::::;::!;:;! Lake-lwttom erosion greater than 1 ft 100 200 300 Feet 100 Iwletars Contours in feet Positive numbers indicate accretion Negative numbers indicate erosion Contour interval 1 foot 115000N 644000E Camp Logan headland Southern limit of 1992 and 1995 mapping Figure 2-25 Isopach map of 1992-1995 lake-bottom changes in the Spring Bluff nearshore. The 1995 bathymetry was collected prior to dispersion of the 1995 beach nourishment and thus does not reflect input from this nourishment. 83 Table 2-6 Summary of erosion and accretion volumes in the Spring Bluff nearshore.^ 1987-88 1988-89 1989-90 1990-91 1991-92 1992-95^ Accretion (+) cu yds 134,000 40,600 41.800 40,100 14,900 35,600 Erosion (-) cu yds 2.200 67.400 79,100 39,400 97.500 97,400 Annual net change (cu yds/yr)^ +131.800 -26.800 -37,300 +700 -82,600 -20,600 Normalized annual net change (cu vds/vr/shoreline ft)^ +51 -10 -14 -32 -8 ^ All volumes are computed for lake-bottom elevation changes In excess of 1 ft and occurring below Low Water Datum (LWD). Volumes are rounded to the nearest 100 cu yds. ^Three-year comparison; erosion and accretion volumes are for the three-year summation and annual net change is a three-year average. Total net change for 1992-1995 equals -61,800. ' Net accretion is indicated by a positive number and net erosion is indicated by a negative number. * Shoreline distance (2600 fl) is based on measurement along a north-south line bounded to the north and south by the defined limits of the nearshore reach. Numbers are rounded to the nearest virtiole number. Net accretion is indicated by a positive number and net erosion is indicated by a negative number. Summary of 1987-1995 nearshore changes The major erosional and accretional areas between 1987 and 1995 occun-ed in the shallow nearshore within about 300 to 500 ft of the shoreline. The net. eight-year lake-bottom change across the Spring Bluff nearshore was erosion totaling 76.000 cu yds (Table 2-6). The record of annual sediment gain and loss from this nearshore area has been complex due to the influence of updrifl beach nourishment. Table 2-6 summarizes these annual nearshore changes. In the interval 1987-1988, the Spring Bluff nearshore had a net gain of sediment. This resulted from the influx of sediment from the north derived from the discharge and downdrift dispersion of sediment dredged from the marina basin. During the following two years (1988 to 1990), net erosion dominated across the nearshore. However, the area was still receiving considerable Input of sediment from erosion occuning to the north along the nearshore and emergent part of the fan delta. Some of this sediment was temporarily deposited across the Spring Bluff nearshore, but the majority was apparently transported southward beyond the Camp Logan headland. From 1988 to 1990, the annual loss of nearshore sediment was neariy twice the annual gain. Thus, net erosion removed a volume of sediment greater than that gained between 1987 and 1988. 84 In the interval 1990-1991, the erosion and accretion were nearly balanced at about 40,000 cu yds each. At this time there was a reduced input of sediment supply from updrift because shore protection had been placed along the fan delta shoreline in 1989. The near balance of gain and loss is also partly due to beach nourishment at the North Unit nourishment site. In late summer 1990, approximately 150,000 cu yds of sand was stockpiled at the nourishment site. Erosion into this stockpile would have provided a considerable influx of sediment to the Spring Bluff nearshore during the 1990-1991 interval. The 1991-1992 interval documents net erosion of 82,600 cu yds/yr. This is the most severe annual rate of net erosion documented for this nearshore. This severe erosion can be attributed to a diminished sediment supply from updrift because of reduced erosion at the fan delta combined with depletion of much of the 1990 beach nourishment. In the 1992-1995 interval, the three-year average net change was erosion of 20,600 cu yds/yr. During this interval, 32,000 cu yds of beach nourishment was supplied in 1994, and this likely reduced the rate of net erosion that might othenAnse have occun-ed. An additional 20,000 cu yds of nourishment was added to the stockpile in 1995, but little of this was yet dispersed at the time of the 1995 bathymetric survey, and thus It does not contribute significantly to the 1992-1995 accretion. The important conclusion drawn from this eight-year record along the Spring Bluff nearshore is that, summing all annual changes from 1987 to 1995, the nearshore had a net loss of 76,000 cu yds. In general, the nearshore lakeward of the 1995 12 ft LWD contour is about 1 ft deeper than it was in 1987, while between the 7 and 12 ft LWD contours there has been 2 to 3 ft of erosion. Closer to shore, the comparison is complicated by the relative positioning of bar-trough pair fnDm year to year. However, landward of the 1995 bar, lake-bottom elevations are generally 0.5 to 2 ft lower than they were in 1987. Thus, despite all the inputs from updrift erosion and dispersion of sediment from the North Unit nourishment stockpile, net erosion has prevailed across the Spring Bluff nearshore. The sediment supply from the nourishment stockpile Is contributing to a reduction in the rate of shoreline recession and nearshore erosion. This sediment supply, however, is insufficient to produce a balanced sediment budget and thus eliminate net erosion in the nearshore and net shoreline recession. IV Spring Bluff Shoreline Changes Si nce Marina Construction Extreme and rapid shoreline changes have occurred between the marina and Camp Logan since marina construction. Initially there was extensive beach and nearshore accretion due to discharge of sediment dredged from the marina basin (Appendix B; Maps B-1 and B-2). Subsequently, there was extensive beach and nearshore erosion as this sediment was 85 transported further southward and there was no appreciable supply from updrift to offset or diminish a net loss (Appendix B; Maps B-3 through B-5). Because of concerns for the conservation of dunes and the natural setting along this segment of the North Unit, questions have been raised by park users conceming the degree to which erosion has adversely affected this shore since marina construction. Parte users observing shoreline changes in the years since the marina was constructed have been able to witness extreme and rapid shoreline erosion, particulariy between 1989 and 1990. As discussed in the previous section, net erosion has occurred since 1989. However, a possible misconception is that there has been a loss of beach area compared to what existed prior to marina construction. Comparison of map data and ground and aerial photography document that although substantial erosion has occurred, as of 1995 there still remains a net gain of beach width compared with 1987. The following discussion focuses on the data that document this net gain. Shoreline changes (Figure 2-26A) Figure 2-26A compares shorelines between the NPM south breakwater and the Camp Logan headland. Annual shorelines are shown for 1987 through 1992, and a 1995 shoreline documents most recent conditions. All shorelines are based on mapping by the ISGS while doing annual bathymetric surveys. Some of the differences in shoreline position result from differences in lake level, but because a steep slope characterizes much of the lower beach and shallow nearshore, the shoreline changes primarily reflect lateral accretion and erosion. Comparing the 1987 shoreline with subsequent shorelines, the maximum lakeward shoreline position occun-ed in 1989 along the fan delta about 300-400 ft south of the south breakwater. Some erosion of this shoreline had already occurred by the time of shoreline mapping by ISGS in June 1989. Thus, the most lakeward shoreline position occun-ed sometime prior to this mapping, probably in late fall 1988 when dredging was completed. The hook-like feature in the 1989 shoreline (Fig. 2-26A) results from the erosion that was then ongoing beyond the southern limit of riprap protection (also see Fig. 1-5). The stability of the shoreline lakeward of the south partying area since 1990 results from the shore defense placed here In 1989 and expanded in 1990. Within the North Unit, the most lakeward shoreline occun-ed in 1989 at the marina/state parte boundary. 1989 con-esponded to the time of maximum beach width and most lakeward position of the shoreline between the south breakwater and a point about two thirds the distance to the Camp Logan headland. In contrast, along the southern 1500 ft of Spring Bluff shoreline, maximum beach width occurred in 1990 (Fig. 2-26A). The one-year difference apparently reflects the time needed for the locus of sand accretion to move southward along the beach. 86 Although shoreline recession has been the general rule since 1989, the shoreline position has not yet reached landward of its 1987 position. The nearest approach occurs just south of the marina/park boundary where the 1987 and 1995 shorelines plot 45 ft apart (Fig. 2-26A). To emphasize that this beach area nnaintains a net gain compared to 1987, a stipple pattem has been added landward of the 1987 shoreline. No subsequent shoreline crosses into this stippled area. Between the marina/state park boundary and the Camp Logan headland, between 1987 and 1989, marina-related accretion added 13 acres of beach area. As of 1995, 5 acres of this accretion remains. Thus there has been a persistent net erosion since 1989, but the beach maintains a net gain. The average rate of loss between 1989 and 1995 has been approximately 1 .3 acres/yr. Zero contour changes Figure 2-26B shows the zero-ft LWD contours for 1987 through 1992, and for 1995. The advantage of comparing the zero contours Is that these are elevations relative to a datum that is independent of lake level. Changes in the position of the zero-ft LWD contour solely reflect accretion or erosion. The 1987 lake level was higher than average, and thus comparing the zero contours makes use of a common datum. Opposite the south parking area, the maximum lakeward position of the 0-ft LWD contour occurred in 1989 when it was about 525 ft lakeward of the 1987 location (measured perpendicular to the 1987 zero contour). Since 1991 the 0-ft LWD contour has been positioned along the line of shore defense that protects the south parking area. South of the marina/park boundary, the maximum lakeward position of the 0-ft LWD contour occurred in 1988 about midway between the marina/park boundary and the Camp Logan headland. This was apparently caused by the early southward dispersion of dredge discharge, and by the initial placement of the discharge pipes at the southem end of the discharge area (Moffatt & Nichol Engineers, 1990). Closer to the marina/park boundary, the 1989 0-ft LWD contour occupied the most lakeward position (Fig. 2-26B). 87 B Figure 2-26 Maps showing 1987-1995 changes in position of the shoreline (A) and the Low Water Datum (LWD) zero-depth contour (B) between the North Point l\/larina south brealcwater and the Camp Logan headland. 88 From 1990 through 1992, landward recession of the zero contour occurred along most of the nearshore. However, the nearshore immediately updrift of the Camp Logan headland continued to accrete and the 0-ft LWD contour moved lakeward as pulses of sediment were transported along the nearshore from updrift erosion. Unlike the shoreline comparisons, there has been an intercept of the pre- and post-construction 0-ft LWD contours (Fig. 2-26B). The 1992 zero contour crosses landward of the 1987 zero contour along a 200-ft segment of nearshore located 250 ft south of the marina/park boundary. The 1995 zero contour also crosses the 1987 zero contour, along 300 ft of nearshore centered about 1150 ft north of the Camp Logan headland (Fig. 2-26B). Factors influencing beach area and shoreline position As previously discussed, the Spring Bluff nearshore had net erosion between 1987 and 1995 (Table 2-6). In contrast, comparing 1987 and 1995 shorelines (Fig. 2-26A) indicates that the Spring Bluff beach maintains a net gain. Updrift beach nourishment has been the prime factor in this gain. The initial addition of sand occun-ed between 1988 and 1989 from the southward dispersion of fan-delta sand from updrift (Rg. 2-26A). Subsequent updrift nourishment occun-ed in 1990 (150,000 cu yds), 1994 (32,000 cu yds), and July 1995 (20,000 cu yds). Southward dispersion of the 1990 nourishment was probably responsible for beach expansion at the south end of Spring Bluff between 1991 and 1992. Southward dispersion of the lesser volume of 1994 nourishment had the localized effect of slowing the change in shoreline position between 1992 and 1995 at the north end of Spring Bluff. Additional nourishment supplied in December 1995 postdates this map comparison. An Interesting factor responsible for reducing shoreline recession rates is the presence of ruins of former concrete bulkheads and other shore defense that presently reside in the nearshore. These ruins are in shallow water located in the northern part of the Spring Bluff nearshore. These structures were buried prior to 1990 by the accretion of sand resulting from the marina dredging. Shoreline recession and nearshore erosion first exposed parts of the ruins in 1990. Since that time nearshore erosion has exposed the entire assembly of mins. These structures influence nearshore wave dynamics such that shoreline recession has decreased landward of the stmctures compared to areas immediately updrift or downdrift (Fig. 2-27). In effect, the ruins are acting as a submerged breakwater that is reduces the available wave energy impacting the shore. The fact that a net gain in area still exists for Spring Bluff beach as of 1995 should not diminish concerns for beach erosion along this reach. All of the Spring Bluff shoreline and nearshore data dearty indicate net erosion. Within the near future, shoreline recession will eliminate most of the 89 beach area that was gained since 1987. Beach nourishment volumes that have been added to this shore have been insufficient to negate the net erosion. In terms of shoreline recession, this is the most seriously eroding reach of shoreline in the study area. The following section discusses a model for shoreline erosion trends at Spring Bluff and the adjacent NPM south parking area. x^'.ilsL; — -:' f J. Figure 2-27 View showing the influence of submerged nearshore ruins on shoreline configuration just south of the marina/state pari^ boundary. The view is southward toward the Camp Logan headland in the distance. Location of the nearshore mins is indicated by one rock protmding above water level. Note the lakeward perturbation of the shoreline opposite the mins which are acting like an offshore breakwater (Photo date July 17, 1995). 90 VI Model for Shoreline Recession at Sp ring Bluff Beach Between 1988 and 1995, shoreline recession along the Spring Bluff beach was greatest just south of the marina/park boundary and minimal at the Camp Logan headland (Fig. 2-26A). The concrete cube and riprap revetment at the south parking area and the steel sheetpile at the Camp Logan headland act as "hard points" that anchor the shoreline at these two locations. The maximum amount of shoreline recession near the marina/park boundary is coincident with the first occurrence of undefended shoreline south of the marina. If erosion was allowed to continue unimpeded, the ultimate extent of shoreline recession along this reach of shore can be estimated using established models of sandy coastline development between hard points. In map view, the ultimate shoreline configuration will be a concave- lakeward "hook" or "half-heart" shape approximating a logarithmic spiral. This type of shoreline configuration is technically refen-ed to as a "logarithmic-spiral" shoreline, or simply a "log-spiral" shoreline (Schwartz, 1982). Log-spiral embayments develop along sandy coasts between hard points such as rocky headlands or coastal stnjctures. The predominant (strongest) waves approaching the coast are responsible for initially forming and then maintaining the log-spiral shape. The more oblique the approach of the incoming predominant waves, the better the capability of the waves to create this shape. For the Illinois shore, the predominant waves are from the northeast quadrant. The obliquely approaching waves refract around the updrift hard point and erode the shore into the log-spiral shape. The distance between the updrift and downdrift hard points determines how far landward the embayment extends. The Idealized log-spiral shape occurs when all wave fronts ultimately approach the beach parallel to the shoreline. As a result, there is little or no lateral movement of beach sediment, and a stable shoreline configuration develops and is maintained. An excellent example of a log-spiral shoreline developed between 1988 and 1989 opposite the south parting area (Fig. 1-5). In the case of the south parking area, the southern end of the riprap along the fan delta acted as the updrift hard point around which northeriy waves refracted to erode the fan delta beach and create a hook-shape shoreline in plan view (Terpstra and Chrzastowski, 1992). Figure 2-28 schematically illustrates three potential shoreline configurations along the reach between the NPM south breakwater and the Camp Logan headland. In all three models, the Camp Logan headland forms the southern (downdrift) hard point; shorelines would converge towards this headland. Shoreline "A" v^rauld develop if the updrift hard point was the NPM south breakwater, and if the south parting area and associated shore defense were absent. Shoreline "B" would develop if the updrift hard point was the south end of the concrete-cube revetment built to protect the south paricing area. Shoreline "C" would develop if the updrift hard point was the 91 boulder and cobble armor that presently extends downdrifl from the south parking area revetment. This rock debris has a low crest elevation that would allow wave over-topping and it would thus be a pooriy defined hard point for wave refraction. However, this feature is used to illustrate that a reduction in the distance between the updrift and downdrift hard points reduces the maximum landward position of the log- spiral shoreline. The erosional shorelines in Figure 2-28 are schematic. To accurately construct potential shoreline configurations would require a rigorous mathematical analysis to determine and quantify the parameters of the log-spiral equation for this shoreline reach. The distribution of rock and debris in the nearshore would complicate quantifying the influence of wave dynamics and the actual erosion/accretion patterns. These obstructions would cause the ultimate shoreline configuration to be a variation on an ideal log-spiral shape. Figure 2-28 illustrates that all three potential shorelines would extend landward of the present vegetation line and reach into forested wetlands. Shorelines "B" and "C" are the models most applicable to the Spring Bluff beach assuming that the south partying area will remain a defended shore. Along shoreline "B," erosion would move the shoreline as much as 350 ft landward of the vegetation line; along shoreline "C," erosion would move the shoreline as much as 200 ft landward of this line. Comparing the locations of log-spiral shorelines "B" and "C" to the location of the vegetation line, the potential erosion of forested wetlands (excluding beach) would amount to about 15.5 acres for shoreline "B" and 7.4 acres for shoreline "C". Development of log-spiral configurations occurs at several scales and would not be limited to this segment of the state park. The log-spiral configuration represents an ultimate equilibrium shoreline configuration that would develop between many of the hard points along the shoreline between the state line and Waukegan Hartaor if the erosional process was allowed to proceed naturally. Once a fully developed log-spiral shoreline is attained, the shoreline is in a dynamic equilibrium with wave dynamics, sediment supply, and lake level. If no major changes occur in these three conditions, ideally, no further erosion will occur. Two important conclusions relate to this model for shoreline recession: 1) Shoreline defense at the NPM south pari o Southern limit of beach nourishment on 6/29/95 Lake Michigan Joins Fig. 2-30b -6/29/95 ■7/15/95 -9/B/95 •11/8/95 643000 643100 643200 643300 Easting (ft), Illinois State Plane coordinates E o ^ -D c c 0) ro .^ D) O E O (u W -Q E "J ii ^ c CO 0) II I— t= O Z) C P O (u (U (U "oi^ ro ^ (- T3 (U 0) O x: D ■c TJ o C c o (U o r (/> ro o ^ >^ WJ (1) CD ^ C D ro W x: m o CM u— o (1) ^ ro — ) F 0) E 13 1- U) ro in en < o CO CM 0) 98 o 21019CX) 21018CX) 2101700 - 2101600 2101500 - 2101400 2101300 2101200 2101100 2101000 2100900 2100800 2100700 4-i— 642900 Joins Fig. 2-30a Lake Michigan ■6/29/96 -7/15/95 ■9/8/95 -11/8/95 m South bathhouse boat ramp 643000 643100 643200 Easting 643300 0> £1 u ^1 m :9 D T3 ■^ -D C C 0) 03 1^ <-> £ O (1) To -Q eB ^ c V) 0) II c o O 0) CO Xi 0) Q) -c x: it: JC E 3 o ■D c o 3 o O o 5 -C CD O CNJ o ^ TO ^ to 03 CD CT) CD o CO I CM 0) 99 Figure 2-31 View looking southward along the South Unit nourishment site three days before completion of the June/July 1995 nourishment. Photograph is taken from the steel sheetpile (lower left) that defends the north end of the South Unit shoreline. Approximately 10 ft of recession has already occurred (Photo date July 11, 1995). Figure 2-32 Same view as Figure 2-31 showing what remained of the South Unit nourishment stockpile in November 1995. The scarp crest has receded approximately 35 ft compared to the photo in Figure 2-31 to re-expose the riprap that defends this reach of shore (Photo date November 8, 1995). 100 REGIONAL COASTAL MONITORING General The coastal monitoring at North Point Marina and Illinois Beach State Parte is designed to address erosion and coastal-management concems specific to these individual coastal segments. To adequately evaluate the gain, loss, and transport of littoral sediment along these two coastal segments, It is necessary to examine coastal processes from a regional perspective. Ideally, this regional emphasis Involves examination of a complete coastal system or compartment bounded by littoral-transport bamers. The WI-IL state line is a political boundary important to coastal management, but also con-esponds to a partial littoral-transport barrier caused by the intercept, dredging and removal of sediment at Prairie HartDor Yacht Club. The jetties and entrance channel at Waukegan HartDor form a partial to near-total barrier to net southerly littoral transport, and thus marie the terminal point for a large percentage of the sediment transported southward along the marina and state parte nearshore. Thus, the coastal reach from the state line to Waukegan HariDor is appropriately treated as a regional coastal compartment having an origin (sediment crossing the state line), a littoral sediment transport pathway, and a terminus (sediment trapped at Waukegan Harbor). Beach and Nearshore Profile Scheme The profile scheme to be used in this study to evaluate regional long-temi changes to the beaches and nearshore will rely on a series of 22 beach and nearshore profile lines spaced from 1700 to 2200 ft apart along the shore. The scheme Is shown in Appendix H. The eight primary lines (Range Lines 1-8) are at locations first established by the U.S. Lake Survey in 1872-73. These range lines were subsequently reoccupied on several occasions in the earty to mid-1 900s by federal and state agencies (U.S. Army Corps of Engineers, 1951; Illinois Division of Waterways, 1954; Illinois State Geological Survey, 1988). Reports by these agencies contain profile data and profile comparisons with previous years. A series of 14 new intemiediate or "secondary" range lines (Appendix H; alpha-numeric line designations) will be established by ISGS for this study to provide more detailed coverage. The objective of collecting profile data using this scheme is to compare present-day profiles with the historical data to evaluate long- term erosion and accretion trends. Profile data were not collected using this range-line scheme during 1995. Field worie focused on determining the X-Y coordinates of the range-line origins and on establishing these points along the marina and state parte shore. The profiles will be run in 1996. 101 Compilation of Dredge Records The volume of sediment dredged from harbors or trapped updrift of coastal barriers is a valuable data source for the development of coastal sediment budgets. In 1995, data were compiled for dredging at the Commonwealth Edison Waukegan Generating Station near the south end of the study area, and at Waukegan Harbor which forms the south end of the study area. Waukegan Generating Station Dredge records for the Waukegan Generating Station are available for a 37-year period from 1958 through 1995 (Table 2-7). During this time dredging was done, on average, every two years. The dredging was confined to the cooling pond and to the water Intake-discharge channels along the jetty to its lakeward end (approx. 8 ft LWD). Entrapment In the dredge areas represents a fraction of the local littoral transport because sand and gravel can readily bypass the facility. The dredge volumes thus provide a minimum estimate of littoral transport. The 32- year record from 1958 to 1990 is used here because a temporal equivalent dredge record is available for Waukegan Harbor. Between 1958 and 1990, the total dredge volume at Waukegan Generating Station was 1,207,100 cu yds. This is a 32-year average of 37.700 cu yds/yr. 1 ■■ — — Table 2-7 Dredge volumes for Commonwealth Edison Waukegan Generating Station 1958-1995."'^ Calendar year Dredge volume Calendar year Dredge volume 1958 120,000 1979 47,000 / 90,000^ 1961 125,000 1981 20,000 1963 50,000 1983 42,000 1965 100,000 1986 102,700 1968/69 120.000 1987 50,700 / 92,700^ 1972 100,000 1988 7,000 1973 103,000 1990 35,500 1976 78.700 1992 114,000^ 1977 105.500 1995 100,000^ ^ Volumes are rounded to the nearest one hundred cu yds. ^ Volumes were provided by T.B Piatt, Regulatory Compliance Engineer, Commonwealth Edison. ^ Records show two possible dredge volumes. The lesser volume is used in the summation of dredge volumes. ^ Volumes were accurately determined from stockpile sun/eys following placement. 102 Waukegan Harbor The jetties, shore-attached breakwater, and the deep-water entrance channel combine to make Waukegan Harbor the largest barrier to littoral transport on the northem Illinois coast, and one of the largest littoral transport barriers in the Great Lakes Region. Dredging to maintain a hariDor at Waukegan spans a 105-year period from 1889 to 1994 (Chrzastowski and Trask, 1995). Prior to 1977, and again in 1982, sediment dredged from the harbor was discharged into deep water about 2.5 miles lakeward of the hartDor entrance. First in 1977, and consistently since 1984, material dredged has been discharged into a nearshore disposal area about three-quarters of a mile south of the harbor. Table 2-8 summarizes the dredge records for Waukegan Harbor from 1958 through 1994. In general, dredging at Waukegan HartDor occun-ed every one or two years. The primary dredging area was the channel between the jetties and the lakeward approach to this channel. Historical bathymetric data verifies that some natural bypass of the hartjor jetties occurred during this time (Chrzastowski and Trask, 1995). Thus, the dredge record at Waukegan Harbor provides a minimum estimate of littoral sediment transport at the south end of the study area. The dredge data from 1958 through 1990 are of interest since the time period corresponds in duration to the 32-year record at Waukegan Generating Station (Table 2-7). The summation of dredge volumes from 1958 through 1990 is 981,473 cu yds. For this 32-year record, an average of 30,700 cu yds/yr was thus captured by the Waukegan Harbor entrance channel. 103 Table 2-8 Dredge volumes for Waukegan Harbor 1958-1994.^ | Fiscal year Dredge volume^ Fiscal year Dredge volume^ 1958 108,200 1976 34.691 1960 12.629 1977 130.000 1961 39.900 1982 85,396 1963 47.191 1984-1985 81,000 1964 50.812 1985 26,180 1965 41.279 1988 100,996 1966 49.370 1990 49,513 1967 32.491 1991 79,482 1969 33.456 1993 66,597 1974 -10.000 1994 44,879 1 1975 -48.369 1 ^Dredge data were obtained from annual reports of the U.S. Army Corps of Engineers and from data on file at the offices of the Chicago District. All dates from 1958 to 1975 are for federal fiscal years July through June; dates from 1 976 to 1 994 are for federal fiscal years October through September. ^ Volumes are bin measures which are a measure of both sediment and water. Estimated water volume is 1 to 20 percent. The dredqe volumes are not corrected for water content. Regional Littoral Sediment Budget A considerable amount of data yet needs to be collected and evaluated to provide a littoral sediment budget for this coast that is better than a first approximation. Subsequent annual reports will continue to refine the budget analysis. For ongoing coastal management, it is valuable to summarize the budget as it is presently understood. The budget presented here primarily considers average trends over the past three years (1992- 1995). This time frame is used since the sediment budget data are derived primarily from 1992- 1995 bathymetric comparisons. Four components of the overall budget can be approximated with some certainty. These are: 1) the minimum volume of littoral sediment moving south across the WI-IL state line 2) the minimum volume of littoral sediment moving south past the Camp Logan headland 3) the volume contribution from beach nourishment 4) the minimum volume of littoral sediment in transport at the south end of the IBSP South Unit 104 1) Sediment volume crossing the state line The volume of littoral sediment crossing the state line can be estimated based on the accretion volumes documented by bathymetric changes. Three distinct areas of accretion can be attributed to a sediment supply moving south across the state line. North Beach/north breakwater nearshore accretion The NPM north breakwater acts as a partial barrier to the southward transport of littoral sediment. The nearshore updrift of the north breakwater has been an area of persistent sediment entrapment since the breakwater was constructed (Figs. 2-6 and 2-7). Between 1992 and 1995. the net accretion in this nearshore area was 31,000 cu yds which is an average net accretion rate of 10,300 cu yds/yr (Table 2-1). Marina entrance accretion The 1992-1995 bathymetric comparison identifies an accretional wedge along the lakeward side of the north breakwater and extending into the marina entrance. This is a pathway for sediment to bypass the breakwater and contribute to accretion within the marina basin. Accretion within the marina basin is widespread and has been ongoing since the time of construction. Bathymetric comparisons suggest that at least in the eariy post-construction history, some of this sediment supply came from the south (Appendix B; Maps B-2, B-3). For the limited marina entrance accretion data for the past three years (Fig. 2-10), it is not known what percentage was derived from either the north or south. Within this area, net accretion between 1992 and 1995 was 9,200 cu yds. If it is conservatively assumed that half of this accretion was supplied by sediment bypassing the north breakwater, the component from the north was 4,600 cu yds. This is an average annual contribution of 1,500 cu yds/yr from a source area updrift of the marina entrance. Nearshore accretion lafteward of the marina entrance The 1 992-1 995 bathymetric comparison documents an accretional lobe lakeward of the marina entrance. This lobe has an orientation and configuration suggesting that it was supplied by sediment bypassing the north breakwater (Fig. 2-13). The volume of this accretional feature is 7,200 cu yds. Assuming this accretion does represent a supply of sediment that originated updrift of the north breakwater, the updrift contribution over the past three years is 2,400 cu yds/yr. The summation of the annual accretion volumes updrift of the north breakwater (10.300 cu yds/yr), accretion in the marina entrance from updrift sources (1,500 cu yds/yr), and accretion in the nearshore lakeward of the marina entrance (2,400 cu yds/yr) yields a total annual accretion rate of 14,200 cu yds/yr. This is a minimum estimate for the annual volume moving south across 105 the state line. What is not known is the volume of sediment coming across the state line and continuing in southward transport past the north breakwater and marina entrance. This sediment that bypasses the marina supplies the nearshore downdrift of the marina. 2) Littoral transport rate past the Camp Logan headland Comparison of 1992-1995 bathymetric data across the nearshore from the NPM marina entrance to the Camp Logan headland documents that net erosion dominates (Figs. 2-13; 2-15; and 2-25). This erosion is an important contribution to the littoral sediment volume in transport past the Camp Logan headland. Net erosion averaged 6,600 cu yds/yr in the south breakwater nearshore (Table 2-3), 10,600 cu yds/yr in the south parking area nearshore (Table 2-4), and 20,600 cu yds/yr in the Spring Bluff nearshore (Table 2-6). The summation is 37.800 cu yds/yr. This is a minimum estimate since it does not account for the unknown volume of sediment that may be bypassing the marina and continuing southward past Camp Logan. Also, the estimate is based solely on nearshore changes and does not consider sediment volumes above LWD. 3) Contribution from beach nourishment Beach nourishment along the IBSP North Unit and South Unit has been sporadic and has fluctuated significantly in annual volume. Thus, to asses how nourishment contributes to the budget, rt is necessary to average the nourishment over as long a time frame as possible. Using data prior to 1992 is problematic because of the large influx of nourishment from dredging of the marina basin in 1987-1988, and from the stockpiling of sediment derived from dredging at Prairie Harbor Yacht Club in 1990. Between 1992 and November 1995, nourishment at the North Unit totaled 52,000 cu yds (32,000 cu yds. 1994; 20,000 cu yds, July 1995); nourishment at the South Unit had a minimum volume of 24,000 cu yds (July 1995; additional nourishment data needs to be compiled). These 1992-1995 nourishment data combined with nourishment data over the next two to three years will have greater application to the budget. 4) Minimum littoral transport rate at south end of IBSP The dredge data (1958-1990) for Waukegan Generating Station and Waukegan Hartjor can be summed to provide a long-term minimum estimate of littoral transport at the south end of the IBSP South Unit. A long-temn average is advantageous in applying the dredge data because this minimizes influence caused by short-term perturtaations in dredge volumes and dredge frequency. For a 32-year dredge record for these two sites (1958-1990; Tables 2-7 and 2-8), the combined average dredge volume is 68,400 cu yds/yr. A minimum of 68,400 cu yds/yr is therefore in transport at the south end of the South Unit. 106 Budget overview Figure 2-33 provides a graphical summary of the littoral sediment budget. The following are key components of the budget. • Littoral sediment is moving south across the WI-IL state line at a minimum rate of 14,200 cu yds/yr. This sediment supply contributes to accretion on the updrift side of the NPM north breakwater, within the marina basin, and lakeward of the marina entrance. The principal accretion area has been updrift of the north breakwater which fomris a partial barrier to southward littoral transport. Accretion updrift of the breakwater averaged 10,300 cu yds/yr between 1992 and 1995. • It is not known what volume of sediment coming south across the state line bypasses the marina. • Between 1992 and 1995, net erosion dominated across the nearshore from near the marina entrance southward to the Camp Logan headland. The net erosion volume across this reach was 37,800 cu yds/yr. This provides a minimum estimate for the sediment in transport past the Camp Logan headland. • Between 1992 and 1995, beach nourishment updrift of the Camp Logan headland totaled 52,000 cu yds, or about 17,000 cu yds/yr. As of 1995, much of this sediment, and particularty the 1995 nourishment, is apparently still in residence updrift of Camp Logan. Ultimately this volume will add to the volume In transport past the Camp Logan headland. • The nearshore between the state line and the Camp Logan headland can be divided into a northern net accretional zone from the state line to the marina entrance (minimum net accretion of 14,200 cu yds/yr), and a southem net erosional zone from the marina entrance to the Camp Logan headland (minimum net erosion 37,800 cu yds/yr). The downdrift net erosion exceeds the updrift net accretion by a factor of at least 2.5. • This study has not yet evaluated data between the Camp Logan headland and the northern half of the IBSP South Unit, but past studies have documented that net erosion predominates along this reach (U. S. Army Corps of Engineers, 1953; State of Illinois, Division of Waten^rays, 1958; Tetra Tech, Inc., 1980). A minimum of 37,800 cu yds/yr enters the updrift end of this reach at the Camp Logan headland. Beach and nearshore erosion along the reach add to the littoral transport volume. Beach nourishment at the northem end of the South Unit (24,000 cu yds during 1995) also adds to the volume. 107 Previous studies have documented that along the beach and nearshore between the state park lodge and the mouth of Dead River, a transition occurs in long-term coastal processes. Net erosion dominates to the north of the lodge and net accretion or a stable shore dominates to the south of Dead River. The first major sediment trap encountered south of Dead River is at the cooling water channels and basin at the Commonwealth Edison Waukegan Generating Station. Entrapment here reduces the littoral transport volume by 37,700 cu yds/yr. The Waukegan Harbor entrance channel Is the next downdrift entrapment area, and a partial to near-total banier to continued southward transport. Entrapment here reduces the littoral transport volume by an additional 30,700 cu yds/yr. An unknown volume of sediment bypasses the harbor entrance and is possibly deflected offshore. The beach and nearshore between the Commonwealth Edison Waukegan Generating Station and Waukegan Harbor is stable or accretional. The 30,700 cu yds/yr entrapped at Waukegan Harbor is sediment that bypassed the Commonwealth Edison facility. Summing the dredge volumes from Commonwealth Edison (37,700 cu yds/yr) and Waukegan Harbor (30,700 cu yds/yr) provides a minimum estimate for the supply approaching the Commonwealth Edison facility from updrift. The proximity of the state park means that this is also a minimum estimate for the littoral transport along the southern end of the IBSP South Unit. This minimum transport volume at the south end of the IBSP South Unit is 68,400 cu yds/yr. Several conclusions result from this preliminary sediment budget: 1) A sediment supply Is coming south across the state line, but the marina Is acting as a partial barrier to the southward transport of a component of this sediment. At this time the volume bypassing the marina is not known and the percentage of entrapment and bypass can not be determined. 2) The beach and nearshore between the marina and Camp Logan headland is a major net erosional area that is providing much of the littoral sediment supply for the rest of the coastal reach south to Waukegan Hartjor. The temi "erosion hot spot" is commonly used to identify coastal areas of severe and persistent erosion. This reach between the marina and Camp Logan headland is such a setting. 108 3) The estimated minimum littoral sediment transport at the south end of the IBSP South Unit suggests that the "carrying capacity" of the littoral system along this shore is at least 68,400 cu yds/yr. If the potential exists for such a volume to be in transport between the marina and the Camp Logan headland, then a minimum of 68,400 cu yds/yr of nourishment would be needed south of the marina to produce a balanced sediment budget and thus have no net loss of littoral sediment. 4) The "natural state" littoral transport along this reach was computed by the U.S. Anriy Corps of Engineers (1953) to be 90,000 cu yds/yr, based on rates of shore erosion and accretion against barriers to littoral drift. The minimum estimate of 68,400 cu yds/yr presented here will likely be revised upwards as additional data are evaluated in future reports of this four-year study. 109 Minimum littoral transport cubic yards /yaar > 14.200 Transport across stale line Bypass of marina > 37,800 Transport past Camp Loyan headland Figure 2-33 Preliminary graphical littoral sediment budget between the WI-IL state line and Waukegan Harbor. The budget relies on data from bathymetric comparisons in the marina vicinity between 1992 and 1995. and dredge records at the south end of the study area between 1958 and 1990. All volumes are minimum estimates. 110 PART 3: STUDY APPLICATIONS AND SUMMARY RECOMMENDATIONS North Point Marina 1) Planning for future maintenance dredging at North Point Marina should consider the possibility of dredging the North Beach bar and the adjacent nearshore. Accretion In this area has established a pathway for sand that comes across the state line to move southward, bypass the north breakwater, and feed Into the marina entrance. If this bypass is not interrupted, it may result in increased rates of shoaling within the marina entrance. Removal of sediment from the North Beach bar and adjacent nearshore will create a depositional sink capable of storing several years of new accretion before the bypass pathway is re-established. However, any dredging on the updrift side of the north breakwater should not approach the toe of the north breakwater. To avoid any potential stability problems along the north breakwater as a result of nearby dredging, the best option may be to dredge a catchment basin on the Illinois side of the state line that would trap littoral sediment and be dredged periodically. 2) Planning for the proposed constmction of a submerged reef {i.e., submerged breakwater) to protect the south paridng area needs to Include provisions for post-construction monitoring. The primary concem is the crest elevation of the reef. The lake bottom on which this proposed staicture would be built is presently eroding, and erosion will continue after the structure is built. This erosion could cause shifting and settling of the reef stone which would lower the crest elevation. As the crest elevation is lowered, the stmcture will become less effective in reducing incoming wave energy. Monitoring of the structure will determine if and when additional stone may be needed. 3) A bathymetric survey is wananted for the entire marina basin to compare present depths with constmction depths. Accretion has been identified for some time at the marina entrance and in the channel leading to the recreational basin. This study documents that between 1992 and 1995 about 1 ft of accretion occun-ed just Inside the marina entrance, over 3 ft of accretion occun-ed locally at the approach to the commercial basin, and up to 6 ft of accretion occun-ed related to the northward advance of a shoal area on the north tip of the south breakwater. A thorough, basin-wide survey is needed to fully assess the accretion trends. 4) A survey is wan-anted to compare present crest elevations along the north and south breakwaters with the "as built" crest elevations that existed soon after construction. The 111 possibility exists that subsidence has occurred along several segments of both breakwaters. A systematic monitoring of reference points on selected stones is recommended to determine changes in location and elevation. A diver survey of unden^^ater conditions is recommended. North Unit / Illinois Beach State Park 1) If It is desirable to maintain beach width in the Spring Bluff area, then either structural measures {i.e., breakwaters, artificial headlands, etc.) or a commitment for continued beach nourishment must be considered. No significant sediment supply reaches this shore from updrift. Without maintaining a nourishment program, severe shoreline recession will continue. 2) As of 1995, the best estimate for the nourishment volume needed at the updrift end of Spring Bluff beach to create a balanced sediment budget is 68,400 cu yds/yr. This should be considered a minimum estimate and a preliminary estimate that will be refined with the collection of additional annual data. An annual nourishment volume at the North Unit nourishment site of less than 68,400 cu yds/yr will slow the rate of both shoreline recession and nearshore erosion, but net erosion will persist. 3) Long-term coastal management options should be considered for the shore between the marina/state parte boundary and the Camp Logan headland. If beach nourishment is continued, vy^hout an increase in the annual nourishment volume, loss of beach area will continue. If the beach is allowed to erode into an equilibrium shape, provision must be made for some erosion extending landward of the existing vegetation line. Construction of shore structures are an option. Offshore structures could be built to reduce incoming wave energy and reduce erosion. One or more constructed headlands could be built to allow the shore to erode into a series of arcuate embayments. The closer the spacing of the constructed headlands, the more the maximum landward extent of shoreline recession will be reduced. South Unit / Illinois Beach State Park 1) The South Unit nourishment site used in 1995 has the advantage of easy access for truck delivery of nourishment, but the site places restrictions on the width and height of the nourishment stockpile and results in a stockpile that is vulnerable to rapid erosion even in moderate wave conditions. Most importantly, because of Its location, this nourishment site benefits a shorter reach of the DNR shoreline. If a given volume of nourishment is to be divided between the North and South Unit nourishment sites, a 60/40 or 75/25 split would be prudent with the major contribution going to the North Unit. With time, the benefits from this North Unit nourishment will translate downdrift to the South Unit beaches and nearshore. 112 SUMMARY OF YEAR-1 KEY FINDINGS The primary goal of this four-year study is to develop a sediment budget for the beaches and nearshore along the coast at North Point Marina and Illinois Beach State Pari^. Achieving this goal will provide the scientific basis for designing and implementing beach nourishment, shore defense, and other coastal management strategies for the long-term conservation of the area's coastal resources. A minimum of four years of data collection and analysis is needed before a thorough sediment budget can be developed. In the interim, data collected on an annual basis has value for ongoing coastal management concems. Follov^/ing is a summary of the major findings resulting from the first-year data collection and synthesis. North Point Marina Overall Erosion/Accretion Trends 1) Comparison of 1992 and 1995 bathymetric data indicates that during these three years, the lake bottom in the marina vicinity was divided into two distinct net accretional areas, and one extensive net erosional area. Net lake-bottom accretion dominated between the state line and the north breakwater, and in the marina entrance. Net lake- bottom erosion dominated from the marina entrance southward to the south partying area, and beyond. The most severe lake-bottom erosion between the state line and the marina/state park boundary occunred opposite the south parking area. North Beach 1) The shoreline position and total beach area at North Beach have been fairiy constant since the marina was constructed. However, persistent accretion has occurred in the shallow nearshore and in association with the development of a nearshore bar. The erosion/accretion reconj from 1992 to 1995 for the nearshore between the state line and the north breakwater indicates that, on average, net accretion occun-ed at a rate of 10,300 cu yds/yr. Marina Entrance 1) Bathymetric data obtained by ISGS is limited within the marina basin, but available data in the marina entrance area document that there has been persistent sediment entrapment since eariy post-construction. A major accretion feature is a broad shoal area built around the north tip of the south breakwater. Between 1992 and 1995, lateral accretion extended the shoal northward about 60 ft and resulted in up to 6 ft of vertical accretion. Vertical accretion of 1 to 3 ft occun-ed in the approach to the commercial basin. The three-year average accretion rate for the marina entrance was 3,100 cu 113 yds/yr. Bathymetric comparisons suggest that, between 1992 and 1995, one of the sources of sediment coming into the marina was from the north by way of bypass around the north breakwater. Additional sediment may have originated in erosion areas south of the marina entrance. South Parking Area 1) A spatial analysis was performed to determine the volume of sand and gravel beneath the south partying area that was derived from 1987-1988 dredging of the marina basin. This volume is 295,100 cu yds. Because of shore defense at the south partying area, this sediment is being prevented from ever re-entering littoral transport. Based on an estimated total dredge volume of 1.5 million cubic yards, the 295,100 cu yds beneath the parking area represents about 20 percent of the total volume dredged from the marina basin. If this volume was available for addition to the littoral transport stream, it would be adequate to supply sediment to the nearshore for about 3 to 4 years. If this sediment had not been protected, and if no other nourishment had been added to the area, this sand reservoir would likely have been depleted by 1995. 2) Between 1992 and 1995, lake-bottom erosion was pervasive across the nearshore opposite the south partying area. The most severe erosion was concentrated close to shore at depths less than 12 ft LWD. Most vertical erosion was in the range of 1 to 2 ft, with a maximum of 5 ft. In 1995, about 80 percent of the lake bottom landward of 12 ft LWD had been eroded to depths greater than those present in 1992. Between the shoreline and 400 ft offshore, erosion had lowered the lake bottom at an average rate of 0.25 ft/yr. Submerged Riprap 1) Comparison of 1991 and 1995 bathymetric data in the vicinity of the submerged riprap Indicate that, in general, this rock has been a beneficial feature contributing to reduced lake-bottom erosion lakewanj of the south partying area. However, the interaction of waves with this submerged riprap has contributed to localized erosion between the riprap and the shore resulting in a trough-like depression. Most of the riprap continued to subside between 1992 and 1995 as erosion lowered the surrounding lake bottom. As of 1995, elevation of the riprap crest is to 2 ft LWD in the north where it rests on the slope of the south breakwater, and 3 to 5 ft LWD to the south where it rests on sand. The submerged riprap remains a navigational hazard for any vessel with a draft exceeding about 2 ft. Breakwaters 1) Visual inspection of the north and south outer breakwaters suggests that localized 114 sagging has occurred. One of the most pronounced sags occurs along the north breakwater where it faces northeast, which is the direction of predominant wave approach. A lake-bottom depression has been a dynamic feature along the base of this breakwater since 1988, and may relate to a winnowing of sand from beneath the stmdure. In 1991 and 1992, this depression was 3.5 ft below the base elevation of the breakwater. Illinois Beach State Park / North Unit Overall Erosion/Accretion Trends 1) Bathymetric data collected in 1995 extended from the marina/park boundary to the Camp Logan headland. Comparison of 1992 and 1995 data indicates net erosion dominated across the lake bottom during these three years. Net shoreline recession also occun-ed, but the 1995 shoreline is still lakeward of the 1987 (pre-construction) shoreline. Beach Nourishment 1) In September 1994, 34,000 cu yds of fine gravel ("pea gravel") was placed at the North Unit nourishment stockpile. The majority of this material had been dispersed southward by summer 1995. The leading edge of this dispersion indicates that this material moved downdrifl along the beach at a minimum annual rate of about 0.6 mile/yr. This provides a minimum estimate for transport of fine gravel moving along the beach. More rapid transport would occur for sand such as the nourishment placed here In July 1995. 2) Nourishment totaling 20,000 cu yds of fine to medium sand was placed at the North Unit nourishment stockpile in July 1995. Wave action had dispersed most of this nourishment by November 1995. A single storm on September 7-8 accounted for neariy all of the dispersion. Nearshore profile data indicate that, once the sand was eroded from the stockpile. It moved downdrift in a nan-ow band within about 250 ft of the shoreline. Thus the nourishment was residing in the shallow nearshore as intended and not being dispersed or lost into offshore areas. Shoreline Changes 1) Shorelines between the marina/pari< boundary and the Camp Logan headland were compared for 1987 through 1995. The 0-ft LWD contours were also compared. As of 1995, this reach still has a net gain in beach area compared to 1987 (prior to marina construction). This is a result of the large volume of sand added to the area by southward dispersion of dredged material between 1987 and 1989. Since shoreline recession resumed in 1989, the 13-acre net gain In beach area between 1987 and 1989 115 has been reduced to 5 acres as of 1995. Between 1989 and 1995, the average rate of loss in beach area was approximately 1.3 acres/yr. Illinois Beach State Park / South Unit Beach Nourishment 1) In June and July 1995, beach nourishment totaling 24,000 cu yds of fine to medium sand was placed along 2,400 ft of riprap-defended shore at the north end of the South Unit. The storm of September 7-8 removed all but a remnant of the stockpile. By the end of November, dispersion was essentially complete. As was the case in the North Unit, the sand dispersion occurred in a narrow band in the shallow nearshore. Regional Coastal Monitoring Preliminary Sediment Budget 1) A minimum estimate of 14,200 cu yds/yr can be made for the volume of littoral sand moving southward across the WI-IL state line between 1992 and 1995. This estimate is based on the summation of annual net accretion in the North Beach nearshore (10,300 cu yds/yr), a conservative estimate for the accretion in the marina entrance derived from bypass of the north breakwater (1,500 cu yds/yr), and accretion in a nearshore feature lakeward of the south breakwater (2,400 cu yds/yr) . This is a minimum estimate since it is not known how much littoral sediment bypasses the marina. The volume crossing the state line would be greater if not for the dredging and updrift disposal of sand that is captured in the entrance channel to Prairie Harbor Yacht Club. 2) The summation of net erosional volumes for 1992 to 1995 across the nearshore between the North Point Marina entrance and the Camp Logan headland provides a minimum littoral transport rate of 37,800 cu yds/yr passing the Camp Logan headland. 3) Dredge records compiled for 1958 through 1990 from the Commonwealth Edison Waukegan Generating Station and Waukegan Hartaor provide a minimum littoral transport rate estimate of 68.400 cu yds/yr at the south end of the IBSP South Unit. 4) If the wave energy along this entire coastal reach is sufficient to transport a minimum littoral sediment volume of 68,400 cu yds/yr, the annual nourishment supplied to the North Unit needs to be a minimum of 68,400 cu yds/yr to create a balanced sediment budget along the North Unit nearshore. 116 ACKNOWLEDGMENTS Logistical support during field studies was provided by numerous members of the staff at North Point Marina and Illinois Beach State Park. Special thanks are due to Jim LaBelle, General Manager at North Point Marina, Charies Price, Harbormaster at North Point Marina, and Robert Grosso, Site Superintendent at Illinois Beach State Park. For contributions to much of the 1987 through 1992 North Point Marina data collection and maps presented in this report, special recognition is due to Paul Terpstra, Christine Fucciolo, and Christopher Rompot of the Illinois State Geological Survey (ISGS). Field studies at North Point Marina between 1988 and 1992 were part of a cooperative study focusing on coastal erosion in southern Lake Michigan funded by the U.S. Geological Survey Center for Coastal Geology. Technical assistance in 1995 field studies and In the compilation of map data were provided by Douglas Mulvey of the ISGS. Special appreciation is extended to T. B. Piatt, Regulatory Compliance Engineer at the Commonwealth Edison Waukegan Generating Station, who provided data on dredging at the station. This ongoing study is funded by a cooperative effort within the Illinois Department of Natural Resources (DNR). Primary funding was provided by the DNR Office of Capital Development; additional funding was provided by the DNR Illinois State Geological Survey (ISGS). REFERENCES Booth, J. S., 1994, Wave climate and nearshore lakebed response, Illinois Beach State Parte, Lake Michigan: Joumal of Great Lakes Research, v. 20, no. 1, pp. 163-178. Chrzastowski, M. J. and C. B. Trask. 1995. Nearshore geology and geological processes along the Illinois shore of Lake Michigan from Waukegan Harbor to Wilmette Hartjor (Contribution to the U.S. Army Corps of Engineers Illinois Shoreline Erosion Interim IV Study): Illinois State Geological Survey, Open File Series 1995-10, Champaign, Illinois, 93 p. Circe, R. and D. Blackwood, 1991, Undenvater observations of breakwaters at North Point Marina, Winthrop HartDor, Illinois: U.S. Geological Survey Open File Report 91-327, 31 P- Illinois State Geological Survey. 1988. Coastal Atlas, Illinois shore of Lake Michigan, revised 1987-1988: Contract report for Illinois Department of Transportation, Division of Water Resources, Obligation WR08819. Illinois State Geological Survey, Champaign, Illinois, 59 maps, scale 1:4800. 117 Jennings, J. R., 1990, 150-year erosion history at a beach ridge and dune plain on the Illinois Lake Michigan shore: Program and Abstracts, International Association of Great Lakes Research, 33rd Conference, May 13-17, 1990, Windsor, Ontario, p. 67. Larsen, C. E., 1985, A stratigraphic study of beach features on the southwestern shore of Lake Michigan, new evidence of Holocene lake level fluctuations: Illinois State Geological Survey, Environmental Geology Notes 112, Champaign, Illinois, 31 p. Moffatt & Nichol Engineers, 1986, Potential impacts of the Illinois Beach State Parte Marina on littoral processes: Moffatt & Nichol, Engineers, Walnut Creek, Califomia (prepared for Epstein Civil Engineering, Inc., Chicago), 132 p. Moffatt & Nichol Engineers, 1990, On the shoaling of the entrance to North Point Marina: Moffatt & Nichol Engineers, Walnut Creek, Califomia (prepared for Epstein Civil Engineering, Inc. Chicago), 40 p. Patrick Engineering Inc.. 1993, Program analysis report / erosion protection, south end North Point Marina, Lake County, Illinois (CDB Project No. 102-350-002; ICX)C File No. 2-93- 03): Report to the Illinois Department of Conservation, Patrick Engineering Inc., Glen Ellyn, Illinois, 29 p. plus exhibits. Patrick Engineering Inc.. 1995, Joint application for Section 404 perniit / erosion protection, south end North Point Marina, Lake County. Illinois (CDB Project No. 102-350-002; IDOC File No. 2-93-03): Patrick Engineering Inc.. Glen Ellyn, Illinois, 14 p. plus photographs, drawings, and appendices. Schwartz. M. L. (ed.). 1982. The Encyclopedia of Beaches and Coastal Environments: Hutchinson Ross Publishing Co., Stroudsburg, Pennsylvania, 940 p. State of Illinois, Division of Waten^/ays, 1958, Interim report for erosion control, Illinois shore of Lake Michigan: State of Illinois, Department of Public Worics and Buildings, Division of Waterways, Springfield, Illinois, 108 p., 27 plates, 13 exhibits. Terpstra. P. D. and M. J. Chrzastowski, 1992, Geometric trends in the evolution of a small log- spiral embayment on the Illinois shore of Lake Michigan: Journal of Coastal Research, V. 8, no. 3, pp. 603-617. Tetra Tech Inc., 1980, Summary plan for Lake Michigan shore erosion protection. Appendix 6, Shore protection plan for Illinois Beach State Parte: Draft report submitted to Division of Water Resources, Illinois Department of Transportation, Tetra Tech, Inc.. Pasadena. Califomia, 57 p. U. S. Army Corps of Engineers. 1953. Illinois shore of Lake Michigan, beach erosion control study. 83rd Congress. 1st Session. House Doc. No. 28. 137 p. plus 21 sheets. 118 APPENDIX A Annual Bathymetric Contour Maps of North Point Marina Vicinity 1987-1992 The series of six annual bathymetric maps of Appendix A are based on fathometer surveys completed by the ISGS. Mapping in 1987 was funded through State of Illinois expenditures related to construction of North Point Marina. Mapping in 1988 through 1992 was funded in part through a cooperative study with the U.S. Geological Survey, examining coastal erosion in southern Lake Michigan. The 1987 bathymetric map documents the pre-construction nearshore bathymetry. All contours are in feet referenced to Low Water Datum (LWD). Contours are based on east- west fathometer profiles. Profile locations and numbers are shown on the right margin of each map. These maps were plotted using the ARC/INFO Geographic Information System (GIS) at the ISGS. 119 120 Map A-1 >- >- in tr- Qi ' 2 y. ^ M ~ O C3 s c ■> z >- < p 1 T' < s q; CQ s tti n < CQ U ? IX cc — ' o o t- ^ ■ ! ; ^ 2 Q_ 7-i c O Q. O < E >- :i: ^ z Z % t- o s CO 05 c 121 Map A-2 •?' < 2: m lC < m M S 2; < c i^ "^ rr -=1 ^ y. o a; ^^ ~ M X z z •x. S; CO u c^ J IJIil i ^ • 3 1 1 .11;! ? • 1 1 iljf 1 1 j 1 i i 1 ! 122 Map A-3 123 Map A-4 - ^ o 2 IT, ^ " M ^^ s > — ,-— ■< — •^ ~ 2 ^ -^ i: a; c CQ :2 u: c ^ 1^ :=L 2 Ll. r. - ^ c; en - ^ -^ < = ^ CiJ x: ~ z 2 ^ ^ * Qi c c^ - L I iV f I ' i ! I I : s ' . I 5 ; ! =: f« i I ? I I J j f J ' . 'I ' I i I ! ^M 1 j ' n = a 124 Map A-5 >- >- t— — >- < — ^ < _ a: m Qi O < g M ^ a: c Z u_ OT Qi ^-- ^ < '-- ^ W i- z ='. -'ill-' . J iillililiiiil 1 : i 1 H ; !- H i- 125 Map A-6 APPENDIX B Annual Lake-Bottom Change Maps for North Point Marina Vicinity 1987-1992 The series of five annual lake-bottom change maps of Appendix B are based on comparison of bathymetry recorded in the successive bathymetric maps presented in Appendix A. Erosion and accretion areas are distinguished by different map patterns. Only areas of lake-bottom change greater than 1 foot are shown. Contours are in 1-foot increments. The maximum contour value is shovAni as a reference in the map areas where congested or numerous contours prevent good contour resolution. The table below summarizes major lake-bottom changes depicted on each map. Summary of annual lake-bottom changes in the vicinity of North Point Marina (1987-1992). 1987-1988 Map B-1 Overview This interval spans from the initiation of basin dredging to the advanced stage of breakwater construction. This interval captures earliest lake-bottom changes associated with marina construction. Erosion Maximum erosion contours were 3 and 4 ft in patchy areas lakeward of the south breakwater. This erosion was possibly a localized lake-bottom adjustment to the breakwater construction. Accretion Accretion extended along the nearshore from the south side of the marina to Camp Logan. The maximum accretion contour was 8 ft at the fan delta. Net change Accretion 1988-1989 Map B-2 Overview The prominent lake-bottom change was an accretional band extending from the marina entrance to Camp Logan. This resulted from both northward and southward dispersion of sediment from the fan delta. Erosion A maximum erosion contour of 6 ft occun-ed along the state park North Unit. This relates to lateral shift of the axis of a nearshore bar. A 5 ft erosion contour occurred adjacent to the northeastern skJe of the north breakwater. This erosion is suggestive of wave-induced scour along the breakwater toe. Accretion Maximum accretion contours of 8 ft occurred at the fan delta and marina entrance. Net change Accretion 126 1989-1990 MapB-3 Overview Erosion dominated between 1989 and 1990. This was also the interval of greatest erosion in the initial six years (1 987-1 992) of data collection. Discharge of dredge material to the fan delta ceased in 1988; the nearshore was rapidly eroding by 1990. Some erosion was rapid and severe due to the instability of the discharged dredged sediments and steep and unstable underwater slopes. Erosion Maximum erosion contours of 6 and 7 ft occurred in the shallow nearshore downdrift of the fan delta. Accretion Two major accretion areas occurred. One related to development of the North Beach bar extending from the WI-IL state line to just south of the southern end of the north breakwater. The other occurred in the marina entrance adjacent to the north end of the south breakwater. Net change Erosion 1990-1991 MapB-4 Overview Both accretion and erosion occurred between the state line and the north breakwater. This is interpreted as a response to both new sediment coming across the state line and redistribution of existing sediment. Between the marina and Camp Logan, changes were less extensive and less extreme compared to 1989-1990. Beach nourishment placed south of the south parking area In 1990 was a sediment source that likely alleviated erosional trends across the nearshore. Erosion A maximum erosion contour of 6 ft occurred landward of the submerged riprap. This area had been above lake level in 1989. but as the riprap subsided, erosion occurred landward of this riprap. A 5 ft erosion contour occurred off the northeast skie of the north breakwater and also off the breakwater's southem end. Erosion also occurred along the shoreline and shallow nearshore south of the south parking area. Accretion Accretion off the southem end of the north breakwater corresponded to infilling of an erosional area that was present in the 1 989-1 990 comparison. Between the south parking area and Camp Logan, accretion occurred in a series of nearshore bars. Net change Erosion 1991-1992 MapB-5 Overview Changes between the state line and the north breakwater were less pronounced in 1991-1992 compared to 1990-1991. The 1991-1992 erosion lakeward of the south parking area was less severe than in 1 990-1 991 , but south of the parking area the erosion was more v^espread and severe. Erosion Erosion dominated between the marina south parking area and Camp Logan. A nearly continuous band of erosion occurred in the shallow nearshore where maximum erosion contours were 6 ft. Accretion Both erosion and accretion occurred near the marina entrance, but accretion dominated. A maximum accretion contour of 7 ft occurred west of the north end of the south breakwater. Net change Erosion 127 >- »_ a: 2 ud C3 <3; < < •X. u E- X "3 O a. a. o K o X E- 7- m cd o s < 2: o q; o o < 2 O o 128 Map B-1 ^ O F- OJ Z 'n 00 CO ^^ C2 cc > z < -J -J in z a a: < c m < 2 < u P X 2 c_ r^ H- Cl l— ri X 2 m [— rr * LxJ c it; 129 Map B-2 O 0^ >- ro 1- 2 ro 00 o o^ > 2 < -J -J 2 o - U3 E- 2 v: 5§ O 05 > — < ^ £ rr O q; o z < S < o Z Q. s ^ o o 9 K E- Q- x ^ £- O I 2 cn ^ S 1 ce ^ w o :^ 2 < 131 Map B-4 > z o £ i u o o q; < u 132 Map B-5 APPENDIX C Profiles across the submerged riprap 1990 - 1995 The five profiles (Profiles 36 through 41) are based on fathometer data. The index map below shows the location of the five profiles. All five profiles show that the net lake-bottom change is erosion, and this has consistently occurred above 15 ft LWD. The 1990 profiles have their landward end against the riprap which was then still emergent. By the time of the 1991 survey, sufficient subsidence had occurred for fathometer data to be collected across the riprap. Profile horizontal reference Easting 644,000 ft South breakwater ISGS Profile number 36 37 38 39 40 41 L Concrete-cube and riprap revetment 133 i o £ a. «i o Profile 36 (Northing N2119000; Illinois State Plane Coordinates) Distance east of Easting E644000 (feel) 500 600 700 800 900 100 150 200 250 Distance east of Easting E644000 (meters) 300 350 Mlinott SOU OwMoglcal Survay E 1300 O <* ■ 1 "S 400 Profile 37 (Northing N2118900; Illinois State Plane Coordinates) 150 200 250 Distance east of Easting E644000 (meters) 300 350 Illinois Stat* Geological Survey 400 w Profile 38 (Northing N211B800; Illinois State Plane Coordinates) Distance east of Easting E644000 (feet) 500 600 700 800 900 E 1300 SO 100 150 200 250 Distance east of Easting E644000 (meters) 300 350 Illinois sot* OMtoglcal Survey 400 134 w a. « o Profile 39 (Northing N2118700; Illinois State Plane Coordinates 18 4 100 200 300 400 Distance east of Easting E644000 (feet) 500 600 700 800 900 1000 1100 1200 2 ' ' 1 1 1 1 1% X A ■ \u jA 6 ■ v^^^f!^tm. 8 XIT %r^ 10 [ -S.-^ ^^fe^ 12 ^^'^^iSar-^^ 14- ^^^^^5^*=- 16 ■ V.E. =20x 50 100 150 200 250 Distance east of Easting E644000 (meters) 300 350 Hlinots Stat* O«otogtcil Survey Profile 40 (Northing N21 18600; Illinois State Plane Coordinates) Distance east of Easting E644000 (feet) 500 600 700 800 900 E 1300 150 200 250 Distance east of Easting E644000 (meters) 300 350 Illinois stata OMtoglcal Survay w 2 + Profile 41 (Northing N21 18500; Illinois State Plane Coordinates) 100 200 300 400 Distance east of Easting E644000 (feet) 500 600 700 800 900 1000 1100 1200 E 1300 50 100 150 200 250 Distance east of Easting E644000 (meters) 300 350 Illinois Stat* O«ologlcal Survay 400 135 APPENDIX D Maps related to proposed shore defense at the NPM south parking area Maps D-1 and D-2 relate to plans for proposed shore defense at the south parking area. A new revetment along the shore is proposed in cx>njunction with a submerged, nearshore reef. Map D-1 shows the proposed location of the reef superimposed on the local 1995 bathymetry. Map D-2 shows the location of the reef superimposed on an isopach map of 1992-1995 lake-bottom changes. Project plans for the reef were obtained from Patrick Engineering, Inc. (1995). 136 1995 NEARSHORE BATHYMETRY North Point Marina - South Parking Area Basea on Batnymelnc Data Collected July 15-20. 1995 — — .._.^..._ July 1995 (Post-Nourishment) Shoreline Cultural Features (1992) — I — Illinois State Plane Coordinates North Point Marina 2.119,200 N 8 9 10 11 12 13 2.117,300 N- Feet Contour interval 1 foot 100 200 300 644.000 E 50 Meters 100 137 Map D-1 1992-1D95 LAKE-BOTTOM CHANGES North Point Manna - Soulh Parking Area Based on Comparison o( Balhymctnc Data From September 1992 and July 16-20, 1905 North Point Manna September 1992 Shoreline September 1992 0' LWD Contour July 1995 (Post-Nourishment) Shoreline July 1995 (Post-Nourishmenl) 0' LWD Contour Cultural Features (1992) Limits to Data Comparison Illinois State Plane Coordinates 2.1 17.300 N 644,000 E Feet 100 200 300 Contour interval 1 foot 50 Meiers 100 138 Map D-2 APPENDIX E Diver report from 1990 survey along the NPM north breakwater The following text and illustration are taken from: U.S. Geological Survey Open File Report 91-327 Underwater Observations of Breakwaters at North Point Marina, Winthrop Harbor, Illinois by Ronald Circe and Dann Blackwood 1991 Dive Site Number 1 : Lakeward Side of North Breakwater Sand waves were observed In water depths of 1 to 2 meters. The sand waves had heights of at}out 5 cm and lengths of about 250 cm. The field of sand waves extended lakeward of the breakwater to a distance of 6 meters. Using an underwater compass, rt was noted that the crests of the sand waves were oriented in a NW-SE direction (Fig. 1). BoukJers from the rubble-mound t>reakwater were obsen/ed as fer as 6 meters lakeward of the breakwater. These displaced rocks were first thought to have moved relative to the breakwater. After reviewing copies of the blueprints for construction of the breakwater, it is possible that these rocks are in, or close to, their intended location. Sediment at the base of the t)reakwater was generally well-sorted, fine sand, but pebbles and cobbles (up to 20 cm in diameter) were also present. These pebble/cobble beds were typically confined to a 1 -meter wide zone adjacent to the breakwater. Evidence of scouring was seen along the entire t}ase of the breakwater within the area of observation. Figure 1 shows a diagrammatic cross-section of the North Breakwater with the sand waves, possible displaced rocks, and the scour area. Due to limited visibility, no underwater photographs were taken at this site. Appendix E / Figure 1 (from Circe and Blackwood, 1991) 139 APPENDIX F Profiles across the 1995 nourishment stockpile at IBSP North Unit 25 1995 IBSP North Unit Nourishment Stockpile Profile 48 -5 V.E =10x Illinois State Geological Survey -d, 6/30/95 -• 7/28/95 -■ 9/8/95 -O 10/13/95 -• 1 1 /9/95 Approximate lake level, summer 1995 Low Water Datum -50 50 100 150 200 Distance in feet east of Easting 643825 250 300 140 1995 IBSP North Unit Nourishment Stockpile Profile 49 V.E. =10x Illinois State Geological Survey -6 6/30/95 -♦ 7/28/95 -• 9/8/95 -0 10/13/95 -• 11/9/95 Approximafe lake level, summer 1995 Low Water Datum -50 50 100 150 200 Distance in feet east of Easting 643825 250 300 141 o iu 25 1995 IBSP North Unit Nourishment Stockpile Profile 50 20 - -a 6/30/95 -* 7/28/95 -■ 9/8/95 -0 10/13/95 -• 1 1 /9/95 • - • Approximate lake level, summer 1995 Low Water Datum -5 V.E. =10x Illinois State Geological Survey -50 50 100 150 200 Distance in feet east of Easting 64382S 250 300 142 25 1995 IBSP North Unit Nourishment Stockpile Profile 51 -t6 6/30/95 -•^7/28/95 -■ 9/8/95 -0 10/13/95 -• 11/9/95 — Approximate lake level, summer 1995 - • Low Water Datum -5 V.E. =10x Illinois State Geological Survey -50 50 100 150 200 Distance in feet east of Easting 64382S 250 300 143 APPENDIX G Profiles across the 1995 nourishment stockpile at IBSP South Unit 15 1995 IBSP South Unit Nourishment Stockpile dio ^ 5 North Bathhouse Profile ^. iFirst three survey i I points located on { i access road j - - — Approximate lake level, summer 1995 Low Water Datum • • — Approximate slope on rip-rap, 6/1S/95 V.E. =10x Illinois State Geological Survey -5 ^ 50 100 150 200 250 Distance in feet east of temporary l>enctiinarfc at Nortli Batlihouse 300 350 144 1995 IBSP South Unit Nourishment Stockpile Grass Knoll Profile 100 150 200 250 Distance in feet east ol temporary benchmark at Grass Knoll 15 1995 IBSP South Unit Nourishment Stockpile South Bathhouse Profile 10 - ■^ o First three ' ^ survey points ? ' located on | i o -1 ^ access road «< -A 6/29/95 • 7/11/95 -a 8«/95 -■ 9/8/95 -*— 10/12/95 • • — Approximate lake level, summef 1995 - - • ■ Lcm Water Datum + Not surveyed on 1 1/8/95 V.E. =10x Illinois State Geological Survey -5+- 50 100 150 200 250 Distance in leet east ol temporary benchmark at South Bathhouse 300 350 145 APPENDIX H Regional scheme for nearshore profiling from WI-IL state line to Waukegan Harbor In •'Wisconsin Rang* hnes astablishad by U.S. Army Corps of EnginMrs witn profile data originating in 1872 Intermadiate rang* lines to be astablisbed in 1996 by INifwis State Geological Survey 10.000 Feet 2 Mies _l 3 Kilometers eaw nae: us. Ortngm 9wv