l ges in Rates of Shore Retreat, Lake Michigan, 1967-76 aatiee by oO : Y i Edward B. Ha nds ICAL PAPER NO. 79-4 _ DECEMBER 1979 {80uEs99 9101} spoon, ude i ISL Oo UGI E R Ri Reprint or republication of any of this material shall give appropriate credit to the U.S. Army Coastal Engineering Research Center. Limited free distribution within the United States of single copies of this publication has been made by this Center. Additional copies are available from: National Technical Information Service ATTN: Operations Division 5285 Port Royal Road Springfield, Virginia 22161 The findings in this report are not to be construed as an official Department of the Army position unless so designated by other authorized documents. i INA HT shite i I 0 03 IN UNCLASSIFIED SECURITY CLASSIFICATION OF THIS PAGE (When Data Entered) READ INSTRUCTIONS REPORT DOCUMENTATION PAGE BEFORE COMPLETING FORM T. REPORT NUMBER 2. GOVT ACCESSION NO|| 3. RECIPIENT'S CATALOG NUMBER TP 79-4 - TITLE (and Subtitle) 5. TYPE OF REPORT & PERIOD COVERED CHANGES IN RATES OF SHORE RETREAT, LAKE MICHIGAN, 1967-76 Technical Paper 6. PERFORMING ORG. REPORT NUMBER 8. CONTRACT OR GRANT NUMBER(«) - AUTHOR(s) Edward B. Hands 10. PROGRAM ELEMENT, PROJECT, TASK AREA & WORK UNIT NUMBERS D31235 12. REPORT DATE 13. NUMBER OF PAGES 15. SECURITY CLASS. (of thia report) UNCLASSIFIED 15a. DECLASSIFICATION/ DOWNGRADING SCHEDULE Approved for public release; distribution unlimited. 9. PERFORMING ORGANIZATION NAME AND ADDRESS Department of the Army Coastal Engineering Research Center (CEREN-GE) Kingman Building, Fort Belvoir, Virginia 22060 = . CONTROLLING OFFICE NAME AND ADDRESS Department of the Army Coastal Engineering Research Center Kingman Building, Fort Belvoir, Virginia 22060 14. MONITORING AGENCY NAME & ADDRESS(if different from Controlling Office) 16. DISTRIBUTION STATEMENT (of this Report) . DISTRIBUTION STATEMENT (of the abstract entered in Block 20, if different from Report) - SUPPLEMENTARY NOTES . KEY WORDS (Continue on reverse side if necessary and identify by block number) Erosion prediction Lake levels Profiles Great Lakes Lake Michigan Submer gence 20. ABSTRACT em reverse aide if necessary and identify by block number) Shorelines tend to retreat landward as water levels rise. Less than 20 per- cent of the shore, lost as Lake Michigan rose between 1967 and 1976, was due to direct inundation; the remaining 80 percent was due to increased erosion in re- sponse to the higher lake levels. A simple correlation of lake level change and simultaneous shore retreat ignores the inevitable lag between process and re- sponse, but still accounts for 50 percent of the variance in shore retreat. A graphic summary of field data is presented to estimate effects of future lake level changes in similar coastal environments. Qualitative guidance is provided DD AS es 1473 ~—s EDITION OF ! NOV 65'S OBSOLETE UNCLASSIFIED a SECURITY CLASSIFICATION OF THIS PAGE (Wren Data Entered) UNCLASSIFIED SECURITY CLASSIFICATION OF THIS PAGE(When Data Entered) on how and when these estimates should be adjusted to reflect differences in environmental settings. Complete adjustment of the shore will be underestimated by the empirical relationship; but where lake levels change constantly, there will be many such instances of incomplete shore response. 2 ae OES et SECURITY CLASSIFICATION OF THIS PAGE(When Data Entered) UNCLASSIFIED PREFACE This report is published to provide coastal engineers with informa- tion on rates of shoreline recession and on changes in these rates during the most recent episode of high water levels on the Great Lakes. This interim report is part of a study to develop and evaluate a profile re- sponse model to explain the effects of rising water levels on shore erosion. The work was carried out under the sediment hydraulic inter- action program of the U.S. Army Coastal Engineering Research Center (CERC). This report was prepared by Edward B. Hands, under the general super- vision of Dr. C.H. Everts, Chief, Engineering Geology Branch, Engineering Development Division, CERC. Assistance of the following individuals and organizations is grate- fully acknowledged: the staff at Mears State Park who were extremely helpful during various data collection periods; P. Wood who assisted in the recovery of survey markers and generously provided transportation in the Silver Lake Dune area; the Tide and Water Level Branch, National Oceanic and Atmospheric Administration (NOAA), Rockville, Maryland, for providing lake level data; the Permit Branch, U.S. Army Engineer District, Detroit (NCE), for help in procuring aerial photography; the field office in Grand Haven, Michigan, for surveying bench marks in 1976; the 30th Engineering Battalion, Fort Belvoir, Virginia, for the 1976 profile survey; the National Ocean Survey (NOS), NOAA, for 1971 and 1975 profile surveys; and the Great Lakes Environmental Research Laboratory, NOAA (formerly the U.S. Lake Survey) for initiating shore-normal profiling in 1967 and 1969 at most of the sites used in this study. The author appreciates the manuscript review and the helpful comments by Dr. C.H. Everts, Dr. C.H. Carter, and the U.S. Army Engineer Division, North Central. Comments on this publication are invited. Approved for publication in accordance with Public Law 166, 79th Congress, approved 31 July 1945, as supplemented by Public Law 172, 88th Congress, approved 7 November 1963. ED E. BISHOP Colonel, Corps of Engineers Commander and Director II IV VI APPENDIX A CONTENTS CONVERSION FACTORS, U.S. CUSTOMARY TO METRIC (SI). TNGRODUGIAON Gy et enn Iv. Buspose. 2. Background . ae , 3. Previous Erosion Reports 6 FIELD METHODS. . 1. Study Area . 2. Survey Periods . 3. Profile Procedures . TERMINOLOGY. DATA PRESENTATION. 1. Shoreline Retreat. 2. Recession. 3. Encroachment DATA INTERPRETATION. 1. Spatial Variation in Retreat Rates 2. Temporal Variations in Average Retreat Rates 3. Effects of the Recent Lake Levels on Shore Retreat Rates 4. The Timelag Between Tee Teel Pencuebacion “and the Reestablishment of Profile Equilibrium. 5. Comparison of Recent and Historic Changes. 6. The Need to Adjust Recession Rates CONCLUSIONS. LITERATURE CITED A PROCEDURE FOR ADJUSTING RATES OF SHORE RETREAT TO COMPENSATE FOR WATER LEVEL DIFFERENCES , NEARSHORE PROFILE CHANGES. TABLES 1 Major increases in annual mean lake level - 2 Survey dates and shoreline positions 3 Comparison of historic with recent recession rates. 45 55 15 36 13 14 15 16 7 18 CONTENTS --Continued FIGURES Location of study area . Historic changes in annual mean water levels on the Great Lakes, 1860 to 1978 Profile stations in vicinity of Pentwater jetties. Station location in the study area . Terminology of retreat Terminology of vertical and horizontal shoreline changes Different formats depicting changes in the shoreline adjacent to the Pentwater jetties Change in shoreline position . Time changes in positions of various contours intersecting the beach face. A fairly typical inner profile . Encroachment versus recession as a cause of shoreline retreat. A shoreline indentation opposite the Little Sable Point light in August 1975 introduces variability in shore retreat as it migrates alongshore . Views of shoreline undulations which sometimes form where the inner bar merges with the shore . Distributions of measured retreat rates. Lake Michigan hydrograph showing changes in lake level between survey periods. Submergence versus retreat Historic shoreline changes in the vicinity of Pentwater jetties. Comparison between rates of historic and recent recession. Page 36 oS CONVERSION FACTORS, U.S. CUSTOMARY TO METRIC (SI) UNITS OF MEASUREMENT U.S. customary units of measurement used in this report can be converted to metric (SI) units as follows: inches 25.4 millimeters 2.54 centimeters Square inches 6.452 square centimeters cubic inches 16.39 cubic centimeters feet 30.48 centimeters 0.3048 meters square feet 0.0929 square meters cubic feet 0.0283 cubic meters yards 0.9144 meters square yards 0.836 square meters cubic yards 0.7646 cubic meters miles 1.6093 kilometers square miles 259.0 hectares knots 1.852 kilometers per hour acres 0.4047 hectares foot-pounds 1.3558 newton meters millibars 1.0197 x 1073 kilograms per square centimeter ounces 28.35 grams pounds 453.6 grams 0.4536 kilograms ton, long 1.0160 metric tons ton, short 0.9072 metric tons degrees (angle) 0.01745 radians Fahrenheit degrees 5/9 Celsius degrees or Kelvins! 1T> obtain Celsius (C) temperature readings from Fahrenheit (F) readings, US® stopmmlas (© = (5/9) CF 52), To obtain Kelvin (K) readings, use formula: K = (5/9) (F -32) + 273.15. CHANGES IN RATES OF SHORE RETREAT, LAKE MICHIGAN, 1967-76 by Edward B. Hands I. INTRODUCTION 1 Purpose. Since 1967 the Coastal Engineering Research Center (CERC) has moni- tored beach profile development associated with a recent episode of sus- tained rising water levels on the Great Lakes. Ten profile stations were initially surveyed in 1967 by the U.S. Lake Survey (now a part of the National Oceanic and Atmospheric Administration-NOAA) for a littoral transport investigation at Pentwater Harbor, Michigan. Subsequently, the number of stations was expanded to encompass a 55-kilometer stretch of shore between Summit and Meinert Parks on the eastern shore of Lake Michigan (Fig. 1); 34 stations were surveyed by CERC at various 1-week to 4-year intervals between 1967 and 1976 to, determine the nature of the long-term beach changes. This period of data collection overlaps a period of above-average precipitation in the Great Lakes Basin when the mean annual elevation of Lake Michigan rose 0.8 meter between 1967 and 1973. This report presents a summary of the changes in rates of shore retreat associated with this long-term increase in lake levels. Study Area “ONTARIO Figure 1. Location of study area. De Background. Alternating periods of sustained rise and fall are characteristic of the annual mean surface elevations on the Great Lakes (Fig. 2). The cumulative effect of these persistent changes in lake levels frequently shifts monthly and annual mean surface elevations as much as a meter in a few years (Table 1). Table 1. Major increases in annual mean lake level. 1925-29 1949-52 1964-73 | (m) (ft) (m) (ft) (m)_ (ft) Ontario : Q,68 Bed 0.91 Erie 0 W555 1.8 1.14 Michigan-Huron c W583. ZsO 1.45 _ Superior ‘ 0.19 0.6 0.26 Although the changes in water level on the Great Lakes (Table 1) may not appear large relative to tidal ranges at many ocean beaches, the long-term, gradual nature of the lake level fluctuations increases their effect on shore erosion and property loss. During years of low water, new property owners easily acquire a false sense of shore stability and, as a result, often build structures too near the shore. Storm erosion during years when the mean lake levels are high accelerates the rate of shore retreat, and causes considerable destruction to shore property on the Great Lakes. The long duration of high water periods also allows time for a relatively broad area of the nearshore zone to adjust to the elevated water surface (Hands, 1976). This adjustment involves offshore transport of large volumes of beach material and, as a consequence, greater shore retreat. After lake levels have declined sufficiently to reverse conditions, the waves transport some material from offshore back on the beach at most localities. Rates of shore erosion fluctuate in response to the long-term hydro- logic cycle. The impact of high lake levels, while relatively strong on all the lower lakes, is relatively weak on Lake Superior where the variations in water level are small (Fig. 2) and rocky shorelines are common. No comprehensive survey of shore damage has been made on the U.S. side of the Great Lakes during the present episode of high water levels. The recent lake level rise is, however, similar to the previous rise which peaked in the early fifties (Fig. 2). A survey of economic loss sustained over a 12-month period coincident with the high levels of 1951-52 attri- buted $50 million worth of damage to wave erosion of U.S. property on the Great Lakes (U.S. Army Engineer Division, North Central, 1965). Consid- ering inflation and recent shoreline development, it has been estimated that a recurrence of 1951-52 storms and high water levels would cause a minimum of $120 million damage (Great Lakes Basin Commission, 1976). Lake Superior Power Diversion around ; Long Loke Diversion Storted St. Marys Ropids Full Regulation Started Ogoki Diversion Started Chort Dotum 600.0 ft (182.88 m) Lake Michigan-Huron An Average of 500 ft/s Into Chicago Diversion Increases Average Chicago Diversion Mississippi River at Chicago to 10,000 ft/s 3,200 ft3/s Chort Datum 576.8 ft (174.25 m) ———— Lake Erie Chort Dotum 568.6 ft (173.31 m) Lake Ontario St. Lawrence Seaway Gut Dam Completed Gut Dam Removed Completed 5 4 3 1.0 (m) (ft) 2 0.5 ! O}— Chort Datum 242.8 ft (74.01m) TSOAnT Sita 0 1860 1880 1900 1920 i940 1960 1978 (Yr) Figure 2. Historic changes in annual mean water levels on the Great Lakes, 1860 to 1978, Recurrently, during periods of extreme shore damage, there has been public pressure to increase control over lake level fluctuations. A recent investigation considered the feasibility of regulating the entire Great Lakes system (International Great Lakes Levels Board, 1973). Al- though regulation of all five lakes is an engineering possibility, bene- fits were not found to be commensurate with costs. Some control over lake levels already exists. Outflows from Lake Superior and from Lake Ontario have been controlled since 1921 and 1958, respectively (Fig. 2). However, regulation which reduces the range of levels on Lake Superior tends to increase fluctuations on Lakes Huron and Michigan. On the other hand, reduction of the range of levels on Lake Ontario can presently be accomplished without affecting the other lakes, because the major inflow is via Niagara Falls and the outflow is to the St. Lawrence Seaway via a series of control structures. Conse- quently, during the 1973-74 high water period, the outflow from Lake Ontario was increased 46 percent above its average flow to alleviate erosion and flooding problems. In spite of these improvements, both the water levels and the erosion problems remained significantly above their long-term average (Haras, 1975). Because uncontrollable natural variations in water supply are so large, it is impractical to attempt to maintain a constant volume of water in any of the lakes. Regulation plans, nevertheless, continue to be re- viewed to determine if modifications to currently controlled flows would reduce the total lake level-related damage to all concerns. Knowledge of how water level fluctuations affect erosion rates is important for deter- mining how changes in regulation plans will affect riparian interests. Knowledge of fluctuations in lake level and their effect on rates of shore retreat is also important in the design of coastal construction projects, in recommending coastal setback, for planning proper beach-fill operations, and in evaluating the usefulness of short-term erosion measurements as a basis for extrapolating to longer periods on the lakes. 3. Previous Erosion Reports. A number of previous studies related to shore erosion on the Great Lakes have been published by CERC. Shore changes were measured monthly from 1970 to 1974 at 17 sites widely scattered over Lake Michigan's eastern shore; results of the first 3 years of this study, reported by Davis, Fingleton, and Pritchett (1975) and Davis (1976) identify seasonal cycles in bluff retreat related to seasonal changes in lake level and storminess. Large, unexplainable spatial differences in bluff retreat were noted. It was hypothesized that these large differences might re- flect the influence of offshore bathymetry on shoaling waves. Well- developed, multiple longshore bars dominate the eastern lake-shore bathy- metry out to depths of about 6 meters (20 feet). Bars absorb part of the wave energy incident on the shore before it reaches the shore. The cross- sectional geometry, areal patterns, and textural composition of the long- shore bars are described in Hands (1976). On the basis of surveys spanning a 4-year period, Hands also briefly discussed gradual changes 10 in bar position, rates of shore retreat, and lake level change. A final CERC report (Hands, in preparation, 1979) will integrate bar migration, shore retreat, and lake level changes over the full period from 1967 to 1976; another final report will discuss the changes in shore retreat over the whole eastern shore between 1970 and 1974 (Birkemeier, in prepara- tion, 1979). Earlier CERC reports (Berg, 1965; Berg and Duane, 1968) cover long- term shore erosion and lake level changes and concern the behavior of beach fill at Presque Isle Peninsula on Lake Erie. Guidelines for moni- toring the effect of shore protection works in the Great Lakes are pre- sented in Coastal Engineering Research Center (1975). Publications discussing wide aspects of Great Lakes shore erosion include State and Federal Government reports, journals, and student theses too numerous to review here. A compilation of published and unpublished data on erosion of the U.S. shoreline was prepared by Armstrong, Seibel, and Alexander (1976). An atlas by Haras and Tsui (1975) persents data on land use, historic flood and erosion damage, ownership, value, and phys- ical characteristics for all the erodible Canadian shoreline of the Great Lakes. In 1976, the Canadian Government also initiated a 5-year program involving annual and poststorm surveys at approximately 160 stations on the Canadian shore of the Great Lakes. II. FIELD METHODS nee Study Area. Profiles taken in 1969 near Pentwater Harbor on the eastern shore of Lake Michigan revealed little variation in beach profiles beyond alter- nate ridge-and-runnel development. However, when the 1969 profiles were compared with profiles taken during an evaluation of longshore transport made 2 years earlier, a significant landward shift of the whole active profile became evident. To evaluate the apparent long-term profile evolution, surveys have been repeated at the 10 stations originally es- tablished in 1967 within a kilometer of the Pentwater Harbor (Fig. 3) and also at 24 additional stations spread over adjacent 55 kilometers (Fig. 4). 2. Survey Periods. Profiles are available for six different time periods: (a) summer of 1967, (b) spring of 1969, (c) fall of 1969, (d) spring of 1971, (e) fall of 1975, and (f) fall of 1976. Because the frequency of profiling and the extent of the study area changed progressively as previously collected data were analyzed, and the scale of changes was better understood, any given station may have been profiled up to four times during one of the survey periods. Over the years, as the shore continued to retreat, a series of reference monuments was established at most profile stations to provide local control if and when the base monument at that station was lost. The reference monuments were established above and landward of the original base monuments on the extended range azimuth for that particular A A Front and rear reference monuments on profile line Reference survey monuments | not on any profile line Scale in Meters 200 Figure 3. Profile stations in vicinity of Pentwater jetties. SS ae 0123545 at 4A Ss! Figure 4. Station location in the study area. station. Table 2 gives the dates when each of the 34 stations were pro- filed, along with the daily mean lake level, and the distance of the shoreline from the base monument. Negative numbers in the table indicate that the shoreline had passed landward of the base monument by the given date. 3. Profile Procedures. In 1967, the profiles were measured by a leveling cart. A leveling rod was attached to a four-wheel cart which was winched ashore. Every 5 meters the cart was halted and the elevation determined by an engineer's level located onshore. When the cart reached shore it was pulled by a Jeep down the beach to the next station and towed back offshore by boat. This method limited coverage to depths of less than 5 meters and required a moderately wide, unobstructed beach for efficient operations. In sub- sequent years, echo sounding was used to measure the outer part of the profile to a depth of 15 meters, but instrument leveling was still used to give overlapping coverage in shallow water and extend the profile to the dry beach. Since this report is concerned only with changes in shore erosion, no further discussion is made of the echo soundings or outer profiles. Instead, inner and outer profiles will be combined in a later report addressing the manner in which the entire active profile adjusted as lake levels rose (Hands, in preparation, 1979). After 1967, the elevations on the inner profile were determined at the top and toe of the bluff (if one existed), the upper and lower limit of the swash zone, and at 5-meter intervals between the dune and the first longshore bar, using the engineer's automatic level. Horizontal control was by tag line, except in 1976 when distances were obtained from stadia intercepts using the "three-wire technique''--the procedure most commonly followed by military topographic surveyors. Reference monuments were tied to existing bench marks and second-order control stations surveyed by National Ocean Survey (NOS) in 1973. Additional vertical reference was obtained during profiling operations using a system of water level gages, water surface rod-readings, and a portable stilling well placed near the shoreline at each station. Profile accuracy in the horizontal is on the order of 1 meter from the base monument along the original.azimuth. Ver- tical profile accuracy is about + 5 centimeters. III. TERMINOLOGY Precise definitions are given that refine the meaning of several familiar terms used in this report. Submergence refers to the sinking of a coastal area relative to the mean water surface regardless of cause. Submergence can result from either subsidence of the shore or increases in the elevation of the water surface. Emergence refers to the opposite relative displacement, and when expressed numerically, both emergence and submergence refer to length measurements in the vertical. Coastal planners and property owners are often more interested in the resulting horizontal change in shoreline position: shoreline retreat is any land- ward migration of the shoreline; advance is the lakeward migration of the shoreline. Table 2. Survey dates and shoreline positions. Station Lake level (m) (mn) 27 Sept. 1976 176.72 -7.6 6 Aug. 1975 176.95 -16.7 8 May 1971 176.70 -1.7 28 May 1969 176.57 3.0 27 Sept. 1976 176.78 -3.0 26 Aug. 1975 176.92 -11.7 3 May 1971 176.61 13.0 15 May 1969 176.55 8.2 14 Sept. 1976 176.71 -24.3 11 Aug. 1975 176.91 -22.5 June 1971 176.80 -5.5 N com May 1969 176.57 19 July 1967 176.33 ' Ww 25 Sept. 1976 176.71 -30. 14 5 Aug. 1975 176.93 -3 14 July 1967 176.33 15 NN ue Sept. 1976 176.71 30 Apr. 1969 176.45 11 Sept. 1976 176.76 T - aa : . Re eee - ——— a inne Negative values indicate the shoreline was by that date landward of the base monument. 1.0 0 8 1.0 0 5 1.0 .0 5 1.0 -0 .0 2.0 .0 a 2.0 .0 5 2.0 2 5 2.0 -0 oil 3.0 -0 .0 3.0 -0 4 3.0 .0 .9 3.0 5 0 of 3.0 7.5 0 3 3.5 0.7 .0 .6 3.5 2.0 -0 .3 3.5 0.5 -0 5 4.0 0.0 .0 4 4.0 S Aug. 1975 176.94 -19.5 16.0 8 4.0 5 May 1971 176.64 -5.0 16.0 8 4.0 21 May 1969 176.54 5.8 16.0 .0 4.0 21 July 1967 176.33 5, 7/ 17.0 .6 4.5 25 Sept. 1976 176.71 15.5 17.0 .6 4.5 S Aug. 1975 176.94 4.5 17.0 .0 4.5 21 July 1967 176.33 14.4 17.0 .0 5.0 25 Sept. 1976 176.71 24.5 19.0 0 5.0 5 Aug. 1975 176.94 12.4 19.0 5 5.0 4 May 1971 176.66 11.2 19.0 .0 5.0 21 May 1969 176.54 18.5 19.0 5 5.0 21 July 1967 176.33 20.5 20.0 7 6.0 12 Sept. 1976 176.77 5.1 20.0 .6 6.0 13 Aug. 1975 176.93 -5.2 21.0 5 6.0 5 May 1971 176.68 -0.6, 21.0 2 6.0 16 May 1969 176.53 8.6 23.0 3 6.0 27 July 1967 176.33 14.5 23.0 5 6.5 12 Sept. 1976 176.69 -5.4 23.0 .0 6.5 13 Aug. 1975 176.95 3.0 24.0 5 6.5 22 July 1967 176.33 22.3 24.0 6 7.0 12 Sept. 1976 176.77 -9.0 24.0 .0 7.0 13 Aug. 1975 176.92 -13.0 24.0 5 7.0 5 May 1971 176.67 3.8 26.0 .6 7.0 16 May 1969 176.53 8.0 26.0 a) 7.0 13 Aug. 1967 176.33 19.8 26.0 of 7.5 23 Sept. 1976 176.69 -8.5 26.0 5 705 15 Aug. 1975 176.93 -23.4 27.0 8 7.5 22 July 1967 176.33 9.0 27.0 3 8.0 11 Sept. 1976 176.77 -14.6 27.0 5 8.0 11 Aug. 1975 176.91 -23.7 28.0 5 8.0 6 May 1971 176.68 -6.1 28.0 4 8.0 21 May 1969 176.54 4.0 29.0 3 8.0 22 July 1S67 176.33 4.0 29.0 .8 9.0 12 Sept. 1976 176.77 4.8 29.0 al 9.0 13 Aug. 1975 176.92 -6.4 29.0 .0 9.0 5.0 0 25 0.0 12 -0 2 = The shoreline is the intersection of the beach with the stillwater surface or, if specified, some other datum (e.g., 176.33-meter shore- line). The relative elevation of the stillwater level can change with time. Submergence causes the shoreline to retreat by direct encroachment of the water over the land. Withdrawal of the water during emergence advances the shoreline. Total horizontal migration of the shoreline can be more or less than that caused by encroachment, depending on whether erosion or deposition prevails at the shoreline. The lateral migration of a spectfted contour is referred to as progradatton if the contour moves toward the center of the basin, and as recesston if the contour moves away from the basin. Shoreline retreat (Fig. 5) is thus an inclusive term referring to the total landward horizontal shift or the algebraic sum of encroachment (a function of submergence) plus recession (a function of erosion). Shore- line retreat implies that either local recession or encroachment has occurred, but is unspecific as to which (or both) is responsible for the landward shift in shoreline position. Retreat ern ~~ Recession I NJ Encroachment Submer Ao Se Me ws gence A se es Ss — ‘Profile adjusted to Submergence = Initial Profile Toa =. _—. — SF eS eee = Figure 5. Terminology of retreat. Retreat = encroachment + recession encroachment = AZ cot a. In geology, the terms transgression and recesston have definitions closely related to those discussed above. In fact, these terms were used by Hands (1976) to mean exactly the same thing as encroachment and with- drawal. The reason for substituting encroachment and withdrawal in this study is to avoid using two terms that sound so similar (recession and regression) in reference to two opposite shoreline changes. The meaning and hierarchy of the terms used in this report are shown in Figure 6. IV. DATA PRESENTATION 1. Shoreline Retreat. Although a simple procedure is used for plotting all shore retreat and recession data in this report, the format is slightly different from the usual method. A short step-by-step explanation is given below to 16 WITHDRAWAL WATER LEVEL FALLING f (Emergence) UPLIFT f (Land Rising) SUBSIDENCE f (Land Sinking) RECESSION (L] f (Erosion) PROGRADATION [L] f (Deposition) WATER LEVEL RISING ENCROACHMENT f (Submergence) a. Vertical Changes b. Horizontal Changes Figure 6. Terminology of vertical and horizontal shoreline changes. accustom the reader to viewing the data from the slightly different per- spective employed here. The most direct way to represent retreat of the shoreline would be to superimpose a set of shoreline maps. However, to depict on a page even a small part of the present study area, distances normal to shore would have to be exaggerated; otherwise, even where the shoreline has retreated 35 meters, the change would not be evident. For example, note that in the aerial photo at the bottom of Figure 7, all shoreline positions for the last 10 years would overlie one another and be indistinguishable at this scale. Expanding the scale perpendicular to shore pulls the shore- lines apart as shown above the photo. Note the expansion also greatly distorts shoreline shape. Since the primary interest is in shoreline re- treat, not shape, all attempts to show shoreline shape could be abandoned and all shorelines referenced to their position on either the initial or- the final survey. Because the year of initial surveying differs among stations, shoreline positions are referenced at the top of Figure 7 to their final positions (as determined in October 1976). Figure 7 shows a two-step transformation of shoreline data (from map view), first to exaggerated distance from base line, then to exaggerated distances from the 1976 shoreline. Because the shoreline protrudes about 10 kilometers lakeward in the vicinity of Little Sable Point (Fig. 4), it is infeasible to depict both shoreline shape and changes in shore position for the entire study area on the same figure; therefore, in all the re- Maining plots the shoreline and contour positions are referenced to their final positions as determined in the 1976 survey at each station. The plots will also have the same exaggeration of scale perpendicular to shore as shown at the top of Figure 7. Shore retreat throughout the study area is shown in Figure 8. Note that two different horizontal scales are used to permit comparison of the closely spaced measurements near Pentwater Harbor with measurements from the more widely spaced stations elsewhere. The straight lines con- necting 1967, 1969, and 1975 data points are plotted to quickly identify | 70 >) Offshore Distances from 1976 Shoreline (m) 17 X Alongshore ) Offshore Distance Expanded to HOLS honesoccle notes Unexaggerated 1976 Aerial View 0 -1,000 -500 0 500 1,000 Distance from South Jetty (m) LEGEND 1976 -——~ 1975 #—-— 1S) tae 17 X Alongshore Distance Figure 7. Different formats depicting changes in the shoreline adjacent to the Pentwater jetties. The top format (also used in Figures 8 and 9) was obtained from the unexaggerated aerial | view by a two-step transformation (A to B, then B to C). 18 °Z 8Tqe] UT USATS o1e sajzep AdAINS 4DeXJ “*p 9INBTY UT ST 9AIn}eAIND oUTT -9LOYS Ont} {pousjYysterys ATTeTITFIAAS ST SUTTSTOYS 9/6T SY, *UOTITSOd [TeUTF sjt JO PIEMOYeT SEM OUTTOTOYS dy} uvoW SoNTeA aATATSOd 9/6] UT poanseou uot ATSOd [eUTJF AITOY} O} paoUsTeFoI 91e SAaUTTeLOYS JeT{IeY ‘uotyTsod sutTeroys ut eBueyD °*g oIN3TY (WH) J@UUDYD J94OM}USq WO1J UOIZOeIIG AjJaYJJON D UI as0YysbuUo|y painsoaW eouUD4sSIG 0! G | om 0) Gi |= G- Ol i! OG] “OG=- Os SS (On fo) 1 fo) T \ | ¥402 soese@e s-@e29@ J OG| \ + - --+- —- 696) |; x i261 | — —e—r— 6/6! —_o—_0o— 9/6 | O€ e) el Al 9zez 10¥ (W) UOIFISOd (926]) JOUl4 WO1} 89UD4SIG FON SJOQWNN UOILD4S 19 measurements from common survey periods; the lines do not necessarily re- flect actual shoreline position between measurement stations. Large positive ordinate values in Figure 8 indicate points where the shore retreated a large distance. Extreme negative values indicate where the shoreline advanced a large distance lakeward. Where data points cluster closely, the shore remained relatively stable. The net retreat of the shoreline between 1969 and 1975 averaged 18 meters or 2.9 meters per year, but as Figure 8 shows, longshore varia- bility in retreat was extreme, ranging from 0.3 to 5.7 meters per year. Because of the large variation observed between adjacent stations, knowl- edge of the 6-year retreat at a single point by itself would be of little help in estimating the rate of retreat at another point a kilometer away. This is not to disparage the calculation of an average rate based on several measurements up and down a particular stretch of shore. Confi- dence in such an estimate of the mean can be increased without limit by increasing the number of measurements. The uniformly small rates measured at four stations adjacent to the Pentwater jetties indicate that partic- ular stretch of shore suffered less retreat than did the surrounding 50 -kilometers. In fact, the shore experienced a net advance at two stations in Mears State Park just north of the jetties and nowhere else except at station 17, on the tip of Little Sable Point (see Fig. 4). The shoreline remained remarkably stable at stations 24 and 12. At station 9 (about 2 kilometers in a northerly direction from station 12), there was negligible net retreat, but this resulted from early retreat being compensated by progradation sometime during the last 13 months of study. Falling lake levels and progradation during the last year of the study advanced the shore at 24 of 30 stations. Though the additional beach width gained during this last year was considerable, only three stations (4.5 and 5, just north of the jetties, and 17 on the tip of Little Sable Point) advanced enough to regain their 1969 shoreline. The average gain (each station given equal weight) between 1975 and 1976 was 4.3 meters or about 20 percent of the retreat which had occurred during the previous 6 years. 2. Recession. The effect of declining water levels and of sediment deposition was discussed previously as both contributing to the advance of the shore and the partial recovery of former beach widths during the last 13 months of study. However, the relative importance of the two distinct processes was not identified. Progradation refers to displacement of a certain topographic contour toward the lake; recession refers to displacement of that contour toward the land. The exact magnitude of recession at the specified datum will depend on the elevation of the datum specified as well as the position along the shore where the measurement is made. 20 Even in the usual simplified model (Fig. 5) the magnitude of recession depends on the elevation where the recession is measured. Shore recession by definition is measured at the elevation of the final water level, be- tween the points where the final water level intersects the initial and the final profiles. In the model, recession also occurs at all other elevations where the initial bottom sloped at a greater angle than the effective angle of profile adjustment; i.e., where the slope was greater than the ratio of vertical to horizontal displacement of the idealized profile (Fig. 5). At those elevations where the bottom sloped at an angle less than this effective angle, the contours would move lakeward, even as the whole profile and features on it move up and landward. In nature, shore profiles are not smooth and do not always increase in depth lake- ward. Thus, progradation may occur at several elevations, while recession (net erosion) occurs elsewhere and the overall profile migrates landward. The profile shape can also change as the shore recedes. The degrees to which these natural complications increase the variability in measurements of contour migration is shown in Figure 9. In the first plot at the top of Figure 9, the progressive recession of the 176.92-meter contour (at the average elevation of the lake surface during the 1975 survey) can be read on the vertical axis. Between 1969 and 1976, the average net recession at 176.92 meters was 10.5 meters and the maximum net recession was 28 meters at station 14. Progradation occurred at stations 4.5, 5, and 17. The 176.92-meter recession is very similar to that of total shore retreat (see Fig. 8). The total shore retreat is, of course, a little larger (averaging 12 meters and with a maximum of 34 meters) because it includes transgression resulting from 0.2 meter of submergence. The areal patterns of recession and retreat are, however, virtually identical. At slightly lower elevations (shown . in succeeding plots in Fig. 9) the overall pattern of recession remains much the same, though the magnitudes of recession progressively depart from the magnitudes of shore retreat. This simply means that the overall pattern of shore retreat, which could theoretically have been obtained from aerial photos, reflects the overall pattern of actual recession of the upper beach face, which could not be obtained without repeated ground surveys. If progressively lower elevations are observed in Figure 9, the sim- ilarity between recession and retreat deteriorates rapidly. Recession at an elevation of 176.33 meters (the level of the lake surface during the 1967 survey) is shown at the top of the second column of plots in the figure. The spread of recession values encountered at the different sta- tions has increased, but the same overall pattern remains recognizable; zones of maximal net recession occurred at the south end of the study area, around Little Sable Point and at two points a few kilometers north and south of the harbor. At still lower elevations, the increased long- shore variability overwhelms similarities between recession and shoreline retreat. Not only would the magnitude of recession change drastically if measured at slightly different elevations on the lower beach face, but even the longshore pattern (i.e., the area of most and least severe erosion) would be obscured in measurements made only at these lower 2| Figure 9, Station Numbers 176.92-m Contour (Average Elevation of Lake During 1975 Survey) EEE -40 -30 -20 -10 -1.0 0 1.0 10 Dietance Measured Alongshore in a Northerly Direction Station Numbere 2927 24 21 19 16 14 10 § 76686 4 3 | 176.72-m Contour (Average Elevation of Lake During 1976 Survey) m nr a 1976 —-—_.——_ a -! (MoE NN ie 5 540 -30 -20 -10 -1.0 t) 1.0 10 IYO S ASSSs Distance Measured Alongshore ina Northerly Direction VEX TiceoCa OUD Csc Station Numbers 2927 24 21 19 16 1410 9 756554 3 |! 176.67-m Contour (Average Elevation of Lake During 197) Survey) Net Distance from Final (1976) Position 176.54-m Contour (Average Elevation of Lake During 1969 Survey) 40 -30 -20 -10 -10 0 1.0 10 Distance. Measured Alongshore in a Northerly Direction Time changes in positions of various contours intersecting the beach face. Distance of selected contours from their final position was determined in the October 1976 survey. Positive ordinate values indicate net recession between the indicated year and the final (1976) survey; negative values indicate net progradation for the indicated period. 22 Figure 9. 176.33-m Contour (Average Elevation of Lake During 1967 Survey) -30 a es ee -40 -30 -20 -10 -1.0 0 1.0 10 Distance Measured Alongshore ina Northerly Direction Station Numbers (m) 176.10-m Contour -10 1976 ——o—o— -20 1975 —--e—2— 1971 x -30 1969 --»— 4 -—— 1QGT ++ O---O-ee ee -50 Cy -40 -30 -20 -10 -1.0 t) 1.0 10 Distance Measured Alongshore ina Northerly Direction Station Numbers 2927 24 21 19 +16 14 10 9 79655 4 3 | 175.80-m Contour (Chort Datum) Net Distance from Final (1976) Position t a So -40 -30 -20 -10 lon mc05 LUN! 10 Distance Measured Alongshore ina Northerly Direction Time changes in positions of various contours intersecting the beach face. Distance of selected contours from their final position was determined in the October 1976 survey. Positive ordinate values indicate net recession between the indicated year and the final (1976) survey; negative values indicate net progradation for the indicated period.--Continued. 23 elevations. All the foregoing reference contours fall on the beach face; i.e., intersect the profile between the berm and the first longshore trough (Fig. 10). 184 183 182 18 | Lake Leve/ 180 a9 178 Elevation (m) liane 176 175 174 1) EO YO NSO) 80. GO. SO. 20. IO “O° FO =4) =o “0 Distance from Base (m) Figure 10. A fairly typical inner profile. Note the various reference ele- vations (International Great Lakes Datum - IGLD) at which contour migration was measured to determine rates of beach face recession plotted in Figure 9. 3. Encroachment. Encroachment refers to the loss in shore width due directly to sub- mergence. Given only the initial profile, the encroachment which would result from a subsidence of Az is exactly Az x cot « (where « is the slope of the profile between initial and final mean water elevations). This simple calculation may be sufficient to indicate the extent of po- tential flooding problems along low-lying coasts. The same approach has also been used in the scientific literature to estimate long-term effects of sea level rise, but this is a severe oversimplification because en- croachment by the sea is only one aspect of shore retreat. Submergence will usually increase erosion rates causing extensive shore recession which contributes to further shore retreat. Between 1969 and 1975, a period of persistent submergence on Lake Michigan, the overall retreat of the shore exceeded the encroachment by a factor of 5 (the total 24 retreat averaged from all stations was 17.9 meters, of which only 3.4 meters was due to encroachment). Furthermore, the amount of encroachment at a given station, while predictable, would have given no clue to the final amount of shore recession (Fig. 11). The amount of recession depends on the exposure and resistance of the beach to erosive forces. Within the range of conditions observed on the lake, the flatter foreshores showed no tendency to recede more or less than steeper foreshores. Moreover, shore recession continued in some cases even after the water levels began to decline. Hence encroachment, depending only on steepness of the foreshore and the change of water levels, is a poor measure of total shore retreat. V. DATA INTERPRETATION 1. Spatial Variation in Retreat Rates. The average rate of shore retreat for the whole study area was 2.9 meters per year (1969 to 1975), but there were wide variations (see Fig. 8). The maximum rate of retreat (4.6 meters per year) was observed at station 16; progradation caused the shoreline to advance at three stations (maximum of 6 meters at station 5). Two of the stations where the shore- line advanced lakeward are in Mears State Park, just north of the Pentwater jetties. The park personnel employ a number of shore protec- tion measures at this locality. Each fall a series of snow fences is installed in multiple rows along the shore to catch and hold windblown sand during the winter. Each spring the fences are removed, the beaches are graded, and the sand that had blown inland and accumulated in the camping area and parking lots is scrapped up and added to the beach op- posite the swimming area. Since 1973 park personnel have also been nourishing the beach with a small part of the 50 to 70X10°% cubic yards which is dredged annually by the U.S. Army Corps of Engineers from the Pentwater Channel. In each of the years 1973, 1974, and 1975, about 5,000 cubic yards of the sand removed from the channel by bucket dredge was dropped across the north jetty onto park property. The park staff widened the beach in the bathing area using the dredged sand together with about an equal amount of sand removed from inland dunes, (G. Zeine, Mears State Park Supervisor, personal communication, 1977). In 1976 the channel was deepened with a hydraulic dredge, and about 7,000 cubic yards was pumped onto the beach between the north jetty and station 4.5. In addition to these steps, three rockfilled gabion groins were installed near station 4.5 in 1973 as part of the Michigan Demonstration Erosion Control Program (Brater, et al., 1977). Concern for swimmers' safety led to replacement of the outer ends of the wire gabions with sandbags the next spring. The effects of these various shore protection efforts at Mears State Park, together with the protection the jetties afford by blocking some of the beach from southern exposure and acting as a terminal groin for the fill, are judged responsible for causing the shore to prograde lake- ward at stations 4.5 and 5 while for the same 7-year period the adjacent 25 25 (m) o i] ! J 20 ! rst ie \ o|s we |e 15 qi s I o| 3 ‘! Retreat 2 yr 3=3.0 ——- 2 10 (0) 10 4 yr s=15 6 yr i ¢ 821.4 Qelgmrro Ts 0.50 120 yr 3-05 Y) 5 Figure 14. Distributions of measured retreat rates. Note the spread in the rates of shoreline change decrease as the period of ob- servation lengthens. The histogram of 120-year rates is not strictly comparable to the others as it is based on a larger number of observations and includes effects of variations encountered around the entire perimeter of the lake (Powers, 1958); however, it still illustrates a continued reduction in the spread of retreat rates as the time interval lengthens. MONTHLY MEAN LAKE LEVEL 581 177.0 580 176.5 Surface Elevation (m) uo Ss Pe) Surface Elevation {#%) 176.0 576 1967 1968 1969 1970 197) 1972 1973 1974 1975 1976 (977 Figure 15. Lake Michigan hydrograph showing changes in lake level between survey periods. 3] ; mean lake level remained essentially stable in 1974 (i.e., repeated the sequence of record high monthly means set in 1973), then began dropping slowly in 1975. During the last half of 1976, precipitation in Lakes Superior and Michigan basins was down 40 and 45 percent, respectively, from their long-term averages, and Lake Michigan levels began to fall rapidly. The relationship between shore retreat and lake level changes is in- dicated in Figure 16. The ordinate value of each point is the difference between the daily mean lake levels from one survey to the next, several years later; the abscissa is the distance the shore retreated between surveys. The general tendency of shore retreat to be proportional to the change in water level and the deviation of individual measurements from this trend are evident. ba H = S 20 i °4 a i << H ° 2 °3 oe 2 7 acetate Meh os) oa s° 70 ° : ° (otters Alagoa essen Ue 915.8 deine wb, Ge tartare oa ae “12 4 O° eee e ° % Co pate tiret<® 3 . oo, hs Se " °6.5 as) 0 0 rr) fe “ ne a 2 on atdel se © : 2 i e ; i ° H = _20 ‘4 eikes ies 3 2 ; & 0 oe 7) 003.6 8 6.5 . °27 014 : 14 °o16 16 : -40 ! Se eee -0.75 -0.50 0.25 0 0.25 Submergence Emergence Figure 16. Submergence (the rise in water level) versus retreat (the landward migration of the shoreline). Data on emergence were obtained between 1975 and 1976, several years after the lake levels began to decline slowly. Outliers far from the evident linear trend are identified by station number. As expected, local variations in wave exposure, in the nature and orientation of the shore, in offshore topography, etc., cause the indi- vidual measurements to scatter widely about the mean predicted solely on a2 the basis of the lake level change. Those retreat values that deviate most from the linear relationship are identified by station number. These stations are mostly located either in the immediate vicinity of the Pentwater jetties or on Little Sable Point--areas of anomolous recession as already pointed out in the discussion of spatial variations in retreat. The points to the right of the vertical axis in Figure 16 all represent changes during the last year of study and just before the rapid fall in lake levels. Note that the shoreline at most stations shows an advance for the first time during the period 1975 to 1976. Apparently the recent cycle of accelerated shore retreat was complete, or nearly so, given the lower water levels in 1976. Because water levels are rarely stable for long periods of time, almost any increase or decrease in erosion measured on the Great Lakes may be partially attributable to a difference in water levels. In many instances, the data shown in Figure 16 could serve as a basis for esti- mating how much of the recession could have been caused by water level changes and how much must be due to other causes. Examples of this appli- cation are given in Appendix A. 4. The Timelag Between Lake Level Perturbation and the Reestablishment of Profile Equilibrium. A tentative model describing the general response of the shore to rising lake levels was proposed by Hands (1976). Based on what appeared to be a more rapid adjustment of the offshore bathymetry, and an over- steepening of the upper profile in surveys through 1971, Hands suggested that the lakeshore would continue to recede about another 11 meters before regaining equilibrium, even if the lake level stabilized at the 1971 elevation. Actual lake level fluctuations were such that annual surveys beginning in 1973 would have precisely identified any lag in shore response that occurred after the end of the rise in lake levels. Unfortunately, the stations were not surveyed until 1975 (Fig. 15). The measurements that were collected, nevertheless, have some bearing on the proposed lag be- tween lake level changes and shore adjustment. The data points in the emergence region of Figure 15, based on the changes that occurred between August 1975 and September 1976, reveal that shore retreat had abated by that time. Since progradation was not occurring everywhere (see Fig. 8) the shore was probably still in a transition stage in 1976, though it is possible that most of the recession may have occurred shortly after the waters peaked in 1973. Given the available data, the critical question in this regard is whether the recession rates between 1971 and 1975 show a significant reduction below the 1969 and 1971 rates. If so, this would indicate the profiles had nearly regained equilibrium and the hypothesized lag in shore response would probably be shorter than 2 years (the time between the 1973 peak and the 1975 survey). Given the great variation in rates of recession at different sites, any examination of changes with time should be based on measurements from 33 a common set of survey stations. The inclusion of a single, rapidly re- treating station in one period but not the other would drastically influ- ence the difference in the means for the two periods. Recession data are also less variable on the relatively high part of the beach face (see Fig. 9). Using these two considerations, the average recession of the 176.92-meter contour was calculated for the 14 stations which included that contour on surveys for all 3 years. The resulting average rate of recession between 1971 and 1975 was not less, but 37 percent greater than during the previous period. Thus, the mean recession rate measured be- tween 1971 and 1975 increased even though the mean lake level had been falling slowly during the last 2 years of this 4-year period. Rates of recession calculated at the other principal elevations (see Fig. 9) also increased for the 1971-75 period. The reason recession increased is unknown; however, wave activity may have been more intense during this period. Johnson and Hiipakka (1976) report that two unusually destruc- tive storms in the 1972-73 storm season evidently removed 1.5 times as much bluff material as had been eroded during the preceding 2.5 years at the site of a temporary harbor near Bridgman, Michigan, 160 kilometers south of the present study area. The cumulative effect of storm vari- ability in the present study area is unknown, but since recession rates did not decline there is no evidence that the beaches were approaching equilibrium before 1975. The data are, therefore, consistent with the concept that the shore lags several years behind in its response to the termination of a rapid rise in water levels. The magnitude of the lag in terms of how much additional shore re- cession actually occurred between the time when lake levels stabilized and the time when profiles finally equilibrated, can not be calculated directly because there was no survey during the year when levels first stabilized. A good estimate, however, of the "latent recession" (i.e., the response to the inherited stress which was not relieved by profile adjustment until after the lake level had peaked) can be obtained by assuming shore recession continued until the water level peaked in July 1973 at the same rate as existed between 1969 and 1971 (1.91 meters per year). By subtracting the estimated recession before peak levels (1.91 meters per year X 25/12 years = 3.98 meters) from the known recession for June 1971 to August 1975 (14.38 meters averaged from the same stations), the remaining difference (10.6 meters) should be the recession which occurred after the July 1973 peak in order to bring the profiles to the near-equilibrium conditions interpreted for 1975. Given the uncertainties involved, primarily the large variation in recession between stations and the less than desirable timing of peak water levels between widely sep- arated surveys, plus the general nature of the original prediction which was based simply on early profile steepening, the extremely close agree- ment between the 10.6 meters of calculated recession and the 11 meters predicted must be largely ascribed to chance. The close agreement cer- tainly supports the prediction procedure, but should not be taken as indicative of the precision to be expected with this method. Additional detailed, long-term studies of profiles, which are adjusting to new water 34 levels, and of wave energy variations during the period of adjustment are desirable to refine methods of predicting shore response. In the interim, if it is known that persistent longshore bars have migrated landward faster than the adjacent shore retreated, after some coastal submergence, then it should be assumed that the shore will continue to recede, even after subsidence ceases, until such time as the original spacing between bars and the shore is reestablished. 5. Comparison of Recent and Historic Changes. The 50-year retreat of the shoreline in the immediate vicinity of Pentwater Harbor was determined by plotting the 1919 shoreline (based on a survey by the U.S. Army Engineer District, Milwaukee) and the 1969 shoreline (based on aerial photos) to a common scale using a zoom-trans- fer scope. The average shore retreat was then estimated by planimetering the area between the two shorelines and dividing by shore length (450 meters). The 1969 lake level stood 0.46 meter below the 1919 level, so that shore recession was actually greater than shoreline retreat. There- fore, the observed shoreline retreat was reduced by the estimated with- drawal that would have accompanied a decline in water levels to their 1919 elevation. The magnitude of such a withdrawal was estimated using the average profile shape at stations 4.5, 5, 6, and 6.5 in this 450-meter stretch (Fig. 17). The estimated 50-year mean rate of shore recession obtained (see inset in Fig. 17) was 0.30 meter per year. During the 1967- 76 period of high water the average rate of recession in this area was only 0.25 meter per year (top part of Table 3). Thus, the rate of re- cession for this stretch of shore actually decreased during the recent period of high lake levels. As discussed in Section VI, various influences combine to stabilize the shore in the vicinity of the harbor; consequently, recent rates of retreat near the harbor are not typical of retreat on the adjacent unpro- tected beaches. It is interesting to note that if measurements had only been made in the vicinity of the harbor, they would have produced no evidence of the increase in recession rates that actually accompanied recent high lake levels. This may be far from an isolated case, because before the present concern for environmental preservation most studies of long-term beach changes on the Great Lakes were conducted near jettied inlets or at sites of critical erosion where efforts were made to stabi- lize the lakeshore. To the extent that these efforts were effective, they tended to reduce the range of recession rates observed through time and, therefore, to also obscure the correlation between lake levels and shore recession. One data set which does not concentrate on areas of critical erosion was compiled by Powers (1958) who resurveyed a part of the shore bluff near section corners along most of the entire perimeter of Laké Michigan. Two of his stations which fall within the present study area are shown as P85 and P86 in Figure 4; two more stations were located just south of the study area, 4 and 15 kilometers, respectively, from station 29. The rates of bluff recession at these four points between 1838 and 1957 averaged BS) @) y| = / | Plonimetered Shore Retreat ij | 8.5(m) in 50 yr =0.17m/yr=0.6 ft/yr / i ys / a Uo fx | | Withdrawal of Shoreline Due to Lower Water- Vi | level ; Y —=,. : uit in 1882-88; ssaisaeee ui | Station 177.12 10 176.66(m) elevation Removed in 193) : $/ 45 115m Rubble-Mound Extension, 1960 / 2 ae m Jom &>. 65 3.2m Average - 67m Withdrowal — Recession = Retreat + Wiihdrowol Isom => 85 + 67m 15.2 fo0 yr = 0.304 m/yr =I ft/yr Between 1919 and 1969 Built in 1867-8: lh 3 i Pd Bs = me | er RR swing bridge installed I868 wr) | £ git | Removed unknown date ea 4 G18) * | € | & ENR is [5 : 1 \ SB fo} \ : ote 2 \ S| mS | ¢! LS A . iJ ~ ald. | jies 5 | s | = foi] o+8 ; le z 1é y é ON Vp: | is | Scolein Meters ay 305 0 305 610 91.4 * Scola in Feet 100 0 100 200 300 12 a ” ig Y iS iat Historic shoreline changes in the vicinity of Pertwater harbor. r year was calculated using 1919 and A 50- year recession rate of 0.3 meter p2 1969 shoreline positions and correcting for withdrawal of the lake away from the shoreline due to lower lake levels in 1969. Figure 17. Comparison of historic with recent recession rates. wae ahecente Table 3. So ee US COR Ce Area extending 290 meters to either side of the Ventwater jetties Planimetered acrial photos Surveyed recession at four adjusted for encroachment f stations I (1967 - 1976) (1919 - 1969) Progradation on the north side of Progradation on the north; recession the jetties; recession on the yon the south. south side. Avg. recession rate: 0.30 m/yr $0-kilometer study area, excluding sites of extensive modification by man 0.25 m/yr Surveyed shore recession at 14 Surveyed bluff recession (1838 - 1957) stations monitored from 1969 - 1975 Power's (1958) Net recession | (m) stations 35 65 ‘66 39 0.43 m/yr 2.54 m/yr 0.43 meter per year, compared to a 2.54-meter per year average rate for profile stations between 1969 and 1976. Stations where bulkheads had been installed were omitted in the determination of the recent recession rate. Property owners also made a variety of other attempts to reduce erosion at many of the remaining stations, but these efforts apparently had only a minor effect and measurements from such stations were retained in the calculation of the recent recession rate. The 2.54-meter per year rate is, therefore, an estimate of the recent rate recession on a rela- tively unprotected shoreline. The older historic measurements also re- flect natural recession, unaffected by man's interference. The recent recession rate of more than six times the historic average reflects the effect of high lake levels in accelerating shore erosion. The 1969-75 period represents the most intense phase of erosion during a lake level cycle, whereas the 119-year rate includes the effects of sev- eral episodes of both high and low levels (Fig. 2). Four measurement stations do not constitute a large sample on which to base an estimate of long-term recession rates for this 60-kilometer stretch of shore; however, each measurement does cover a 119-year period, and variations in retreat rate do decrease as the period of observation increases (see Fig. 14). In fact, these four measurements are about as efficient for estimating the long-term mean rate as the larger number of measurements made during this investigation are for estimating the shorter term mean rate. The difference between bluff and shore erosion over the 50-year period should be inconsequential compared to the 600-percent in- crease in recession during the recent period of high water. The four historic measurements near the present study area, and other available information on historic rates along Lake Michigan's eastern shore are shown in Figure 18. These historic rates are based on net changes in bluff position surveyed in the 1830's and again in the 1950's (Powers, 1958). Over such a long period of time the error involved in assuming equilibrium becomes small; i.e., recession of the bluff tends to approach recession measured at any other point on the upper profile in the sense that any differences become small relative to the total dis- placement of the profile (in this case an average of 52 meters). The relatively uniform low rates of historic recession along most of the eastern shore further indicate that the estimate of 0.43 meter per year cannot be too far from the true rate of historic recession for the present study area. Changes in the rates of shore retreat between various time intervals at severely eroding localities on Lake Michigan are given by Seibel (1972) and by Hands (1976). Net changes presented in those references were meas- ured over periods of several years; some periods coincided with episodes of high water, others with episodes of low water. The rates of successive periods at given locations commonly varied by 200 to 600 percent. Thus, although the surveys for this study covered only a part of a lake level cycle, sufficient historic evidence is available to indicate OMG Extentof Recent Survey Historic Recession Rate Over a 120-Yeor Pertod Recent Recession Rate Over a 6-Year Period Recession Rate (m/yr) MICHIGAN CITY, IND. PENTWATER , MICH. FRANKFORT , MICH. Distance Measured Northerly along the Eastern Shore of Lake Michigan (km) Figure 18. Comparison between rates of historic (1830's to 1959's) and recent (1969-75) recession. Positions of survey stations on the eastern shore of Lake Michigan are referenced to the Porter-La Porte County boundary in Indiana. that recession rates during the period of rising water rose to six times their longer term average. 6. The Need to Adjust Recession Rates. The large temporal changes observed in recession rates make it diffi- cult to determine a "natural erosion rate" to be expected for a given beach on the Great Lakes. Changes in the mean recession rate by a fac- tor of 3 to 4 after some intervention by man (which may be related to shore protection, etc.) can not definitely be ascribed to that interven- tion, unless the water levels were essentially the same before and after the action in question. An untested general rule for comparing recession rates determined under different water level conditions would be to sub- tract a factor presumably attributable to the difference in lake levels from the "after-action" recession rate. Figure A-1 in Appendix A can be used as a guide for determining this factor. If the resulting ''ad- justed estimate” does not differ substantially for the "before-action" rate, then the conservative conclusion would be that there is insufficient evidence to establish whether the action in question played any part in increasing recession rates. This approach would be most directly applicable when the lake level has changed from stable to rising. If the before-action survey interval ends about the time the rise in lake levels ends, then it should be remem- bered that the recession rates may remain high in response to that rise for several years until erosion has brought the profile back into adjust- ment. As the effects of high water may persist into the after-action survey period, the full "correction factor" should not be applied unless the shore has regained equilibrium. Examples are given in Appendix A. 38 More study is needed on profiles returning to equilibrium under constant water levels to establish procedures for estimating the adjustment time, and also to establish the outer limit of the responding profile as this factor controls the physical work required to adjust the profile after perturbation by a lake level change. VI. CONCLUSIONS When water levels rise or a coast subsides, shorelines tend to re- treat. Retreat in response to submergence is particularly important on the Great Lakes where climate and hydrologic variations cause significant water level fluctuations. The process of shore retreat and eventual sta- bilization is examined by using beach profiles obtained at 34 stations surveyed in 1967; 1969, 1971, 1975, and 1976 on the eastern shore of Lake Michigan. The annual mean elevation of Lake Michigan during this century has gone through several cycles during which water levels rose for several years in succession (e.g., 1964-73), and then declined for a similar period. However, the net rate of shore recession during the last 100 years is small relative to the rates measured during the end of the re- cent rising phase. Landward sand transport and shoreline accretion during the intervening years of declining lake levels cause the shore to advance, thus lowering the overall historic recession rate. The mean water level elevation is the principal factor establishing a potential erosion rate for a given shore type; the extent of erosion actually realized will then depend on the available energy. The actual retreat of the shore can be divided into two components: (a) encroachment of the water due to sub- mergence of the beach, and (b) recession due to erosion as the beach ad- justs to the new water surface elevation. Given a change in water level, the encroachment can be predicted exactly; the recession, which may be several times more important in terms of ultimate shore retreat, can only be crudely predicted at present. After a rise of 0.8 meter in annual mean lake level between 1967 and 1973, recession rates remained well above the historic average through 1975. By the fall of 1976, however, shore erosion had ceased at most survey stations. After retreating an average of 24 meters from 1967 to 1975, the shore may have finally regained approximate equilibrium with the (by then) slowly falling mean water surface. Large variations in retreat were observed at adjacent stations. Areas on Little Sable Point and at the south end of the study area suf- fered the greatest net retreat; the area in the immediate vicinity of the Pentwater jetties suffered the least. Less loss around the jetties re- flects the effects of various shore protection measures employed there. An explanation for the generally high rate of retreat in the other areas is not evident at this time. The spread of retreat rates among the different stations decreased as time progressed through the study period. This trend shows that only 39 a few profile lines need to be resurveyed after an elapse of many years to provide an estimate of the long-term retreat rate, which is equally as efficient as an estimate of a short-term rate based on a larger number of profiles. The 119-year rate of recession based on four stations origi- nally surveyed in 1838 was 0.43 meter per year. The rate of recession between 1969 and 1975 (based on measurements at 20 stations) was more than five times greater than this historic average. This acceleration of re- cession was brought on by the recent high water levels. The correlation between water levels and recession rates is poorly defined at localities where shore protection measures are adopted at times of greatest potential loss; however, data from the relatively undisturbed stations monitored in this study show that shore recession was roughly proportional to the increase in water levels. Although local variation was considerable, the shore retreated on the average of 4 meters for each 0.1 meter of submergence. Surprisingly, this 40 to 1 ratio also gives a good approximation to the average advance of the shoreline as water levels declined during the last year of the study. Encroachment of water on the shore as lake levels rise causes only a small part of the total retreat of the shoreline. Ero- sion and accretion are nearly an order of magnitude more important than encroachment in terms of how far the shore is actually displaced. The period of adjustment following a change in the mean water level elevation may last for several years depending on the magnitude of the water level change, the type of beach material, the geomorphology of the shore, and the availability of wave energy to redistribute material. The capability to generalize recession predictions will improve when the balance of sedi- ment volumes shifting back and forth over the entire active profile is better understood. Recession of a particular contour is one convenient way of expressing the amount of shore erosion. The actual contour or elevation selected, however, will affect the outcome, and all contour changes do not give equally representative estimates of the regional recession. In this study, all recession lines significantly above the lowest water level gave relatively good indications of at least the regtonal pattern of shore recession; however, measurements at the higher elevations more efficiently estimated the actual mean recession for a stretch of shore. Recession lines near and below lake level not only were inefficient as estimates of the mean recession for the area, but also failed to reveal even the gen- eral pattern of regional shore retreat. This is because lower contours may prograde lakeward while the higher beach face is eroding. In general, to obtain stable and reliable estimates of recession from a few measure- ments, the measurements should be taken where the beach profile slopes steeply so that small changes in elevation do not cause large changes in contour position. The effect of water level changes on recession must be considered if historic changes .in the rate of shore retreat are to be properly ascribed to other causes. A graph of the retreat that accompanied sumbergence is 40 given in Figure A-1 which can be used as a guide to determine whether observed changes in retreat rates medsured after a given event are ac- tually due to that event, or might more simply be attributed to the dif- ferent water levels during the interval between surveys. 4 LITERATURE CITED ARMSTRONG, J.M., SEIBEL, E.A., and ALEXANDER, C., "Determination of Quantity and Quality of Great Lakes U.S. Shoreline Eroded Material," International Joint Commission, Chicago, I11l., Sept. 1976. BEACH EROSION BOARD, "Beach Erosion Study, Lake Michigan Shoreline of Milwaukee County, Wisconsin," H. Doc. 526, 79th Cong., 2d sess., U.S. Army, Corps of Engineers, Washington, D.C., Apr. 1946. BERG, D.W., "Factors Affecting Beach Nourishment at Presque Isle Peninsula, Erie, Pennsylvania," Proceedings of the Ninth Conference on Great Lakes Research, 1965, pp. 214-221 (also Reprint 3-66, U.S. Army, Corps of Engineers, Coastal Engineering Research Center, Washington, D:C., NTIS AD 631 520). BERG, D.W., and DUANE, D.B., "Effects of Particle Size and Distribution on Stability of Artificially Filled Beach, Presque Isle Peninsula, Pennsylvania," Proceedings of the 11th Conference on Great Lakes Research, 1968, pp. 161-178 (also Reprint 1-69, U.S. Army, Corps of Engineers, Coastal Engineering Research Center, Washington, D.C., NTIS AD 694 204). BERG, R.C., and COLLINSON, C., "Bluff Erosion, Recession Rates, and Volumetric Losses on the Lake Michigan Shore in Illinois," Environ- mental Geology Notes No. 76, Illinois State Geological Survey, Urbana, I11., July 1976. BIRKEMEIER, W.A., "The Effect of Structures and Lake Levels on Bluff and Shore Erosion in Berrien County, Michigan, 1970-74," U.S. Army, Corps of Engineers, Coastal Engineering Research Center, Fort Belvoir, Va.(in preparation, 1979). BRATER, E.F., et al., ''The Michigan Demonstration Erosion Control Program in 1976,'' Technical Report No. 55, Michigan Sea Grant Program, University of Michigan, Ann Arbor, Mich, 1977. DAVIS, R.A., "Coastal Changes, Eastern Lake Michigan, 1970-73;"" TP 76-16, U.S. Army, Corps of Engineers, Coastal Engineering Research Center, Fort Belvoir, Va., Oct. 1976. DAVIS, R.A., FINGLETON, W.G., and PRITCHETT, P.C., ''Beach Profile Changes: East Coast of Lake Michigan, 1970-72,'' MP 10-75, U.S. Army, Corps of Engineers, Coastal Engineering Research Center, Fort Belvoir, Va., Oct. UW/S. GREAT LAKES BASIN COMMISSION, ''Shore Use and Erosion," App. 12, Great Lakes Basin Framework Study, Ann Arbor, Mich, 1976. HANDS, E.B., "Observations of Barred Coastal Profiles Under the Influence of Rising Water Levels, Eastern Lake Michigan, 1967-71,"' TR 76-1, U.S. Army, Corps of Engineers, Coastal Engineering Research Center, Fort Belvoir, Va., Jan. 1976. 42 HANDS, E.B., "Some Data Points on Erosion and Flooding for Subsiding Coastal Regions," Proceedings of the Sympostum of Anaheim, 1976 - Land Subsidence, International Association of Hydrological Sciences, 1977, pp. 629-645 (also Reprint 78-11, U.S. Army, Corps of Engineers, Coastal Engineering Research Center, Fort Belvoir, Va., NTIS AD A051 796). HANDS, E.B., "Prediction of Shore Retreat and Nearshore Profile Adjust- ments to Rising Water Levels on the Great Lakes," U.S. Army, Corps of Engineers, Coastal Engineering Research Center, Fort Belvoir, Va., (in preparation, 1979). HARAS, W.S., "Canada/Ontario Great Lakes Shore Damage Survey," Technical Report, Ontario Ministry of Natural Resources, Oct. 1975. HARAS, W.S., and TSUI, K.K., eds., ''Canada/Ontario Great Lakes Shore Damage Survey, Coastal Zone Atlas," Ontario Ministrv of Natural Re- sources, 1975. HULSEY, J.D., "Beach Sediments of Eastern Lake Michigan," Ph.D. Thesis, University of Illinois, Urbana, I1l., 1962. INTERNATIONAL GREAT LAKES LEVELS BOARD, "Regulation of Great Lakes Water: Levels,'' International Joint Commission, Chicago, I11., Dec. 1973. JOHNSON, C.N., and HIIPAKKA, L.W., "Sand By-Pass and Shore Erosion, Bridgeman, Michigan,"' Proceedings of the 15th Coastal Engineering Conference, American Society of Civil Engineers, 1976, pp. 1361-1376. POWERS, W.E., "Geomorphology of the Lake Michigan Shoreline," ONR Final Report on Project NR 387-015, Northwestern University, Evanston, I11., Mar. 1958. RESIO, D.T., and VINCENT, C.L., "Design Wave Information for the Great Lakes; Report 3, Lake Michigan," Technical Report H-76-1, U.S. Army Engineer Waterways Experiment Station, Vicksburg, Miss., Nov. 1976. SAVILLE, T., Jr., "Wave and Lake Level Statistics for Lake Michigan,"' TM-36, U.S. Army, Corps of Engineers, Beach Erosion Board, Washington, DaGo, Were, OSS. SEIBEL, E., "Shore Erosion at Selected Sites Along Lakes Michigan and Huron,'' Ph.D. Thesis, University of Michigan, Ann Arbor, Mich., 1972. COASTAL ENGINEERING RESEARCH CENTER, "Guidelines for Monitoring Shore Protection Structures in the Great Lakes,'' U.S. Army, Corps of Engineers, MP 2-75, Fort Belvoir, Va., Feb. 1975. U.S. ARMY ENGINEER DIVISION, NORTH CENTRAL, "Water Levels of the Great Lakes," Report on Lake Regulation, Chicago, I11., Dec. 1965. 43 a Stas awl ey pie Pat, 4 aa OG ROAR MO arnt 4 bey i : y ays a Ay ahs’ > Bry = \ ee ‘ Tega ‘ Gai hi vai Sc Pike eee Ey) by be ar th) Tithe h i, Nitty, ij he N i thts ae ba,” fvorte t a ene 45 a er eee) Sit 0) A av mi int on Aiea, oe wa wee iy aie ik 3 ua iy f Ve a 45 TS : nae “ahd: : NL RN bins ai, eh fey nee | bn ree epee ty DV aE ws otk TRY: arpa iN a suited iM edit ; he /BS ay, Copne or eres ae hy . lav caradaindl yu gic: ae, ‘a mop gtiteel , bemoe | 1G! ae i ¥ ‘ ie 7) re dey APPENDIX A A PROCEDURE FOR ADJUSTING RATES OF SHORE RETREAT TO COMPENSATE FOR WATER LEVEL DIFFERENCES 1. Problem. The rate at which a particular beach retreats will depend on inter- actions among a large number of factors. These factors can be grouped into categories, such as (a) the characteristics of littoral materials that determine their mobility or resistance to erosion, (b) the intensity of waves and currents, and (c) the degree to which the littoral materials are in or out of adjustment with the potentially erosive forces. Some-' times the activities of man can have a drastic and obvious influence on retreat rates. In other instances man's impact, though substantial, is difficult to assess because it cannot be isolated from the total effect due to the interaction of many varying but poorly known factors. It is well known that a long-term increase in water levels on the Great Lakes promotes rapid shore retreat. There have been a number of attempts to quantify certain aspects of the relationship between lake levels and erosion (Beach Erosion Board, 1946; Davis, 1976; Berg and Collinson, 1976). Because water levels are always varying on the lakes, it would often be helpful if the effect of water levels on erosion rates could be removed from measured rates so that the impact of the other factors would be clearer. The following is a description of how shore retreat measurements made on eastern Lake Michigan between 1969 and 1976 can be used to estimate the minimum amount of shore retreat in response to various lake level changes. Bo IDENEELS The data base consists of shoreline changes measured over 1- to 6- year intervals at 33 stations along a 50-kilometer reach centered on Little Sable Point, Lake Michigan. Station locations, survey dates, and lake level elevations are described in the text. Figure A-1 gives an estimate of the mean shoreline change due to the long-term net differ- ences in lake levels. Figure A-1 is similar to Figure 15 except (a) measurements made within 1 kilometer of Pentwater are deleted as unrep- resentative of the natural processes on an unobstructed coast, and (b) all measurements of shoreline retreat have been reduced by 3 meters (the residual retreat presumably not due to any water level effect) so that the predicted response is zero when there is no water level change. The amount of recession is a function of many factors. Submergence explains roughly half the variance in the test data. The mean shore response to submergence by a given amount will probably fall within the bounds shown in Figure A-1. These bounds indicate the nominal 95-percent confidence limits for the mean recession based on a least squares fit (r = 0.75) to the 105 data points. The fact that most of the points fall outside the confidence band illustrates the greater difficulty of 45 ‘(3X93 995) SuoT}eNIS UTeIIed UT asuodsez ar1oys uBou ay} B3UTIeUTISAa Ios apInd e se aatas SOUTTYSEp ay, “StoqJow UT aie SquauaInseeul YIOg *sdUaTLOWGns JO SjUNOWR JUeLezZIp FO uoTIOUNZ eB SB }¥aIJEI aI0US epi EEEE| JINIIAINENS ‘[-V ean3sty MEOW eye eWge WHxIS9eWwWy TA veZOW 46 predicting recession at a single point as compared to predicting the mean recession along some stretch of shore. The curves in Figure A-1 are intended to serve as guides for making a conservative estimate of the mean response due solely to lake level changes, as illustrated in the following examples. Because the assumptions required to make a strict statistical inference may not be justified, the curves are not intended to support probability-like statements as to exactly how reliable such a correction would be. Reference to the upper or lower curve simply indi- cates a safe or conservative interpretation. In some cases, as in example 1 below, this will be sufficient basis for a decision. 3. Engineering Application. Two examples are given to illustrate applications of Figure A-1 to field problems. a. Example 1 - Has a Coastal Project Increased Erosion on An Adja- cent Beach? The effect of a project on shore erosion, and the extent of its influence in an alongshore direction are uncertain. Assume that during the 50-year period just before the project, the shore retreated 150 meters (for an average rate of 3 meters per year). In the 4 years since completion of the project, the same stretch of shore receded 24 meters (6 meters per year; Table A-1). Further assume these rates are well established by measurements at a number of points along a particular beach. Do these data provide clear evidence that the project accelerated erosion on that beach? If so, is the difference in retreat (3 meters per year X 4 years = 12 meters) a reasonable estimate of the beach loss caused by the structure? Table A-1. Data and adjustment for example 1. Later interval Earlier survey interval [Reiner 150 In 50 yr Rate Measured retreat Adjustment -O m Remaining retreat 150 Adjusted rate Suppose that lake levels began rising a few years before project com- pletion, and during the 4-year survey period after construction the annual mean lake level rose 0.4 meter. The preceding 50 years had been marked by several cycles of rising and falling water levels with no sig- nificant net change in lake level elevation. (Such conditions would not be unusual on the Great Lakes.) The higher water levels certainly played some role in increasing shore retreat during the latter period. Can the amount of additional erosion due to high water be estimated? Assuming the response of the shore will be directly proportional to the amount of water level change, and that the retreat measured on the 47 eastern shore of Lake Michigan can serve as a guide in the present sit- uation, Figure A-1 suggests 14 to 19 meters of retreat occurs in response to 0.4 meters of submergence. It is expected that individual measure- ments from points up and down the problem beach would show greater vari- ation as indicated by the scatter of points beyond the confidence band for the mean in Figure A-1. However, at least 14 meters of retreat would be a conservative estimate of the mean response due solely to the in- creased lake level; it is the mean response which is of concern here. Assuming the overall conditions of the problem site aré similar to those around Little Sable Point (as indicated by similarities in sand size, nearshore bathymetry, wave exposure, etc.), the estimated lake level effect is subtracted from the measured postproject retreat, and the re- sulting adjusted rate becomes (24 minus 14 meters = 10 meters in 4 years) 2.5 meters per year. The adjusted recession rate is even less than the historic average before the project (Table A-1). Thus, the increase in lake levels is more than enough to explain the observed increase in shore retreat. There is no evidence that the project itself resulted in any increased shore retreat. In this example many variables that may have influenced the rates were not measured, so it is still possible that the project itself tended to increase erosion and that this tendency was overshadowed by other factors. However, once adjusted, the available re- cession rates are not sufficient to suggest the project has had any det- rimental effect on the beach in question. b. Example 2 - Evaluating a Shore Protection Device. The second hypothetical case involves the determination of how well a shore protec- tion device has performed. The device was installed along a shore which had experienced erosion during a recent period of high water. The average beach width had decreased 40 meters in 7 years. Lake levels had risen 0.4 meter during the first 5 years, but had remained stable during the 2 years just before installation of the shore protection device. The project was monitored for 2 years after installation, and no further shore retreat was observed. Based on this information, how well did the device seem to perform? Again, Figure A-1 suggests that the average distance the shore would have receded in adjustment to the 0.4-meter increase in lake level is from 14 to 19 meters. Subtracting this from the measured average retreat leaves 21 to 26 meters unexplained (Table A-2). A conservative claim would be that even after taking water level differences into account, the Table A-2. Data and adjustment for example 2. Earlier survey interval Later interval Retreat 40 m In apy 2 Sy Rate 5.7 m/yr stable Measured retreat 40 m Adjustment -19 m No change in mean water level No retreat 0 m/2yr Remaining retreat 21 m Adjusted rate 3 m/yr 48 rate of shore retreat seems to have decreased from 3 meters per year during the 7 years before installation to zero after the device was in- stalled. The question would then be whether the 2-year monitoring period included representative conditions, or whether any other factors during that period could have been responsible for the reduced erosion, and whether the apparent benefits outweigh the known costs. 4. Discussion. Figure A-1 can at best help the engineer to evaluate one factor in what would probably be a multifaceted problem. If this important factor is taken into account by extrapolation from actual measurements, the other items can then be dealt with in their usual manner. The data in Figure A-1 are estimates of actual retreat during a period of submergence. The actual retreat may be less than the ultimate retreat necessary to reestablish equilibrium, both because of conditions under which the data were collected and the simplistic manner in which the data were analyzed. In the use of Figure A-1, lake level changes and shore response should refer to net displacements over periods on the order of 2 to 10 years. The applicability of Figure A-1 will also depend on the degree of similarity between the problem site and the site where data for Figure A-1 were collected. The environmental summary at the end of this appendix will assist the engineer in comparing the problem area to the present site. If a significant difference between sites exists, then the qualitative effect this would have can be determined by considering sediment balance. If the problem site has a deficiency of sand-sized material in the back- shore, either because of low relief or the preponderance of very fine grained material, then the retreat required to reestablish equilibrium, with a unit increase in lake level, will be greater than Figure A-1 in- dicates. The same will be true if there are longshore or offshore sedi- ment sinks, or if the active profile is broader than in the study area. More turbulence or lower nearshore gradients would increase the breadth of the active profile and the anticipated retreat. Conversely, less re- treat than predicted in Figure A-1 would be expected when the problem area has a narrower active profile, higher or coarser backshore sand de- posits, or a net inflex of sediment from external sources. 5. A Summary of Environmental Conditions in the Study Area. The shore throughout the study area consists of unconsolidated de- posits. As along most of the eastern shore of Lake Michigan, the shore type alternates between sections of morainal bluffs and dune-covered plains. During the study, waves primarily attacked modern foredune ridges which were present even where bluffs of glacial drift formed the backshore. The presence of a shallow stiff clay was observed at a couple of points of exposure both onshore at the base of bluffs and offshore along the deeper sections of troughs between longshore bars. These 49 scattered clay outcrops are the most resistent formations in the study area. One of the more extensive areas of actively migrating dunes on the Great Lakes marked the central part of the study area (see Figs. 4 and A-2). Examples of shore forms throughout the study area are shown in Figures 12, A-3, and A-4. Shore profiles at each station are shown in Appendix B. A typical nearshore profile is shown in Figure A-5; de- tails of nearshore geometry are described in Hands (1976). In consid- ering the likely differences in response to high water on two separate beaches, the shape of the nearshore profile may be the single most im- portant comparison, since it reflects aspects of both the level of tur- bulence which the coast is exposed to and the materials of which it is composed. Silver Lake Figure A-2. Aerial view looking across Little Sable Point from Lake Michigan toward Silver Lake. Profile station 14 is in the right foreground. Where glacial bluffs are being eroded by direct wave attack, several kilometers north of the study area, gravel and cobbles are prominent on the upper beach. Throughout the study area, however, the beaches are sandy with a mean grain size in the upper 2 centimeters of the swash zone ranging from 1 to 2.5 phi (0.50 to 0.18 millimeter). Longshore trends in mean grain size on the beaches are mirrored by similar trends in the finer sands along the crests of the longshore bars. Additional factors repre- sentative of conditions in the study area, and references to more detailed descriptions are itemized in Table A-3. 50 Station 3.5, September 1976. Station 16, September 1976. Figure A-3. Wave erosion of foredunes resupplies the beach with more well-sorted fine sand. Counterclockwise from the upper left, surveyors measure profiles at stations B.55 IO, IS, ainel 22. 5! Station 13, August 1975. Station 22, September 1976. Figure A-3. Wave erosion of foredunes resupplies the beach with more well-sorted fine sand. Counterclockwise from the upper left, surveyors measure profiles at stations 3.5, 16, 13, and 22--Continued. 52 Figure A-4. The top photo shows a 65-meter-high bluff (behind profile station 25) which is composed primarily of sand, but was protected from wave attack during the study period by a narrow beach and foredune that survived this period of erosion. The lower photo shows the highest dune under- mined by wave erosion. By 1976, the unstable slip face extended almost to the top of this 37-meter-high dune at profile station 19. 8) 160 € i=) rc) — 170}- s = cS tu 160 0 500 1000 1500 Distunce from Shore (m) Figure A-5. A typical example of the nearshore bathymetry in the study area. Table A-3. Environmental parameters in the study area. Nearshore bathymetry Distance of the 10-m isobath from shore | 800 to 975 m |} Hands (1976) Number of persistent longshore bars 4 to S Distance of outer bar from shore 300 to 500 m Depth over the crest of the outer bar 4to6 m Surface grain-size distribution In the swash zone < 2.5 | Hands (1976) On the crest of the third bar From backshore dunes < 0.4] Hulsey (1962) Wave climate Wave height exceeded once per month 0 Saville (1953) Wave height exceeded once per year Wave height exceeded once per 5 years Resio and Vincent (1976) _ 54 APPENDIX B NEARSHORE PROFILE CHANGES 55 Elevation (m) Elevation (m) 182 181 180 179 178 177 176 175 184 183 182 181 180 179 178 176 175 STATION 1 27 Sept. 1976 6 Aug. 1975 —-— 12 Moy 1971 ------- 28 May 1969 Distance from Base ( m) Station 2 14 Sept. 1976 Il Aug. 1975 3 Moy 1971 15 May 1969 50 40 30 20 10 Distance from Base (m) 56 184 183 162 Station 3 (alae |4| Sept.l976 ———— — —— | Aug. 1975 —~———- - -—-— 5 June 1971 ——------ 28 May 1969 B(!60 eeeee cece oofQ July 1967 é $179 - o 3 uw) 178 177 1967 176 ee ee ee ee Ce eS eS ee ot 90 @0 70 60 50 40 30 20 10 (0) -10 -20 -30 -40 -50 -60 Distance from Base (m) 183 182 181 Station 3.5 25 Sept. 1976 180 —— — 5 Aug 1975 coocccccee cal July 1967 179 G 6178 _ i=] 3 W177 90 80 70 60 50 40 30 20 10 (0) -10 -20 -30 -40 -50 -60 Distance from Base (m) 57 184 ~~ 183 If ‘oS | \ Station + 181 ————— 25 Sept. 1976 —- — —-—_ 5 Aug. 1975 —_——- 5 May {S71 (@OWet eras a ee 21 May 1969 e ccccccecccesee2! July 1967 $179 a o > £ W178 ete 176 175 174 80 70 60 50 40 30 20 10 0 -10 -20 -30 -40 -50 -60 -70 Distance from Base (m) 184 183 182 Station 4.5 181 25 Sept. 1976 ———_ — ——-. 5 Aug. 1975 ceeccccco ceee2! July 1967 180 iG S179 ad os > A 4178 177 Frequently Regraded Public Beach 176 175 174 cares dae eesereee Lasse ae esssaees ieee lees ase ered assssreend 110 100 90 ‘80° 70 60 (0) 40 30 20 10 0 -10 -20 -30 -490 Distance from Base (m) 58 180 Station 5 (79 -25 Sept. 1976 — —- —_ 5 Aug. 1975 — 4 May 1971 7 5) ey ee ee ea 21 May 1969 cele) sieleiele eles «+21 July 1967 Frequently Regraded Public Beach x re Elevation (m) 173 172 171 170 P 120 0 100 90 80 70 60 50 40 30 20 10 0 -10 -20 -30 Distance from Base (m) 184 y a Sao 183 f ' 182 | ! 181 Station 6 / 12 Sept. 1976 u ——— 13 Aug. 1975 | 0) 5 May 1971 i & | -------- 16 May 1969 $179 ercrccccccces27 July 1967 <./ ° > 2 “176 177 176 = _—a ioc oatie Le aeia/s hee 175; = 80 70 60 50 40 30 20 10 (0) -10 -20 -30 -40 -50 -60 -70 Distance from Base (m) 59 181 Station 6.5 NGOS ——————> 748), Soaps (3 —— — — 1/3 Aug. 1975 coccccccecccce 22 July 1967 179 178 Elevation (m) “ <>) ie) 100 90 80 70 60 50 40 ‘ 30 20 10 (0) Distance from Base (m) 183 Station 7 182 12 Sept. 1976 Pe Ao, (v8 181 ——" 5 Moy 1971 =-—-—-——-- 16 May 1969 cecccceeove22 July 1967 180 Windblown from Elevation (m) = (es) X 2 ° = ° i—J ne 100 90 60 70 60 50 40 30 20 10 0 -10 -20 -30 -40 -50 ; Distance from Base (m) 60 134 183 182 18) 180 ~“ o Elevction (m) 178 177 183 182 Elevction (m) Station 7.5 23 Sept. 1976 —— — — 15 Aug. 1975 coccccccccceoo ae VUly 1967 50 40 30 20 Distance from Base (m) STATION 8 11 Sept. 1976 —-— | Aug. 1975 ihe y 19¢1 Soconse 12 May 1909 smomocrcom: ~ 22 July 1967 10 Distance from Base (m) 6| -40 Elevation (m) Elevation (m) 175 184 STATION 9 ( is 3 oe : a ee ug. wa==--- 30 April 1969 Blowkeut 182 ! 18! 180 2 — windblown 179 4 Accretion 178 177 176 17400 90 80 70 60 50 40 30 20 10 9 -10 -20 -30 -40 -50 Distance from Base (m) 183 STATION 10 (1 Sept. | —-— 13 Aug. 7 May tS 22 July 182 (81 180 179 178 ald I 3 100 90 80. 70 60 50 40 30 20 10 0 -10 -20 -30 -40 -50 Distance from Base (m) 62? 183 STATION 14 182 ——-—— 13 /Aug1975 SOON 26 Moy 1969 Soa = 23 Oct 1969 181 / 180 | Fill 179 / 178 Breokwol | ft 4 tA Elevation (m) | 7 Built in July 1969 tf 177 176 100 90 80 70 60 50 40 30 20 10 0 -10 -20 -30 -40 -50 Distance from Bosa (m) STATION 12 162 10 Sept. 1976 Sere RRS 181 180 179 178 Elevation (m) 177 176 175 174 100 90 80 70 60 50 40 30 20 10 0 -10 -20 -30 -40 -50 Distance from Bose (m) 63 Elevation (m) 190 fae STATION 13 188 4 zeeee la May 1863 185 184 183 182 181 180 179 178 177 176 175; ee ee eS eee Se eee eee 80 70 60 50 40 30 20 10 (0) -10 -20 -30 -46 -50 -60 -70 Distonca from Base (m)} 188 197 186 185 ' 184 aL 1976 183 -— — —— 12 Aug. 1975 =—-—— —-— 30 Apr. 1969 176 175 174 173 100 90 80 8670 60 50 40 30 20 10 0 =O 20M —3 0 4 Om 50 Distance from Base (m) 64 Elevation (m) Elevation (m) “ ~ 177 183 182 181 180 “N wo “N a 177 176 (75 174 184 183 182 161 Station 15 24 Sept. 1976 === 12 Aug. 1975 90 80 70 60 50 Station 16 24 Sept 1976 —— —— 2 Aug. 1975 CaO 30 Apr. 1969 90 80 70 60 50 40 30 20 10 Distance from Base (m) 40 30 20 10 Distance from Base (m) 65 -10 -20 Elevation (m) Elevatian (m) 188 187 166 185 184 183 182 161 180 179 178 Station 17 10 Sept. 1976 —_— —_ 7 Aug. 1975 —— -——— 7 Moy |97! SSS SSS 26 May 1969 177 176 175 174 173 90 80 70 60 50 40 30 20 10 fe) -10 -20 -30 -40 -50 -60 Distance from Base (m) 163 230 Smaller scale view showing 40-m bluff failure 162 181 180 200 Station 19 23) Septalone | 3 —— 7m, VS 179 190 —-. 10 May 1971 @-—-—----- 28 Oct. 1969 177 176 110 100 90 80 70 60 50 40 30 20 10 0 -10 -20 -30 -40 : Distance from Sase (m) 66 185 184 Station 20 163 24 Sept. 1976 —— — — 3 Aug. 1975 ------- 26 May 1969 Elevation (m) = Les} 177 176 175 174 173 90 80 70 60 50 40 30 20 10 0 -10 <-20 -30 -40 ~50 -60 Distance from Base (m) 190 STATION 24 188 20 Sept. 1976 —-— 13 Aug. 1975 186 164 182 180 Elevation ( m ) 178 177 176 100 90 80 70 60 50 40 30 20 10 - O -10 -20— -30 -40 -50 Distance from Base (m) 67 Elevation (m) Elevation (m) 165 184 183 182 Station 23 181 20 Sept. 1976 — -— 2 Aug. 1975 180 -—--—--—-— 7 June 1969 179 178 (77 176 ee petellnd eA: oss awe 7 Ss 175 Va -7 174 173 172 100 90 80 70 60 50 40 30 20 10 0 -10 -20 -30 -40 -50 Distance from Base (m) 183 182 Station 24 20 Sept. 1976 181 —- —- —— 2 Aug. 1975 —— - —— 14 May 1971 ere 7 June 1969 180 179 (78 177 176 Buried ship Wreckage 106 90 80 70 60 50 40 30 20 10 0 -10 -20 -30 -40 -50 ‘ Distance from Base (m) : 68 Elevation (m) Elevation (m) 162 181 180 Station 26 20 Sept. 1976 179 ee eel 2 Aug 1975 —— +: —— 1/4 Moy 1971 SIF S35 9 June 1969 178 “N “ 176 175 10 100 90 80 70 60 50 40 30 20 10 0 -10 -20 -30 -40 Distance from Base (m) Station 27 20 Sept. 1976 SP — 0 Ao, (us OF ee 9 June 1969 100 90 80 70 60 50 40 30 20 10 (0) -10 -20 -30 -40 -50 Distance from Base (m) 69 Elevation (m) Elevation (m) 184 183 162 161 178 177 (76 175 174 100 162 181 180 179 178 177 176 175 174 173 172 110 90 100 Station 28 20 Sept. 1976 — - —/2 Aug. 1975 80 70 60 50 Station 29 20 Sept. 1976 —— - — 2 Aug. 1975 —— - — 14 May 1971 CROP 2S 3 June 1969 90 . 80 70 60 40 30 20 10 Distunce from Base (m) 50 40 30 20 Distance from Base (m) 70 {™ t | ! “Earth Fill Redistributed after Cottage Construction in 1976 -10 -20 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