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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

ISL Oo

UGI

E R

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- 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

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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)

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. 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

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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.

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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

APPENDIX

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.

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

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

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

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).

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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.

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

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.

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

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

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

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

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ENCROACHMENT WITHDRAWAL

( due to rising water) (due to falling water )

Figure 11. Encroachment versus recession as a cause of shoreline retreat.
The total retreat of the shore is the sum of encroachment by
the higher waters plus recession of the beach. Given only the
initial profile shape, the encroachment that would result from
different water level changes can be predicted exactly. Re-
cession would be more difficult to predict, and though sometimes
neglected, recession was by far the more significant of the two
components contributing to retreat on the sandy lake shore.

beaches retreated an average of 14 meters.

No adequate explanation can be given at present for the other instance
of a net lakeward advance of the shoreline at station 17, located on the
tip of Little Sable Point. The shore opposite station 17 alternately
prograded, receded, and then prograded between the 1969, 1971, 1975, and
1976 surveys. On the final survey, the shoreline was 3.6 meters lakeward
of its initial position, but it had fluctuated through a range of 16.3
meters. Photos taken at the time of the 1975 and 1976 surveys at station
17 (Fig. 12) suggest that as a result of the longshore passage of a sand
wave (i.e., a lakeward protrusion of the shoreline, sometimes referred to
as shore rhythm, crescentric planform, beach pod, etc.), the shore can
alternately prograde and recede over such distances. Additional shore-
line protrusions occurred where bars merged, at their updrift ends, with
the beach face (Fig. 13). Smaller shoreline undulations marked the lo-
cation where the inner bar frequently forms a cellular pattern in plan
view. Shoreline undulations seem slightly more prominent on Little Sable
Point than elsewhere throughout the study area; however, this is not a
complete explanation for the shoreline behavior at this locality. While
station 17 showed net progradation, stations 14, 16, and 19 on Little
Sable Point were among the most rapidly retreating during the study
period. Shore retreat would probably have been extensive at station 18,
had the property owners not rebuilt and maintained an earlier (1950-51)
timber bulkhead to protect a cottage near the edge of the high bluff which
backs this site. Likewise, terrace erosion reported by Davis (1976) for
a point between stations 16 and 17 (D6 in Fig. 4) was among the highest
he determined in a 1970 to 1973 study of 17 sites spread over almost the
entire length of the eastern shore of Lake Michigan. Other measurements
in the vicinity of the point also showed rapid retreat during recent
years. It is not known why Little Sable Point during the last several
thousand years has been the site of massive sand dune accumulation (ap-
parently fed by a convergence of littoral transport from both the north
and south) should now be the site of the most rapid shore retreat. It is
equally difficult to explain why a shorter section of shore on Little
Sable Point (represented by station 17) alternately prograded and re-
ceded, producing only a small net change in the midst of this presently
rapidly receding section of shore.

2. Temporal Variations in Average Retreat Rates.

Engineers are sometimes criticized for placing too much reliability
in average retreat rates derived from a limited number of measurements
widely spaced along the shore. If the dynamics of beach cusps, rip cells,
or the possible effects of edge waves were of interest, then obviously
the temporal and spatial scales of these processes would have to be con-
sidered in planning the response measurements. More often, however, the
practicing engineer is interested in overall conditions affecting a large
section of shore, and in long-term results affecting the lifetime of a
project or structure (e.g., 30 years). It is worth pointing out that as
the temporal scale increases some of the problems that originally contan-
inated data tend to cancel one another rather than accumulate as the

aS Adgust 1975

1 69-17-A
es __. September 1976

A shoreline indentation opposite the Little Sable
Point light in August 1975 introduces variability
in shore retreat as it migrates alongshore.

Figure 12.

fy cm "

Figure 13. Views of shoreline undulations which sometimes
form where the inner bar merges with the shore.

time between observations is extended.

A problem frequently faced by engineers is to choose a sampling in-
terval adequate to determine a mean recession rate for a given beach.
The precision of the estimated mean recession will depend on the inherent
longshore variability of recession which can be large (see Fig. 11); e.g.,
4 meters of advance and 34 meters of retreat were measured over the same
7.4-year period at two stations less than 2 kilometers apart. It is well
known that for a fixed level of longshore variability, the precision of
the estimated regional mean can be improved by increasing the number of
survey stations. Less well recognized is that inherent variability
usually does not increase greatly with time. Thus, the probable error
of mean rates and the percent error in mean recession tend to decrease
with time. The variance of these estimates would also tend to decrease
(thus, the precision increase) in direct proportion to the number of years
between surveys.

The claim that longshore variability in recession does not increase
with time nearly so fast as does recession itself, is supported by ob-
serving the spread among individual recession measurements from a fixed
set of stations over 2-, 4-, and 6-year intervals (1969-71; 1971-75;
1969-75). While the mean recession grew from 5 to 12 to 17 meters, the
standard deviations of the measurements only increased from 6.2 to 7.1 to
7.6 meters. Nearly constant variability may be partially related to sand-
wave migration, etc., which tends with time to merely distribute the same
variability uniformly along the shore.

The clear improvement with time in the precision of the estimated
mean rate is shown by the histograms of retreat rate measurements in
Figure 14. Note at the top of the figure that the variability in retreat
rates based on net change over a 5-month period is relatively large. An
estimate of the true rate of recession would require a relatively large
number of measurements, even if the need is only to typify the mean re-
cession for this short period. As the length of time between observations
increases, the individual measurements more closely cluster about their
mean, and thus an estimate from a fixed number of measurements tends to
better represent the true mean rate for that section of beach.

Variability need not always decrease with time, nor with number of
observations, if the character of the processes themselves changes. This
is where the engineer's judgment must be applied in selecting appropriate
historic data to fit the specific case at hand. Various aspects of how
lake level changes affect the process of shore erosion are discussed later
in this report.

3. Effects of the Recent Lake Levels on Shore Retreat Rates.
The annual cycle of high lake levels in summer and low lake levels in
winter was superimposed on a fairly steady rise in mean level that began

several years before the first profiles were taken and ended at a record
high annual mean elevation for this century in 1973 (see Fig. 15). The

Peviod of Distribution of Standard
Observation Rates of Retreat Deviation (m/yr)

Advance <———___+_.> 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.

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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

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

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

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:
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= me | er RR swing bridge installed I868
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ea 4 G18) * |
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ay 305 0 305 610 91.4
* Scola in Feet

100 0 100 200 300

12
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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.

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

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

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.

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.

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.

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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

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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

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

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

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.

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.

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.

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.

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) _

APPENDIX B

NEARSHORE PROFILE CHANGES

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)

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)

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)

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)

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)

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)

-40

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

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)

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)

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)

-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)

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)

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) :

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)

Elevation (m)

184
183
162

161

178
177
(76
175

174
100

162
181
180
179
178
177
176
175
174
173

172
110

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)

{™

“Earth Fill
Redistributed
after Cottage Construction in 1976

-10 -20 -30 -40 -50

Meinert Pork
Upper Beach Frequently
Regraded

Elevation (m)

184

183

182

180

“N
o

Station 32
25 Sept. 1976
i Aug 1975

40 300m
Distance

20 10
from Base (m)

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Full text of "Changes in rates of shore retreat : Lake Michigan, 1967-76"

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