Coastal Engineering, 2 (1978)41--53 41 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

ANALYSIS OF COASTAL EROSION AND STORM SURGE HAZARDS

ROBERT DOLAN, BRUCE HAYDEN and JEFFREY HEYWOOD

Department of Environmental Sciences, University of Virginia, Charlottesville, Va. (U.S.A.)

(Received July 18, 1977; accepted December 2, 1977)

ABSTRACT

Dolan, R., Hayden, B. and Heywood, J., 1978. Analysis of coastal erosion and storm surge hazards. Coastal Eng., 2: 41--53.

Prediction of shoreline erosion and storm-surge penetration is essential for coastal planning and management in the United States. Historical aerial photography provides the best data base for information that can be used in establishing hazard zones along and across the coast. In this paper we summarize a new methodology for deriving risk proba- bilities. The method is tested and applied along a highly developed (90 km) reach of the New Jersey coast.

INTRODUCTION

Along the Atlantic coast of North America hurricanes and severe winter storms are responsible for frequent and sometimes dramatic landscape modifi- cation. In addition to the physical and ecological changes that occur, private land holdings are destroyed, communication and transportation facilities are disrupted, and the loss of life is not uncommon. In spite of the obvious hazards, development has proceeded at a rapid pace (Fig. 1).

To marine scientists and coastal engineers, the shore zone has long been recognized as an element of a highly dynamic physical system. The informa- tion base essential for good planning and management includes the current state of this system and its rates of change through time. Information of this type is now required in the United States for various sections of the National Environmental Policy Act of 1969, the Coastal Zone Management Act of 1972 and 1976, the National Flood Insurance Act of 1968, and the Flood Disaster Protection Act of 1973.

DATA COLLECTION

The analysis of shoreline dynamics for the purposes of establishing coastal hazard zones requires repetitive sampling of the shoreline and storm-surge penetration line, both spatially and temporally. Information of this type can

42

Fig. 1. Coastal development at Ocean City, Maryland.

be obtained from: (1) ground surveys; (2) maps and charts; and (3) aerial photographs. Based upon our research, we are convinced that the use of metric aerial photography is the best and most feasible data source for a nation-wide mapping effort. For this reason we have developed an Orthogonal Grid Address System (OGAS) to systematize the analysis of shorezone dynam- ics and to provide a uniform data base for establishing hazard zones.

The OGAS method provides for the rapid and systematic acquisition of shoreline information from historical aerial photographs at 100-m intervals along the coast (Dolan et al., 1978). Comparison of the data derived from different years permits the definition of statistical properties of shoreline change, storm-surge penetration, and the risks and hazards associated with development within these zones. The value of defining the statistical variabil- ity of beach profile data was emphasized recently by Hale (1977).

In brief, standard 1:5,000 scale base maps of the study region are prepared. These maps are produced by photo-enlargement of 7½ min series United States Geological Survey topographic maps (1:24,000), which provides an area 3,500 m by 2,100 m. The frame of each base map is oriented with the long axis parallel to the coastline and positioned over the active portion of the coast. One long edge, lying entirely over the ocean, serves as the base line from which all measurements are made.

The historical aerial photographs are then enlarged to the exact scale of the base map through the use of a reflecting projector. On a transparent over-

43

lay placed over the base map, the shoreline and the storm-penetration line are traced from the projection. These tracings prepared from l:5,000-scaie projections of the sequence of historical photographs constitute the raw cartographic data base from which subsequent measurements are made. In this study, the shoreline is operationally defined as the high-water line. The storm-surge penetration line is defined as the line that separates the active, non-vegetated sand areas from the areas of continuous stands of grass and shrub.

On each tracing a transparent grid is overlaid -- the grid is rectilinear with 100-m spacings. Any coastal location is thus specified by base map number and co-ordinates of the grid. The position of the shoreline and other lines of interest, with respect to the base map base line is then measured to the nearest 5 m. These data are punched on IBM cards for subsequent analyses (Dolan et al., 1978).

RESULTS

Among the several information sets generated by the OGAS program is a graph of the mean rate of change of shoreline calculated over all time periods and plotted with an envelope of + one standard deviation (Fig. 2). This is a very useful graph for determining areas along the coast which have been sub- jected to the greatest change. It visually presents a measure of the erosion rates, stable areas, and areas along the coast that are more vulnerable to erosion. Numerical values for the mean and standard deviation at each of the 100-m transects are listed. The standard deviation lines are of additional value because they couple the historical trend with episodic changes associated with extreme storm events.

These statistics, as with any statistics of natural systems, must be applied with caution. Although there is a wealth of geological information to confirm that shoreline change has been underway for many coasts of the world on a more or less continuous basis for centuries, aerial photography is available for only three or at most four decades. The resulting statistics are thus based upon samples of short periods of a much longer trend. One must therefore assume that the past 30 to 40 years of shoreline change is representative of the longer trend, and thus indicative of the future. Complementary informa- tion from other disciplines, including oceanography, climatology, and geo- morphology can be important in establishing confidence in the statistics.

In addition, the selection of individual photographic flights can bias the statistics. In our analysis, for example, we included among our five sets of photography a flight taken soon after the Ash-Wednesday storm of 1962 (125-year return interval). This storm alone is responsible for a considerable amount of statistical variance in our data set.

44

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45

APPLICATION

Three separate lines are related to the question of risks and hazards in the shorezone (Fig. 3): (1) the shoreline; (2) the landward limit of the major destruction zone; and (3) the landward limit of the surge damage zone. Since each of these lines vary in position over time, the risk or probabili ty

l l tAJOR DE3TRUCTION ZONE --JIl I - " ~ S ~ q t Q I ~ Z O N E ;- "~ -- FLOOOt I~S Z O N E . . . . .

Fig. 3. Hazard zones across the coast .

that a place or point will fall within one of the zones defined by these lines likewise varies over time. These probabilities can be estimated from the data base provided by the OGAS program.

Future positions of the shoreline and the landward limits of the major destruction and surge damage zones (Fig. 3) may be calculated for specified time intervals and desired probabili ty levels. The information required for these calculations is: (1) the rate of shoreline change (ds/dt); (2) the standard deviation of shoreline change rates (os); (3) the rate of change of the storm- surge penetration line (dv /d t ) ; and (4) the standard deviation of the rate of change of storm-surge penetration line (Or).

The landward limit of the shoreline (AS) for a design time interval (At) specified for a given probabili ty level (p) is given by:

A = + kos (1) A t,p \ d t

where h is the number of standard deviations appropriate to the specified probabili ty level.

The landward limit of the major damage zone (AMDZ) for a design time interval (At) specified for a given probabili ty level (p) is given by:

AMD = ABZ + At kos (2) A t,p - ~

where ABZ is the width of the existing active beach zone. In other applica-

46

tions and if data are available, temporal variation of ABZ may be incorpo- rated into eq. 2.

The landward limit of the surge damage zone (ASDZ) for a design time interval (A t) specified for a given probability level (p) is given by:

Zl ( d r ) ASD = A t ~-~+ko v (3)

At, p

These relationships may also be solved for k and thus p, the probability that a given geographic location will fall within a given zone. For example, the equation for the major damage zone (AMDZ) may be solved for k:

ds AMDZ-- A B Z - A t - -

dt k = (4)

At(Os)

where AMDZ is the difference between the current position of landward limit of the major damage zone and the coastal location of concern; ABZ is the active beach zone, A t is the design time interval, ds/dt the rate of change of the shoreline and o s the standard deviation of the shoreline rate of change. Once k is calculated, the probability may be found. Using solutions of this type the probability for any location may be calculated.

The relationship between probability levels (p) and equivalent k values is:

p: 0.99 0.95 0.90 0.85 0.80 0.75 0.70 0.65 0.60 0.55 0.50

k: 2.33 1.65 1.28 1.04 0.84 0.67 0.52 0.39 0.25 0.13 0.00

Since k is merely the number of 6 desired, thus p is accordingly specified.

THE NEW JERSEY COAST

Under sponsorship of the Federal Flood Insurance Program of the Depart- ment of Housing and Urban Development, we conducted a demonstrat ion project using the OGAS program along the New Jersey coast. Changes in the position of the shoreline and storm-surge penetration line were mapped from Cape May to Little Egg Inlet, a distance of 90 km (Fig. 4), from five sets of aerial photography spanning the period from 1930 to 1971.

The New Jersey coastline is not an unbroken reach; it is segmented into eight individual "islands" by a series of inlets. Our analysis shows that rates of change in both the shoreline and storm-surge penetration line for islands I V and VI (Fig. 5) have very low variance along the coast, so the means can serve as good predictors. In contrast, the means for islands II, VII, and VIII give poor estimates because the variation is high; islands III and V have modest variation. Thus it is clear that island averages are not necessarily the best choices for establishing risk and hazard zones along the New Jersey coast and that stratification into smaller segments would be preferable.

47

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48

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i ~ l I 1 7 i ~ T l 1 --20 --IO O +IO @-20 --20 --I0 0 -blO +20

1930 TO 1971 MEAN RATE OF CHANGE (M/YR):

Fig. 5. Mean rate of change in storm-surge penetration line and shoreline for the southern New Jersey coast from 1930 to 1971.

Sample calculations for formulas 1 and 3 are provided in Tables I, II, III, and IV for each o f the eight islands included in the N e w Jersey demonstra- t ion s tudy - - changes in pos i t ion of the def ining lines o f the various hazard zones are given. Table I presents data for the mean (p = 0.5) shorel ine posi-

49

T A B L E I

Forecasted m e a n s h o r e l i n e p ~ i t i o n ( i n m ) r e l a t i v e t o c u r r e n t s h o r e l i n e ~ = 0 . 5 ; k = 0 ) *

Island Forecasted posit ion for: number At = 30 yr. At = 66 yr.

I - 9 0 - 1 9 8 H +27 +59 I I I --21 --46 I V --75 --165 V +18 +40 V I +21 +46 VII +18 +40 VI I I +81 +178

*Probability that the shoreline will be at or seaward of the distance given. A plus sign denotes a seaward direction of change (accretion), and a minus sign indicates a landward direction of change (erosion).

TABLE II

Forecasted shoreline posit ion (in m) relative to current shoreline (p = 0.84; k = 1.0)*

Island Forecasted posit ion for: number At = 30 yr. At = 66 yr.

I --177 --389 H --129 --284 I I I --102 --224 I V --108 --238 V --72 --158 V I --9 --20 VII --111 --244 VII I --528 --1,162

*Probability that the shoreline will be at or seaward of the distance given. A plus sign denotes a seaward direction of change (accretion), and a minus sign indicates a landward direction of change (erosion).

t i o n f o r 30 a n d 66 y e a r s h e n c e . In T a b l e I I t h e p r o b a b i l i t y c o n s t r a i n t is r a i s e d tO 0 . 8 4 , o r o n e c h a n c e in seven t h a t t h e s h o r e l i n e wi l l b e l a n d w a r d o f t h e , l i ne d e f i n e d . T a b l e s I I I a n d IV p r o v i d e e q u i v a l e n t i n f o r m a t i o n f o r t h e l a n d w a r d l i m i t o f t h e su rge d a m a g e z o n e .

DISCUSSION: THE BASE LINE PROBLEM

T h e r i sk o f o c e a n f r o n t f l o o d i n g s t o r m - s u r g e d a m a g e d e c r e a s e s i n l a n d f r o m t h e s h o r e l i n e , b u t t h e s h o r e l i n e is n o t a f i x e d f e a t u r e o f t h e c o a s t . R e c e s s i o n

50

TABLE III

Forecasted mean position of the landward limit of the storm-surge damage zone relative to current storm-surge penetration line (p = 0.5; k = 0)*

Island Forecasted mean position (in m) for: number a t = 30 yr. ~ t = 66 yr.

I --81 ]78 / / +36 +79 I I l : -42 9 2 I V --60 132 V +15 +33 VI 0 0 VII +48 +106 VIII +138 +304

*Probability that the storm-surge penetration line will be at or seaward of the distance given. A plus sign denotes seaward migration, and a minus sign indicates landward migra- tion.

TABLE IV

Forecasted landward limit (in m) of storm-surge damage zone relative to current storm- surge penetration line (p = 0.84; k = 1.0)*

Island Forecasted limit for: number a t = 30 yr. ~ t = 66 yr.

I --168 --370 H 105 231 III 171 376 I V --102 .... 224 V --108 - 2 3 8 VI - 2 4 --53 VII --159 -350 VIII --264 581

*Probability that given. A plus sign tion.

the storm-surge penetration line will be at or seaward of the distance denotes seaward migration, and a minus sign indicates landward migra-

and a c c r e t i o n o c c u r t h r o u g h o u t t h e y e a r as t h e b e a c h r e s p o n d s t o waves ,

t i des , a n d sea l eve l c h a n g e s , so t h e m a n n e r in w h i c h t h e r i sk d e c r e a s e s i n l a n d

f r o m t h e s h o r e l i n e var ies in t i m e . T h e r e f o r e , t h e b e s t base l i ne f o r e s t ab l i sh -

ing spec i f i c h a z a r d z o n e s is t h e l ine w i t h t h e l eas t v a r i a t i o n on t i m e scales o f

less t h a n t h e a n n u a l s u m m e r - - w i n t e r c l i m a t i c c y c l e , y e t s ens i t i ve t o l o n g e r

t e r m v a r i a t i o n in t h e s h o r e l i n e c a u s e d b y c h a n g e s in sea leve l o r loss o f sedi-

m e n t . O n m a p s a n d c h a r t s t h e s h o r e l i n e is u s u a l l y d e f i n e d b y m e a n h igh w a t e r o r

m e a n sea leve l . N e i t h e r o f t h e s e l ines are r e c o g n i z a b l e in t h e f i e ld o r o n aer ia l

51

photographs. As such they are not suitable for base lines upon which to define the risks of coastal hazards.

Of the several linear features of the shorezone, aerial photo interpretors have relied upon the high water line (HWL) as the best delineator of the shoreline. Under most conditions this line is relatively stable over the tidal cycle (Stafford, 1968), and for the purposes of field surveys, the HWL is clearly recognizable for several hours following high tide. In addition, the HWL is the most consistently recognizable linear feature of the shoreface on aerial photographs. While the HWL is relatively constant over the tidal cycle, it is subject to the following variations which must be recognized and planned for.

High tide variation

Over the course of the lunar month the height of tidal maxima varies between the extreme high spring tide maximum and the extreme low neap tide maximum. The position of the HWL on the beach face varies accordingly. Appropriate selection of the survey time within the lunar cycle or adjustment using tide tables is adequate correction for this variation.

Wave height variation

The distance of wave run-up on the beach face is in part determined by wave height. The higher the breaking wave the greater is the run-up. The impact of this contr ibution can be minimized by restricting survey periods to surf conditions of less than some specified height. Such a restriction eliminates survey on days of storm wave condition.

Beach slope variations

Beach slopes vary over time especially during and immediately following storms when large amounts of sediment are exchanged between the beach and the offshore. Variations in beach slope also are evident over the annual cycle. This variation in HWL due to temporal beach slope variation may be accounted for by: (a) eliminating surveys immediately following storm events; and (b) by seasonal survey time with and without required adjustments.

SUMMARY

Mean rates of change in shoreline and storm-surge penetration line for the eight islands of the southern 90 km of the New Jersey coast are summarized in Table V; standard deviations of rates of change are also presented. The magnitude of both the trends and the extremes are evident; even within areas with an accretion trend (island VIII) there have been periods of erosion. Therefore, in order to establish meaningful zones, consideration of both the

52

TABLE V

Mean rate of change and standard deviation of rate of change in shoreline and storm-surge penetration line for eight barrier islands on the southern New Jersey coast (All figures in m/yr.)*

Island number

Mean rate of change Standard deviation of rate of change

shoreline SSP line shoreline SSP line

I --3.0 2.7 2.9 2.9 H +0.9 +1.2 5.2 4.7 III -0 .7 1.4 2.7 4.3 I V 2.5 2.0 1.1 1.4 V +0.6 +0.5 3.0 4.1 VI +0.7 0 1.0 0.8 VII +0.6 +1.6 4.3 6.9 VIII +2.7 +4.6 20.3 13.4

*A plus sign denotes seaward migration, and a minus sign indicates landward migration.

averages and the var iances is essential . Given these s tat is t ics for the shorel ine and the line o f s to rm-surge p e n e t r a t i o n , p robabi l i t i es o f the var ious hazards m a y be derived.

The p r o b l e m s of risks and hazards in coasta l e n v i r o n m e n t s d i f fe r f r o m the equ iva len t f lood plain p r o b l e m s because o f secular var ia t ion due to shorel ine change. This p rec ludes es t imates of s to rm-surge re tu rn intervals for assigning haza rd probabi l i t ies . Accord ing ly , given a loca t ion on the coas t , the haza rd due to a " o n e - h u n d r e d - y e a r s t o r m " increases sys t ema t i ca l ly wi th t ime on an e rod ing coast . Since there m a y be several causes fo r shorel ine change, the p r o b l e m c a n n o t be resolved b y fu r t he r inves t igat ion of s t o rm f requenc ies and magn i tudes . The on ly a l te rna t ive is empir ica l eva lua t ions of his tor ical data . To insure t h a t an adequa t e i n f o r m a t i o n base is available, a sys t ema t i c p h o t o reconna i s sance o f the coas t should be ini t ia ted. As the sample size in eros ion s tudies increases, c o n f i d e n c e levels in the der ived stat is t ics will improve .

Our inves t iga t ion also resul ted in this final obse rva t ion . In those areas a long the New Je r sey coas t t h a t have been engineered to stabil ize the shorel ine (groins) and to p reven t s to rm-surge p e n e t r a t i o n (seawalls), the m e a n ra tes o f change have been great ly r educed - - one m e t e r or less per yea r in m a n y areas; howeve r , in these same areas, the s t andard devia t ions of the ra tes of change are high. This suggests t ha t a l though the engineer ing works have succeeded in s tabi l izing the shorel ine sys tem, when e x t r e m e s t o rms do occur , damage is o f t e n grea ter wi th in the s tabi l ized areas. Thus , the haza rd o f sys t ema t i c e ros ion damage is decreased, b u t the r isk o f episodic s t o r m d a m a g e due to s to rm-surge p e n e t r a t i o n is increased.

53

ACKNOWLEDGEMENTS

We wish to give special acknowledgement to our research assistant, Ms. Kathy Schroeder, who performed the aerial photograph interpretation for the historical erosion and storm-surge penetration data. The research was made possible through funding from the Department of Housing and Urban Develop- ment, NASA-Goddard Space Flight Center, and The National Park Service. Aerial photography for the New Jersey study was provided by the New Jersey Office of Shore Protection. Finally we wish to thank the Chesapeake Bay Ecological Program Office for providing services and imagery essential to the development of our method of measuring coastal change.

REFERENCES

Dolan, R., Hayden, B. and Heywood, J., 1978. A new photogrammetric method for deter- mining shoreline erosion, Coastal Eng., 2:

Hale, J.S., 1977. Coastal sediments, Los Angeles county. Coastal Sediments '77, Fifth Symposium of the Waterway, Port, Coastal and Ocean Division of ASCE.

Stafford, D.B., 1968. Development and Evaluation of a Procedure for Using Aerial Photographs to Conduct a Survey of Coastal Erosion. Report prepared for State of North Carolina, Dept. of Civil Engineering, North Carolina State University, Raleigh, North Carolina. (Also unpublished PhD Thesis).