Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
2 Coastal Erosion: Its Causes, Effects, and Distribution INTRODUCTION This chapter discusses how beaches are formed and factors that determine coastal erosion, stability, or accretion. It also contains a summary of U.S. coastline characteristics, which serves to empha- size the diversity of shore types that must be considered in erosion management policies. Historical shoreline changes along the coasts of the United States range from highly erosional to accretional. Superimposed on these Tong-term trends, however, can be rapid, extreme erosion caused by coastal storms from which the shore may or may not recover. In addition, the high likelihood of significant increases in sea level also has the potential to affect future shore erosion trends (National Research Council, 1987a). A quantitative understanding of these short- and long-term shoreline changes is essential for the establishment of rational poli- cies to regulate development in the coastal zone. Shoreline changes can be due to natural causes or they can be human-induced. Several common causes of human-induced shoreline change are poses, regions, construction or modification of inlets for navigational pur- construction of harbors with breakwaters built in nearshore 20
CAUSES,EFFECTS,AND DISTRIBUTION construction of dams on rivers with steep gradients, sand mining from riverbeds in the near coastal area, and extraction of ground fluids resulting in coastal subsidence. 21 The human-induced causes are particularly relevant for policy makers to consider. Beaches can change on various time scales from short-duration, dramatic changes to slow, almost imperceptible evolution that over time yields significant displacements. An important part of the FEMA program implementation is determination of the long-term trend of shoreline change. Unfortunately, storm-induced short-term beach variations can be so large that they may mask Tong-term trends. Another complicating factor is that at some locations the shoreline change trend rate itself has changed during the past several decades; quite often these changes are human-induced. Table 2-1 summarizes the possible natural contributions to shoreline change. REGIONAL VARIATION Types of Beaches The United States has three general types of beaches: pocket, mainland, and barrier beaches. Beaches are composed of loose sed- iment particles, ranging in grain size from fine sand to large cow blest Pocket beaches form between erosion-resistant headlands and are usually quite small. Pocket beaches are common along the rocky coast of New England and the cliffed coasts of California and Oregon. Because the sediment that constitutes pocket beaches is trapped by adjacent headlands, these beaches respond to prevailing waves; there is little movement of littoral sediment to or from adjacent beaches. Mainland (also called strandplain) beaches are the most common type along the Pacific coast and on the Great Lakes, where the adjacent bluffs often are over 100 feet high. These beaches develop anywhere that ample sediment supply allows for accumulation along the shoreline. The beach usually is derived from the adjacent erodible chit material. Mainland beaches backed by high eroding bluffs are well displayed along outer Cape Cod, Massachusetts. Elsewhere, mainland beaches can be quite low, such as those found in northern New Jersey and Delaware and along parts of the Gulf coastal plain. The mainland beaches of Holly Beach, Louisiana, are particularly Tow lying and susceptible to storm flooding.
22 a V A_ o ~ ~ E o Ct Z o U) Cal to C) Cal an: C) ._ ~ C) U) ~ Cal o ~ Ct ~ o ~ ~ U ~ ~ C ~r~ ~ ~ ~ ~ ~ ~ ~ i ~ U 7 ~ ' ~ ; '; ' , ' ~ ~ , o ~ ~ ·~ C) ,c~ ~ o.= ° =~ ~ ~ E E E E E ~ ~ ~ ~ E c ~ ~ U. ~ ou ~ ~ _ ~o _ _ _ _ o o o _ o ~ ~ ~ u~ u~ u~ ~q u: c~ ~ ~ .~ ~ £ -= ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ C,) ° o o o o o o o C) o o ~ ~ ~ ~ V ~ =~= ~ ~ ~ ~ ~ ~ ~ ~V U' U' e~ ~.o e ~ _ ~ c C ~ c c ° ° C) O O O ~ O O _ G) C~ O 0 C; ~ O O O O O -= 0 0 0 () C) 0 0 0 _ ~ O 0 0 0 0 `:Q ~ <: ~ ~ Z = = ~ ~ ~ :; r~ ~ 3 ~ o c ~ C ' 3 o ~ _ ~ c U) V) V) U) ~ U) ~ ~ ~ ~ ~ O ~ ~
CAUSES, EFFECTS, AND DISTRIBUTION 23 Slope instability is a major concern along erodible mainland coasts. Slope instability is largely controlled by the local geology, water level, wave action, and ground water movement. Bluff failure, concomitant loss of land, and sometimes houses are a continual prob- lem along outer Cape Cod (Leatherman, 1987), the western shore of the Chesapeake Bay (Leatherman, 1986), and parts of the California coast (e.g., San Diego and Los Angeles county beaches). Extreme instability problems occur along the Great Lakes, where nearly 65 percent of the 16,047-kilometer-Iong shoreline is designated as having significant erosion (Edil, 1982~. Barrier beaches are perhaps the most dynamic coastal land masses along the open-ocean coast. These land forms predominate the U.S. East and Gulf coasts from Long Island, New York, to Texas. Barrier beaches can extend continuously for 10 to 100 miles, inter- rupted only by tide] inlets. Physically separated barrier islands often are linked by the longshore sediment transport system, so that an engineering action taken in any one beach area can have major im- pacts on adjacent downdrift beaches. For example, the south shore of Long Island, New York, is considered a single littoral cell. The eroding headlands and mainland beaches at Montauk Point to the east supply a portion of the sand that moves westward along the bar- rier beach chain (Southampton/Westhampton beaches, Fire Island, Jones Beach, the Rockaways) and is eventually deposited in New York harbor (Leatherman, 1985~. Barrier islands are typically Tow lying, flood prone, and underlain by easily erodible, unconsolidated sediments. Thus, these land forms are especially difficult to develop because they are so dynamic. BEACH PROCESSESTHE NATURAL SYSTEM Natural beaches are formed by the accumulation of loose sed- iment, primarily sand, along the U.S. coasts. Their morphology is the result of antecedent conditions and sediment supply as well as the forces of waves, tides, currents, and winds. Beaches respond to changes in these forces and conditions on time scales ranging from hours to rn~lennia. A discussion of the formation and processes of beach change follows. Beach Sands: Sources and Sinks Beaches are formed by an accumulation of sediment at the shore- line. The factors that determine coastal change are the rate of rise
24 MANAGING COASTAL EROSION or fall in sea level relative to the land, the frequency and severity of storms, and the total volume of sand size and coarser sediments available in the sand-sharing system. Many coastal regions can be segmented into compartments; the boundaries are defined by the geologic features and processes that isolate the transport of littoral sediments from adjacent coastal compartments. Each compartment normally is composed of one or more sand sources and sand sinks, and the beach and nearshore serve as a conduit for the flow of sand between the sources and sinks. Many factors are involved In the natured processes that provide sandy sediment to the coast. Often, the sand sources are local and transport distances are short; however, sometunes sedunents are carried great distances before deposition occurs. There are five general sources of beach sediment: (1) terrestrial, (2) headlands, (3) shoreface, (4) biogenic production, and (5) the inner shelf. Their contributions vary with geographic location. Terrestrial erosion and runoff provide rivers with large quanti- ties of sediment of widely varying grain size and composition. These coarse-grained sediments then are carried toward the coast and may eventually reach the shore and be dispersed to adjacent beaches by littoral transport processes. However, to be significant sources of sand, rivers must have fairly high gradients. Many rivers along the U.S. Pacific coast were major contributors of sand, but dam bui23- ing has greatly reduced the sediment that reaches the beaches. In contrast, most rivers along the Atlantic and Gulf coastal plains have Tow gradients and limited capacities to transport coarse sediment. Nearly all of the sand entering these rivers and transported seaward is deposited In their flood plains or estuaries and never reaches the open-coast beaches; the Mississippi River in Louisiana is an obvious exception. Headland and linear bluff areas along coasts offer another major source of beach sand; wave undercutting and slumping make available large volumes of sediment for redistribution by wave action. Sand- size and coarser materials are carried by longshore currents along the beach, while the finer sifts and clays are transported seaward into deep water. These finer materials also may be deposited in backbarrier lagoons if mIets are present. The shoreface (i.e., the subaqueous portion of a beach) is another source of coastal sediment. Wave action erodes sand from a beach and shoreface, and longshore currents transport it downdrift. In this manner sand is recycled continually as the shore retreats.
CAUSES, EFFECTS, AND DISTRIBUTION 25 Calcium carbonate (CaCO3, such as found in shelIs) accounts for much of the beach sand in Tower latitude tropical and subtropical regions and can contribute to beaches in middle- and even high- latitude areas as well. In most cases the shell debris is provided by organisms living in shallow areas close to the beach. The majority of carbonate particles are derived from disintegration of calcareous hard parts of invertebrate fauna such as molluscs, brachipods, corals, echinoderms, and foraminifera. Some beaches, however, also contain significant contributions of shell derived from older estuarine or la- goonal deposits that crop out and are eroded along the shoreface. The inner shelf offshore of wide and gently sloping coastal plains also can be an important source of beach materials where there is an abundance of relict sand on the sea bottom. During the gradual rise in sea level over the past 15,000 years since glacial times, marine sand bodies have been eroded and the sediment redistributed by coastal currents. Over the long term, sand may be moved landward across the shelf where it can be incorporated eventually into the littoral system. In contrast to sediment sources, littoral sinks function to re- duce the volume of sand along the coast. The most common sinks to beaches are landward transport of sand through tidal inlets to form flood-tide shoals, storm-generated overwash deposits, landward- migrating sand dunes, losses down submarine canyons that extend close to shore (Pacific coast only), Tosses from sediment abrasion (largely the CaCO3 fraction), and losses from human-induced causes such as mining, dredging, and breakwaters and jetties (Dolan et al., 1987~. Seasonal Fluctuations Beaches respond quickly to changing wave conditions. In par- ticular, steep (i.e., storm) waves formed by a combination of large wave heights and short wave periods tend to result in seaward sed- iment transport and shoreline recession. Thus, stormy winter (and hurricane) waves generally cause erosion, whereas milder and longer period summer waves promote beach recovery. Thus, beach width fluctuates on a seasonal basis for many U.S. beaches. These natu- ral, interannual changes in shoreline position should not be confused with net long-term changes. Relatively poor documentation exists to quantify seasonal beach changes around the U.S. coastline; beach width may fluctuate by 100 feet or more (Johnson, 1971), but the national average is probably about half of this amount.
26 A~4NAGING COASTAL EROSION Storm-Related Beach Changes Storm surges also contribute substantially to the beach erosion process. These above-normal tides are caused primarily by the high winds (i.e., shoreward-directed wind stress) and the reduced baro- metric pressures associated with major tropical or extratropical (i.e., low pressure) storms. Along the Atlantic and Gulf coast shorelines, the Goodyear return period storm surges are approximately 12 to 15 feet above mean sea ferret. The Goodyear storm surges along the Pa- cific shore are much smaller because of the narrow continental shelf. The largest documented storm surge along the U.S. coast was caused by Hurricane Camille in 1969, when the water was elevated 22.4 feet above normal at Pass Christian, Mississippi. The three most important factors contributing to beach and dune erosion during storms are (1) storm surge heights, (2) storm surge duration, and (3) wave steepness (ratio of wave height to length). Almost all hurricane-induced erosion is limited because the time scale of the erosion process is shorter than the duration of the near- peak storm tides. Therefore, only a percent of the potential erosion actually may be realized. "Northeasters (i.e., severe storms coming from the northeast along the AtIantic Coast) can last for days and therefore can achieve their full erosional potential. Trends of Shoreline Change The long-term trends of shoreline change depend on a number of factors, and all the causative processes cannot be quantified at present. Relevant factors include the antecedent topography (geo- morphology) and the geologic rise of sea level, which has caused the shoreline to shift landward across the present-day continental shelf during the last 15,000 years. In some areas submerged sand on the inner shelf still is being transported shoreward and thus contributes to overall shoreline stability or accretion (Williams and Meisburger, 1987~. In other areas there are no offshore sources of sand, and the slowly rising sea level induces beach erosion. Local land subsi- dence caused by natural or human-induced processes also can cause shoreline recession. Finally, the equilibrium beach profile is not well established along some (particularly glaciated) coasts, and sand is transported seaward for this reason alone even if there are no other causes. Beach stability also must be considered in terms of alongshore discontinuities, which can cause areas of long-term erosion (e.g.,
CAUSES, EFFECTS, AND DISTRIBUTION 27 headlands) to be in close proximity to areas of long-term accretion (e.g., sand spits). For example, headland erosion along the outer Cape Cod shoreline supplies the sand necessary for the continued accretion of Provincetown spit (Leatherman, 1987~. Elsewhere, ero- sion may be pervasive on one flank of a coastal land form, such as the severe erosion on the northern section at Cape Hatteras, North Carolina, while the adjacent southern flank experiences long-term accretion. Because of these complexities, the only reliable basis at present for determining long-term shoreline changes or stability is through analysis of site-specific data. The methods of obtaining such data are described in Chapter 6. Natural Subsidence Numerous examples of naturally occurring subsidence can be seen around the nation. For example, in the Mississippi River delta the weight of the accumulating sediment causes continued com- paction and sinking. Earthquakes can result in rapid downward displacement of the land surface. An example of this type of tec- tonic subsidence occurred during the 1964 earthquake at Homer Spit, Alaska. This severe earthquake caused differential subsidence amounting to about 3 feet near the headland attachment and nearly 7 feet at the tip of the spit (Smith et al., 1985~. Subsidence usually results in a similar impact on the shore: beach erosion. When nearshore elevations drop, it is equivalent to a sea level rise of the same magnitude; the beach profile is thrown out of equilibrium by the creation of a sand sink offshore, and this induces offshore sediment transport and shore recession. Stinson Beach, north of San Francisco, California, demonstrates that beach erosion does not always follow land subsidence if am- ple sand supplies are available from other sources. During the 1906 earthquake, the land dropped as much as 1 foot and moved 13.5 feet horizontally (Ritter, 1969~. Residents reported that waves over- topped the spit far more frequently after the earthquake than before, but because adequate quantities of sand are supplied from contiguous regions, the spit is relatively stable on average. Beach erosion at Stinson Beach was severe during the winter of 1982/1983 because of a series of large storms, several of which occurred when tides were abnormally high (i.e., spring tides). E! Nino also contributed to high water conditions. Near the end of
28 MANAGING COASTAL EROSION FIGURE 2-1 Several homes in jeopardy and vertical scarp, western end of Seadrift, Stinson Beach, California, January 27, 1983. SOURCE: Photo pro- vided by Robert E. Wiegel. ~ _ ,. ~ FIGURE 2-2 Riprap largely covered by accretion of sand, Seadrift, Stinson Beach, California, November 15, 1987. SOURCE: Photo provided by Robert E. Wiegel.
OA USES, EFFECTS, AND DISTRIBUTION 29 January 1983, several homes were nearly lost (Figure 2-1), and an emergency-engineered seawall was built. The beach has gradually recovered, and much of the quarry-rock seawall has been covered naturally by sand (Figure 2-2~. This example further illustrates the dynamic nature of the beach, which varies on a daily, monthly, and yearly basis. HUMAN-INDUC1DD CHANGES ~lets, Jetties, and Dredged Entrances Natural channel entrances have a substantial capacity to modify sediment transport in their vicinity. However, artificially dredged channel entrances, structurally modified for navigational purposes, have a much greater potential for affecting the adjacent shores. These impacts can have a different magnitude depending on the character- istics of the particular entrance (Table 2-2~. Effects can extend miles from the entrance and are greatest where there is substantial net longshore sediment transport. The following three examples illus- trate how human-induced changes affect beaches. O C EAN C ITY INLET, MARYLAND The barrier island breach that later became Ocean City Inlet was caused by a major hurricane in September 1933. The inlet was stabilized by jetties soon after the breach occurred. Net longshore sediment transport is toward the south and estimated at 140,000 m3/year. No sustained effort has been made to carry out a sand- bypassing program. The impacts on the adjacent shoreline have been substantial: the immediate downdrift (i.e., south) shoreline has migrated landward a distance equal to the complete width of the barrier island in the last 50 years (Leatherman, 1984~. Figure 2-3 shows changes in the Ocean City shoreline from 1931 to 1972 and presents a sediment budget for the area. All of the factors listed in Table 2-2 have contributed to these changes, with the exception of dredge disposal In deep water. ST. MARY'S ENTRANCE, FLORIDA The natural river entrance at St. Mary's was stabilized by jetty construction between 1881 and 1902. The jetties, which are low and permeable, allow flood waters to flow over them into the inlet.
30 MANAGING COASTAL EROSION TABLE 2-2 Mechanisms by Which Modified Inlets Can Affect the Sediment Budget of Adjacent Shorelines Mechanism Net Deficit to Adjacent Shorelines? 1. Storage against updrift jetties 2. Ebb tidal shoal growth 3. Flood tidal shoal growth 4. Dredge disposal in deep water 5. Leaky jetties No, balanced by downdrift erosion Generally Yes Yes (very large sand quantities have already been permanently lost) Yes SOURCE: Modified from Dean, 1989. During ebb flow the ocean tide is low, and the seaward-directed flow is confined between the jetties. As a result, there has been a major alteration of the ebb tidal shoals, which are large depositional features formed seaward of the inlet by sand transport of the ebb tidal currents. A total of 90 million cubic meters of sand was removed from the nearshore and displaced farther offshore in the ebb tidal shoals (Olsen, 1977~. P ORT CANAVERAL, F LORIDA This inlet was cut in 1951, the jetties were constructed in 1953 and 1954, and a beach nourishment project was carried out in 1974. The net longshore sediment transport has been estimated by the U.S. Army Corps of Engineers (1967) to be 270,000 m3/year toward the south. Results of shoreline change rates over the periods 1877-1951 (Ion" term before entrance), 1955-1987 (postentrance establishment to prenourishment), and 1974-1986 (postnourishment) are presented in Figure 2-4. Primary impacts include interruption of the longshore sediment transport, impoundment of the north jetty to capacity, and offshore disposal of maintenance dredging material. Sand Disposal Onshore Disposal of beach-quality dredged sand offshore often results in a significant adverse impact on adjacent shorelines. The primary erosional effects occur on the downdrift shoreline, but erosion also
CAUSES, EFFECTS, AND DISTRIBUTION Legend 1 972 1931 0 2000m 1 1 1 1 Bay Shoals .~0~ l ," (+1.0x 106m3) ~ '(t I) ~ A: 1: ~ ~1 ~/l/ -~/~} ·~; 1 ·:~.1 1. 1 N:.. A,: Upward Growth of Assateague Island due to Sea Level Rise over a Distance of 10,000 m ( + 0.6 x 1 06m3) 31 N 1 11 \ 1 / , 1 1 1 Westward Migration of I Assateague Island Bay Shoreline over a Distance of 10,000 m ( + 0.6 x 1 06m3) )-: · ~ N.. k/ it\ ~ I't1\ - / - -- Fillet Impoundment at North Jetty over a 2150 m Distance (+2.0x106m3) (x ) I ~ ~ ~ ( + 5.9 x 1 06m3) Recession of Eastern Shore of Assateague Island over a 10,000 m Distance (-13.4 x 1 06m ) NET FROM SEDIMENT BUDGET ANALYSIS = + 1.5 x 1 06m3 FIGURE 2-3 Components of sediment budget analysis, Ocean City, Maryland. SOURCE: Dean and Perlin, 1977. can occur on the uplift shoreline if jetties are not present or are leaky. The beaches and nearshore system in the vicinity of a natural tidal entrance to a bay can be considered as a "sand-sharing system," and the ebb tidal shoal is a vital part of this system. If a portion of this ebb tidal shoal is lowered by dredging, a sand sink is created and the remainder of the system responds by providing sand from
32 a) =>, 1 0 LL IIJ o CD z I Z -10 llJ Or O -15 I On 5 ~ 15 IL G z I z 10 5 o 5 MANAGING COASTAL EROSION a) l o 1877-195 1 (Preentrance) _ _ - - 1955-1974 (Postentrance, Prenourishment) 10 20 30 DISTANCE SOUTH FROM PORT CANAVERAL ENTRANCE (miles) Port Canaveral Entrance b) 1974-1986 (Postnourishment) 40 Sebastian Inlet Entrance ~ I ~ -1 0 i~ 1955-1974 (Postentrance, Prenourishment) -15 ~ l / o 10 20 30 DISTANCE SOUTH FROM PORT CANAVERAL ENTRANCE (miles) 40 FIGURE 2-4 Effects of establishment of Port Canaveral, Florida, entrance and subsequent nourishment project on downdrift beaches. Top: ERects of channel entrance on downdrift beach stability, compared to preentrance condition. Bottom: Shoreline changes following 1974 nourishment project. SOURCE: Dean, 1989.
CA USES, EFFECTS, AND DISTRIBUTION 33 the beach to attempt to reestablish equilibrium. If sand removed from the channel is deposited offshore rather than back into the sand-sharing system, erosion can be the only result. Wholesale losses of beach sand have been caused by the indis- criminate dumping of inIet-dredged material onshore. Figure 2-5 estimates the amount of beach-quality sand dredged and deposited offshore from Florida's east coast inlets. This 56 million cubic yards would be sufficient to advance the sandy beach about 25 feet seaward along the entire 375-m~le east coast of Florida. Sand Mining The loss of sand from beaches because of mining for construction and other purposes can be considerable. The effects of sand Coning are similar to the effects caused when dredge material is deposited offshore. California illustrates the scale and impacts of sand and gravel mining. For example, about 145 million tons of sand was mined in California in 1972; about 2 percent of this material was derived from beaches and dunes (Magoon et al., 1972~. It is further estimated that about 9 million tons has been taken from the south end of Monterey Bay alone since the inception of the industry; about 500,000 cubic yards per year is taken from this area (Oradiwe, 1986~. In many other regions sand is mined from riverbeds, and this material might otherwise be transported to beaches during periods of flooding. This mining also can produce a sand deficit, accelerating beach erosion along the coast. HI Induced Subsidence Human-induced subsidence can be caused by extraction of hy- drocarbons; water extraction for industrial, agricultural, and ~nari- cultural use; and loading by earth overburden. An example of ex- treme subsidence caused by hydrocarbon extraction is found in the Terminal Island-Long Beach region in California (National Research Council, 1987a) where the ground surface above the center of of! production subsided about 9 meters (29.5 feet) over 27 years. The subsidence was finally arrested through the use of water injection, and some rebound occurred, which in conjunction with pump pres- sures lifted the surface as much as 33.5 cm (1.1 feet) over an 8-year interval.
34 MANAGING COASTAL EROSION . _ Berm ( 1 ~= / a St. Mary's Entrance 12.5 St. John's Entrance 20.8 . ~ \ at-- ~ ~ ''. ~r-: ~.~: ~ 1 on ~ ) .1: v' ,: ~ -; - 0 Boom 0 00o ~ Port Canaveral Entrance 10+ Am_ Sebastian Inlet 1.8 i_ Fort Pierce 2.7 ,] o Ohio -_ I sky Worth Entrance R~ ) _ I Hillsboro Inlet 0.5 Port Everglades Entrance 2.8 ~ Bakers Haulover 0.2 - - O (1) Quantities in Millions of Cubic Yards (2) Total in Excess of 56 Million Cubic Yards FIGURE 2-5 Estimated quantities of maintenance dredging disposed of in the deep water off the east coast of Florida. SOURCE: Dean, 1989. Coastal land subsidence is a worldwide problem. Combined wa- ter and hydrocarbon extraction has been responsible for subsidence in the Po River Delta, Italy (Carbognin et al., 1984), and in the Galveston Bay-Houston, Texas, area. As much as 5 feet of subsidence occurred in the Galveston region between 1943 and 1964 (Gabrysch, 1969~. The south end of San Francisco Bay subsided about 3 feet
CAUSES, EFFECTS, AND DISTRIBUTION 35 between 1934 and 1967; subsidence a few miles south of the bay was as much as 8 feet during the same interval, which was caused by ground water withdrawal for agricultural use (Seltz-Petrash, 1980~. Dams Rivers are a major source of sand for U.S. Pacific coast beaches. The rate at which sand is transported to the coast increases rapidly with increased river flows; therefore, infrequent large floods are re- sponsible for supplying most of the sand to the coast. During floods, rivers deposit sand in a river mouth delta, and waves and currents act on this episodic source, gradually transporting the sand along the coast. The building of dams for flood and debris control, water supply, and hydroelectric power reduces the supply of sand available to the coast. By preventing floods, dam operations substantially decrease the supply of sand to beaches (National Research Council, 1987b). The Santa Clara River in southern California illustrates the effects of dams on sediment transport downstream. The average annual trans- port of sand and grave] from 1928 to 1975 was estimated to be 0.96 million tons (about 600,000 cubic meters). During the years when dams operated (1956 to 1975), the estimated average annual deficit of beach material was 270,000 tons (170,000 cubic meters), about a 28 percent decrease (BrownTie and Taylor, 1981~. Thus, water resource planners and national decision makers should consider carefully the effects of dams on the supply of sand to beaches. Increased research and development of economical means to transport sand to beaches and alternative operations of dams would provide the information needed to make future decisions wisely. Groins, SeawalIs, and Breakwaters An array of coastal engineering structures have been used with varying degrees of success to stabilize beaches and control erosion around U.S. coasts. Their design and utility are discussed in Chap- ter 3; the problem, however, is that many of these rigid structures may induce downdrift beach erosion. Because no new sand is cre- ated, their purpose is to redistribute sand along and across the beach profile or to prevent further erosion of the coast. The use of prop- erly engineered structures has proven to be useful where properly designed, constructed, and maintained; however, their effects on ad- jacent shores must be carefully evaluated.
36 MANAGING COASTAL EROSION U.S. COASTLINE CHARACTERISTICS The U.S. continental coast is highly variable in character because of differences in the geology and coastal processes. The Pacific coast (including Hawaii and Alaska) is tectonically active and is subject to earthquakes, volcanic eruptions, and tsunamis in contrast to the Atlantic and Gulf of Mexico coastal plains and the Great Lakes. The continental shelf is narrow and in places essentially nonexistent, which results in minimal attenuation of deep-water wave energy. Also, the continual passage of low-pressure cells, centered along the northern Pacific Ocean, generates large oceanic swells. The resulting wave energy is significantly higher on average than for the Atlantic and Gulf of Mexico coastlines. Atlantic Coast The Atlantic coast ~ composed of two parts: the glacial north- east coast and the southern coastal plain extending from New Jersey to Florida. Barrier islands are the dominant coastal land forms in the southern portion. The glaciated coast extends from Maine to northern New Jersey. Scattered small pocket beaches can be found at shoreline reentrants along the erosion-resistant crystalline rock of northern New England. In contrast, the cliffs along southern New England and New York are erodible glacial deposits, with some notable exceptions such as the rocky headlands at Point Judith, Rhode Island, and Cape Anne, Massachusetts. These glacial sediments have been shaped by waves and currents into sandy barriers across embayments. Therefore, the New England coast is highly irregular, the outer shoreline becoming more smoothed by headland erosion and barrier beach accretion. For instance, the relatively small state of Massachusetts has over 1500 miles of open-coast shoreline due to its complex coastal configuration. This results in a wide range of beach types and susceptibility to storm waves and surges. In fact, Massachusetts has the largest number of recreational beaches in New England, but those along the Rhode Island coast are more urbanized and have suffered severe darnage during hurricanes. The mid-Atlantic coast, which extends from New York to Vir- ginia, is the most urbanized shore in the country except for parts of Florida and southern California. The recreational beaches in New
CAUSES, EFFECTS, AND DISTRIBUTION 37 York and northern New Jersey serve as playgrounds for the 15 mil- lion people in the greater New York metropolitan area. The New York to Washington metropolitan corridor exerts heavy demands for coastal recreational opportunities. As a result, land prices have soared, and there has been a coastal building boom for the last three decades. Beach erosion is a chronic problem, perhaps averaging 2 to 3 feet of recession per year along these sandy beaches. Numerous shoreline engineering projects have been attempted, particularly in northern New Jersey, to stop the shore recession. Now, however, planners are shifting emphasis away from rigid structures (e.g., sea- walis, groins, etc.), and relying more on "soft" techniques such as beach nourishment. The U.S. southeastern coast (i.e., from North Carolina to Florida) is the least urbanized along the Atlantic Coast, but this area has sig- nificant growth potential because of the availability of beachfront property. The Outer Banks of North Carolina are a long chain of barrier islands with development spread out along the shoreline. Al- though an increasing number of multistory condominiums are being built, the traditional building is a wooden single-family house that can be readily moved. Therefore, in this area retreating from the shore often is more attractive than beach stabilization. This alter- native is plausible to a lesser extent in South Carolina and Georgia, but many islands already are too urbanized for this approach (e.g., Hilton Head, South Carolina). Also, the barrier islands in the Geor- gia bight (southern South Carolina to northern Florida) are generally higher in elevation, much wider, and more stable than the microtidal barriers found elsewhere along the U.S. Atlantic coast (Leatherman, 1989~. Florida should be considered separately from the other south- eastern states. Its long coastline is perhaps the most important in the United States as it serves as a national and even international resort area. Recreational beaches are a major source of revenue for Florida, and state officials are considering spending tens of millions of dollars each year for beach nourishment. The Miami Beach project, completed in 1980 at a cost of $65 million for 10 miles of beach, represents the scale and magnitude of potential future projects along this rapidly urbanizing coast, which is becoming dominated by high- density, high-rise-type developments. The southern two counties, Dade and Broward, have nourished over 68 percent of their total (45 mile) shoreline.
38 MANAGING COASTAL EROSION Gulf Coast The Gulf Coast is the lowest lying area in the United States and consequently is the most susceptible to flooding. One of the earli- est extensive beach nourishment projects undertaken in the United States was in Harrison County, Mississippi, in the 1950s. The beaches have narrowed substantially since this time, and renourishment was required after Hurricane Camille in 1969 and again after Hurricanes Elena and Kate in 1985. Louisiana has the most complex coastline in the region and also holds the distinction of having the most rapid rate of coastal erosion in the nation. This is largely a result of regional subsidence. The state of Louisiana has only two recreational beaches: Grand Isle and Holly Beach. Although Grand Isle recently was nourished, it is unlikely that the economics (i.e., the relative high cost of sand fill versus the value of property to be protected) will make such future projects feasible. Texas has the most extensive sandy coastline in the Gulf, but much of the area is not inhabited nor easily accessible. Clearly, the city of Galveston will be maintained; the nearly century-old seawall and landfi~! generally have been effective in protecting this urbanized area. Elsewhere, retreat from the eroding beaches probably is the most viable alternative because land on the barrier islands is usually available for relocation. Pacific Coast The Pacific coast can be divided into two sections: southern Cali- fornia and the rest. Southern California, which extends roughly from Santa Barbara to San Diego, may be the most modified coastline in the country (although some could argue the same is true of northern New Jersey). This semiarid area has been transformed into one of the largest population centers in the United States, and explosive growth still is occurring. Because of extensive and widespread nour- ishment projects, many beaches reportedly are wider today than they were a century ago. The long-term trend of shore recession has been reversed successfully through coastal engineering projects (largely beach nourishment), primarily as a by-product of harbor construction. Considering the value of this real estate, the potential for continued growth, and the history of coastal projects, these public recreational beaches undoubtedly will be maintained in the future (Herron, 1980~.
CAUSES, EFFECTS, AND DISTRIBUTION 39 The northern California, Oregon, and Washington coasts can be divided into regions exposed directly to the waves and currents of the Pacific Ocean and those within tidal estuaries (e.g., San Francisco Bay arid Grays Harbor), tidal lagoons, and the Puget Sound-Strait of Georgia area. The Pacific coast is mountainous and the continental shelf is narrow (Inman and Nordstrom, 1971~. The major sand beaches are associated with large rivers. There are also hundreds of miles of rocky headlands and rugged mountainous regions with small steep rivers and small narrow beaches, but barrier beaches are very limited. The beaches may consist of fine sand, coarse sand, or cobbles, and some are composed of sand with cobbles underneath. The coast of Oregon is mostly mountainous rugged shoreline, but there are some sand beaches in the south. The central coastline is characterized by narrow sand beaches, low cliffs, and marine terraces. Alternating regions of both rock coast and beaches continue to the north, where wide, flat-sIoped, fine sandy beaches are associated with the Columbia River. The coast of Washington can be divided into three geomorphic regions: southern, central, and northern. The southern region is composed of wide beaches; the sand is transported as littoral drift from the Columbia River mouth. The central region consists of Tong beaches backed by steep seacliffs. Some sand is derived from the Columbia River mouth, but most is supplied by five other rivers that discharge into this region. The northern region is rugged, with high seacTiffs and small pocket beaches of pebbles and cobbles. Earthquake faults play an important role in the coastal geology of the Pacific coast. For example, the San Andreas Fault crosses the coastline just south of San Francisco and then crosses the coastline again and forms Bolinas Lagoon. The fault continues northwest, forms Tomales Bay, and then moves out to sea and back across land, forrn~ng Bodega Bay. The region is tectonically active, which affects the relative change in mean sea level. Also, earthquakes can be a major factor in cliff erosion as is evident from the reconnaissance survey by members of the U.S. Geological Survey, made a few days after the October 17, 1989, earthquake (Richter Scale 7.1) centered in the Santa Cruz mountains (Flinn, 1989~. The U.S. Pacific coast can be divided into a series of littoral cells, such as the Santa Monica and San Pedro cells. Sources of sand within littoral cells can be quite complex. Local rivers and coastal bluff erosion are obvious sources, together with some biogenic material. There are two major types of sinks of beach sand along the
40 MANAGING COASTAL EROSION Pacific coast: submarine canyons, such as the Monterey Canyon and the Scripps Canyon (Shepard and WanIess, 1971), and sand dunes (Cooper, 1967~. Natural events sometimes occur that have a major anp act on the sources of sand in a littoral cell; for example, the Los Angeles River flowed through Ballona Gap until 1825, when it was diverted to the south during a severe flood, joining the San Gabriel River discharging into San Pedro Bay. During severe floods in 1862 and 1884, some of the Los Angeles River waters again flowed to the sea via Ballona Creek, but since then it has discharged only into San Pedro Bay (Kenyon, 1951~. Historical changes in this river course also changed the location of sand discharge from the Santa Monica Cell to the San Pedro Cell. Great [ekes The Great Lakes coasts are composed of a wide variety of shore types, ranging from high rock bluffs to low plains and wetlands. The general character of the coasts is related directly to erosional and de- positional influences of the last period of glaciation. Stream mouths and shore lakes are a distinctive feature of the coastal corridor bor- dering the Great Lakes. Stream mouths generally are associated with low gradient streams and in many cases form freshwater estuaries. Some shore lakes are drowned river mouths formed by the great melt of the last Ice Age; others are erosional and/or depositional features from the same period. River mouths and shore lakes contribute little sediment to the Great Lakes littoral system. Most sediment com- prising Great Lakes beaches and transported in the littoral system comes directly from erosion of coastal bluffs and dunes. A detailed inventory of shore types that compose the U.S. Great Lakes has been prepared (Great Lakes Basin Comrn~ssion Framework Study, 1975~. Classification of shore types was based on shore height, slope, composition, and erodibility. Shores were simply divided into nonerodible (e.g., rock bluffs) and erodible (e.g., glacial deposits and sand dunes) categories. Rates of bluff and dune erosion along Great Lakes shores vary from near zero to tens of feet per year because of annual changes in wave climate and lake level. The U.S. coastline of Lake Superior has approximately 400 miles of nonerodible shore with areas of steep rock cliffs such as Pictured Rocks National Lakeshore. The remaining 487 miles of Lake Superior
CAUSES' EFFECTS, AND DISTRIBUTION 41 shore is erodible and varies from low-lying clay and grave! bluffs to sandy bluff-backed beaches. Lake Michigan's 1,362 miles of shorelands has every shore type characteristic of the Great Lakes. Most impressive is the expanse of sand dunes that extend almost continuously from the Indiana Dunes National Lakeshore on southern Lake Michigan northward along Michigan's western shore to the Leelanau Peninsula. Large areas of high erodible bluffs exist along both the Michigan and Wisconsin shores, which all too often are used as prime building sites because of their exceptional natural view. By contrast, Lake Huron's 565 miles of coastline is characterized by rocky and boulder areas with some high cliff-backed beaches; elsewhere, the shore is sandy with low dunes and bluffs. Lake Erie with 342 miles of coastline is predominately high and low erodible bluff. The southwestern area contains wetlands and a Tow erodible plain. This shore type changes to a low bluff and sparse dune area in western Ohio, before becoming a high erodible bluff in central and eastern Ohio. Approximately 12 percent of Lake Erie's shore is artificial fin. Lake Ontario's 290 miles of coastline consists of bluffs of glacial material and rock outcrops at the shore. A bluff shore type fronted with narrow grave} beaches predominates along the southern shore of Lake Ontario. Bluff heights range from 20 to 60 feet and are occasionally broken by low marshes. A short reach of low dunes and barrier beaches separates this erodible bluff type shore from the erosion-resistant rock outcrops extending northward to the St. Lawrence River. SUMMARY The U.S. coastline exhibits a great diversity of shore types, and these variations must be considered when establishing an erosion management program. Differences in the level of development, use, and engineering structures at the shore complicate this natural di- versity. Sediment sources and sinks, which are highly susceptible to human activities at the shore and in adjoining rivers and waterways, are also a major concern for erosion zone management. As a result of these multiple factors, it is necessary to consider both local con- ditions and broad regional issues when establishing a coastal erosion zone management program.
42 MANAGING COASTAL EROSION REFERENCES Brownlie, W. R., and B. D. Taylor. 1981. Coastal Sediment Delivery by Major Rivers in Southern California. Sediment Management for Southern Califor- nia Mountains, Coastal Plains and Shoreline, Part C, California Institute of Technology, Environmental Quality Laboratory, EQL Report No. 17-C. Carbognin, L., P. Gatto, and F. Marabini. 1984. Guidebook of the Eastern Po Plain (Italy): A Shore Illustration About Environmental and Land Subsidence. Published for use at the Third International Symposium on Land Subsidence, Venice, Italy, March 19-25, 1984. Printed by Officio Stampa, Comunicazione e Informazione, Comune di Madera. Cooper, W. S. 1967. Coastal Dunes of California. Geological Society of America, Memoir No. 104. Dean, R. G. 1989. Sediment interaction at modified coastal inlets: Processes and policies. Pp. 412-439 in Hydrodynamics and Sediment Dynamics of Tidal Inlets, Lecture Notes on Coastal Estuarine Studies, 29, D. Aubrey and L. Weishar, eds. Berlin: Springer-Verlag. Dean, R. G., and M. Perlin. 1977. Coastal Engineering Study of Ocean City, Maryland. Pp. 520-542 in Proceedings of ASCE Specialty Conference on Coastal Sediments '77. Dolan, T. J., P. G. Castens, C. J. Sonu, and A. K. Egense. 1987. Review of sediment budget methodology: Oceanside littoral cell, California. Coastal Sediments '87, ASCE, Vol. II, pp. 1289-1304. Edil, T. B. 1982. Causes and Mechanics of Coastal Bluff Recession in the Great Lakes. Proceedings of Workshop on Bluff Slumping, Michigan Sea Grant Report 901, pp. 1-48. Flinn, J. October 26, 1989. Beaches Imperiled by Weak Cliffs. San Francisco Examiner, pp. A1 and A9. Gabrysch, R. K. 1969. Land-Surface Subsidence in the Houston-Galveston Region, Texas. Land Subsidence: Proceedings of the Tokyo Symposium, September 1969, Paris: UNESCO, Vol. 1, pp. 43-54. Great Lakes Basin Commission Framework Study. 1975. Shore Use and Erosion, Appendix 12. Herron, W. J. 1980. Artificial beaches in Southern California. Shore Beach 48:3-12. Inman, D. L., and C. E. Nordstrom. 1971. On the tectonic and morphologic classification of coasts. J. Geol. 79:1-21. Johnson, J. W. 1971. The significance of seasonal beach changes in tidal boundaries. Shore Beach 39:26-31. Kenyon, E. C., Jr. 1951. History of Ocean Outlets, Los Angeles Flood Control District. Proceedings of First Conference on Coastal Engineering, Long Beach, California, October 1950. Edited by J. W. Johnson, Council on Wave Research, The Engineering Foundation, pp. 277-282. Leatherman, S. P. 1984. Shoreline evolution of North Assateague Island, Mary- land. Shore Beach 52:3-10. Leatherman, S. P. 1985. Geomorphic and stratigraphic analysis of fire island, New York. Marine Geol. 63:173-195. Leatherman, S. P. 1986. Cliff stability along western Chesapeake Bay, Maryland. Marine Tech. Soc. J. 20:28-36.
CAUSES,EFFECTS,AND DISTRIBUTION 43 Leatherman, S. P. 1987. Reworking of glacial sediments along outer Cape Code: Development of Provincetown spit. Pp. 307-325 in Treatise of Glaciated Coasts, D. M. Fitzgerald and P. S. Rosen, eds. New York: Academic Press. Leatherman, S. P. 1989 (in press). Coasts and beaches. In Heritage of Engineer- ing Geology: The First Hundred Years. Geological Society of America. Magoon, O. T., J. C. Haugen, and R. L. Sloan. 1972. Coastal Sand Mining in Northern California, U.S.A. Proceedings of the Thirteenth Coastal Engineering Conference, July 10-14, 1972, Vancouver, Canada, ASCE, Chapter 87, pp. 1571-1597. National Research Council. 1987a. Responding to Changes in Sea Level: Engi- neering Implications. Washington, D.C.: National Academy Press. National Research Council. 1987b. River and Dam Management: A Review of the Bureau of Reclamation's Glen Canyon Environmental Studies. Wash- ington, D.C.: National Academy Press. Olsen, E. J. 1977. A Study of the Effects of Inlet Stabilization at St. Mary's En- trance, Florida. Pp. 311-329 in Proceedings of ASCE Specialty Conference on Coastal Sediments '77. Oradiwe, E. N. 1986. Sediment Budget for Monterey Bay. M.S. thesis. U.S. Naval Postgraduate School, Monterey, California. Ritter, J. R. 1969. Preliminary Studies of Sedimentation and Hydrology in Bolinas Lagoon, Marin County, California, May 1967-June 1968. U.S. Geological Survey Open-File Report. Seltz-Petrash, A. 1980. Subsidence a geological problem with a political solu- tion. Civil Eng. 52:60-63. Shepard, F. P., and H. R. Wanless. 1971. Our Changing Coastlines. New York: McGraw-Hill. Smith, O. P., J. M. Smith, M. A. Cialone, J. Pope, and T. L. Walton. 1985. Engineering Analysis of Beach Erosion at Homer Spit, Alaska. U.S. Army Corps of Engineers, Coastal Engineering Research Center, Misc. Paper CERC-85-13. U.S. Army Corps of Engineers. 1967. Beach Erosion Control Study on Brevard County, Florida. Jacksonville, Fla. Williams, S. J., and E. P. Meisburger. 1987. Sand Sources for the ~ansgressive Barrier Coast of Long Island, N.Y.: Evidence for Landward Transport of Shelf Sediments. Proceedings, ASCE Specialty Conference on Coastal Sediments '87, New Orleans, Louisiana.
