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s Erects of Sea Level Rise in the Coastal Zone Sea level rise wiD have different effects along various portions of the U.S. coastline depending on conditions such as sediment type and coastal planform. It Is possible to divide the coasts into physiographic regions for consideration of their response to relative sea level rise. For instance, the conditions in Louisiana do not apply to the coast of Mane because the Mississippi delta region is very flat, undergoing pronounced compaction and subsidence, while northern New England is characterized by nonerodible cliffs and portions are experiencing neotectonic uplift. The present rise in water level is a complex phenomenon, including local, regional, ~d global components, as detailed pre- viously. Shoreline position will respond to the cumulative effect of vertical motions, termed the relative mean sea level rise, regard- less of their cause. However, it is instructive to divide the coasts into regions that will behave In a similar manner due to particular processes and materials. The U.S. continental coastline is highly variable in character but certain regional trends are apparent. Tectonic mapping clearly indicates the reasons for the prominent differences between the Pacific coastal range as compared to the Atlantic and Gulf coastal prams. The tectonically active Pacific run ~ a coast where one plate is being subducted below another (Inman and Nordstrom, 40

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EFFECTS O1? SEA LEVEL RISE IN THE COASTAL ZONE 41 1971), resulting in a narrow continental shelf and an essentially nonexistent coastal plain. This coast is characterized by headlands and intervening pocket beaches. By comparison, the Atiantic and Gulf coasts have long been tectonically stable and constitute trailing edge and marginal sea coasts, respectively. The AtIantic coastal plain, which extends over 100 miles in- land, is characterized by a gently sloping surface with gradients of only several feet per mile near the shore. The prominent landforms from Long Island, New York to Miami Beach, Florida along the Atlantic coastal plain are barrier islands. The Gulf coastal plain exhibits the lowest average relief and gentlest gradients. Barrier islands are again the dominant coastal landforms, but the chain is less continuous than on the Atlantic Coast, as broken by the Mississippi River deltaic sediments of Louisiana, the marshy out- crops along the northeast Gulf coast of Florida (Tanner, 1960), and the broad outflowing of the Everglades along the limestone rocky coast of the southern peninsula of Florida. These three coasts can be further subdivided into physio- graphic regions on the basis of geologic history and coastal mor- phodynam~cs. Basically 11 types of coasts can be defined for the U.S. contment (Figure 5-1) using a modification of the ciassifica- tion by Shepard and WanIess (1971~. The glaciated coast extends from northern New Jersey to Mane. This physiographic region can be further subdivided into the erosion-res~stant crystalline (gran- ite) rock of northern New England and the mostly unconsolidated glacial tiD of southern New England. While coastal barriers have developed by spit growth across many embayments, the main- land is often fairly high near the shore, forming cliffs. The low areas, which are subject to storm surges, are clearly marked on topographic maps. The coastal compartment barrier chains (Swift, 1968) of New Jersey and the Delmarva Peninsula are characterized by four sec- tors: (1) terminal north spit, (2) low, eroding headland, (3) long barrier islands backed by open lagoons, and finally to the south (4) short, stubby barriers with marsh-fired embayments. Within these compartments, there is variable shoreline vulnerability in response to cliffering rates and patterns of shoreline erosion (Leatherman et al., 1982), storm surge flooding, and inlet breaching potential. The south shore of Long Island, New York can also be included in this sector since the glacial outwash plain there has a similar physiography to that of the Atiantic coastal plain.

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42 RESPONDING TO CHANGES IN SEA LEVEL /1 ~ ~' \ 1 W~ If. ~ -at/ ; ~ _ /% Cuspate Coast ~ 1 )0f \~) w~~W 'A W./ WL ~ \\ __~__~ / ~ 4 MesoSIdal Coast \ - - '` ~ ~ ~ \~\ 5 Stralght Barrler ~ ~~ ~~ Florlda 8 \ L ~\ Islands 0 2~ 4 >\ SO 9 DelSalc ~ Panhandle ~ ~ I ~ ~ l N~ Hi / Stralght ~ CoasS Panhandle ~~ ~ . Barrlers At\ =lA-~_ x~ V miles Guii 7~ti j 6 Florlda Keys \ | Peninsula \ \ .~ FIGURE 5-1 The Atlantic and Gulf coastlines classified according to geo- logical and morphological criteria. Source: Adapted from Shepard and Wanless (1971~. The cuspate coast, which is best exemplified by Cape Hatteras, North Carolina, extends from Norfolk, Virginia to Cape Romain, South Carolina (Figure 5-1~. The Outer Banks" type barrier islands form a string of sand that protrudes far from the mainland coast along the northern part until merging with the mainland at Myrtle Beach, South Carolina. The historical record shows that the Outer Banks of North Carolina have been breached by many inlets (Swift, 1968~. The mesotidal coast of the Georgia bight extends from south- ern South Carolina to northern Florida. This physiographic region has the only tide-dominated barrier islands along the U.S. Atlantic and Gulf coastal plains. These islands tend to be relatively short and stubby with marsh-fi~led lagoons; they often display a "drum- stick shaper (Hayes, 1979~. The famed Sea Islands (Hoyt and Henry, 1967) are also present in this region. These coastal land- forms are distinctive by being composed of a Pleistocene core (often with land surface above the Midyear flood level). Holocene

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EFFECTS OF SEA LEVEL RISE IN THE COASTAL ZONE 43 (modern) sediments are plastered onto the seaward face of the old Pleistocene barrier or separated from it by small salt marshes and tidal creeks. The Sea Islands and mesotidal barriers are more stable than their m~crotidal counterparts due to the fact that the tidal inlets are located in pre-Holocene drainage channel, and their locations shift within narrowly defined limits (Oertel, 19793. That these in- lets are tidally adjusted to better accommodate storm surge flood- ing, in combination with the antecedent topography and possibly some present day river~ne sources of coarse sediment, makes these islands more stable than barriers along the rest of the Atlantic and Gulf coasts. Straight, narrow barrier islands persist from northern Florida to the Florida Keys, with Cape Canaveral being the only anomaly. The lagoons are narrow and shallow, and the presence of existing inlets, maIly of which are stabilized, largely precludes future in- let breaching. The coast is low and sandy, with only occasional outcrops of erosion-res~st ant limestone (Anastasia coquina rock formation) in the beach face. The Florida Keys are founded on coral reefs, often capped with coral rubble and, in lirn~ted areas, faced with sandy beaches produced by the hurricane destruction and wave-abrasion of living or dead corals in the surf zone. Coral reef platforms are more resistant to erosion than barrier islands, but their low elevations make them especially vulnerable to increased flooding and over- wash with relative sea level rme. However, coral reefs can continue to grow vertically, which is nature's response to sea level rise, as long as anthropogenic pollution does not kill these ecosystems. The Florida Gulf peninsula contains a diverse coast of sandy barrier beaches and swamps. The Everglades empties directly into the Florida Bay along a wide front. This physiographic section is largely controlled by Pleistocene limestone rock, which in some places outcrops near sea ferret to shelter and allow formation of coastal swamps and In others lies tens of feet below the present water surface (Evans et al., 1985~. The swampy coast is little developed, but the intermittent, low elevation, m}crotidal barrier islands have been highly urbanized near such population centers as Tampa. The Florida panhandle barrier-island system actually extends to the Mississippi delta off Gulfport, Mississippi. The barriers are largely Holocene in origin, except for a Pleistocene core, which

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44 RESPONDING TO CHANGES IN SEA LEVEL constitutes the eastern, bulbous end of Dauphin Island, Alabama. These beach-ridge dorn~nated barrier islands indicate periods of past geological accretion and are presently experiencing strong westward rn~gration due to littoral drift. The deltaic coast of Louisiana is the most vulnerable to rela- tive sea level rise of the entire U.S. continental area. The sed~rnents are largely fine grapnel (silts and clays), very organically rich, and subject to compaction (and hence suffer subsidence) and erosion along the shoreline. Much of the Louisiana coast, except for the small area of active delta building, is retreating landward on the order of many meters per year (May et al., 1983~. The natu- ral problem of compaction, predominantly a result of loading by sediments deposited in the Mississippi delta region, is greatly com- pounded by the artificial withdrawal of subsurface fluids. Hence, some areas are sinking at rates of 1 cm/yr or more, drowning salt marshes and pushing the small sandy barriers as thin sand wedges over the adjacent back-barrier serpents. The Chancleleur Islands and Isles Dernieres barrier chains are being fragmented by hur- ricanes, and it appears that these islands will be lost during the next 100 years even under the present conditions of relative sea level rise (PenIand et al., 1985~. The straight barrier coast of Texas has been wed studied (McGowen et al., 1977~. The small delta of the Brazos River is the only major interruption of these very long, but fairly wide barrier islands. The existing inlets front the mouths of large bays or rivers emptying into the sea (e.g., Brazos and Rio Grande rivers). Generally, the great widths (several miles across) of the Texas barriers, mucrotidal conditions, and shallow lagoons preclude most inlet activity. Unlike the condition on most of the Atiantic and Gulf coasts, rivers are still providing some coarse-grained material (sand) to the open-ocean coast, so that the barrier islands are not nearly as sand-starved as those found elsewhere. Locally, erosion is a problem, as at Sargents Beach (Herbich, 1975) and as evidenced by the lack of beach in front of portions of the Galveston sea wall. The Pacific Coast of California, Oregon, and Washington is quite irregular and diverse. There is little to no coastal plain, and cliffs of resistant hard rock or unconsolidated river-fi~} sediments predominate along this tectonically active Pacific rim. Within small embayments, sandy to gravelly spits can grow, but these landforms are ephemeral, geologically speaking, and are highly

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EFFECTS OF SEA LEVEL RISE IN THE COASTAL ZONE 45 unstable features over the short term (Komar, 1976~. The geo- morphic diversity over short distances largely precludes the typing of this coast into natural units as physiographic regions. Generally, there are three sections. From the Mexican border to Point Conception, California, the coast is a nearly continuous, albeit very narrow, beach interrupted by a few headlands. From central California to the Columbia River, Oregon, headlands predorn~nate. From the Columbia River to the Olympic Peninsula, Wash- ington, the fine-grained, flat-sIoped, sandy beaches are composed of river-derived material. The Alaskan Coast can be roughly divided into four physio- graphic regions (Shepard and WanIess, 1971~: fjords of the south coast, Aleutian hard rock islands, permarost-dominated lowlands of the west coast, and low barriers, spits, and permafrost mainland of the north coast. ~ assessing the vulnerability of a coastal area to sea level rise, the best guide is to consider the nature of the sedunents (erosion- resistant bedrock or unconsolidated sands, gravels, and clays) and the topography (high to low clips versus low sandy barrier spits). Additionally, the degree of existing erosion may serve as an index if future problems result from an accelerated sea level rise. SANDY COASTLINE:S Mean sea level is one of the principal determinants of shoreline position. Swift et al. (1972) suggested that a relationship exists among several factors: sediment supply, wave energy, sea level, and shoreline position. Rising relative sea level tends to cause shoreline recession, except where this trend is offset by an influx of sediment. The primary reason that a sea level rise would induce beach erosion ~ that natural beach profiles are concave upward; this geometry results in the wave energy being dissipated in a smaller water volume than without sea level rise, and thus the turbulence generated within the surf zone is greater. The profile responds

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46 RESPONDING TO CHANGES IN SEA LEVEL by conforming to a more gentle nearshore slope, which requires additional sedunent to be eroded from the beach. Most sandy shorelines worIdw~de have retreated during the past century (Bird, 1976~. Progradation has been restricted to coastal areas where excess sediment is supplied by river sources or where the land is being elevated due to tectonic uplift or iso- static glacial rebound. Human interference cannot be considered a primary cause of erosion worldwide since retreat also occurs on sparsely populated and little-developed sandy coasts (Bird, 1976~. Such recession could result from an increase in storminess, but this trend would have to be almost worldwide to account for erosion on geographically dispersed sandy shorelines. Therefore, in view of the demonstrated general relative rise of sea level along the U.S. shoreline, the link between shore retreat and sea level rise is based on more than circumstantial evidence; it can be stated that the relationship is causal in nature. III some areas, it is clear that human actions have caused sum spatial erosional pressures. Undoubtedly the principal contribu- tar has been the construction of jettied inlets and the deepening of channel entrances for navigation. Along shorelines with high rates of longshore sediment transport, these constructed features trap sediment at the uplift jetty and, if material dredged from the navigation channel is not placed on the downdrift beaches, cause an amount of downdrift erosion equal to the reduction In trans- port. At some Florida entrances, tens of millions of cubic yards of dredged material have been placed in water depths outside the lit- toral system. This has resulted In very high erosion concentrated downdrift of the entrances. Some of these shorelines were stable or accret~ng in their natural condition, prior to mIet modification. Geologic Indicators The geologic record of the Atlantic and Gulf coastal plains over the last 10 million years indicates that sea levels have fluctuated by 200 m or so during this time period. Five distinct transgressive coastal systems have been identified on the DeLnarva peninsula from geomorphic and subsurface data. Each was produced during interglacial high sea levels and range in age from more than 1 minion years to 60,000 years (Demarest and Leatherman, 1985~. Sedimentologica1 and historical evidence for four minor transgres- sive phases or pulses, with sea level fluctuations of less than 1 m,

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EFFECTS OF SEA L,EYEL RISE IN THE COASTAL ZONE 47 during the last 2,000 years have been found along the Eriesland barrier Glands in the Netherlands (Bakker, 19813. The modern transgressive pulse during the overall Holocene transgression pre- sumably began in the eighteenth century, when history indicates an increase In storm surge damage and coastal flooding. Long periods of sedimentary accretion resulting in beach ridges have been arrested or the trend reversed during the past cen- tury. Teichert (1947) reported that beach ridge formation ceased slightly more than 100 years ago, and the western Australian corall~ne shore is now subject to erosion by the sea. In Nigeria, Pugh (1954) noted that earlier progradation had similarly given way to retrogradation on sandy shorelines. Bogue Banks, along the Outer Banks of North Carolina, Is a barrier island composed of parallel sets of beach ridges, which have prograded seaward during the past 3,000 4,000 years, but now the beaches are nar- row and dunes are actively wave cut during annual winter storms (Steele, 1980~. Similar reversals in trend, from long-term accretion to recession, have been noted by many investigators working along sedimentary coots in the United States (e.g., Tanner and Stapor, 1971) and worse (Davies, 1957~. Onset of the present transgressive pulse, attested to by marked beach and dune erosion, has varied geographically depending upon local differences In sand supply ~d wave energy. Information from 73 correspondents in 39 coastal countries showed that less than 10 percent of the length of the worId's sandy shorelines have prograded, more than 60 percent have retrograded, and the balance have been relatively stable or have shown no consistent trend during the past century (Bird, 1976~. Other geologic indicators of shore retreat are woodcut cliffs, which occur worldwide (Sunamura, 1983~. Exhumation of salt marsh peat on beach faces indicates upward and landward bar- rier migration. Most barrier island coasts have been retreating for at least the past few hundred years, as clearly indicated by these back-barrier peat outcrops, exposed on the lower beach fore- shore after severe storms. Peat outcrops have been reported in widely dispersed areas Tong the U.S. AtIantic and Gulf coasts, in- cluding Nauset Spit, Massachusetts (Leatherman, 1979b); coastal Delaware (Kraft, 1971~; Assateague Island, Maryland (Leather- man, 1979a); Cape Hatteras, North Carolina (Swift, 19683; and Sargent Beach, Texas (Herbich, 1975~.

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48 RESPONDING TO CHANGES IN SEA LEVEL TABLE 5-1 National Assessment of Shore Erosion (Jules) Total Location Shoreline Erosional Nonerosional North Atlantic 8,620 7,460 1,160 South Atlantic Gulf 14,620 2,820 11,800 Lower Mississippi 1,940 1,580 360 Texas Gulf 2,500 360 2,140 Great Lakes 3,680 1,260 2,420 California 1,810 1,550 260 North Pacific 2,840 260 2,580 Alaska 47,300 5,100 42,200 Hawaii 930 110 820 Total 84,240 20,500 63,740 SOURCE: U.S. Army Corps of Engineers (1971~. Historical Records Historical records also indicate the prevalence of shore reces- sion during at least the past century. The National Shoreline Study by the U.S. Army Corps of Engineers (1971) was the first overall national appraisal of shore erosion problems. Of the over 84,000 miles of United States ocean and Great Lakes shorelines, significant erosion occurs along 20,500 miles or 25 percent of the total (Table ~1~. Excluding Alaska, it shows that 43 percent of the shoreline is undergoing significant erosion. It should also be noted that a significant portion of the shoreline Is categorized by the U.S. Army Corps of Engineers (1971) as noncritical, which does not connote nonserious. ~ these cases the problems appear to be amenable to land use controls and other management techniques rather than relying upon engineering measures to halt erosion. More recently, May et al. (1983) have assembled data derived from aerial photography of the U.S. Geological Survey dating back to the late 1930s, providing a maximum record of 40-50 years. The National Ocean Service of the National Oceanic and Atmospheric Administration (NOAA) also has made efforts to determine the historical rate of shoreline change along portions of the U.S. continental coast using historical maps and charts (NOS "T~ sheets). The data base includes most of the rriid-Atiantic Coast as well as South Carolina and parts of California. These

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EFFECTS OF SEA LEVEL RISE IN THE COASTAL ZONE 49 data aDow for the quantification of historical shoreline changes over 100 150 years of record. The NOS maps show a general pattern of pervasive shore recession except for local anomalies (Everts et al., 1983~. The existing data sets (Table 5-2) have been grouped by state for comparative purposes (May et al., 1983~. The national average (unweighted) shoreline erosion rate is 0.4 m/yr. Along the Atlantic Coast, the average erosion rate is about 0.8 m/yr with the Virginia barrier islands exhibiting the highest rates of erosion (Leatherman et al., 1982~. The Gulf Coast states are distinguished by the high- est average erosion rate in the nation (1.8 m/yr). The delta~c coast of Louisiana is by far the most dynamic (4.2 m/yr erosion; May et al., 1983~. The Pacific coastline is essentially stable, although more than half of the shore is hard rock. The erosion rate can be tabulates} by landform type for comparative purposes (Table 5-3~. Table ~3 may be useful In ascertaining the shoreline erosion rate for a site-specific area, such as along the geomorphically diverse Pacific Coast. Ter~n;ques of Projectmg Shoreline Retreat Due to Sea [eve! Rise Rising sea level ~ accompanied by a general recession of the shoreline due to inundation or erosion. Inundation is the sum mergence of the otherwise unaltered shore, while erosion is the physical remove of beach material. Direct submergence of the land occurs continuously through time and is particularly evident in coastal bays where upland is slowly converted to coastal marsh- lands. Submergence, however, accounts for only a small portion of the net shore recession Tong exposed, sed~rnentary coasts (Hands, 1976~. Several different approaches can be used to mode! the result- ing shoreline configuration as a function of sea level rise. The simplest method uses the drowned-vaBey concept (Figure 5-2), in which preexisting topography along shorelines is considered fixed and combined with increased sea level to project new shorelines (Kane et al., 1984~. Slope is the controlling variable: steep-sloped areas wig experience little horizontal shoreline displacement with each increment of water level rise, while gently sloping shores will undergo a much broader area of dooding for a given sea level rise. This Is the preferred methodology for immobile substrates, such as

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so RESPONDING TO CHANGES IN SEA LEVEL TABLE 5-2 Shoreline Erosion Rate Based on Historical Aerial Photographs by State and Region Standard Average Deviation Extreme Shoreline Change Number of Shoreline of Shoreline Rates (m/yr)a Sample Change Rate Change Rate Maximum Maximum Data b Region (m/yr) (m/yr) Accretion Erosion Points Atlantic Coast -0.8 3.2 25.5 -24.6 510 Maine -0.4 0.6 1.9 -0.S 16 New Hampshire -0.5 -- -0.5 -0.S 4 Massachusetts -0.9 1.9 4.5 -4.5 48 Rhode Island -0.5 0.1 -0.3 -0.7 17 New Yorlc 0.1 3.2 163.8 -2.2 42 New Jersey -1.0 S.4 25.5 -15.0 39 Delaware 0.1 2.4 5.0 -2.3 7 Maryland -1.5 3.0 1.3 -8.8 9 Virginia -4.2 5.5 0.9 -24.6 34 North Carolina -0.6 2.1 9.4 -6.0 101 South Carolina -2.0 3.8 5.9 -17.7 s? Georgia 0.7 2.8 5.0 -4.0 31 Florida -0.1 1.2 5.O -2.9 105 Gulf of Mexico -1.8 2.7 8.8 -15.3 358 Florida -0.4 1.6 8.8 -4.5 118 Alabama -1.1 0.6 0.S -3.1 16 Mississippi -0.6 2.0 0.6 -6.4 12 Louisiana -4.2 3.3 3.4 -15.3 106 Texas -1.2 1.4 0.8 -5.0 106 Pacific Coast -0.0 1.5 10.0 -5.0 305 California -0.1 1.3 10.0 -4.2 164 Oregon 0.1 1.1 5.0 -5.0 86 Washington 0.5 2.2 5.0 -3.9 46 Alaska -2.4 2.0 2.9 -6.0 69 baNegative values indicate erosion and positive values indicate accretion. Total number of 3-min grid cells over which statistics are calculated. SOURCE: May et al. ( 1983). rocky or armored shorelines, or where the wave climate is subdued, as on the sheltered coasts of embayments. Several approaches to shoreline recession that have been em- ployed to date are largely based on the erosional potential of sea level rise: (1) extrapolation of historical trends (Leatherman, 1984b), (2) the Bruun rule (Hands, 1981, Weggel, 1979; Bruun, 1962), (3) the sediment budget method (Everts, 1985), and (4) the dynamic equilibrium model (Dean, 1983~. These methodologies, including applications and limitations, will be discussed in the order outlined.

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EFFECTS OF SEA LEVEL RISE IN TWIT COASTAL ZONE TABLE 5-4 Projected Erosion at Ocean City, Maryland in Meters (ft) of Shoreline Retreat Relative to Its Current Position Current Trends 2000 2025 2050 2075 Bruun~ 5 (16) 11 (36) 17 (57) 23 (75) Everts 21 (68) 47 (153) 73 (238) 99 (323) Leatherman 12 (39) 26 (85) 41 (134) 56 (182) Kriebel/Dean 20 (66) 47 (153) 70 (231) 95 (102) Mid-ran~e Low Bruuna b 7 (22) 22 (72) 43 (140) 70 (231) Bruun adjusted 23 (74) 58 (189) 98 (322) 147 (482) Exerts 26 (84) 73 (238) 132 (434) 215 (205) Leatherman 20 (64) 56 (182) 105 (345) 174 (571) Kriebel/Dean NC 55 (180) NC 140 (460) Mid-ran~e High Bound b 12 (38) 32 (106) 63 (206) 105 (346) Bruun adjusted 27 (90) 68 (223) 118 (388) 181 (592) Everts 29 (95) 83 (273) 156 (511) 268 (878) Leatherman 27 (89) 76 (250) 147 (483) 249 (812) Kriebel/Dean NC 66 (216) NC 168 (550) _ NOTE: NC = not calculated. 3ruun rule includes only the impacts of sea level rise. Brulm rule adjusted includes 2.6 ft/yr due to factors other than sea level rise. Because 2.6 ft/yr is derived from Exerts, Bruun adjusted is equal to Everts for current trends. SOURCE: Titus (1985). 61 rates near the end of the time span considered and substantial potential shoreline changes within the entire time span. The long- shore losses along this portion of the coastline me believed to be due to the presence of a nodal point located at South Bethany Beach (U.S. Army Corps of Engineers, 19803. BLUFF AND CLIFF RETREAT While most of the attention by coastal geomorphologists and engineers has been directed at studying sandy beaches, cliff retreat is a significant problem along large portions of the nation's coast (i.e., the Pacific Coast, the Great Lakes, and parts of the New England and New York coasts). Creases In water level will only

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62 RESPONDING TO CHANGES IN SEA LEVEL accelerate the erosion rate as has been clearly shown by Hands (1981) along the Lake Michigan shore. Elsewhere, the high cliffs of unconsolidated sands and grave] along outer Cape Cod, Massachusetts are eroding at an average rate of 2.2 ft/yr based on more than 100 years of field survey data. Dalrymple et al. (1986) indicate that bluff recession in Chesapeake Bay is related to the heights of the bluffs and their compositions, as well as the available wave energy. Kuhn and Shepard (1981) showed that the unconsolidated sedimentary cliffs of southern California recede in an episodic manner, corresponding to rainfall and storm wave attack during unusually severe winter storms. Thornton et al. (198S) derived an empirical relation between surge level and wave runup and cliff retreat based on studies of Monterey Bay, California. As previously mentioned, cliffs of crystalline rock are essen- tially stable with response times to sea level rise much longer than those of sandy shorelines. Thus, for parts of the Pacific Coast and ahnost Al of the rocky Maine coast, cliff position is unchanged over historical periods of record. Sunamura (1983) provides a review of cliff erosion processes. TIDAL INLETS Along the barrier island coastlines of the United States, inlets provide hydraulic connections between the back-barrier environ- ments and the ocean. In their natural conditions, inlets can mi- grate along the shoreline, whereas when stabilized by jetties, they are fixed in position to provide reliable navigation channels. An inlet can be characterized by its tidal prom, the toted flow Of water through the inlet from low to high tide, and the amount of sand moving locally in the littoral transport system. Inlets with small tidal prisms have little ability to scour and erode sand transported into the inlet Tom the adjacent shores. Often these inlets have very pronounced ebb tidal deltas, shallow enough to permit waves to move sand past the inlet. Inlets with reduced sediment transport environments or large tidal prisms have ebb tidal deltas located in deeper water or farther out to sea. In either case the amount of sand capable of bypassing ~ inlet modified for unproved navigation is very much less, and severe downdrift erosion can result (Bruun and Gerritsen, 1960~. The ebb tidal delta and the flood tidal delta in the backbay

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EFFECTS OF SEA LEVEL RISE IN THE COASTAL ZONE 63 consist of platforms (~wash-platforms) on the ebb tidal delta or ramps (flood ramps), which are separated by channels kept clear by the tidal currents. The size of the ebb tidal delta is roughly proportional to the tidal prism (Walton and Adams, 1976), and often represents the trapping of millions of cubic meters of sand unavailable to the neighboring beaches. The development of new inlets or the stabilization of existing inlets generally results In the development of large or larger ebb tidal deltas, impounding greater amounts of sand, thereby reducing the sand available to the beaches. Artificial sand bypassing, consisting of a Boating or land-based dredge pump which Recharges sand onto a downdrift beach, is used at several inlets (e.g., Lake Worth Entrance, South Lake Worth Inlet, and Hilisboro ~let, Florida) to augment any natural bypassing of sand; however, as a general rule a tidal inlet represents a sink of beach sand. A recent study by the state of Florida (1986) showed that most of the state's eroding areas were next to tidal miets, implying that effective bypassing of sand at the inlets would reduce many of the erosion problems. The basic effects of sea level change on tidal prism and inlet cross-sectional area were discussed in general terms earlier In this report. The magnitude of change in tidal prism in response to sea level rise is highly dependent on conditions along the bay shoreline. Bays surrounded by Pleistocene uplands generally have relatively steep shorelines, so that rising sea level will have only a minor impact on changes in tidal prism. For example, an estimated 1 m rise in local sea level at Indian River Inlet, Delaware may cause only a 2 percent change in prism of Indian River Inlet. Shallow bays surrounded by extensive wetlands will expand rapidly in response to a rise both because of the gentle slope and the deterioration of the marshes in response to water level increases. Barataria Bay, Louisiana has increased its surface area about 1~15 percent over the last century in response to about 1 m of local relative sea level rise in that area. Of perhaps greater importance is the change in sand storage volume of the ebb and flood deltas. If the prism increases, there is likely to be a corresponding increase in the volume of these shoab. Furthermore, as the sea level rises the deltas must grow in elevation to keep up with the rise, implying that any natural bypassing of sand will be reduced and that downdrift erosion will increase.

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64 RESPONDING TO CHANGES IN SEA LEVEL Stabilized inlets will be afl3ected strongly by a large sea level rise. The protective jetties, which retard the ability of the lit- toral drift to enter the navigational channel and reduce the wave climate in the channel, will become less effective as they are sum merged. Also, the stability of the jetties is reduced due to the aforementioned greater wave heights as a result of sea level rise. WETLANDS Wetlands account for most of the land less than 1 m above sea level. These extensive marshes, swamps, and mangrove forests fringe most of the U.S. coastline, particularly along the Atlantic and Gulf coasts. Coastal wetlands serve as nurseries for fish and shrimp, many birds, and fur-bear~ng animals. They are vital to coastal recreation, to the maintenance of water quality, and as a buffer against shore erosion. Their estimated original extent in the United States was 5 million acres or about 20,200 km2 (7,800 rni2) (Hoese, 1967~. This acreage has been significantly reduced through a variety of actions including an early widespread practice of filling marshlands in urban areas. Wetlands loss has also been caused by other human actions, such as the construction of canals and waterways and the diversion of fluvial sediment to the offshore. III response to this loss, several federal and state programs have been designed to prevent wetness destruction. Specifically, Section 10 of the federal Rivers and Harbors Act, Section 404 of the federal Clean Water Act, and Executive Order 11988 on flood- plain management all establish permit requirements for actions affecting waterways and wetlands. In general, the wetlands policy of both the U.S. Army Corps of Engineers and the Environmental Protection Agency (EPA) ~ to discourage issuance of a permit for an activity that would involve alteration of wetlands. However, the effectiveness of this permitting process has been questioned. The congressional Office of Technology Assessment (OTA) con- cluded that permit applications for wetlands alteration are still rarely denied (OTA, 1984~. The continuing human destruction of wetlands should be kept in mind for the proper perspective when considering sea level rise and its potential effects on wetlands de- terioration. Ecological conditions in coastal marshes range from marine to nearly terrestrial. A change in controlling factors, such as water

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EFFECTS OF SEA LEVEL RISE IN THE COASTAL ZONE 65 salinity or tidal and wave energy, will cause a displacement in marsh zonation. Generally, coastal marshes are divided into low and high marsh based on their elevation relative to sea level (Red- field, 1972~. Since marsh plants are attuned to particular mean water levels (e.g., spatting patents, salt meadow grass, grows at mean high tide), a rise in sea level will shift the distribution of plant species proportionally landward. Beyond this fundamental response to variation in relative sea level, however, a more com- plex set of attendant responses may occur, tied to the type of marsh considered. Thus, anticipated changes in coastal marshes must be assessed within the context of the basic marsh types that characterize U.S. coasts. Marshes have been classified on the basis of the flora present (Redfield, 1972) and salinity and floristic relations (Chabreck, 1972), and functionally on the basis of geologic/geomorphic pro- cesses (Stevenson et al., 1986~. Nevertheless, with respect to the future effects of a rise in sea level, coastal marshes may be broadly divided into back-barrier marshes, estuarine (brackish) marshes, and tidal freshwater marshes. Back-Barrier Marshes Back-barrier marshes occur along the bay sides of barrier systems of the Atlantic and Gulf coasts. Studies (e.g., Zaremba and Leatherman, 1986) show that these marshes are formed and destroyed rapidly in such dynamic environments. Maintenance of these marshes is therefore more a function of barrier stability than the pace of upward growth of the marsh surface, since sediment supplies are ample (Letzsch and Frey, 1980~. For barriers rapidly migrating landward, there may be a net decline in back-barrier marshes. This has been found to be the case at north Assateague Island, Maryland, where sediment blockage by jetties has greatly increased the rate of landward barrier migration (Leatherman, 1984a), and the same qualitative result would be anticipated as a result of accelerated sea level rise. Estuarine (Brackish) Marshes Estuarine marshes embrace a wide variety of floristic species in diverse geologic settings where salinities are less than 30 ppt. These marshes, comprising integral components of major estuarine

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66 RESPONDING TO CHANGES IN SEA LEVEL systems such as the Chesapeake Bay, occur in areas of quiescent waters and ample sediment supply. Accretionary budgets differ widely (Table 5-5), but in a dynamic equilibrium condition onsite production of organic materials and influx of mineral sediments cause vertical accretion, balancing the loch rate of sea level rise. In view of the geographic range of the measurement sites and the local variability within coastal marshes, it is rather remarkable that the measured sedimentation rates all fall within the same order of magnitude. Accretion rates are generally found to vary from 1 mm/yr in high marsh at Duplin River, Georgia to 11 mm/yr in the Savannah River estuary, Georgia. The data In Table 5-5 demonstrate that, in general, the mea- sured accretion rates do exceed the locally determined relative rates of sea level rise. Consequently, most marshes do receive ad- equate sediment supply to compensate for current sea level rise. This must have been the case over the last few hundred years since "naturals marsh loss has not historically been reported to be a problem. Three notable exceptions occur at Barn Island, Con- necticut; Blackwater Marsh, Maryland; and in I,ouisiana, where the present short-term rates of marsh accretion are lower than the local rates of sea level rise. ~ Louisiana there is widespread loss of coastal wetlands, in part attributable to a sediment deficit. Marsh deterioration is also known In the Blackwater Wildlife Refuge, Maryland, but no such problems are yet reported at Barn Island. Exceptionally low local rates of sediment accretion appear to be the cause in both Connecticut and Maryland. Tidal Freshwater Marshes Tidal freshwater marshes are located in the upper reaches of estuaries and other areas where ambient salinities are less than 5 ppt. The flora of these marshes is varied and lacks the typical vegetation zonation of open-coast marshes. The effects of rising sea levels will be saltwater intrusion and the eventual dominance of higher salt-tolerant plants. However, the effects of canalization on tidal freshwater marshes in the Mississippi delta demonstrate that dramatic increases in salinity over a comparatively short period exceed the capability of these marshes to adjust so that rapid losses ensue.

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EFFECTS OF SEA LEVEL RISE IN THE COASTAL ZONE Processes of Marsh Loss with Sea [eve] Rise 69 Land losses in most marshes result from a combination of mechanisms. Shoreline erosion at the seaward edge of the marsh, being the most obvious process, could be expected to accelerate with increased water levels. Nationally, however, shoreline erosion probably accounts for about 1 percent of all marsh losses annually. The comparative resistance of marshy shorelines to wave attack suggests that with rapidly rising sea levels, most marshes will be Tong since submerged before extensive shoreline erosion occurs. A more probable catastrophic mechanism of marsh loss with a large increase In sea levels will be the formation of extensive interior ponds allied with general tidal creek bank erosion and headward growth as tidal prisms increase. The rapid enTarge- ment and coalescence of interior ponds in marshes subject to rapid coastal submergence has been amply documented in the Missis- sippi delta (DeLaune et al., 1983) and at the Blackwater Wildlife Refuge (Stevenson et al., 1986~. The magnitude of marsh losses from interior poking is instructive. At the Blackwater Wildlife Refuge in Maryland, over one-third of the total marsh area (about 5,000 acres) was lost between 1938 and 1979 by the growth of in- terior ponds, largely occurring during a midyear period. The phys- iolog~cal mechanism behind the development of interior ponds is believed to be anoxia, and ult~rnate root death of marsh plants, as sea levels outpace the ability of the marsh to maintain elevation. Hn~n-~duced Changes The most dramatic changes ~ wetlands have historically re- sulted from human alterations. Over half the salt marshes in New England have been lost because of dredge and fill activities. Elsewhere, the expanse of marshes has actually been increased by poor land practices. Early settlers felled large tracts of forest for agricultural fields, resulting ~ massive siltation of some bays and estuaries. This pattern Is especially true of the Chesapeake Bay, where the colonial port at Gunpowder River is now separated from navigable waters by several miles of intertidal flats, colonized by marsh grasses. Present human activities are mainly preventing sediments from reaching wetlands areas. Moreover, soil conservation prac- tices through contour plowing, buffer strips, and no-till agriculture have substantially reduced the influx of soil into adjacent water

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EFFECTS OF SEA LEVEL RISE IN THE COASTAL ZONE ~1 bodies and wetlands. Darns and levees on major rivers trap mate- ri~ upstream and prevent over-bank flow of muds and fine sand during flood conditions. This Is a particularly acute problem in coastal Louisiana, where the marshes have been established on deltaic sediments and have continued to accrete upward by sedi- ments delivered during flood stage of the Mississippi River. With- out these levees and other engineered structures, some cities built on floodplains, such as New OrIeans, would be subject to massive and frequent flooding. Finally, wetlands are being lost ~ coastal Louisiana because of pipeline and navigation canals that now lace much of the area, allowing saltwater intrusion. The resulting interactions are com- plicated, but there is no doubt that this practice has significantly contributed to the dramatic loss of wetlands presently being expe- T e rlencec . In Louisiana. Although salt marshes are protected by federal legislation, major losses of estuarme marshes can be anticipated in the fu- ture because of bulkheading along bay shores (Figure 5-4~. With Holocene sea level rise, these salt marshes have been naturally translated landward through tune. With the construction of landward-flank~g bulkheads, which are prevalent along the main- land bay shores of many coastal states, these marshes will literally be squeezed out of existence with a sea level rise (Figure 5-4~. Prospects for Wetlands The prospect for wetlands is bleak in light of existing con- ditions and projected changes. The present situation in coastal I.ouisiana can be used to forecast qualitatively the expected wet- lands changes elsewhere. Due predominantly to subsidence from loading by the Mississippi delta and elirrunation of sediment sum ply by levee construction, the land surface has been subsiding about 1 cm/yr. Without the influx of massive quantities of in- organic riverborne sediments, the marsh surface can only accrete vertically by biogenic production, which is limited. Therefore, marshes are not able to keep pace with relative sea ieve! rise (over 90 percent due to subsidence at present) and are being drowned in place. A rapidly subsiding substrate or accelerating sea level rise can yield similar results. Marsh grasses cannot accrete vertically fast enough to keep pace with sea level rise. This will likely be the fate for extensive estuarine marshes elsewhere in the United States if substantially higher rates of sea level rise are realized in the future.