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COASTAL EROSION

Severity of the Problem


Most coastal hazards are natural processes. While hurricanes, storms, and gradual coastal erosion can affect or change the natural environment, the impacts are generally not considered "disastrous" unless they involve human exposure. Coastal environments typically undergo continual natural change, and shorelines routinely move back and forth as a result of natural processes. In the absence of people and property, these changes are simply examples of the evolution of natural ecosystems.

Often, people and property are not absent. In U.S. coastal areas, where population has exploded over the past 30 years, significantly changing demographics and socio-economic conditions. The nation's 30 coastal states contain approximately 85 percent of our entire population and over 50 percent of the population reside within 50 miles of the coast. In many locations, rustic beachfront "cottages" have been replaced by multi-story, million-dollar residences, and property once considered "swampland" is now sold and developed as prime marshfront real estate. These growth trends are exposing an increasing human population to the potentially destructive impacts of natural coastal processes.

The type of coastal development has also changed significantly in recent decades. Thirty years ago, most areas vulnerable to such hazards have been mostly undeveloped, so that coastal areas designed as vacation communities, cottages, or shacks were easily rebuilt after a major storm. Urban and suburban development was largely confined to unprotected coastal areas. In recent years, however, urban and suburban development has grown and spread rapidly in coastal areas, expanding along the coastline and extending seaward.

Much of the coastal development has been concentrated on barrier islands. One of the biggest challenges facing many coastal communities is to ensure that new development takes into account natural hazard vulnerability. The demand for new development in these highly desirable coastal areas often outpaces a community's ability to adequately address hazard threats.

Map of 1990 Coastal County Population

Below are general descriptions of coastal population trends and shoreline conditions in the various regions of the United States.

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North Atlantic Coastal Region (Maine to Virginia)

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Map of Population Change in Atlantic Coastal Counties (1960-1990)

Description of Coastline

From the Maine-Canada border to Cape Elizabeth, Maine the coastline is rocky, steep, and deeply incised with numerous bays, estuaries, and islands. There are small areas of mudflats, marshes, and shallow areas, but the coast is impacted by high energy conditions and high tidal ranges. The area is less developed, consisting primarily of rugged terrain, scattered resort residences, rural areas, and fishing villages. Where structures are present, ocean and side yard setbacks are minimal so there is a tendency for higher urban density.

South of Cape Elizabeth to Cape Cod, Maine, the shoreline grades from rocky to sandy, especially south of Cape Ann, Maine. Beaches are influenced by high energy conditions. The coastline is heavily populated and extensively developed. This portion of the coastline also contains many other structures, some of which date from the nineteenth century. Both the mainland and the barrier beaches have been developed. Because of the desirability of the properties involved, very small lots with minimal setbacks and little space between residences have resulted. Much of the development of this region either predated construction codes or was constructed under codes with few specific provisions for coastal construction.

The northern New England area experiences a moderate to severe climate, suffering from increasingly harsh winters toward the north. Generally, north of Cape Cod, Massachusetts, the northeaster is the most damaging event and dictates control measures design. These are relatively slow moving storms, with large amounts of precipitation and high winds, and are the primary cause of coastal flooding and erosion. Hurricanes are not as severe a threat as these winter storms.

The coastline from Cape Cod, Maine to Montauk Point, New York, including Long Island Sound, New York, is fairly irregular with several large islands, bays, and sounds. The beaches are mainly sandy and are variously characterized by high energy areas, marshy areas, barrier islands, and dunes.

From Montauk Point, New York to the Virginia-North Carolina border, the coastline contains wide, sandy, high-energy beaches. Extensive marsh areas are protected by a series of barrier islands with dune systems. Estuaries of varying sizes, including the Chesapeake Bay and the Delaware Bay, are found throughout this portion of the coast.

The area between Cape Cod, Maine and Montauk Point, New York is heavily populated and extensively developed. The coastline south of Montauk Point is characterized by resort towns, summer residence communities, and state and federal parklands and refuges. In several areas, primarily near the mouths of major bays and rivers, harbors have been developed that have fostered considerable growth. Such population centers include New York City, at the mouth of the Hudson River, and Hampton Roads, Virginia, at the mouth of the Chesapeake Bay. Between these two population centers, the coastline has a distinctly rural character, with summer tourism the primary commercial activity. Much of the land on which resort towns have developed consists of barrier beaches.

Although nineteenth century structures are at the core of most of the towns, a large amount of construction began after World War II and continues to date. Present day construction primarily consists of high-rise apartments and subdivision development. Generally, larger lot sizes are typical than those in New England; side yard and especially ocean-side setbacks have long been in force, resulting in much less crowded communities than farther north. Historically, the area from Massacusetts through Virginia has been subjected to numerous coastal storms severe enough to inflict significant property damage. This region averages two to three winter storms each year and an occasional hurricane.

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South Atlantic Coastal Region (North Carolina to Southern Tip of Florida)

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Map of Population Change in Atlantic Coastal Counties (1960-1990)

Description of Coastline

The South Atlantic Coastal Region is comprised of three general segments: the Outer Banks from the Virginia-North Carolina border to Cape Fear, North Carolina; the coastline and Sea Islands from Cape Fear to St. Johns River, Florida; and Southeastern Florida from St. Johns River to the southern tip of Florida. The Outer Banks contain numerous long, narrow barrier islands characterized by relatively steep, broad, quartz sand beaches. These barrier islands protect the mainland from the high-energy sea.

The Sea Islands comprise numerous, irregularly shaped small barrier islands that contain wide, low-sloped, quartz sand beaches. In many areas, the difference in beach width at high and low tide often exceeds 300 feet. Behind the islands are expansive tide marshes that are highly dissected by coastal plain rivers and tributaries. The barrier islands of the East Florida segment are long and narrow, and generally front high-salinity lagoons. The beaches are low-lying and composed of calcareous sands. In the southern portion of the Florida mainland, the continental shelf is narrow and high-relief coral reefs dominate the nearshore area. Limestone underlying the Florida mainland extends further south in an undersea ridge that forms the Florida Keys, a chain of 97 low-lying islands. The average ground elevation in the Keys is about three feet above sea level, and only a thin soil cover overlies the limestone in most areas. The South Atlantic region has a temperate climate that is subject to the effects of coastal storms and occasional hurricanes that produce high winds, above-normal tides, and heavy rains. The most frequent storm types are winter cyclonic storms traveling northeastward up the coast. These storms number 8 to 10 per year, and according to local residents, many of them cause minor damage to residences and erode appreciable amounts of beach. Hurricanes, while less frequent, are the major cause of damage to residential structures and thus are the controlling factor in construction design. Although the area is vulnerable to tidal flooding, tides greater than 8 feet above mean low water (mlw) are rare; the highest recorded tide level was 11.2 feet mlw, in August of 1983.

The Outer Banks of North Carolina, the coast near the South Carolina-Georgia border, and the east coast of Florida are the areas within the South Atlantic region most likely to be struck by a hurricane. National building codes base their maximum coastal construction design wind speeds on wind conditions for the Outer Banks and southern Florida areas.

Along the Outer Banks coastline, numerous resort developments have been created within the past 15 to 20 years in areas where there had been little or no development. Thus, an increasing number of residences are being constructed that are susceptible to coastal flooding damage.

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Gulf Coastal Region (Southern Tip of Florida to Texas)

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Map of Population Change in Gulf of Mexico Coastal Counties (1960-1990)

Description of Coastline

The Gulf Coastal Region consists of eight segments that can be characterized by their differences in physiography. Each exhibits varying conditions that determine susceptibility to storm-generated erosion, wave action, and tides. Since this region is routinely a target for hurricanes, it exhibits some of the highest probabilities of tropical storm occurrence in the country.

From Key West, Florida to Cape Romano, Florida, the low relief coastline is dominated by a multitude of small mangrove islands, tidal channels, and extensive swamps. The continental shelf is very broad, extending over 800 miles offshore. Human development is minimal due to predominance of the Everglades. From Cape Romano to Tarpon Springs, Florida, a transition occurs from a mangrove-dominated coastline to sandy beaches and marshy bays characteristic of the northern Gulf of Mexico. Exposed sandy beaches with scattered mangrove stands and rocky areas characterize shoreline features along a series of barrier islands that protect marshy embayments. The continental shelf is broad and regular.

Continuing north to Lighthouse Point, Florida, the shoreline of the Florida Big Bend is rugged and characterized by very wide shallow areas, and extensive seagrass beds, oyster bars, and marshes. The region surrounding the Apalachicola Delta, Florida to Cape San Blas, Florida has an exposed coastline, partially protected by barrier islands and smooth sandy beaches; protected bays are turbid with muddy bottoms. From Cape San Blas to Petit Bois Pass, Alabama, the coast comprises high-energy, sand beaches and an extensive system of dune and barrier islands. The shoreline is relatively steep and the dunes rise sharply.

The Mississippi Delta, extending from Petit Bois Pass west, Mississippi to Vermilion Bay, Louisiana, is characterized by an extensive and wide marsh and barrier island system. The marshes contain many lakes and bayous and are crossed by numerous stream channels. From Vermilion Bay to Galveston Bay, Texas, the coast is identified as a strandplainchenier complex. The shoreline is exposed without substantial barrier islands and is characterized by a marshy plain with a series of long, low, narrow, brushy beach ridges that lie parallel to the coastline. The Texas barrier island system extends from Galveston Bay to the United States-Mexico border. This low-relief section is characterized by an extensive lagoon system bordered by long, narrow, sandy barrier islands. On the upper portion of Corpus Christi, Texas, marshes are common in the bays. The bays of the southern portion have minimal freshwater inflow; hypersaline conditions exist and submerged grass beds are common.

Development along the Gulf Coast is varied. Many areas, because of the inhospitable terrain or shoreline protections (refuges and parks), are relatively undeveloped. These include much of the Texas barrier island system, the Chenier Plain, the Mississippi Delta, the Florida Big Bend, and the mangrove swamps of southwest Florida. Development in these areas usually occurs on the mainland behind the barrier islands (e.g., Corpus Christi, Texas) or on the landward side of extensive marsh systems (e.g., Lake Charles, Louisiana). A notable exception is Galveston, Texas, which is built directly on a barrier island. On the other hand, the central Florida coast, the panhandle of Florida, and coastal Alabama and Mississippi are highly developed with beach communities and resorts.

Two gulf coast areas can be distinctly identified as having high hurricane probability. These are the areas around Galveston, Texas, and Pensacola, Florida. Other areas also appear to be prone to hurricane activity; specifically, the area around the Florida Keys and the area south of New Orleans near Grand Island, Louisiana.

The Florida Panhandle has a moderate climate that is occasionally influenced by hurricanes. There have been nine seriously destructive hurricanes since 1879, the worst in September 1906 and September 1926. The area from Pensacola, Florida, to Pascagoula, Mississippi, has also experienced relatively frequent hurricane damage.

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Pacific Coastal Region (Washington to California)

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Map of Population Change in Pacific Coastal Counties (1960-1990)

Description of Coastline

The Pacific Coastal Region is composed of two basic segments: the southern California coast from the United States-Mexico border to Point Conception, California and the coasts of California, Oregon, and Washington from Point Conception to the United States-Canada border.

The southern segment is characterized by a fairly smooth coastline with long stretches of sandy beaches interspersed with rocky headlands. Both low and high cliffs border the landward side of the beaches while few large islands occur offshore; nearshore algae and kelp beds are widespread.

The northern segment has mainly rocky, high-cliffed beaches with numerous pocket beaches. North to Cape Mendocino, California, extensive algal communities and kelp beds are present. From Cape Mendocino to the Canadian border, the coastline is moderately dissected with numerous rocky islands, small bays and estuaries with mudflats and eelgrass beds.

Although not subject to the same frequency of major storms and hurricanes as the Atlantic and gulf coasts, the Pacific coast does experience more frequent and much higher energy waves during the winter months. The Pacific coast has an occasional tropical cyclones and tsunamis which can originate near Japan and grow as they track across the Pacific. In addition, tropical cyclones that form off the Mexican coast occasionally travel north and affect southern California. Tsunami activity has also been recorded; the most damaging was in 1964, at Crescent City, California.

Storm surges are of limited magnitude on the Pacific coast because of the great ocean depths close to shore. Numerous hurricanes form off the west coast of Mexico, but these tend to move seaward. Only rarely does one of these hurricanes reach the extreme southern California coast, and those that do are weak compared to east coast hurricanes. Their intensity is limited by the cold temperature of the underlying water surface and other factors.

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Great Lakes Coastal Region

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Map of Population Change in Great Lakes Coastal Counties (1960-1990)

Description of Coastline

The shoreline of the Great Lakes is shared with Canada, Michigan, Minnesota, Wisconsin, Illinois, Indiana, Ohio, Pennsylvania, and New York. The physical characteristics of the Great lakes shore range from high bluffs of clay and shale and rock, through lower rocky shores and sandy beaches, to low, marshy clay flats.

The Great Lakes shoreline represents a unique natural resource, rich in aesthetic and ecological values, and their scenic attractiveness, many beaches and access to large water areas provide outstanding recreational opportunities. The shorelands are subject to unique problems of flooding and erosion when subject to unusually high lake levels and storms.

Erosion and flooding problems are caused primarily by the forces of nature which include storms, lake level changes, wave action, frost and ice action, underground water seepage and surface water runoff. Major storms create the largest long term changes in the shore. Seasonal fluctuations average from about one foot to 1.5 feet.

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