Losses Along the US Coast
Of the 30 coastal states in the United States, those on the Gulf of Mexico experience the highest rates of erosion: an average of 6 ft. or more per year. In Maine, a little over half the state’s coastline is subject to erosion. That includes sandy beaches, which represent about 2% of the coastline. It also includes bluffs, some 50 ft. or more in height, along the coast, inner bays, and estuaries. These bluffs make up about half the coastline. The rest of the shoreline is rock, where little erosion occurs.
In general, the erosion rate among the Atlantic coastal states averages about 2–3 feet per year. During a hurricane or other major storm, 100 feet or more of shoreline might erode in just one day. In California, 86% (946 mi.) of the state’s 1,100 miles of shoreline is actively eroding, and this will continue as long as the sea level continues to rise. Erosion rates range from little, if at all, along the granitic rocks of the Monterey Peninsula of central California to more than 6 feet per year in unconsolidated sand dunes just a few miles to the north in Monterey Bay.
“Coastal erosion or retreat is a natural ongoing process that has only become a problem because people have built permanent structures in areas that are prone to erosion or wave attack,” says Gary Griggs, director of the Institute of Marine Sciences at the University of California, Santa Cruz.
Over the years, the attraction of living and playing in the surf and sand has proven irresistible to ever-increasing numbers of people. Weekend shacks and cabins have been replaced by multimillion-dollar oceanfront homes, luxurious high-rise condominiums, and lavish resorts. That has heightened the economic and political pressures to fortify the shoreline in developed areas, even as the cost of doing so goes up. Last year, ABC News reported that in California alone, beaches generated about $12 billion in annual revenue from recreation and tourism.
Last year the Federal Emergency Management Agency issued results of a three-year study of the US coastline prepared by The Heinz Center for Science, Economics and the Environment. Among the findings:
- Each day, 3,600 people move to the coast.
- Development in several of the high-risk coastal areas studied increased by more than 60% over the preceding 20 years.
- About one of every four homes and other structures within 500 feet of the US coastline—about 87,000 buildings in all—could be lost to erosion in the next 60 years. That’s about 1,450 structures a year.
- Annual erosion-related losses–ranging from failed septic systems to condemned or destroyed houses–are likely to exceed $500 million a year. Those living along the Atlantic and Gulf of Mexico coastlines are expected to bear about 60% of the costs.
Why Shorelines Are Eroding
“One of our biggest challenges is getting owners of beachfront property to realize that their beach will change over time and that sooner or later it will erode,” relates Janet Freedman, a coastal geologist with Rhode Island’s Coastal Resources Management Council.
One reason residents along Rhode Island’s shoreline, as well as those in other coastal areas, are seeing their property erode is that the sea level rises as the earth warms and glaciers dwindle in size. Since the last Ice Age, melting ice caps and glaciers have added an estimated 11 million cubic miles of water to the world’s oceans, reports Griggs. When the last Ice Age ended, the sea level was about 425 feet lower than today, he notes. On the Atlantic Coast of the US–a relatively flat, depositional coastline–the shoreline might have been 50 miles or more offshore of its current position. On the uplifted West Coast, with a much steeper continental shelf, the shoreline was probably no more than 10 miles seaward of the present coast.
Currently, sea level is rising at an annual rate of about 2 mm. Many scientists believe, however, that burning of fossil fuels over the last century or so has accelerated global warming. That could increase the speed at which the sea rises. “The best estimate now is that sea level could rise as much as 3 feet by the end of this century,” says Griggs. Such a rise, of course, would have major implications around the world. Already some low-elevation islands in the South Pacific, as well Bangladesh and other low-lying areas of the world, are being threatened.
“Millions of people are living within a few feet of sea level, and that line is constantly changing,” notes Griggs. “Right now, we don’t know how fast and how high it will rise.”
Development inland is also adding to erosion of the US coastline. Dams and reservoirs can hold back sediment that would otherwise flow to the coast and drift along shorelines to build up beaches. A 1995 study found that dams in the San Gabriel River drainage of California had trapped more than 78 million cubic yards of sediment. As Griggs points out, that’s equivalent to more than 260,000 cubic yards of littoral drift per year for 300 years.
Dredging rivers to improve navigation and building levees to control floods can also add to the toll. Louisiana, for example, is losing more than 35 square miles of coastal land a year. At one time, sediment carried by the Mississippi River would be deposited in delta areas near the mouth of the river. As one area built up, the river would change course to build new delta areas and rebuild eroded ones. Erosion and delta formation remained in balance. However, this balance was upset with construction of levees in the early 1900s, which prevented the river from changing course, and dredging at the river’s mouth, which caused the river to deposit its load of sediment there.
Removal of oil, gas, and sulfur deposits along Louisiana’s coastline also led to subsidence of the coastal area as a result of fluid withdrawal from the subsurface. Development of these resources also led to construction of canals to transport crews, equipment, and materials through the marshlands. These canals allowed salt water to penetrate into the marshes, damaging and killing soil-holding trees and other vegetation and leading to increased erosion.
