Coastal & Marine Geology Program > National Assessment of Coastal Change Hazards > Open File Report 03-337

An Overview of Coastal Land Loss: With Emphasis on the Southeastern United States

USGS Open File Report 03-337

by: Robert A. Morton

Summary Introduction Physical Agents of Land Loss: Waves, Currents, & Storm Surges Landslides & Cliff Retreat Sediment Budget Relative Sea Level Climate & Land Loss Role of Shoreline Characteristics: Composition, Induration, & Saturation Coastal Morphology & Vegetation Role of Human Activities: Introduction Transportation Coastal Construction River Modification Hydrocarbon & Groundwater Extraction Climate Alteration Coastal Excavation Wetland Losses Acknowledgments References

Physical Agents of Land Loss: Waves, Currents, & Storm Surges Introduction Winter Storms (Extratropical Cyclones) Hurricanes (Tropical Cyclones) El Niño Events Winter Storms (Extratropical Cyclones) Figure 4a. Cobble washover deposit and properties damaged by waves and wave-hurled cobbles during the 1991 Halloween storm, Wells, Maine. [larger version] In North America, extratropical cyclones are large, mid- to high-latitude, winter storms associated with cold air masses that generally travel from west to east. Their strongest winds occur along fronts separating air masses, which vary greatly in temperature and humidity. These conditions produce several thousand extratropical disturbances each year (Riehl, 1979). However, only a few of those disturbances cause intense coastal storms with high winds and waves. Winter storms driven by masses of cold Arctic air are responsible for significant land losses in the Gulf and Atlantic Coast regions where they are known as northers and northeasters, respectively (Roberts et al., 1987; Pilkey et al., 1989). Although hurricanes periodically ravage the Atlantic coastal states, winter storms typically account for more cumulative losses than hurricanes in this region. This is because intense winter storms occur frequently (several each year) and may last for several days, whereas hurricanes occur less frequently and only last for a few hours as they cross the coast. Extratropical winter storms also cause significant land losses around the Great Lakes, an area that is unaffected by hurricanes. In the Gulf Coast region, frontal-related storms occur about every week to ten days from November to April. These storms act like pumps that cause rapid changes in water levels and associated wave erosion. Preceding passage of a cold front, low barometric pressure generates strong onshore winds that set water up along the coast, flooding open ocean and mainland beaches and exposing the shores to strong wave attack. As the front passes the coast, strong winds are directed offshore driving water onto the backbarrier flats and away from the ocean beaches. The frequent oscillation in water levels and waves erodes both sides of barrier islands as well as mainland and bay shores. Gulf Coast winter storms cause much less land loss or property damage than do hurricanes so they are not ranked or given names like severe northeasters of the Atlantic coast. During the past 50 years, the Ash Wednesday Storm of March 1962 was probably the largest East Coast winter storm in terms of land loss and number of homes damaged or destroyed. This northeaster, which coincided with a spring tide, remained stationary for almost 36 hrs so that beach and barrier flooding lasted over 5 consecutive spring high tides (O'Brien and Johnson, 1963). The strong northeast winds, broad fetch, and high angle of wave approach caused record flooding and beach erosion down the eastern seaboard extending from New England to Florida. Most houses near the beach were destroyed by storm waves and washover unless a wide beach and high dunes protected them (Morton et al., 2003). Another destructive northeaster struck the Atlantic coast in late October 1991 (Fig. 4a) and therefore is known as the Halloween storm. During the storm, high waves lasted more than five days causing widespread beach erosion and washover all along the east coast of the U.S. (Davis and Dolan, 1992). The Halloween storm began as an extratropical disturbance that was later reinforced when it merged with the remnants of Hurricane Grace. In Maine, the Halloween storm constructed thick washover deposits of cobbles (Fig. 4a), and large rocks tossed from the sea damaged some expensive seaside homes. continue to Hurricanes (Tropical Cyclones)

Coastal & Marine Geology Program > National Assessment of Coastal Change Hazards > Open File Report 03-337