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

Role of Shoreline Characteristics: Composition, Induration, & Saturation Figure 9. Collapse of sinkholes in limestone terrane causes catastrophic land loss. [larger version] Lithologic composition and hardness determine the land loss potential of the coast. For example, loose sand is more easily eroded than compacted, stiff mud. Hard crystalline rocks resist erosion and consequently some rocky coastlines in New England and along the Pacific coast have not changed appreciably in recorded history (Shepard and Wanless, 1971; Bird and Schwartz, 1985). Some limestones such as the coral reefs of the Florida Keys also resist erosion, but other limestones may be dissolved by underground springs that cause the land to collapse and form drowned sinkholes (Fig. 9). Some land loss along the west Florida coast near Homosassa Springs is caused by nearsurface dissolution of limestone, or karst terrane (Davis et al., 1985). These karst features are only sparsely developed and therefore no major structural damage is reported. However, these same areas would fail catastrophically like other densely developed sinkhole terranes such as near Orlando, Florida where schools and car lots have been destroyed as sinkholes collapsed. Even height of the water table can influence the erodability of sandy beaches. The beach is less susceptible to erosion when the water level in the beach is lower than the ocean. As waves run up an unsaturated beach, the water flows into the sand. This causes deposition of sand carried by the waves and reduces the backflow of water returning to the ocean. Conversely, erosion is enhanced when the beach sand is saturated and the water level in the beach is higher than the ocean. When this happens, water flowing out of the beach and down the beach face carries sand with it and makes the beach more susceptible to erosion. continue to Coastal Morphology & Vegetation

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