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 Human Activities: Hydrocarbon & Groundwater Extraction Land subsidence can be induced by any one of several different activities that involve large volume extraction of underground resources (water, oil and gas, sulfur, salt). Land loss associated with induced subsidence is actually more common than most people realize, especially where large volumes of fluids are removed from underground formations. This induced subsidence, which is either sub-regional or local in extent, has its greatest impact on flat coastal plains and wetlands near sea level where minor lowering of the land surface results in permanent inundation. For more information about induced subsidence, see Subsidence and Fault Activation Related to Fluid Energy Production, Gulf Coast Basin Project. Subsidence around the Goose Creek Oil field near Houston, Texas was the first evidence that rapid, large volume extraction of hydrocarbons was capable of causing the ground to sink around the producing wells. The induced subsidence, which was discovered shortly after field development began in 1917, indicated that accelerated withdrawal of oil, gas, and associated water from shallow unconsolidated reservoirs could lower the land elevation, cause minor earthquakes, and activate faults around the periphery of producing fields (Pratt and Johnson, 1926). One of the most dramatic cases of land subsidence caused by oil and gas production occurred at the Wilmington Field in Long Beach, California. As the oil reservoirs were depleted, sand compaction caused almost 9 m of land subsidence that flooded streets and wharfs and caused structural damage to bridges, railroads, and other harbor facilities (Poland and Davis, 1969). However, the subsidence did not cause widespread land loss because the subsidence bowl only covered about 35 km2. Induced subsidence in the Houston-Galveston area, which covers more than 12,000 km2, has lowered the land surface as much as 3 m. This subsidence bowl is largely a result of long-term industrial ground-water withdrawal from shallow aquifers (Gabrysch, 1984) and deeper oil and gas extraction (White et al., 1985). Near the center of the subsidence bowl, more than 200 homes in a middle class neighborhood of Baytown, Texas were destroyed after groundwater extracted for the petrochemical industry caused the area to subside, flooding the entire subdivision (Fig. 8). Subsidence induced by fluid withdrawal is usually an irreversible process because it usually involves sediment compaction and dewatering of interbedded clays. In the Gulf Coast region, subsidence induced around large, mature oil and gas fields is also locally concentrated along linear trends that coincide with faults (White and Morton, 1997). Extraction of hydrocarbons and formation water causes a decline in pore pressure within the reservoirs and alters the state of stress near the faults. Because of the slope of the fault plane and its intersection with the land surface, fault-plane subsidence may be surficially expressed more than 2 km away from the producing wells. Relatively little is known about the severity of land loss caused by induced subsidence and the relationship of land loss to production history, fluid composition, local geology, and near-surface conditions prior to hydrocarbon or groundwater production. Except along the northern Gulf of Mexico and in southern California, land losses associated with subsurface fluid withdrawal are minor. But continued withdrawal and concomitant decline in fluid pressure from hydrocarbon extraction and ground-water pumping could eventually cause even greater lowering of land elevations. This would augment the effects of relative sea-level rise and lead to additional land losses near the coast. continue to Climate Alteration

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