Risk assessment and causes of subsidence and inundation along the Texas Gulf Coast




John M
Germiat SJ

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Analysis of tidal-gauge data from stations bordering the Gulf of Mexico indicate that the Texas Gulf Coast is sinking or submerging at a significant rate. This is attributed to a combination of eustatic sea-level rise and undifferentiated subsidence caused by man's withdrawal of pore fluids and by natural consolidation. The sinking is probably exacerbated by decreasing sediment input to the shore by reservoir impoundments. From 1958 to mid 1980's, submergence rates of 11.1 and 11.4 mm/yr are documented for station at Galveston Bay and Sabine Pass, respectively. The Galveston Bay station shows an increasing (second-order equation) rate over its period of record (1908-1986). This equation is almost identical to the Marine Board's (1987) middle estimate of sea-level rise created by the greenhouse effect'', Submergence at the mouth of the Rio Grande is 4.9 mm/yr. Data from relatively stable stations in Florida indicate that a eustatic/tectonic'' rise of approximately 2.4 mm/yr for the Gulf of Mexico. The remaining 2.5 to 9.0 mm/yr is thus due to combined, but undifferentiated, effects of petroleum production, groundwater pumpage, and the natural consolidation of low-permeability, clay-rich sediments. Groundwater withdrawals are well documented in the Houston area, but have been greatly reduced in the last 15 years (Houston has been switching to surface-water reservoirs for its major water supply). Groundwater pumpage is not everywhere a factor in the zones of high subsidence. Petroleum production has apparently created regional depressurization and concomitant subsidence along the uppermost Texas Coast. A risk assessment couples these subsidence effects to three scenarios to future sea-level rise. Conservative analyses indicate that the upper Texas Coast will experience significant shoreline retreat due to undifferentiated erosion and submergence in the next 60 years. One complicating factor is the reduction of sediment input to the coastal areas--the Mississippi River, Brazos River, and Rio Grande now transport 50%, 10% and 5% of their predevelopment sediment loads, respectively




Coastal subsidence, Gulf of Mexico, M2 551.35.061, M2 551.435.31, M2 551.46.24, Marine transgressions, Sea level rise effects on coastal lands, Surface water, Texas,United States