An assessment of future coastal land loss in Galveston, Chambers, and Jefferson Counties, Texas
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Loss of coastal land has been occurring along the Upper Texas Gulf Coast over the historical record. This loss of land will continue into the future, probably at increasing rates due to accelerated sea-level rise caused by global warming (i.e. the "greenhouse effect"). Three scenarios for shoreline retreat and land loss to the year 2050 are developed for the uppermost 200 km of the Texas Coast, between Sabine Pass and the mouth of the Trinity River. The scenarios (baseline, low-rise and high-rise) integrate best available estimates of sea-level rise in the next century with empirical relations between relative sea-level rise and shoreline movement during a baseline period (1930-1974 or 1982, depending on the availability of shoreline movement data) for each of 10 shoreline segments. Loss of coastal land results from both erosion and submergence of the coastline due to relative sea-level rise. Relative sea-level (RSL) rise, in turn, encompasses eustatic rise and land-surface subsidence. Baseline rates of RSL rise at Pier 21 on Galveston are 6.9 mm/yr (1930-1974) and 7.6 mm/yr (1930-1982). Data from nonsubsiding tidal gages along the Florida Gulf Coast are used to define a 2.2 mm/yr baseline rate of eustatic rise within the Gulf of Mexico. Land-surface subsidence accounts for the residual 4.7 and 5.4 mm/yr. Land-surface subsidence within the study area is the result of undifferentiated natural consolidation of clay-rich sediments and regional subsurface depressurization caused by production of oil and gas. Groundwater pumpage is minimal within the area and is not considered a cause of subsidence, although large-scale pumping in the metropolitan Houston area has probably caused subsidence along the eastern shore of Trinity Bay. The baseline scenario assumes a constant rate of RSL rise, resulting in a rise of 0.45-0.49 m by 2050. At 2050, RSL rises of 0.66-0.70 min the low-rise scenario and 0.88-0.92 m in the high-rise scenario are predicted by combining low- and high-rise estimates of eustatic rise developed from a synthesis of nine recently published projections with a constant baseline rate of subsidence. The multiplicative factors of approximately 1.5 for the low-rise and 2.0 for the high-rise scenario are calculated from the ratios of projected RSL rise to baseline RSL rise at 2050. These factors are integrated into the baseline relation between RSL rise and shoreline movement to estimate shoreline displacement and subsequent loss of land by 2050 in each of the 10 shoreline segments. The estimated net change of land area by 2050 for the entire study area coastline is -17.2 km² (4248 acres) in the baseline scenario, -25.2 km² (6224 acres) in the low-rise scenario, and -33.8 km² (8349 acres) in the high-rise scenario. Shoreline retreat and land loss scenarios developed in this study represent conservative estimates. Recent rates of relative sea-level rise (between 1958 and the mid-1980s) at Pier 21 and at Sabine Pass exceed 11 mm/yr, 50-60% higher than the calculated long-term rates of 6.9 and 7.6 mm/yr, used as the baseline for this analysis. Therefore projected RSL rise by 2050 are conservative and may underestimate shoreline displacement and coastal land loss. These projections should be considered in the future development of and use of the Texas Coast.