THE IMPACT OF RAPID SEA-LEVEL RISE ON THE TEXAS COAST

Extending some 600 kilometers, the Texas coast is one of the most vulnerable coastlines to rising sea-level because of its low elevation and gradient. The current rate of sea-level rise (~3-4 mm/yr) for this part of the Gulf of Mexico is roughly six times the average rate (~0.6 mm/yr) over the past 4,000 years. This rate will likely continue to increase, and the impact on coastal systems is expected to be dramatic. During the middle Holocene, the rate of sea-level rise in the northwestern Gulf of Mexico was ~5 mm/yr, which coincides with the low-end of rate projections for the coming century. Along the Texas coast, this was a time of rapid coastal change, as rates of shoreline retreat were up to ~20 m/yr. Several barrier islands were drowned to form sand banks that currently exist ~ 50 kilometers offshore (Sabine and Heald Bank). Additionally, bays along the upper Texas coast experienced flooding events when bay-head deltas rapidly migrated landward. However, several central Texas barriers remained relatively stable throughout the rapid rise of the middle Holocene (e.g. Mustang and Matagorda Islands) due to higher rates of sediment supply (due to longshore current convergence). Elsewhere, barrier islands did not begin to stabilize until the rate of sea-level rise decreased to < 2.0 mm/yr (which occurred ~5,000 yrs B.P.). An examination of sediment cores from the upper Texas coast revealed that net sand transport from erosion of Galveston Island into San Luis Pass, a tidal delta at the west end of the island, is more than double the average flux for the past several millennia. This suggests that erosion of Galveston Island has increased over historic time relative to geologic time. This change is mainly attributed to accelerated sea-level rise. Other barriers show evidence for accelerated retreat in historical time, some of which is due to direct anthropogenic influence and other cases are likely due to accelerated sea-level rise.