Response of the Texas coast to global change: Geologic versus historic timescales
Wallace, Davin Johannes
Anderson, John B.
Doctor of Philosophy
The response of coastal systems to global change is currently not well understood. To understand current patterns and predict future trends, we establish a geologic record of coastal change along the Gulf of Mexico coast. A study examining the natural versus anthropogenic mechanisms of erosion reveals several sand sources and sinks along the upper Texas coast. It appears that hurricane washover and offshore sand deposits are minimal sand sinks, while flood-tidal deltas are areas of significant sand sequestration. Additionally, it appears that damming of rivers has had only a minimal effect on sedimentation along the upper Texas coast. However, hard engineering structures placed on the beach have exacerbated erosion due to trapping sand of that would otherwise be in the longshore transport system. Coastal sand budgets are derived to put geologic events (such as hurricanes and erosion) into context. Sand budgets often use engineering assumptions to establish sand transport within a coastal system. However, a disconnect typically exists between engineering principles and geologic concepts when quantifying these budgets. Geologic principles are relied upon to calculate a sand budget and evaluate published sediment budgets. This reveals that assuming too shallow a depth of closure can result in ∼17% error in the total calculated sediment flux and an error of ∼40% of the total longshore transport flux for the upper Texas coast. This suggests that revised approaches are necessary to accurately represent sand transport within the coastal zone. The long-term probability of hurricane impacts in the western Gulf of Mexico is constructed. For south Texas, an intense hurricane landfall probability of ∼.46% is established for the past ∼5,000 years. Based on published studies, this is similar to the intense hurricane impact probability of ∼.39% for the eastern Gulf of Mexico. Studying the evolution of San Luis Pass provides a unique opportunity to study the response of accelerated sea-level rise and hurricane impacts on the evolution of a natural tidal delta system and adjacent Galveston Island. This study reveals an increased sand flux into San Luis pass tidal delta, and suggests that the erosion along Galveston Island has more than doubled over historic time relative to geologic time.
Geology; Physical oceanography