RECONSTRUCTING THE FORMATION OF AN OVERWASH FAN ON THE TEXAS COAST USING STORM SURGE DATA AND STRATIGRAPHY

Hurricanes create extensive overwash deposits along sandy coastlines, and these deposits are a primary tool for interpreting the frequency and severity of ancient storms. However, hurricane conditions make it hazardous to directly monitor the formation of overwash fans. We use deposit stratigraphy and sedimentology, as well as storm surge records collected by the USGS to analyze the formation of an overwash fan emplaced on Matagorda Peninsula, a Texas barrier island, during Hurricane Ike in 2008. Deposit stratigraphy shows a topset-foreset break that rises 0.48 m in elevation as the deposit builds. We tie this rise to increasing storm surge water levels in the back barrier lagoon, which rose at a rate of 0.03 m hr^-1 as the hurricane approached the coast. The rising trajectory of the topset-foreset break shows that the deposit was emplaced while storm surge was rising, and was finished building before the peak storm surge associated with hurricane landfall. By connecting the rise in the topset-foreset break point with the rate of water level rise, we conclude that the deposit with cross-sectional area of 30.9 m² took 16 hours to build. This formation-time estimate is consistent with the deposit’s sedimentology. Undifferentiated grain fall deposits have a coarse fraction (D₉₀) with a nominal grain diameter of 512 μm, indicating a shear velocity u_* > 64 mm s^-1. Using the modified Meyer-Peter Muller sediment transport formula to fill the deposit’s cross-section, the estimate of formation time is less than 49 hours. We conclude that the timing of fan emplacement deduced using deposit stratigraphy and storm surge records can yield valuable new insight for interpreting hurricane overwash deposits.