MESOSCALE TRANSFER OF SAND DURING AND AFTER STORMS - IMPLICATIONS FOR PREDICTION OF SHORELINE MOVEMENT

Monitoring beach volume changes of the Texas Coast following a major hurricane reveals the impact of storms on sand dispersal and shoreline movement at spatial and temporal scales encompassing tens of kilometers and decades. Beach volume histories at profile sites show the interdependence of sand exchange among adjacent sites and the spatial autocorrelation of sand movement. Beach volume histories also indicate periods when either longshore or cross-shore transport predominate and illustrate the long-term effects of coastal structures on beach mobility. This study confirms that net losses of sand from updrift barriers may not be directly linked with net gains of sand on adjacent downdrift barriers. Instead, sand dispersal within a coastal compartment may depend partly on the dynamics of shoals and temporary sand storage at the intervening tidal inlet. In our study, sand eroded from the updrift barrier (Galveston Island) is deposited in a terminal sand flat of the barrier, whereas sand accreted to the downdrift barrier (Follets Island) is derived from the intermediate ebb-tidal delta (San Luis Pass). Unlike continuous sand bypassing on some microtidal, wave-dominated coasts, sand bypassing at San Luis Pass is episodic, event driven, and inefficient, and sand is not transferred directly from one barrier to the next. Because storms rapidly redistribute beach sediment, they can be the most important factor controlling short-term (< 10 yr) shoreline movement where natural replenishment of beach sand depends entirely on updrift erosion. Large-volume, nearly instantaneous sand transport during storms can locally accelerate rates of shoreline change or reverse the trend of beach movement, thereby significantly altering projected shoreline positions even ten years into the future. Future storms will probably have even greater impact on coastal sand budgets and beach mobility as natural sources of beach sand are eliminated or become unavailable to replenish beaches.