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

Monitoring beach volume changes of the Texas Coast following a major h urricane reveals the impact of storms on sand dispersal and shoreline movement at spatial and temporal scales encompassing tens of kilometer s and decades. Beach volume histories at profile sites show the interd ependence of sand exchange among adjacent sites and the spatial autoco rrelation of sand movement. Beach volume histories also indicate perio ds when either longshore or cross-shore transport predominate and illu strate 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 ba rriers. Instead, sand dispersal within a coastal compartment may depen d partly on the dynamics of shoals and temporary sand storage at the i ntervening tidal inlet. In our study, sand eroded from the updrift bar rier (Galveston Island) is deposited in a terminal sand flat of the ba rrier, whereas sand accreted to the downdrift barrier (Follets Island) is derived from the intermediate ebb-tidal delta (San Luis Pass). Unl ike continuous sand bypassing on some microtidal, wave-dominated coast s, sand bypassing at San Luis Pass is episodic, event driven, and inef ficient, 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 m ovement where natural replenishment of beach sand depends entirely on updrift erosion. Large-volume, nearly instantaneous sand transport dur ing 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 wil l probably have even greater impact on coastal sand budgets and beach mobility as natural sources of beach sand are eliminated or become una vailable to replenish beaches.