ALONGSHORE VARIATION IN ISLAND RESPONSE TO EXTREME STORMS AND SEA LEVEL RISE

The response of barrier islands to sea level rise tends to be considered as a strictly two-dimensional problem. However, the response and recovery of barrier islands to extreme storms, which controls the rate of island transgression, can exhibit considerable variability over short distances. The source of this variation is not well understood in general, and is not easily explained by contemporary dune models that ascribe dune height and extent to regional variations in sediment availability and beachface slope. In this paper, LiDAR data from the Gulf Islands and Padre Island National Seashore are used to characterize the alongshore variation in dune height and extent as a control on barrier island response and recovery to extreme storms. It is shown that the variation in dune morphology and storm impact in these parks is dependent on the alongshore correspondence of sediment supply and transport potential, which is in turn dependent on the which are controlled by transverse ridges on the inner-shelf fronting both islands. It is further argued that dune development in Gulf Islands National Seashore is transport limited by the relatively reflective beachface, leading to the largest dunes and storm surge at the ridge crests. In contrast, the dunes on the more dissipative Padre Island are supply limited, such that the largest dunes are found between ridges where the storm surge is smaller. The role of the ridges in controlling the dune morphology and the storm surge determines the amount of sediment moved to the backbarrier as washover and the amount lost offshore during island transgression.