Alongshore variation in the morphology of coastal dunes: Implications for storm response

Abstract

The geomorphological impact of an extreme storm on a barrier island tends to be modeled using a single cross-shore transect and dependent only on the elevation of the storm surge relative to the height of the dune. The foredune line, however, is rarely uniform and can exhibit considerable variation in height and width alongshore at a range of length scales. The purpose of this modeling study is to determine how alongshore variations in dune height affect barrier island response to extreme storms. The MIKE21 wave and current model is used to predict the morphological response of Matagorda Peninsula, Texas in response to storm surges associated with dune scarping, washover and inundation. The extent and degree of dune-scarping, washover and shoreline erosion is predicted for each storm scenario, with respect to the base morphology of the island and low-pass filtered forms in which small-scale topographic variance is removed. Results suggest that small variations in the height of an otherwise alongshore uniform foredune act as overwash conduits and are unstable, leading to a more variable duneline that is more susceptible to change by subsequent storms. The vertical development of the washover gaps in the duneline is limited and eventually replaced by a lateral expansion that erodes adjacent dunes and leads to a more uniform island elevation. The loss of island elevation is greater for the (original) unfiltered alongshore profile, but relatively uniform duneline is the most unstable and exhibits the greatest morphological change. The different alongshore profile responses suggest that the impact of an extreme storm is sensitive to initial conditions and specifically the pre-storm variability of the crest elevation alongshore. This in turn suggests that the evolution of barrier islands is dependent on storm history until the variability in the duneline elevation reaches a maximum. Further study of barrier island response to storm sequencing with and without post-storm recovery, however, is required to understand the evolution and form for the prototype island.