Exploring the competition between runaway overwash and alongshore smoothing: barrier island stability and characteristic segmentation length scales

Where barrier islands are subjected to strong tidal influence, inlets segment barrier chains over a length scale that depends upon a balance between tidal flows and alongshore sediment transport. However, segmented barriers are also found in regions with small tidal ranges—these barriers typically have a curved shoreline and show little indication of tidal flow between neighboring barrier segments. Here, we quantify the controls on and scales of barrier segmentation in the relative absence of tides. Recent research has emphasized how, on a homogeneous barrier, the onset of overwash can transition a stable barrier into a rapidly overwashing migratory stage. However, if the runaway overwash is localized (due to alongshore heterogeneity such as variable barrier widths or heights), the shoreline will bend, causing deposition by alongshore sediment transport into the overwashing region. If alongshore sediment transport is sufficient to rapidly nourish the overwashing region, overwash slows, and the runaway overwash is halted. Alongshore transport gradients will continue to straighten the shoreline and the barrier will remain intact. However, if alongshore transport fails to halt the runaway overwash, the barrier could segment as continued retreat causes the localized overwashing region to abandon rest of the barrier island. Here, we develop a theoretical framework to estimate the alongshore length scales at which overwashing regions or segmentation due to runaway overwash may occur by applying a Peclet number analysis which compares overwash (advective) and alongshore sediment transport (diffusive) processes along barrier island chains. A second non-dimensional comparison can be made by comparing retreat rates driven locally by overwashing and by alongshore sediment transport gradients along the adjoining curved shoreline. These predicted length scales of segmentation are tested using a numerical model of coupled alongshore and cross-shore barrier evolution. We then compare both theoretical and modeled barrier predictions to segmented barriers found in regions with low tidal ranges, such as the Gulf of Mexico, Alaskan, and Canadian coasts.