Coastal Inlets Research Program. Barrier Island Migration Over a Consolidating Substrate

Barrier islands that overlie a compressible substrate load consolidate the underlying subsurface. Through time, the elevation and aerial extent are reduced, making them susceptible to inundation and overwash. Sand washed over the island and onto back-barrier marsh or into the bay or estuary begins the consolidation process on a previously non-loaded substrate. The result is an increase in the overwash, migration, breaching, and segmentation. This research determined the degree to which consolidation affects the evolution of barrier island systems. A two-dimensional (cross-shore) mathematical model was developed, tested with field data, and then applied to evaluate how a compressible substrate modifies long-term barrier island evolution. The model spans time durations to decades and represents cross-shore evolution of a sandy barrier island due to erosion, runup, overwash, migration, and time-dependent consolidation of the underlying substrate. The implications of two strategies for restoring these islands (a one-time "Initial" large-scale infusion of sand from an external source versus traditional "Incremental" beach nourishment and subsequent smaller maintenance volumes) were tested. Barrier islands overlying compressible substrate are more likely to have reduced dune elevations due to consolidation, incur overall volumetric adjustment of the profile to fill in compressed regions outside the immediate footprint of the island, and experience increased overwash and migration when the dune reaches a critical elevation with respect to the prevalent storm conditions. Initial large-scale infusion of sand from an external source decreased the cross-shore migration rate, consolidation rate, and rate of dune lowering for barrier islands overlying a compressible substrate as compared to the Incremental restoration. The reduction in the migration and consolidation processes for the Initial Method resulted in more stability of the island as compared to the Incremental Method.

Coastal Inlets Research Program (CIRP). The original document contains color images.