Modes of Overwash and Depositional Patterns on Assateague Island

In November 2009, Nor'Ida, a Mid-Atlantic nor'easter that evolved from Hurricane Ida, impacted the Assateague Island shoreline. Overwash processes during strong storms such as Nor'Ida largely control the morphodynamics and retrogradational nature of Assateague Island. Within five days after the storm, we observed and documented the impacts on the island. Four primary categories of overwash were identified: (1) Complete overwash was seen primarily on the northern 10 km of the island. This included fans stretching back into the lagoon. (2) Channelized active overwash was displayed as new deposition of sand as thick as 0.5m in the low areas of former inlets. (3) Ridge-controlled overwash was deposition that occurred as water flowed laterally around existing storm ridges that bracket the former inlets. (4) Ponding of substantial volumes of seawater occurred in swales between storm ridges and within the secondary dune field. The four modes of overwash are digitized from a set of aerial photos taken 3 days after the storm to document the spatial distribution on the island. Infiltration following overwash is a primary mechanism for introducing saline water into the fresh groundwater lens of the island. With accelerating sea-level rise, this process will become more frequent and of greater magnitude. These observations provide further knowledge on the processes and impacts of storm-surge overwash on Assateague Island.