OBSERVATIONS OF OVERWASH ON ASSATEAGUE ISLAND FOLLOWING THE NOVEMBER 2009 NORTHEASTER

The mid-November 2009 northeaster storm that developed from the remnants of Hurricane Ida produced substantial overwash onto and across Assateague Island. Three days of observations less than a week after the storm allowed documentation of the extent of overwash, measurements of the salinity of pooled surface water and shallow ground water, and observations of indicators of surface flow such as wrack lines, bedforms, and fresh sand deposits. These observations were made largely to evaluate the relationships among pre-existing island-surface features, such as ridges, swales and locations of former inlets, the active processes of overwash, and the resulting distribution of saline and brackish ground water in the surficial aquifer. These relationships and the resulting distribution of saline ground water are key components for the interpretation of the hydrogeomorphology of Assateague Island as recently mapped by Krantz (2009). The predictions derived from that map were well supported by field observations after the storm. Much of the northern 10 kilometers of the island was completely flattened by overwash, with several fans extending into or across the back-barrier tidal marsh. Low areas associated with former inlets were particularly susceptible to active overwash flow, often channelized, across most of the width of the island. Water depths during the event in many of these overwash channels were 1 to 1.5 m, and flow extended several hundred meters across the island. Overwash into dune fields with both substantial elevation and lateral continuity typically resulted in large areas of ponded water that subsequently infiltrated into the surficial aquifer. As predicted, almost all sections of the island core (a primary map unit) were not inundated by overwash; rather, existing storm ridges and swales directed overwash flow laterally around the island core, often to adjacent former inlets. Flooding from the bay side of the island was minimal; wrack lines generally were no higher than the high-marsh fringe. One unexpected observation was that many areas of ponded water and shallow ground water beneath obvious overwash flow had salinities less than 5. This was interpreted as the result of the heavy precipitation that followed the period of the storm with active overwash, and diluted or displaced the seawater.