SHORELINE IMPACTS FROM MINING SHOALS OFFSHORE OF ASSATEAGUE ISLAND NATIONAL SEASHORE

Wallops Island is a barrier island on Virginia’s Eastern Shore and NASA’s rocket launch site for Wallops Flight Facility. Because Wallops Island is experiencing chronic beach erosion that threatens the facility’s existence, a storm damage reduction program has been designed. The closest available beach fill material for the project has been located in shoals offshore of Assateague Island National Seashore, 10+ miles away.

A numerical modeling study was set up to analyze the impact of offshore mining on the wave-induced longshore sediment transport along Assateague Island. The procedure involved the refraction of offshore waves over the existing bathymetry into near-breaking depths. Then, the same offshore waves were refracted over bathymetry that had been modified by an appropriate increase in the depth in three potential borrow areas. All sets of resulting near-breaking waves were used to drive a sediment transport model, and with and without mining results were compared. The mining-induced difference in the sediment transport for each shoal was related to the natural variation in the wave climate to determine if it was significant.

Blackfish Bank is the shallowest of the three shoals and the closest to shore. Modeling results showed that removing material from this shoal has shoreline impacts that exceed threshold criteria. The impacts of mining the other two shoals are below threshold criteria.

For all the shoal-mining scenarios, the greatest shoreline impacts were found to be in the vicinity of Tom’s Cove, a narrow isthmus separating a portion of Chincoteague Bay from the Atlantic Ocean. This is a particularly vulnerable area since repeated surveys have shown this narrow strip of land to be thinning over time, mainly due to erosion of the ocean shoreline. Because of the potential for overwash in this area, possibly leading to new inlet formation, it is important that offshore mining not have significant negative impacts in this area. Because of this analysis, Blackfish Bank has been removed from further consideration as a potential borrow site. In November 2009, after the modeling study was completed, but before offshore mining had occurred, a Nor’easter created the first substantial breach along this section of shoreline.