HYDROGEOLOGIC INVESTIGATION OF INFILTRATION RATES IN NATURAL AND MAN-MADE BEACHES AT WALLOPS ISLAND, VIRGINIA

Located on the Eastern Shore of VA, Wallops Island is home to the NASA Wallops Flight Facility (WFF). The island has faced the problem of shoreline erosion for decades. Bulkheads, groins, geotubes, and a rock seawall have been employed to maintain the shoreline. In addition to hard-stabilization, WFF completed a beach replenishment project that added over 3 million cubic yards of sand in front of the seawall. This project elevated the berm, reformed a continuous dune line, and increased the beach width to ~70 m.

This new beach has the capacity to change the hydrogeologic characteristics of the barrier island. The new quartz-dominated sand is a relatively homogenous medium for groundwater flow. The greater permeability of the coarser-grained sand will affect groundwater recharge via precipitation or overwash infiltration. The purpose of this study was to quantify infiltration and porosity values on 8 transects that span the natural and replenished beaches. We measured in situ porosity using 8” push cores which were flooded to determine effective pore volume for all transects. Infiltration measurements were taken in situ by recording the volumetric rate at which water entered the ground at each of our transect sites. Our analysis has shown the lowest overall effective porosity occurs on the natural beach (as low as 10 mL per 56.55 inᶟ of sand) and the highest porosity occurs on the replenished beach. Likewise the lowest infiltration rates (to .028 mL per second) occurred in the natural beach, whereas rates were up to 10x faster in the new beach. As this is the first study to investigate the effect of renourishment on the hydrogeology on Wallops, it is unclear if this difference is temporary or if it will have a more sustained impact.

Since August, 2012, we have observed steepening of the shoreface, lowering and long-shore incision in the berm, as well as shoreline erosion at the south end of the new beach. The lowering of the berm is, in part, due to settling of the new beach. To what extent this compacts the sediment to decrease infiltration needs to be quantified. Sampling in the coming months will help to determine if there is a longer-term impact on porosity and infiltration rates. Groundwater levels in the surface aquifer will be compared to pre-replenishment datasets to evaluate the impact of this new beach on the hydrogeology of the island.