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Paper No. 70-5
Presentation Time: 1:30 PM-5:30 PM
REVISITING BARRIER ISLAND BEACH INFILTRATION RATES AND IN SITU POROSITY MEASUREMENTS FOUR YEARS POST-REPLENISHMENT, WALLOPS ISLAND, VIRGINIA.
In 2012, NASA’s Wallops Flight Facility (WFF) completed a 3.4 million ydᶟ beach replenishment to help reduce shoreline erosion. The project resulted in a 6 km x 3 m high x 70 m wide new beach that became the highest point on the island. It resulted in a new catchment area more than 420,000 m² (.16 miles²). Dredged sand used for replenishment was much coarser than native sand. These pronounced changes to the shoreline altered the hydrogeologic parameters of the island, including in situ porosity, infiltration rates, hydraulic conductivity, and resulted in a modified water table. Since completing the replenishment, WFF has experienced persistent flooding of freshwater wetlands and roadway areas. It is suspected that this is in part due to the flow of water infiltrated through the new beach.
This study follows a preliminary infiltration assessment completed by Heston and colleagues (2013). These authors quantified in situ porosity and infiltration rates along transects of the natural and newly replenished beaches. Data revealed that porosity and infiltration rates were highest on the replenished beach and lowest on the natural beach. The purpose of this study was to evaluate current infiltration and porosity in order to compare with the analyses done in 2012-13. The question at hand is whether the replenished sands changed over the last 4 years; and to what extent the new beach might contribute to observed freshwater wetland flooding.
Data from fall 2016 has shown that infiltration and porosity continue to follow trends observed previously. Porosity remained highest on the replenished beach with an avg. of 4.8mL/inᶟ. It was an avg. of 72% lower on the natural beach at 3.4mL/inᶟ. Similarly, the infiltration rates along the shoreline were higher on the natural beach at 2.6mL/ sec. versus 4.5mL/ sec. on the replenished berm. Given the catchment and avg. rainfall of ~44”/year, the USGS calculator estimates that >15 million ftᶟ/yr will fall on the replenished beach. Given our porosity and infiltration measurements, not including storm surge-related infiltration, we estimate that as much as 30% could effectively infiltrate and yield a higher water table for the island. Moreover, with higher hydraulic conductivities it is likely that some of this water discharges into interior wetlands and contributes to more persistent flooding.