THE MISQUAMICUT STATE BEACH REPLENISHMENT PROJECT: ONE YEAR LATER, MEASURED USING RTK-GPS, DGPS AND BEACH PROFILES

The U.S. Army Corps of Engineers added more than 65,000 m³ of sediment to the berm of Misquamicut State Beach (MSB), a 1 km portion of the Rhode Island south shore in May 2014 at a total cost of 3.1 million dollars. We have been monitoring the replenishment using monthly beach profiles and mapped position of last high-tide swash (LHTS) and quarterly RTK-GPS topographic surveys.

Comparing interpolated surfaces generated using RTK-GPS points indicates that 20% of the placed volume was eroded in the first three months, mostly during periods of high wave energy associated with offshore tropical cyclones. West of MSB, the berm initially widened, suggesting that some of the sediment was transported west via longshore current by the southeast swells during the passage of the cyclones. 27% of the placed sediment volume on MSB has been lost as of May 2015, with little change during the winter/spring of 2014-2015, a period of prolonged fair-weather along this coast. Erosion has not been uniform along MSB, with most of the placed sediment already lost from the central and eastern profiles, with little net change at the west end of the beach. Our working hypothesis is that the shoreface topography focuses wave energy onto the more erosional segments of MSB. Pre-replenishment orthophotography shows the berm is consistently narrower here, and points to the importance of understanding the shoreface processes and local shoreline orientation when designing replenishment projects.

It remains unclear if sediment eroded from the berm is temporarily being stored in the subtidal swash bar, or on the shoreface depositional platform. On-going mapping of the extent of the upper shoreface using side-scan sonar will be compared to mapping completed in 2008 to examine changes in the volume of shoreface sand. Field observations suggest that at least some of the eroded sediment ultimately has been transported east, and has been deposited on the flood-tidal delta of the adjacent coastal lagoon via the jettied inlet. While common elsewhere, replenishment at this scale has been rare in Rhode Island, however, it likely will become a more common practice as shoreline change continues to impact developed shorelines. Understanding the efficacy of these projects is vital to understanding the net benefit and longevity of replenishment in the future.