FLOW PATHS FOR FLOODING ON THE BACK-BARRIER OF SLAUGHTER BEACH, DELAWARE

Coastal storms have caused nuisance and more significant flooding and road closures in the Town of Slaughter Beach, a community on Delaware Bay. Flooding occurs on the west side of Town (backside of an estuarine barrier), in swales in the barrier interior, and on two access roads between the Town and the mainland. In severe storms, flooding can also be from the Delaware Bay side of the barrier, but these events are not discussed here. While most flooding appears to be from tidal surge from the marsh and tidal channels, some could be from saturation overland flow that occurs when the water table rises to the land surface due to storm surges of rainfall events. Also, unmapped drainage pipes between the back-barrier marsh and the interior swale may be significant pathways for flooding. Anecdotal frequent flooding in the time period from 2012 to 2016 appears to be related to breaches in the barrier south of the Town in the Prime Hook National Wildlife Refuge. The breach was closed in 2016 and since that time flooding has been minimal. This study seeks to identify origins and flow paths for back-barrier flooding using existing data and literature and newly collected hydrologic data. Field methods include deploying sensors that measure water level, temperature, and salinity in tidal channels, on the marsh platform, on the back-barrier upland, and in groundwater wells. This methodology is excellent for looking at time series at a point but not for identifying spatial heterogeneity. The second method uses heat as a tracer to observe flow in space and time, simultaneously. During one day or seasonally, the water will be warmer than the land or vice-versa. Thermal contrasts in water from different sources or between water and land document flow paths. Areal and temporal thermal patterns are collected using thermal infrared and visual cameras attached to a helikite. Results will be shown from a deployment in winter 2019-2020.