INFLUENCE OF GEOLOGY AND HISTORICAL MINING AND DITCHING ON THE FLOODING OF AN ESTUARINE BARRIER AT SLAUGHTER BEACH, DELAWARE, USA

Flooding in Slaughter Beach results from tidal storm surge and moderate to large precipitation events. Geology and anthropogenic alterations of the landscape control flooding locations. The Town is on the northern end of an estuarine barrier bounded by a salt marsh to the west and Delaware Bay to the east. A tidal flat exists on the north end of the barrier. Sediments represent a Holocene transgressive sequence of sand migrating landward over a salt marsh. A mud-filled, Pleistocene paleovalley underlies the north part of the barrier and salt marsh. Backbarrier and dune elevations are lower in the north than in the south due to compaction of the paleovalley mud. Cedar and Slaughter canals flow in man-made channels, quasi-parallel to the barrier. The creeks flowed directly into Delaware Bay before the canals were built in the late 1800s. Cedar Creek Canal increased the tidal prism in the salt marsh. Minor flooding occurs on the north side of Town and east of Bay Avenue after precipitation events greater than about 20 millimeters. Poor infiltration there causes ponding in closed depressions and is exacerbated by higher elevation of the main road. The poor infiltration occurs because of the lower permeability of the fill in an historical sand mine that supported construction of a nearby World War I fort (Fort Saulsbury). Minor to major flooding of the backbarrier is sourced from Delaware Bay at the mouth of Cedar Creek. The backbarrier floods preferentially by flow from the canals into Clarks Gut in the north, then Sheps Gut, and then Todds Gut in the south. The dune and backbarrier elevations in the north are more prone to flooding as they have a lower elevation due to compaction of the paleovalley mud by the migrating barrier. Major flooding occurs in large storms when storm surge washes over the dunes. Dune breaching appears to be greatest in the south. The dunes are higher in the south than the north, but a jetty and tidal flat protect the north part of Town from waves that attack the dunes. Critical water elevations were defined for use in monitoring flood risk. Taken together, our findings indicate local flooding patterns in this estuarine barrier community are a result of both its geologic setting and human changes to the landscape. A set of recommendations for the Town was drafted to ameliorate flooding.