Assessment of beach and surf zone modifications due to intermittent beach nourishment on the Delaware Atlantic coast

Abstract

Intermittent beach nourishment projects have been conducted over the last three decades on different beaches along the Delaware Atlantic coast to mitigate the long term beach erosion problem. The objective of this study is to investigate and better understand the beach and surf zone modifications that have occurred due to beach nourishment practices. Numerical simulations were performed in Delft3D to analyze the variations of surf zone wave characteristics pre- and post-nourishment for different years at different locations (i.e. Rehoboth Beach, Dewey Beach, Bethany Beach, South Bethany Beach and Fenwick Island) along the coast. In addition, beach nourishment performance may be related to the nourishment design with regard to sediment placement, berm width, foreshore slope and grain size distribution. We focused on Bethany Beach and South Bethany Beach to analyze the effects on the final morphology by altering the fill configuration (template slope and grain size). The simulation results were validated by comparing model results with measured wave data and available beach profiles. The results from the wave analyses revealed that surf zone wave characteristics change temporarily after major nourishment projects, but tends to return to the prevailing conditions within 1-2 years. No identifiable continuous trend was observed in modeled surf zone wave characteristics (i.e. breaking distance, wave shoaling and breaker type) due to intermittent beach nourishment. Similar changes in the morphology at Bethany Beach and South Bethany Beach were found for beach nourishments with two different grain sizes (d50 = 0.3 mm and 0.2 mm). Beach nourishments with two different design foreshore slopes exhibit similar erosion trends. In general, the volume of beach erosion per meter width was found lower for the design template with milder foreshore slope.