Modelling Cross-Shore and Alongshore Transport on a Meso-Tidal Beach

Sandwich Town Neck Beach (Massachusetts, USA) is a 1300-m long barrier spit on the north shore of Cape Cod that has experienced chronic dune erosion during nor’easters. Tidal range is ~3 m. A small dune nourishment project that placed 67,500 m³ of sand in front of the existing natural dune, at an elevation of ~6.3 m above mean water, was completed in January 2016. By March 2018, more than 80% of that sand was missing from the subaerial beach. Repeated mapping with drones and photogrammetry over the three winter seasons showed that dune erosion varied spatially along the beach, and earlier analyses suggested these variations were controlled by nearshore topography that generates gradients in wave-induced alongshore transport. However, available bathymetric data did not allow us to close the sediment budget, so we were unable to separate the relative contributions to the observed changes of sediment supply, alongshore transport, and cross-shore transport. We approached that problem by simulating the transport and morphology using the wave model SWAN coupled with the Regional Ocean Modeling System (ROMS) in the Coupled Ocean Atmosphere Wave Sediment Transport (COAWST) system. The sediment-transport model includes a newly implemented cross-shore transport scheme that includes effects of wave asymmetry. The wave model was validated against nearshore wave measurements made in 8-m depth. Modeled morphology changes for selected nor’easter storms were compared with measured changes in subaerial beach volume. The alongshore variability in dune retreat was sensitive to mapped variations in nearshore bathymetry (but not roughness) and tuning parameters in the sediment transport model. However, one of the most important contributions to the volume of subaerial sediment loss was the timing of these relatively short (~12-h) storms with respect to tidal stage, which determined how long waves reached the dune toe.