Observations of Sea-Swell Wave Reflection From a Steep, Nearshore Bar

Abstract

Wave reflection near the beach affects the energy reaching the shore, surfzone wave conditions, and sediment transport. Many prior studies have shown strong reflection of infragravity (0.01 < frequency < 0.05 Hz) wave energy from steep foreshore slopes. Here, swell-frequency bores were observed propagating offshore across a shallow sandbar crest at low tide. Reflection coefficients (ratio of outgoing to incoming energy) estimated from pressure and velocity observations were large (∼0.8) at the frequency of the sea-swell peak (0.07 Hz), especially at low tide when the water depth on the bar crest relative to that in the trough is smallest. The high observed sea-swell reflection may have been at least partly owing to interactions of the incident waves with the surfzone bathymetry, which included a steep drop from the bar crest into a narrow bar-trough at the alongshore location where the offshore bore propagation was observed.