REFLECTION OF OCEAN SURFACE GRAVITY-WAVES FROM A NATURAL BEACH

The energy of seaward and shoreward propagating ocean surface gravity waves on a natural beach was estimated with data from an array of 24 b ottom-mounted pressure sensors in 13-m water depth, 2 km from the Nort h Carolina coast. Consistent with a parameterization of surface wave r eflection from a plane sloping beach by Miche, the ratio of seaward to shoreward propagating energy in the swell-sea frequency band (0.044-0 .20 Hz) decreased with increasing wave frequency and increasing wave h eight, and increased with increasing beach-face slope. Although most i ncident swell-sea energy dissipated in the surf zone, reflection was s ometimes significant (up to 18% of the incident swell-sea energy) when the beach face was steep (at high tide) and the wave field was domina ted by low-energy, low-frequency swell. Frequency-directional spectra show that reflection of swell and sea was approximately specular. The ratio of seaward to shoreward propagating energy in the infragravity f requency band (0.010-0.044 Hz) varied between about 0.5 and 3 and incr eased with increasing swell energy. This trend suggests that infragrav ity waves generated in very shallow water, and refractively trapped on the sloping seabed, are significantly dissipated over a 50-km wide sh elf during storms.