DISTRIBUTION OF SURFACE PEBBLES WITH CHANGES IN WAVE ENERGY ON A SANDY ESTUARINE BEACH

A 29-day field experiment in Delaware Bay, NJ identified the dispersion of surface pebbles due to cross-shore transport on a mesotidal estuarine beach composed mainly of sand. Wave heights were 0.04-0.56 m. Dissipation of wave energy on the low-tide terrace increases as water levels become lower, creating a spatial gradient in wave energy delivered to the foreshore. Analysis of surface sediment samples taken daily at 4 m intervals across the foreshore and two experiments using dyed pebbles as tracers document increasing quantities of pebbles with distance downslope on the beach, corresponding to the decreasing energy gradient. There is a second locus of high pebble concentration near the upper limit of swash at high water. Numbers of surface pebbles are at a minimum following high-energy events (wind speed > 8.0 m/s, wave height > 0.3 m); the most conspicuous pebble accumulations result from low-energy conditions (wind speed < 3.0 m/s, wave height < 0.2 m). Pebbles move up and down the foreshore within the beach step. They accumulate just above the low-tide terrace on the falling tide when wave energy diminishes at low water under both storm and nonstorm conditions. Finer particles are moved onshore from this location by low-energy post-storm accretional waves and offshore by outflow from the beach water table, leaving pebbles as a surface lag. A fraction of these pebbles is moved up the beach by the swash of low energy waves or within the beach step in subsequent tidal cycles. There is insufficient sand in the backwash of low-energy waves to bury pebbles; they project into the flow of the swash and have low pivoting angles, increasing the probability of entrainment and movement over the sand particles to the upper limit of swash.