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 dispersio n of surface pebbles due to cross-shore transport on a mesotidal estua rine beach composed mainly of sand. Wave heights were 0.04-0.56 m. Dis sipation of wave energy on the low-tide terrace increases as water lev els 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 peb bles as tracers document increasing quantities of pebbles with distanc e downslope on the beach, corresponding to the decreasing energy gradi ent. There is a second locus of high pebble concentration near the upp er limit of swash at high water. Numbers of surface pebbles are at a m inimum following high-energy events (wind speed > 8.0 m/s, wave height > 0.3 m); the most conspicuous pebble accumulations result from low-e nergy 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 d iminishes at low water under both storm and nonstorm conditions. Finer particles are moved onshore from this location by low-energy post-sto rm accretional waves and offshore by outflow from the beach water tabl e, leaving pebbles as a surface lag. A fraction of these pebbles is mo ved 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 bac kwash 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.