SEDIMENT MIXING-DEPTHS ON A LOW-ENERGY REFLECTIVE BEACH

A field experiment was conducted on a sandy, micro-tidal, estuarine be ach at Fire Island, New York, U.S.A. to determine the maximum depth of sediment mixing by waves and currents on a low-energy, reflective bea ch. Mixing depth was measured during three experiments ming fluorescen t-tracer sands injected across the upper foreshore at high water. Mean wind speeds at the injection times ranged from 5.3 m sec-1 to 11.7 m sec-1; average significant wave heights ranged from 0.09 m to 0.13 m, with peak wave periods from 2.1 sec to 2.2 sec. Measurement of the max imum mixing depth from 192 tube corm revealed average mixing depths of 25 mm, 27 min, and 28 mm for the three experiments. The average mixin g depth for this environment during these experiments was about 22 % o f significant wave height. Wave-based models for high-energy beaches u nderestimate the average, maximum mixing-depth by as much as 400%. The cross-shore distribution of mixing depth shows a maximum at the break point associated with breaker-generated turbulence in the swash.