Sediment Mixing-depths on a Low-energy Reflective Beach

Douglas J. Sherman, Karl F. Nordstrom, Nancy L. Jackson, James R. Allen

Abstract


A field experiment was conducted on a sandy, micro-tidal, estuarine beach 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 beach, Mixing depth was measured during three experiments using fluorescent-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 maximum mixing depth from 192 tube coree revealed average mixing depths of 25 mm, 27 mm, and 28 mm for the three experiments. The average mixing depth for this environment during these experiments was about 22% of significant wave height. Wave-based models for high-energy beaches underestimate the average, maximum mixing-depth by as much as 400%. The cross-shore distribution of mixing depth shows a maximum at the breakpoint associated with breaker-generated turbulence in the swash.


Keywords


Micro-tidal beach; estuarine beach; fluorescent-tracer sands; high-energy beaches; wave-based model; cross-shore sediment transport; turbulence; swash

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