Reynolds Stress and Small-Scale Morphology Measurements During DUCK94

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Abstract
A recently developed Coherent Acoustic Sediment Probe (CASP) was deployed from an instrumented sled during the Duck94 nearshore experiment at Duck, North Carolina, in October 1994. The CASP measured acoustic backscatter profiles and three component velocity vectors in an O(1 cm3) volume 25 cm in front of the downward-looking instrument package. The CASP was remotely positioned to selected heights to form profiles of velocity and sediment concentration through the bottom boundary layer. A vertical stack of 8, two-axis electromagnetic current meters extended the horizontal measurements to the surface. A co-located scanning pencil-beam sonar and tilt sensors were used to map the small-scale morphology across a 1m by 4m area centered on the CASP measurements, quantifying the local roughness and slope elements down to 4cm scales under low acoustic clutter conditions. It is shown that the stress measured near the bed in a gravity oriented vertical reference system is associated with both the turbulent Reynolds’ stresses and a wave stress due the vertical velocity induced by the horizontal wave velocity acting on the sloping bottom being correlated with the horizontal velocity. The wave stress is largest at the bed and decreases towards the surface and can be larger than the turbulent Reynolds’ stresses.

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