An array of five  acoustic Doppler velocimeters (ADV), which produce high quality measurements of the three-dimensional velocity vector in a sample volume with a scale of one centimeter, was deployed from late August through late November of 1997 at a water depth of approximately 4.5 m off Duck, North Carolina (Figure 1).  The sensors were deployed near the sea floor but above the centimeters-thick wave boundary layer, and the sampling scheme was designed to resolve turbulence statistics averaged over tens of minutes, much longer than typical wave periods but shorter than time scales associated with variability of energetic wind-driven and wave-driven alongshore flows.

A part of the SANDYDUCK 97 field program, the experiment addresses turbulence in the shallow nearshore environment, where water depths are on the order of meters, energetic currents are forced by both winds and breaking waves, and the motion of water and sand is of primary scientific interest and practical concern.  Existing models and measurements indicate that turbulence is particularly important in this environment:  the effect of bottom drag, which plays a dominant role in controlling the magnitude of wind-driven and wave-driven flows, is believed to be transmitted through the water column by turbulent Reynolds stresses; near-bottom turbulent stresses are believed to control the entrainment of sediment from the sea floor; and turbulence in the water column is believed to maintain sediment in suspension against the action of gravity.  In spite of its importance, turbulence in the nearshore environment has rarely been measured. Direct measurements of turbulent Reynolds shear stress, in particular, have never been obtained in this environment, because of fundamental problems produced by surface waves.

The objectives of the analysis are (1) to obtain direct estimates of turbulent Reynolds shear stress, by using a novel method involving the difference between velocity measurements obtained by pairs of spatially separated sensors; (2) to obtain indirect inertial-range estimates of the dissipation rate for turbulent kinetic energy; (3) to test, in collaboration with other SANDYDUCK principal investigators, a wave-averaged alongshore momentum equation in which wind stress and cross-shore gradient of wave-induced radiation stress balance bottom stress; and (4) to test a simplified turbulence energy balance in which shear production balances dissipation.
 

Instrumentation :

A low-profile frame was designed and built to provide a platform on which to mount five SonTek1 acoustic Doppler velocity  (ADV) meters.  The instrument array (Figure 2) included three of the 10 MHz field version of SonTek's acoustic Doppler velocimeter (ADVF) and two of the more rugged 5 MHz ocean version (ADVO).  The relative positions of the two sensors within each sensor pair were suitable for providing estimates of Reynolds stress by means of the differencing technique (Trowbridge, 1998) and the sensing volumes were at different heights, so that the measurements can provide estimates of the vertical gradient of Reynolds-averaged horizontal velocity.  The offshore ADVF  was likely too far from the other ADVFs to provide reliable stress estimates, and it was included primarily to provide information about vertical structure.  Measurements from either the pair of ADVOs or the farthest-onshore pair of ADVFs are sufficient to achieve the objectives described in the previous section.

Deployment:

The frame was deployed on 8/25/97 off the coast at the Field Research Facility (FRF) of the U.S. Army Engineer Waterways Experiment Station (WES), Coastal Engineering Research Center (CERC), using the Coastal Research Amphibious Buggy  (see heading figure).  It was positioned in 4.5 meters at 426.07 meters offshore and 830.38  meters along-shore (bathymetry).
 

A data report is available as Woods Hole Oceanographic Technical Technical Report, WHOI 2001-02.

The burst averaged data are available as matlab files (sandyduck.zip). To unpack using a PC use pkunzip.exe or unzip on a unix workstation.

Contact J.J.Fredericks.

This project was supported by the Mellon Foundation, Rinehart Coastal  Research Center and the National Science Foundation.
 
 

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