COI Funded Project: Direct Measurement of Bottom Stress in the Wind- and Wave-forced Nearshore Environment


Project Duration: 6/1/97-5/31/98
Key Words: Duck Coastal Observatory, botton stress, currents, modeling

Final Report

Breaking waves on beaches drive alongshore currents that reach or exceed one meter per second, among the fastest oceanic flows observed. These currents are important because they produce the so-called coastal "river" of sand, typically transporting hundreds of thousands of cubic yards of beach material alongshore past any given position each year. This movement of material provides the potential for dramatic coastal erosion or accretion.

The wave-induced driving mechanism has been known for thirty years, but the processes that counter the effects of breaking waves have remained obscure. These counter forces create a "force balance" that controls the magnitude and distribution of wave-driven currents, Most researchers believe that the dominant ¿counter° effect is drag (friction) on the sea floor. They further argue that this drag force is transmitted throughout the water column by turbulent eddies, which are far weaker than either the breaking waves or the alongshore currents, but are nevertheless an effective agent for transmitting momentum. An array of five acoustic SonTek, Inc. Doppler velocity sensors, (San Diego, CA) was mounted on a bottom-fixed steel frame at a water depth of 5 m. During the three-month deployment period, three instruments were badly damaged, probably by logs or other semi-submerged objects carried alongshore by strong storm-driven flows. Also, severe bio-fouling occurred late in the experiment. However, the remaining two sensors produced measurements sufficient to provide estimates of the force-transmitting role of turbulence during three large storms. Measurements obtained by other SandyDuck investigators provide estimates of wave and wind forcing, which are required to put the velocity sensor turbulence measurements in context.

The results are intriguing. The flow energetics and the relationship between the vertical structure of the alongshore current and the bottom stress were approximately the same as in the classically studied near-wall region of turbulent boundary layers, which indicates that turbulence generated by wave breaking did not have the expected large influence on the near-bottom flow. The drag force inferred from the turbulence measurements was only about half the value required to balance the wind and wave forcing. On-going research focuses on understanding these results and on the design of future measurements.

Funding for the turbulence measurements was provided by a WHOI grant from the Mellon Foundation and by a grant from WHOI Rinehart Coastal Research Center. Funding for the analysis of the turbulence measurements was provided by the National Science Foundation. SandyDuck was funded primarily by the Office of Naval Research, with additional support from the US Army Corps of Engineers, the US Geological Survey, and the Naval Research Laboratory. J. Edson provided the wind data and S. Elgar, T. Herbers and R. Guza provided the wave data. The scientific results of this project were published by Trowbridge & Elgar in the Journal of Physical Oceanography (volume 31, pages 2403-2417).

Sum of wind and wave forcing (Figure 1, top panel) and near-bottom stress (Figure 1, bottom panel) estimated from atmospheric and oceanic measurements obtained during the SandyDuck field program. Gaps in the near-bottom stress estimates are due to intermittent instrument failure. According to the simplest model of wind- and wave-driven currents, the two timeseries should be equal. The correlation between the two timeseries is high (squared correlation coefficient is 0.63), but the magnitude of the near-bottom stress is too small by a factor of approximately two (regression coefficient is 0.53).