Onshore volume transport (Stokes drift) due to surface gravity waves propagating toward the beach can result in a compensating Eulerian offshore flow in the surf zone referred to as undertow. Observed offshore flows indicate that wave-driven undertow extends well offshore of the surf zone, over the inner shelves of Martha's Vineyard, Massachusetts, and North Carolina. Theoretical estimates of the wave-driven offshore transport from linear wave theory and observed wave characteristics account for 50% or more of the observed offshore transport variance in water depths between 5 and 12 m, and reproduce the observed dependence on wave height and water depth.During weak winds, wave-driven cross-shelf velocity profiles over the inner shelf have maximum offshore flow (1-6 cm s Ϫ1) and vertical shear near the surface and weak flow and shear in the lower half of the water column. The observed offshore flow profiles do not resemble the parabolic profiles with maximum flow at middepth observed within the surf zone. Instead, the vertical structure is similar to the Stokes drift velocity profile but with the opposite direction. This vertical structure is consistent with a dynamical balance between the Coriolis force associated with the offshore flow and an along-shelf "Hasselmann wave stress" due to the influence of the earth's rotation on surface gravity waves. The close agreement between the observed and modeled profiles provides compelling evidence for the importance of the Hasselmann wave stress in forcing oceanic flows. Summer profiles are more vertically sheared than either winter profiles or model profiles, for reasons that remain unclear.

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LONG TERM GOAL The long-term goal of this research is to increase our understanding and capabilities of predictive modeling of shortwave averaged nearshore circulation patterns (shoreline to nominally 15m depth), through use of a combination of numerical modelling and data analysis from field and laboratory experiments. OBJECTIVES The project contributes to achieving the long term goal by establishing a cross-shore array of self recording wave and current meters north of the main area of instrument arrays during the SandyDuck experiment. The results from these instruments are expected to enhance our capability of modelling the nearshore circulations in the area of the SandyDuck experiment. APPROACH In order to define wave and current boundary conditions perpendicular to the shoreline at the northern boundary of the SandyDuck experiment four self-recording pressure gage/current meters (PUV) have been be deployed. Two of the gages are of type SeaPac 2100 manufactured by Woods Hole Instrument Systems, the other two are SonTek ADVO These instrument packages will measure absolute pressure, water temperature, north-south current and east-west current at 1Hz for a specified number of minutes 5 times a day for 6 weeks. Since the SonTek ADVO is a fairly new instrument, a SonTek and a SeaPac have been placed close to each other at the central cross-shore position, giving a total of three locations along a cross-shore line. This will give a chance for comparison of the performance of the two types of instruments.

2016-11-03T14:11:40

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