Observations of surfzone alongshore pressure gradients onshore of an ebb-tidal delta |
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| Institution: | 1. United States Geological Survey, Pacific Coastal and Marine Science Center, 400 Natural Bridges Drive, Santa Cruz, CA 95060, USA;2. School of Earth and Environment, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia;3. Theiss Research, El Granada, CA 94018, USA;4. Woods Hole Oceanographic Institution, 266 Woods Hole Rd., Woods Hole, MA 02543, USA;5. Center for Applied Coastal Research, University of Delaware, 301 Du Pont Hall, Newark, DE 19716, USA;6. Department of Geoscience, San Francisco State University, 1600 Holloway Ave., San Francisco, CA 94132, USA;1. Depart. of Civil and Environ. Eng., Louisiana State Univ., Baton Rouge, LA, USA;2. Center for Computation and Tech., Louisiana State University, Baton Rouge, LA, USA;3. Depart. of Mathematics, Louisiana State University, Baton Rouge, LA, USA;1. Coastal Engineer, Coast & Harbor Engineering A Division of Hatch Mott MacDonald, formerly with Johns Hopkins University, 110 Main St, Suite 103, Edmonds, WA 98020, USA;2. Williard & Lillian Hackerman Professor of Civil Engineering, Johns Hopkins University, Baltimore, MD 21218-2682, USA;3. Professor of Civil Engineering, Johns Hopkins University, Baltimore, MD 21218-2682, USA |
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| Abstract: | The relative importance of radiation stress gradients and alongshore pressure gradients to surfzone dynamics is investigated using observations of water levels, waves, and flows measured onshore of a large ebb-tidal delta. Incident wave heights measured along the ~ 11-m depth contour varied about 10% over a 1.2-km alongshore transect, resulting in alongshore wave setup differences on the order of 10 cm over the 600-m extent of the surfzone instrument array in 1.5-m depth. Despite the moderate alongshore variability in wave heights, the southerly alongshore pressure gradient, associated with the alongshore variability of wave-driven set-up, was typically twice as large as the northerly radiation stress gradient forcing, consistent with the observed southerly currents during the week-long experiment. The magnitude of the alongshore forcing and resulting alongshore velocity is reproduced by the two-dimensional depth-averaged numerical model of Shi et al. (JGR-Oceans, 2011). These observations, together with the numerical results, indicate that moderate alongshore wave height gradients (O(10− 4)) outside the surfzone owing to alongshore variations in the offshore bathymetry can result in alongshore pressure gradients that are larger than radiation stress gradients. |
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