Importance of wind, fetch and water levels on waves and wave-generated shear stresses in a shallow coastal lagoon

Wind waves and the bed shear stresses they produce are critical for the morphological and ecological equilibrium of shallow tidal basins. Wave-generated shear stresses are the main mechanism responsible for sediment erosion on tidal flats, and regulate both sediment concentrations in the water column and, together with tidal currents, sediment export to salt marshes and to the ocean. We analyze the response of a system of shallow tidal basins along the Eastern Shore of Virginia, U.S., to wind-wave events, with a specific focus on the interplay of basin morphology, tidal elevation and wind direction on depth, fetch and the resulting wave- generated shear stresses. Our analysis indicates that the potential for erosion is the highest when the salt marshes are submerged. Under these conditions the direction of the wind is critical, with maximum wave heights and erosion potential occurring for winds blowing along the barrier islands of the basin (NNE-SSW). We identify four bottom shear stress regimes produced by wind waves in the Virginia Coastal Reserve as a function of water elevation. For elevations between MLLW and MSL the increase in water depth dominates the increase in wave height thus reducing the bottom shear stresses. For elevations between MSL and MHHW the flooding of the salt marshes increases fetch, wave height and bottom shear stresses, producing the largest resuspesion events in the bay. Finally, for elevations above MHHW, the increase in depth reduces the average bottom shear stresses, thus reducing possible erosion in the tidal flats.