Vorticity and Divergence of Surface Velocities Near Shore

Abstract

The nearshore environment is complex, with many competing dynamical elements. Surface waves and edge waves (a form of surface wave trapped to the shore) can generally be separated from other forms of motion because of their fast propagation speeds. However, other motions such as internal waves, shear waves, density flows, and isolated vortex pairs can move at comparable speeds. A tool to help separate these dynamical elements is decomposition of the surface 2D flow into two parts, one nondivergent and the other irrotational (solenoidal and potential flows, respectively). Here, an efficient algorithm for this separation is developed and applied, and two examples are examined from data taken at Duck, North Carolina, in 1997 as part of the SandyDuck experiment. The first example is a fresher-water density flow propagating downcoast (probably from the Chesapeake Bay). It is seen that 1) the wave-driven alongshore flow leads the flow, generating a ?surge? of offshore surface flow in its wake; 2) the isolation of the irrotational (2D divergent) part of the flow permits estimates of some dynamical characteristics of the flow; and 3) the nondivergent part of the flow indicates a meander in the alongshore flow that moves downcoast with the surge. The second example is a hypothesized form of isolated vortical structure, such as might be generated by a pulsed rip current that detaches from the shore and bottom and coasts offshore some distance before dissipating. A kinematically self-consistent structure is formulated that would have both divergence and vorticity fields associated with it. However, the observations inspiring the hypothesis are inconclusive, so the existence of such a structure has not been verified.