Shear-flow induced secondary circulation in parallel underwater topographic corrugation and its application to satellite image interpretation

This study aims to figure out satellite imaging mechanisms for submerged sand ridges in the shallow water region in the case of the flow parallel to the topography corrugation. Solving the disturbance governing equations of the shear-flow yields the analytical solutions of the secondary circulation. The solutions indicate that a flow with a parabolic horizontal velocity shear and a sinusoidal vertical velocity shear will induce a pair of vortexes with opposite signs distributed symmetrically on the two sides of central line of a rectangular canal. In the case of the presence of surface Ekman layer with the direction of Ekman current opposite to (coincident with) the mean flow, the two vortexes converge (diverge) at the central line of canal in the upper layer and form a surface current convergent (divergent) zone along the central line of the canal. In the case of the absence of surface Ekman layer, there is no convergent (divergent) zone formed over the sea surface. The theoretical results are applied to interpretations of three convergent cases, one divergent case and statistics of 27 cases of satellite observations in the submerged sand ridge region of the Liaodong Shoal in the Bohai Sea. We found that the long, finger-like, bright patterns on SAR images are corresponding to the locations of the canals (or tidal channels) formed by two adjacent sand ridges rather than the sand ridges themselves.