Convergence of lateral flow along a coastal plain estuary

A set of velocity profiles obtained in the James River estuary with an acou stic Doppler current profiler was used in combination with the results of a n analytic tidal model to depict the appearance of surface lateral flow con vergences (av lar) during both flood and ebb stages of the tidal cycle. The bathymetry of the estuary was characterized by a main channel and a second ary channel separated by relatively narrow shoals. Lateral surface flow con vergences appeared over the edges of the channels and were produced by the phase lag of the flow in the channel relative to the shoals. Flood converge nces developed in the late tidal stages and ebb convergences appeared soon after maximum currents. Most of these convergences caused fronts in the den sity field and flotsam lines that also appeared over the edges of the chann el and that lasted <2 hours. The transverse flows associated with the conve rgences were mostly in the same direction throughout the water column. Ln f act, the vertically averaged flow produced the same convergence patterns as those near the surface. The analytic tidal model reproduced well the timin g and location of the convergences as observed in the James River. Model re sults with different bathymetry emulated the results in other estuaries, e. g., axial convergence in an estuary with a channel in the middle. This work showed that the strength of lateral convergences along the estuary was pro portional to the tidal amplitude and the channel steepness. It also suggest ed that the convergences were produced mainly by the tidal flow interacting with the channel-shoal bathymetry, i.e., that they did not require the pre sence of density gradients. However, the analytic model underestimated the magnitude of the convergences and did not account for vertical circulations associated with fronts. The formation of fronts resulted from the interact ion of the tidal flow with the bathymetry and the density field.