Spatial gradients in the flow over an estuarine channel

Acoustic Doppler current profiles were measured for a twelve-hour period on February 21, 1997 across Thimble Shoal channel, lower Chesapeake Bay, for the purpose of determining bathymetrically-induced spatial gradients in the flow and their implications for the lateral momentum balance in estuaries. A least-squares fit to semidiurnal and quarter-diurnal harmonics was used to separate the tidal and subtidal contributions to the observed flow. The period of observation was characterized by onshore winds and subtidal inflo w everywhere along the transect sampled but strongest in the channel. Spati al gradients in both the tidal and subtidal horizontal flows showed that th e greatest lateral shears and convergences were found where the bathymetric changes were sharpest, i.e., on the shoulders of the channel. The ratio of the quarter-diurnal to the semidiurnal tidal amplitudes was greatest over the channel shoulders, for both the along- and across-estuary flow componen ts, and indicated the importance of non-linear effects there. The nonlinear term caused by across-estuary divergence was larger than the Coriolis term over the channel shoulders. The nonlinear contribution was comparable to t he Coriolis acceleration in the subtidal and tidal lateral momentum balance s. For the tidal balance, the local accelerations were also as important as the Coriolis accelerations. Equivalent results in the momentum balances we re obtained with another data set of October 1993. Contrary to the customar y assumption, the across-estuary momentum balance in this area was ageostro phic.