Tidal current shear in a broad, shallow, river-dominated estuary

Approximately one year of current, temperature, salinity and pressure data were collected at a site near the mouth of the Mobile Bay estuary in Alabama. This estuary is broad, shallow (3m) and relatively flat. Diurnal tides account for 85% of the current variance in the lower estuary. Tidal currents have significant vertical shear in both amplitude and direction. The principal diurnal constituents in the lower bay, O1 and K1, have amplitudes that range from 12 to 25 cm/s in the near-surface layer, 9 to 12 cm/s near the bed. The principal axis of the near-surface diurnal ellipse is rotated 30° clockwise relative to the near-bottom ellipse. This strong tidal current shear can exist because the estuary has a strong, persistent, haline stratification maintained by a continuous river input. The cause and strength of the tidal shear depends on a combination of factors, including differential topographic forcing through separate entrances into the bay and spatial and temporal variations in water column stability caused by changes in the magnitude of river flow and wind stress.