Direct stress measurements in a shallow, sinuous estuary

Abstract

Observations from a 4 element mooring array collected in a bend of a shallow, sinuous estuary are used to describe the flow, density structure and momentum balance over a 10-day period. In general, the flow in the lower 3m is stratified on ebb and unstratified on flood and shear is concentrated near the bed on flood and nearly uniform throughout the water column on ebb. At spring tides stratification is reduced and the flows 1m above bottom (mab) are consistently greatest at the downstream end of the bend. The along-channel density gradient is weakest during spring tides owing to zero gradient over most of ebb flow. At neap tides vertical stratification is strong enough to raise the gradient Richardson number well above 0.25 for most of the ebb tide. Currents are weaker and do not display a regular along-channel pattern. The variation in density and current structure is interpreted to result from variations in cross-channel circulation associated with the channel bend. At spring tides, the cross-channel circulation appears to be strong enough to overturn the water column whereas at neap tides stratification is strong enough to halt the overturning.Reynolds stress measured with a Benthic Acoustic Stress Sensor undergoes a four-fold increase between neap and spring tide. The drag coefficient relative to flow at 1 mab is 0.0015–0.0025. Bed stress in the bend is estimated using this drag coefficient and the maximum instantaneous velocity at 1mab over the array. Because of the along-channel variability in current speed, the estimated bed stress is roughly twice as large as the measured Reynolds stress in the middle of the bend. The estimated bed stress is found to balance the horizontal pressure gradient and local acceleration, implying that a depth-averaged linear momentum balance adequately describes the dynamics on the bend when the impact of the cross-channel circulation is taken into account in the estimate of the bottom stress.