High and Variable Drag in a Sinuous Estuary With Intermittent Stratification

In field observations from a sinuous estuary, the drag coefficient C-D based on the momentum balance was in the range of 5-20 x 10(-3), much greater than expected from bottom friction alone. C-D also varied at tidal and seasonal timescales. C-D was greater during flood tides than ebbs, most notably during spring tides. The ebb tide C-D was negatively correlated with river discharge, while the flood tide C-D showed no dependence on discharge. The large values of C-D are explained by form drag from flow separation at sharp channel bends. Greater water depths during flood tides corresponded with increased values of C-D, consistent with the expected depth dependence for flow separation, as flow separation becomes stronger in deeper water. Additionally, the strength of the adverse pressure gradient downstream of the bend apex, which is indicative of flow separation, correlated with C-D during flood tides. While C-D generally increased with water depth, C-D decreased for the highest water levels that corresponded with overbank flow. The decrease in C-D may be due to the inhibition of flow separation with flow over the vegetated marsh. The dependence of C-D during ebbs on discharge corresponds with the inhibition of flow separation by a favoring baroclinic pressure gradient that is locally generated at the bend apex due to curvature-induced secondary circulation. This effect increases with stratification, which increases with discharge. Additional factors may contribute to the high drag, including secondary circulation, multiple scales of bedforms, and shallow shoals, but the observations suggest that flow separation is the primary source. Plain Language Summary In shallow estuaries, bottom roughness is usually a major contribution to the flow resistance. The drag coefficient C-D is a dimensionless number that is typically used to quantify the overall flow resistance. In field observations from a sinuous estuary, C-D was much greater than expected from bottom roughness alone. We find that sharp bends in the channel lead to flow separation and recirculating eddies, and this creates "form drag" that removes energy from the flow. Our analysis links the increased C-D to the evidence of flow separation and also explains tidal and seasonal variations in C-D. This observational study suggests that channel curvature can greatly increase flow resistance and affect the tidal dynamics in similar estuaries.