High and variable drag in a sinuous estuary with intermittent stratification

Abstract

In field observations from a sinuous estuary, the drag coefficient C, much greater than expected from bottom friction alone. C 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 inhibition of flow separation with flow over the vegetated marsh. The dependence of C_D during ebbs on discharge corresponds with 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.