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Abstract In field observations from a sinuous estuary, the drag coefficientbased on the momentum balance was in the range of, much greater than expected from bottom friction alone.also varied at tidal and seasonal timescales.was greater during flood tides than ebbs, most notably during spring tides. The ebb tidewas negatively correlated with river discharge, while the flood tideshowed no dependence on discharge. The large values ofare explained by form drag from flow separation at sharp channel bends. Greater water depths during flood tides corresponded with increased values of, 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 withduring flood tides. Whilegenerally increased with water depth,decreased for the highest water levels that corresponded with overbank flow. The decrease inmay be due to the inhibition of flow separation with flow over the vegetated marsh. The dependence ofduring 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. 

Abstract Idealized numerical simulations were conducted to investigate the influence of channel curvature on estuarine stratification and mixing. Stratification is decreased and tidal energy dissipation is increased in sinuous estuaries compared to straight channel estuaries. We applied a vertical salinity variance budget to quantify the influence of straining and mixing on stratification. Secondary circulation due to the channel curvature is found to affect stratification in sinuous channels through both lateral straining and enhanced vertical mixing. Alternating negative and positive lateral straining occur in meanders upstream and downstream of the bend apex, respectively, corresponding to the normal and reversed secondary circulation with curvature. The vertical mixing is locally enhanced in curved channels with the maximum mixing located upstream of the bend apex. Bend-scale bottom salinity fronts are generated near the inner bank upstream of the bend apex as a result of interaction between the secondary flow and stratification. Shear mixing at bottom fronts, instead of overturning mixing by the secondary circulation, provides the dominant mechanism for destruction of stratification. Channel curvature can also lead to increased drag, and using a Simpson number with this increased drag coefficient can relate the decrease in stratification with curvature to the broader estuarine parameter space. 

Abstract Curvature can create secondary circulation and flow separation in tidal channels, and both have important consequences for the along-channel momentum budget. The North River is a sinuous estuary where drag is observed to be higher than expected, and a numerical model is used to investigate the influence of curvature-induced processes on the momentum distribution and drag. The hydrodynamic drag is greatly increased in channel bends compared to that for straight channel flows. Drag coefficients are calculated using several approaches to identify the different factors contributing to the drag increase. Flow separation creates low-pressure recirculation zones on the lee side of the bends and results in form drag. Form drag is the dominant source of the increase in total drag during flood tides and is less of a factor during ebb tides. During both floods and ebbs, curvature-induced secondary circulation transports higher-momentum fluid to the lower water column through vertical and lateral advection. Consequently, the streamwise velocity profile deviates from the classic log profile and vertical shear becomes more concentrated near the bed. This redistribution by the lateral circulation causes an overall increase in bottom friction and contributes to the increased drag. Additionally, spatial variations in the depth-averaged velocity field due to the curvature-induced flow are nonlinearly correlated with the bathymetric structure, leading to increased bottom friction. In addition to affecting the tidal flow, the redistributed momentum and altered bottom shear stress have clear implications for channel morphodynamics.