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Increased Drag Coefficient in Estuarine Channels with Curvature
  • Tong Bo,
  • David Keith Ralston
Tong Bo

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David Keith Ralston
Woods Hole Oceanographic Institution
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Flow separation has been observed and studied in sinuous laboratory channels and natural meanders, but the effects of flow separation on along-channel drag are not well understood. Motivated by observations of large drag coefficients from a shallow, sinuous estuary, we found in idealized numerical models representative of that system that flow separation in tidal channels with curvature can create form drag that increases the total drag to more than twice that from bottom friction alone. In the momentum budget, the pressure gradient is balanced by the combined effects of bottom friction and form drag, which is calculated directly. The effective increase in total drag coefficient depends on two geometric parameters: dimensionless water depth and bend sharpness, or the bend radius of curvature to channel width ratio. We introduce a theoretical boundary layer separation model to explain this parameter dependence and to predict flow separation and the increased drag. The drag coefficient can increase by a factor of 2 - 7 in “sharp” and “deep” sinuous channels where flow separation is most likely. Flow separation also enhances energy dissipation due to increased velocities, resulting in greater loss of tidal energy and weakened stratification.Flow separation and the associated drag increase are expected to be more common in meanders of tidal channels than rivers, where point bars that inhibit flow separation are more commonly found. The increased drag due to flow separation affects the tidal amplitude and phasing along the estuary and creates potential morphological feedbacks.