Abstract
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.