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 built idealized numerical models representative of that system. We found 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, quantified as 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 in bends, 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 reduces tidal amplitude and affects velocity phasing along the estuary and could result in morphological feedbacks.
Over the course of a year, we conducted three field deployments in South San Francisco Bay to examine seasonal variability in bottom drag. Our data consisted of turbulence measurements both within and outside the bottom boundary layer and benthic characterization surveys adjacent to our study site. Our results suggest that canopies of benthic worm and amphipod feeding tubes, which were denser during summer, can increase the drag coefficient by up to a factor of three relative to the smoother beds found in winter and spring. The extent of the drag increase varied depending on the measurement device, with the greatest increase inferred by measurements taken further from the bed. The small scale and temporally varying population densities of these living roughness elements pose significant challenges for hydrodynamic models, and future work is needed to begin incorporating benthic biology statistics into drag coefficient parameterizations.
more » « less- Award ID(s):
- 1736668
- NSF-PAR ID:
- 10452065
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geophysical Research Letters
- Volume:
- 47
- Issue:
- 15
- ISSN:
- 0094-8276
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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