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High and variable drag in a sinuous estuary with intermittent stratification
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  • Tong Bo,
  • David Keith Ralston,
  • Wouter Michiel Kranenburg,
  • W Rockwell Geyer,
  • Peter A. Traykovski
Tong Bo

Corresponding Author:tongbo@mit.edu

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David Keith Ralston
Woods Hole Oceanographic Institution
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Wouter Michiel Kranenburg
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W Rockwell Geyer
Woods Hole Oceanographic Institution
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Peter A. Traykovski
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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 CD, 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 CD during flood tides. While CD generally increased with water depth, CD decreased for the highest water levels that corresponded with overbank flow. The decrease in CD may be due to inhibition of flow separation with flow over the vegetated marsh. The dependence of CD 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.
Oct 2021Published in Journal of Geophysical Research: Oceans volume 126 issue 10. 10.1029/2021JC017327