Kranenburg Wouter M.

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Kranenburg
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Wouter M.
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  • Article
    Reversed lateral circulation in a sharp estuarine bend with weak stratification
    (American Meteorological Society, 2019-06-13) Kranenburg, Wouter M. ; Geyer, W. Rockwell ; Garcia, Adrian Mikhail P. ; Ralston, David K.
    Although the hydrodynamics of river meanders are well studied, the influence of curvature on flow in estuaries, with alternating tidal flow and varying water levels and salinity gradients, is less well understood. This paper describes a field study on curvature effects in a narrow salt-marsh creek with sharp bends. The key observations, obtained during times of negligible stratification, are 1) distinct differences between secondary flow during ebb and flood, with helical circulation as in rivers during ebb and a reversed circulation during flood, and 2) maximum (ebb and flood) streamwise velocities near the inside of the bend, unlike typical river bend flow. The streamwise velocity structure is explained by the lack of a distinct point bar and the relatively deep cross section in the estuary, which means that curvature-induced inward momentum redistribution is not overcome by outward redistribution by frictional and topographic effects. Through differential advection of the along-estuary salinity gradient, the laterally sheared streamwise velocity generates lateral salinity differences, with the saltiest water near the inside during flood. The resulting lateral baroclinic pressure gradient force enhances the standard helical circulation during ebb but counteracts it during flood. This first leads to a reversed secondary circulation during flood in the outer part of the cross section, which triggers a positive feedback mechanism by bringing slower-moving water from the outside inward along the surface. This leads to a reversal of the vertical shear in the streamwise flow, and therefore in the centrifugal force, which further enhances the reversed secondary circulation.
  • Article
    High and variable drag in a sinuous estuary with intermittent stratification
    (American Geophysical Union, 2021-09-29) Bo, Tong ; Ralston, David K. ; Kranenburg, Wouter M. ; Geyer, W. Rockwell ; Traykovski, Peter A.
    In field observations from a sinuous estuary, the drag coefficient C based on the momentum balance was in the range of 5-20 X10-3, much greater than expected from bottom friction alone. C also varied at tidal and seasonal timescales. CD was greater during flood tides than ebbs, most notably during spring tides. The ebb tide CD was negatively correlated with river discharge, while the flood tide CD showed no dependence on discharge. The large values of CD 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 the inhibition of flow separation with flow over the vegetated marsh. The dependence of CD during 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.
  • Dataset
    North River estuary 2017 dataset
    (Woods Hole Oceanographic Institution, 2021-03-07) Geyer, W. Rockwell ; Ralston, David K. ; Kranenburg, Wouter M. ; Garcia, Adrian Mikhail P. ; Bo, Tong
    These are the observational data collected in 2017 from the North River estuary. Data files include the long-term (LT) CTD and Aquadopp measurements from April to July, the short-term (STI from April to May and STII in late July) CTD measurements, eight shipboard CTD and ADCP surveys in April, May and July, the ADV measurements in late July, the North River mid-estuary region bathymetry, and the North River discharge (from USGS measurements).