Hydraulic roughness of tidal channel bedforms, Westerschelde estuary, The Netherlands
Van den Berg, J.H.; Asselman, N.E.M.; Ruessink, B.G. (1995). Hydraulic roughness of tidal channel bedforms, Westerschelde estuary, The Netherlands, in: Flemming, B.W. et al.Tidal Signatures in Modern and Ancient Sediments. Special Publication of the International Association of Sedimentologists, 24: pp. 19-32
Resistance to flow was studied in a 6-7-m deep straight tidal channel of the Westerschelde estuary. For this purpose long-term current velocity and water level measurements were carried out at three locations. Vertical current and sand concentration profiles were also measured. Bed morphology was characterized by 0.2-0.4-m high dunes which showed complete reversal of asymmetrical shape during a spring tidal cycle. Hydraulic roughness was calculated in three ways: (i) by solving the one-dimensional momentum equation; (ii) by fitting log-normal regressions to vertical current velocity profiles; and (iii) by fitting regressions to best-fit Rouse distributions of vertical sand concentration profiles. Consistent values of hydraulic roughness were obtained only by the momentum equation. During a tidal cycle the values of hydraulic roughness varied by one order of magnitude. The data indicate an inverse relationship between the Nikuradse roughness parameter k, and the water depth. This conforms to findings of an earlier study in the Conwy estuary, North Wales. Also, an inverse relationship between k, and flow velocity is evident. At maximum tidal flow the k_.-value is equal to about half the dune height.
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