Influence of compound bedforms on hydraulic roughness in a tidal environment |
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| Authors: | Alice Lefebvre Verner B Ernstsen Christian Winter |
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| Institution: | 1.MARUM—Center for Marine Environmental Sciences,Bremen,Germany;2.Department of Geography and Geology,University of Copenhagen,Copenhagen K,Denmark |
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| Abstract: | The effect exerted by the seabed morphology on the flow is commonly expressed by the hydraulic roughness, a fundamental parameter
in the understanding and simulation of hydro- and sediment dynamics in coastal areas. This study quantifies the hydraulic
roughness of large compound bedforms throughout a tidal cycle and investigates its relationship to averaged bedform dimensions.
Consecutive measurements with an acoustic Doppler current profiler and a multibeam echosounder were carried out in the Jade
tidal channel (North Sea, Germany) along large compound bedforms comprising ebb-oriented primary bedforms with superimposed
smaller secondary bedforms. Spatially averaged velocity profiles produced log-linear relationships which were used to calculate
roughness lengths. During the flood phase, the velocity profiles were best described by a single log-linear fit related to
the roughness created by the secondary bedforms. During the ebb phase, the velocity profiles were segmented, showing the existence
of at least two boundary layers: a lower one scaling with the superimposed secondary bedforms and an upper one scaling with
the ebb-oriented primary bedforms. The drag induced by the primary bedform during the ebb phase is suggested to be related
to flow expansion, separation, and recirculation on the downstream side of the bedform. Three existing formulas were tested
to predict roughness lengths from the local bedform dimensions. All three predicted the right order of magnitude for the average
roughness length but failed to predict its variation over the tidal cycle. |
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