Abstract
Detailed field measurements during the 1998 flood of the Rhine River in The Netherlands show that both Manning n and Darcy–Weisbach friction factor f increase with discharge. The changes in bedform roughness height and friction factors are attributed to the increased dune height during floods. There is a near-peak hysteresis in the dune height measurements. At a given discharge, dunes are significantly larger after than before the peak discharge. The trend is most apparent for the Bovenrijn with weaker variations for the Waal. The methods of Engelund and Vanoni–Hwang provide similar estimates of form drag. When combined with van Rijn’s method to estimate grain resistance, both methods tend to overpredict the measured bed friction factor after the peak discharge. These methods perform best when field bedform measurements are available to estimate form drag. The composite effect of primary and secondary dunes should be considered in the analysis of resistance to flow.
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