IN COLLABORATION WITH

sediment characteristics aimed at a direct determination of the changes in bed resistance to flow during floods. Hourly stream gauge measurements at regular stations complemented a local laser altimetry survey near the peak discharge. The migration of bedforms could be tracked on a daily basis with complete bathymetric profiles measured from single- and multibeam echosounders. The observed growth and decay of dunes during the rise, peak and fall of the flood hydrograph confirmed earlier studies on the change in bedform geometry during the floods of the Rhine river branches. Field measurements of flow depth, flow velocity and water surface slope clearly show that bed resistance in terms of the Darcy-Weisbach friction factor f and Manning n increases with discharge. The increase in bed friction factor is attributed to the increased dune height during floods. The method of van Rijn provides reasonably good estimates of the bed friction factor as long as field measurements of large dune height and wavelength are considered. The methods of Engelund and Vanoni-Hwang provide similar and realistic estimates of form drag. When combined with van Rijn’s method to estimate grain resistance, however, both methods overpredict the measured bed friction factor. There is also a near-peak hysteresis effect with a calculated peak bed friction factor occurring two days after the measured peak