Sediments, Hydraulics and Morphology

Data of flume experiments with bed forms are used to analyze and compare different roughness predictors. In this study, the hydraulic roughness consists of grain roughness and form roughness. We predict the grain roughness by means of the size of the sediment. The form roughness is predicted by three approaches: Van Rijn (1984), Vanoni & Hwang (1967) and Engelund (1966). The total roughness values (friction factors) are compared with the roughness values according to the Darcy-Weisbach equation. Results show that the different methods predict different friction factors. In future research uncertainties in the hydraulic roughness will be taken into account to determine their influence on the computed water levels. Introduction In the Netherlands, the heights and strengths of dikes and other flood defense systems are based on computed water levels which occur during a certain extreme discharge, i.e. the design discharge. The uncertainty in the hydraulic roughness of the river bed is one of the main sources of uncertainty in these computed water levels (Van der Klis, 2003). The purpose of the present research is to compare different state-of-the-art roughness predictors and examine the influence of the roughness predictor on water levels. We use the same approach as Julien et al. (2002). The overall aim of this study is to gain knowledge on the size and type of uncertainties in the hydraulic roughness and their influence on computed water levels. Material and methods Flume experiments were conducted by Blom et al. (2003) in the sand flume facility at WL|Delft Hydraulics in the Netherlands (1997-2000). The experiments were performed under steady uniform flow conditions and sediment from the Waal River (near the Pannerdensche Kop) was used. The experiments were aimed at conditions with bed forms. Their heights ( ) and lengths ( ) were measured, as well as the hydraulic radius (R), flow depth (h), flow velocity (u) and the energy slope (i). We derive the friction factors by means of two different methods. The first method gives the reference values. It uses flow data and the DarcyWeisbach equation: