Dynamic roughness in rivers during floods (TCB.6222)

Accurate predictions of water levels during floods in rivers are essential in order to design an appropriate flood defence system or to determine effects of changes to the river system. Water levels during floods are largely influenced by bed roughness in the main channel of the river, formed by large bed forms on the riverbed (e.g. dunes; see Figure 1). Dunes develop during floods as a result of the increasing discharge, leading to a dynamic roughness (e.g. Ten Brinke et al., 1999). Still, a lot is unknown about these bed forms and the resulting roughness, let alone about their development during floods. Incorrect implementation of roughness in hydraulic models will lead to incorrect predictions of water levels and therefore either to unnecessary measures to the flood defence system (if water levels are lower than predicted) or - even worse - to inundation of land behind dikes, resulting in damage to homes, businesses, live stock or other properties and possibly loss of lives (if water levels are higher than predicted). The present proposal aims at improving understanding of dynamic roughness in rivers during floods and developing a model concept to predict dynamic roughness during floods, which can be incorporated in existing hydraulic models. This will yield more accurate predictions of water levels during floods. The proposed research consists of two main parts: i) increasing the understanding of the process of dune development and dune behaviour during floods and modelling this process. The modelling may be based on existing models for sand waves (i.e. the marine equivalent of river dunes; e.g. Morelissen et al., in press; Nemeth, 2003) or on an existing model for bedform development based on probabilities of deposition and entrainment over disturbances on the bed (e.g. Nio et al., 2002). Part i) also includes verification against (laboratory and/or field) data and possibly improving the model by use of data-assimilation. ii) identify the relation between dune development and dynamic hydraulic roughness and develop a model concept to implement dynamic roughness in hydraulic models. This concept will be applied for at least one case in order to determine the effects of dynamic roughness and its uncertainties on water levels. The model results will be verified against field observations with the ultimate goal to model dynamic roughness during floods in an appropriate way, i.e. as simple as possible but accurate enough to yield the required accuracy in water level predictions.

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