Hydraulic Resistance of Vegetation in River Flow Applications

Proceedings of the Seventh International Conference on Hydroscience and Engineering, Philadelphia, PA, September 2006. http://hdl.handle.net/1860/732If vegetated regions become part of a river’s flow field, the hydraulic resistance of vegetation affects the overall conveyance. Several descriptions exist to describe this type of flow; among them are empirical relations and relations that are process-based. In the current work three expressions are considered that have equal input parameters, similar levels of complexity but different theoretical backgrounds. The performance of the three methods is evaluated by comparison with flow measurements (collected from literature), and limits are given for their practical use

Getting grip on complex water issues? : a case study: Rotterdam Mainport, Appropriate Assessment Wadden Sea

In Dutch water systems many human interventions are carried out. These interventions are designed to achieve management goals, like increase protection against flooding, improve environmental quality and/or stimulate the national economy. Decision-makers involved in these kinds of plans have to deal with ecological effects, physical effects, economic costs and benefits and technical feasibility. Furthermore, they operate within a complex web of interactions between policy, regulations, and social and political processes. This PhD-project aims at assisting decision-makers in constructing assessments of the impact of future human interventions in water systems

The generation of offshore tidal sand banks and sand waves

A simple morphological model is considered which describes the interaction between a tidal flow and an erodible bed in a shallow sea. The basic state of this model describes a spatially uniform tide over a flat bottom where the flow vector is represented as a tidal ellipse. The linear stability of this solution is analysed with respect to bed form perturbations. Results are presented for both a uni-directional and circular tide. In the former case the wave-length and the orientation of the fastest growing bed mode agree well with those of tidal sand banks. However, this model only predicts the growth of large-scale sand ridges. With a simplified numerical model we tentatively show that the effects of secondary currents on the sediment transport trigger the formation of instabilities at an essentially smaller scale, viz, sand waves. Another limitation of a model with uni-directional tides is that no selective modes found are the first to become unstable if the model parameters are varied. In the case of a circular tide, critical model parameters are found below which the basic state is stable. We conclude that this provides a starting point for the development of a weakly non-linear analysis, which will yield information on the amplitude behaviour of marginally growing bed forms

Identification and quantification of uncertainties in a hydrodynamic river model using expert opinions

Hydrodynamic river models are applied to design and evaluate measures for purposes such as safety against flooding. The modelling of river processes involves numerous uncertainties, resulting in uncertain model outcomes. Knowledge of the type and magnitude of uncertainties is crucial for a meaningful interpretation of the model results and the usefulness of results in decision making processes. The aim of this study is to identify the sources of uncertainty that contribute most to the uncertainties in the model outcomes and quantify their contribution to the uncertainty in the model outcomes. Experts have been selected based on an objective Pedigree analysis. The selected experts are asked to quantify the most important uncertainties for two situations: (1) the computation of design water levels and (2) the computation of the hydraulic effect of a change in the river bed. For the computation of the design water level, the uncertainties are dominated by the sources that do not change between the calibration and the prediction. The experts state that the upstream discharge and the empirical roughness equation for the main channel have the largest influence on the uncertainty in the modeled water levels. For effect studies, the floodplain bathymetry, weir formulation and discretization of floodplain topography contribute most to the uncertainties in model outcomes. Finally, the contribution of the uncertainties to the model outcomes show that the uncertainties have a significant effect on the predicted water levels, especially under design conditions. \u

Explaining artificial side channel dynamics using data analysis and model calculations

Side channel construction is a common intervention to increase both flood safety and the ecological value of the river. Three side channels of Gameren in the river Waal (The Netherlands) show amounts of large aggradation. We use bed level measurements and grain size samples to characterize the development of the side channels. We relate the bed level changes and the deposited sediment in the side channels to the results of hydrodynamic computations. Two of the three side channels filled mainly with suspended bed-material load. In one of these channels, the bed level increased enough that vegetation has grown and fine suspended load has settled. In the third side channel, the bed shear stresses are much smaller and, in addition to the suspended bed-material load, fine sediment settles. Based on the side channel system at Gameren, we identify two types of side channels: one type fills predominantly with suspended bed-material load from the main channel and a second type fills predominantly with fine suspended load. This gives an indication of the main mechanisms that lead to the aggradation in artificial side channel systems.<br/

On the influence of suspended sediment transport on the generation of offshore sand waves

Sand waves are bed-forms occurring in shallow seas. Although their characteristics are mainly affected by bed load transport, during rough weather suspended sediment transport can influence their characteristics. As a first step to model these influences, we added suspended sediment transport to a numerical 2DV model that was specifically developed for simulating sand waves. In this paper, results are presented for initial, small amplitude, sand waves. Incorporating suspended sediment transport increases the growth rate of sand waves significantly while their wave length is more robust. Furthermore, we found that the results are sensitive to flow conditions, as expected, and sediment diffusivity, which needs a more advanced description