University of Twente Student Theses


The morphological modelling of river interventions

Oldenhof, M. (2021) The morphological modelling of river interventions.

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Abstract:Rivers fulfil an important function in the natural ecosystem. They are a source of drinking water, a popular place for recreation, and they enable transport over water. To maintain these functions and guarantee safety against floods many river projects have been executed. The construction of river interventions affects the river morphodynamics. The interventions generally reduce the discharge that is conveyed within the main channel resulting in a smaller sediment transport capacity, and subsequent aggradation in the main channel. The objective of this research is to gain insight into the morphological effect of river interventions in a quick way. The equilibrium state is split up into a static component and a dynamic component. The static component of the equilibrium is found by a space-marching method. This means that the solution is found by stepping through space without the necessity of computing the transient phase. It significantly reduces the computation time compared to the more ”traditional” models. An abridged version of the Backwater-Exner Model is used to find the dynamic component of the equilibrium state. We verified the rapid method with field measurements and the results of a Delft3D computation. We studied the morphological effect of five different types of river interventions: the construction of a side channel, lowering of the floodplains, lowering of the groynes, widening of the main channel and dike relocation. We chose the size of the river interventions as such that the time-averaged bed level change is similar. However, we see large differences in the dynamic component of the equilibrium state between the river interventions. River interventions that change the geometry of the main channel cause the smallest fluctuations. River interventions in the floodplains are only activated during peak flowwhen the floodplains are inundated. Once the floodplains are inundated the changes in discharge caused by the river interventions are large, resulting in large bed level fluctuations. Besides the deterministic model approach, the rapid method makes it possible to look into the uncertainties in morphological modelling and their sensitivity on the bed level. A Monte Carlo Simulation (MSC) is used to quantify the range of bed level fluctuations under seasonal variation in discharge, yearly sediment transport and varying hydraulic roughness. The relationship between discharge and bed level change can be expressed as an exponential fit for which the maximum bed level change reaches a limit by increasing discharges. River interventions that change the geometry of the main channel and the groynes have a small distribution of bed level changes. The distribution of bed level fluctuations is the largest for river intervention in the floodplains. It is recommended to use a MCS with varying discharges to quantify the range of bed level fluctuations for these type of river interventions. In a deterministic approach, the probability of peak flows is very small. Therefore the size and frequency of bed level fluctuations are strongly related to the hydrograph and the frequency of peak flows. The bed level is less sensitive to variations in yearly sediment transport and hydraulic roughness. An increase in these two parameter values results in a slight increase in the size of bed level variations of a couple of centimetres. We suggest the rapid method as a useful tool in the inventory phase of river projects. The method can be used to make quick estimations of the morphological effect of different types of river interventions separately and combined. Combining river interventions with an opposite morphological effect can reduce the negative effect of a single intervention. The rapid method makes it possible to get insight into the single effect of the river interventions to understand the combined morphological effect of a complex river project, like the Room for Living Rivers. In thisway, the rapid method thatwe developed is a useful tool in river management to gain insight into the morphological effect of river interventions in a quick way.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Programme:Civil Engineering and Management MSc (60026)
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