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Sediment nourishments in the River Waal to mitigate bed degradation : a numerical modelling study

Lange, B.M. de (2022) Sediment nourishments in the River Waal to mitigate bed degradation : a numerical modelling study.

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Abstract:Over the past century, the bed level of the Waal has degraded by 1-2 metres. By itself, this poses problems such as a decrease of stability of structures, cables & pipes on the river bed and a decrease in the groundwater level in the vicinity of the river. Moreover, the rate of degradation is not equal along the river, which causes problems with regards to the navigability of ships over the river. Additionally, since degradation is not equal in all branches of the Rhine, the discharge distribution at the bifurcation points is affected. This decreases flood safety during high discharges and limits the supply of fresh water to the North and East of the Netherlands during low and middle discharges. This study confirms that without future human interventions, the bed slope of the Waal will continue to decrease over the next 50 years. Consequently, the Boven-Waal and the largest part of the Midden-Waal degrade with an average degradation rate of 1.3 cm/year. Within the ‘Integral River Management’ (IRM) programme of Dutch local and national governments, solutions are sought to stop bed degradation in the Waal. This can be done by decreasing the erosivity of the flow or by increasing the bed level and increasing the resistance of the bed through sediment nourishments. The latter is investigated in this research. The aim of this research is to determine how sediment nourishments can be used to mitigate bed degradation in the River Waal. The research question that is answered in this study is: “What are the morphological effects of sediment nourishments in the Waal and how can they be used to mitigate bed degradation?” The development of the bed is modelled by simulating the morphological development of the Dutch Rhine branches in a 1-D model. Sediment nourishments with various characteristics are implemented in the model to evaluate their effect on the development of the bed. The characteristics that are varied are the sediment composition, the initial location of the nourishment, the nourishment distribution and the nourishment volume. Sediment nourishments are found to be able to increase the bed level over a longer section than the initial length of the nourishment. Upstream of the nourishments, a reduction in bed degradation of up to 4 cm is found through the backwater effects of the nourishments. This reduction increases when the nourishment is placed further upstream or when the nourishment volume is increased. Downstream of the original nourishment location, erosion is reduced through downstream translation of the nourishment. While the nourishment propagates downstream it disperses, meaning that its height reduces while its length increases. After 50 years, the nourishments still locally reduce erosion by an order of centimetres. The larger nourishments evaluated in this study are capable of reducing erosion by at least 5 cm over almost the complete degrading reach of the Waal. The propagation speed and dispersion rate of the nourishment are mostly dependent on the sediment composition and volume of the nourishment. When a nourishment has a coarser sediment composition than the original bed surface, the nourishment induces additional erosion downstream of the nourishment through a reduction in the mobility of the bed. The magnitude of the additional erosion is dependent on the sediment composition and the volume of the nourishment. Although the additional erosion found in this study is limited to 5 cm at maximum, this is an unwanted effect in an already degrading reach. It is found that by using a distributed nourishment scheme, in which multiple nourishments are placed several kilometres apart, additional erosion downstream of the nourishment can be prevented. Distributing the nourishment over multiple parts can also enhance the performance of the nourishment in reducing bed degradation. Initially, the maximum reduction of erosion that is achieved by a distributed nourishment is smaller but the length over which this reduction is achieved is larger. When the parts of the nourishment are sufficiently large, on the long term the maximum reduction of erosion caused by a distributed nourishment is equal to when the nourishment is not distributed, while the length over which the nourishment reduces erosion is still larger. The net effect of a distributed nourishment is therefore found to be larger than that of a non-distributed nourishment. It is found that all nourishments simulated here travel into the aggrading section of the Waal after 8 to 15 years. After this, the nourishments contribute to additional sedimentation in an already aggrading reach. The initial location of the nourishment mainly determines the period over which the nourishment is able to reduce erosion in the degrading section of the Waal. By placing the nourishment further upstream, the period over which the nourishment reduces erosion in the degrading section of the Waal can be extended. Moreover, it is found that increasing the volume of a nourishment can enhance the effects of a nourishment by a larger factor than with which the nourishment volume is increased. By increasing the volume by a factor 2.7, the maximum increase in bed level caused by the nourishment is increased by a factor 3, while the length over which the nourishment reduces erosion by more than 5 cm is increased by a factor 4. Nourishing a larger volume of sediment at once is therefore found to be beneficial. Finally, it is found that discharge variability due to an uncertainty in discharge conditions does not change the trend of the bed level change. However, it may induce local and temporal changes of up to 0.7 m. This is 0.2 m larger than the maximum height of the nourishments simulated in this research. The bed level changes induced by discharge variability are therefore found to be significant. Additionally, varying discharge conditions affect the propagation speed and dispersion rate of a nourishment. This shows the importance on evaluating the bed level development and the effects of interventions under various discharge conditions. Currently, the IRM programme is looking for ways to stop bed degradation and increase the bed level of the Waal. Sediment nourishments are one of the solutions that are needed to reach this goal. There are still a lot of unknowns when it comes to defining a nourishment strategy. The knowledge on the behaviour of nourishments gained in this study can be used as a basis to define a nourishment strategy based on practical possibilities and limitations.
Item Type:Essay (Master)
Clients:
Sweco
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering and Management MSc (60026)
Link to this item:https://purl.utwente.nl/essays/93400
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