The geomorphic and hydraulic response to complex weir removal: Evaluating the risk in river restoration
thesisposted on 08.11.2021, 10:31 by Milly Bulcock
For over 200 years, weirs have been built on UK rivers for reasons that include supplying water mills and to mitigate flood risk. It is estimated that there are more than 25,000 weirs in the UK. Following the introduction of the Water Framework Directive (WFD) in 2000, weirs were highlighted as one of the reasons for the poor ecological status of many rivers. This has led to weir removal being at the forefront of river restoration, but the response of a river to removal remains understudied. This thesis examines the hydraulic and geomorphic impact of weir removal, particularly where there is a high density of weirs within the channel, primarily using a numerical modelling approach.
The broad framework of the research was met through four studies. Impacts associated with weir removal were initially explored for a simple, idealised channel containing ten weirs, modelled in HEC-RAS. Individual weir removals led to decreases in water depth and increases in water velocity upstream of the removed weirs, and erosion of sediment upstream and deposition downstream of the removed weirs. Changes to both modelled hydraulics and morphology were largely constrained to the local channel by the upstream and downstream weirs. There was little difference in the response of the river as a function of where in the reach a weir was removed, and removal of an increased number of weirs did not increase the changes to water depth, velocity or shear stress or increase morphological change at reach scale. The numerical experiments in this study also suggested that the largest changes to hydraulics and morphology occur when two consecutive weirs are removed.
The second modelling study also explores the hydraulic impacts of single and multiple weir removals but uses the River Dove as a case study and examines the scale of the change and impacts of flow. The results are consistent with the findings of the first study, but also show that despite the same general changes to hydraulics, the river responds slightly differently to each weir removal. Also, despite large, localised changes to hydraulics, the hydraulics of the whole reach were not affected by weir removal. Finally, these experiments showed that the impacts of weir removal on hydraulics were more prominent at lower flows and decreased with increasing flow levels.
A third modelling study used 2D modelling in HEC-RAS to examine, in greater detail, the spatial and temporal scale of changes related to weir removal and the associated changes in the potential for sediment transport, using a short section of the River Dove as a case study. The findings were consistent with the previous two studies but showed how the localised changes to hydraulics decreased with distance away from the removed weir. It also highlighted the impact that consecutive removals have on hydraulics and potential for sediment transport, concluding that an intact downstream weir reduces the changes to hydraulics caused by weir removal.
Finally, a field study documented the immediate and short-term impact of two weir breaches on the River Dove. Responses were similar response to those modelled in the modelling studies, with water depth decreasing and velocity decreasing upstream of the breach sites. Much of the bed in the study reach was also coarsened as a response to the breaching.
The results from the four detailed studies provide an improved understanding of how a river may respond to weir removal and breaching in situations where weirs are situated close to one another, as on the River Dove. The findings from this thesis, for example the knowledge that an upstream and downstream weir mitigates changes, and that the removal of two consecutive weirs increases changes to hydraulics, potential for sediment transport and widens the area of change, could help inform stakeholders and help in the decision-making processes surrounding future weir removal projects.
Central England NERC Training Alliance (CENTA)
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- Social Sciences and Humanities
- Geography and Environment