title: Modelling the Hydraulic and Sediment Dynamics of Leaky Barriers in Relation to Natural Flood Management
Creators: name: Mcparland, Matthew
Publisher: name: University of Liverpool Repository
Year: 2023

description: The risk of fluvial flooding in the UK is set to increase over the 21st Century, both in terms of the number and magnitude of flood events. This is due to both anthropogenic climate change but also the mismanagement of river systems and catchments. It has been recognised that traditional approaches to flood management (also referred to as hard engineering methods) such artificial embankments, levees, channelisation, diversion spillways and dredging alone are going to be unable to protect against heightened future flood risks. As such, a set of new flood management techniques have been developed that aim to ameliorate the impacts of flooding by increasing the water holding capacity of the landscape by enhancing natural process. These new methods, collectively referred to as Natural Flood Management (NFM) hold a great deal of promise as NFM techniques are not only able to reduce flood risk but also restore environmentally degraded riverine ecosystems, providing multiple benefits such as carbon sequestration and increasing biodiversity. NFM techniques include remaindering straightened rivers, creating or reconnecting rivers to floodplains, planting riparian vegetation and creating large woody debris dams (LWDs or leaky barriers). However, the field of NFM is still in its infancy and whilst there is anecdotal evidence as to is effectiveness, there is a little quantitative evidence to evaluate the efficacy of NFM for reducing flood risk. There are also large gaps in the underlying science, especially when it comes to determining the most effective method for modelling NFM measures. Combined, these problems mean that there is little guidance or tools available for flood management practitioners to aid with the implementation of these measures. As such, there are large uncertainties regarding the implementation and effectiveness of these techniques. For LWDs, one of the most popular and widely used NFM techniques, there is little evidence robust enough to show that woody barriers reduce flooding and flood risk. The main uncertainties surrounding its application are how to calculate the effects of LWDs on both the flow and for sediment transport. As such, this thesis had developed a 1-D model that can be used a tool for NFM practitioners for calculating the effects of LWDs on hydraulic and sediment dynamics. As such, the model can be used to help develop and design LWDs as well as help provide much needed quantitative evidence. The first stage of research focused on creating a hydraulic model to compute the changes in flow depth, discharge and flow velocity that occur both upstream and downstream of an LWD. The changes in flow properties were then used as inputs for sediment transport equations to estimate the resultant changes in erosion and deposition. The LWD model was tested using hypothetical prismatic and non-prismatic channels as well as a using a dataset created by taking in-situ measurements at an LWD that was installed in the Sankey Valley catchment. It was found that the LWD hydraulic and sediment transport was able to replicate the expected behavior of LWDs in the hypothetical tests. When applied to field data the LWD model was able to replicate the changes in flow velocity. However, the 1-D model was not able to account for the complex geomorphological changes that occurred around the LWD. As such, it was possible to demonstrate the LWD model constructed as part of this thesis provided a good basis from which more complex representations of LWDs can be developed.