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Modelling woody debris dam form to function to location for flood purposes

Furnues, David 2023. Modelling woody debris dam form to function to location for flood purposes. PhD Thesis, Cardiff University.
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With traditional flood defences seen as costly and no longer able to mitigate the effects of climate change, Natural Flood Management (NFM) has become the new paradigm, to complement hard engineered flood defences in a less expensive and more environmentally sustainable way. Unlike hard engineered flood defences which speeds the flow through communities, NFM slows the flow through upper catchments and reduces the flood peak. NFM could involve planting riparian and floodplain woodland, installing water storage ponds, re-meandering water courses and installing engineered woody debris dams (WDDs). However, the evidence base of NFM is relatively new and many gaps remain, especially about its effectiveness in large flood events. To give confidence in the implementation of NFM, a robust scientific evidence base is needed and to this end the United Kingdom (UK) governments have invested in NFM projects for research. Engineered WDDs are installed in upper catchments as part of nature-based solutions to reduce fluvial flood risk. They are designed to slow the flow through the catchment, enhance channel floodplain connectivity and increase temporary water storage, so attenuating the flow. As the effect of an individual WDD in slowing the flow is small, a large number is required at the catchment scale. Hydro-environmental modelling has become increasingly popular to predict the effectiveness of multiple WDDs in attenuating the flow at the larger scale. However, with no standardised tool to simulate them and a lack of quantifiable empirical data to validate results, uncertainty remains in representing WDDs in the modelling domain. This research investigates the efficacy of a network of WDDs, in attenuating peak flow. To address this aim, this thesis examines the effectiveness of WDD structural design, on sediment transport dynamics, to inform the development of a suitable hydraulic unit for use in a hydro-environmental model. As local scour is fundamental for investigating WDD design, providing insight into the hydraulics at WDDs, flume experiments with an erodible bed, were conducted to investigate bathymetric changes in respect to structural design. Structures were composed of three horizontal wooden key members stacked vertically one above the other, representative of WDDs installed at Wilderhope Brook, Shropshire, UK. WDD design criteria plays an important role in establishing the extent of scour and deposition with key member heights above the bed and gaps between key members altering flow hydraulics. To verify experiments conducted in the flume, the flume was simulated in Flood Modeller v6.1. using 1D, 1D-2D linked and 2D models. Hydro-environmental modelling was used to ascertain, bed / boundary shear stress distributions, velocity profiles and flow depths whereby statistical analysis was performed to examine the accuracy of the hydraulic unit that was used to represent WDDs. To further examine WDD induced morphological effects in the real-world, a field survey at Wilderhope Brook (2019) was conducted which involved examining channel planform adjustment by comparing the surveyed water course to historical maps. To examine how effective the WDDs are at attenuating peak flow in the real-world, catchment descriptors obtained through the field survey enabled 1D hydro-environmental modelling of the catchment. WDD effectiveness in attenuating peak flow was simulated for two storm events with results verified using field data. This thesis helps to inform optimal WDD design criteria for the desired function in this specific location, while offering a hydraulic structural representation which can be manipulated to account for seasonal changes. Additionally, this thesis, in part, showcases the optimum WDD design for the desired function and provides a hydraulic structural representation unit which addresses the research gap into how best to simulate WDDs in the modelling domain enabling improved understanding of the potential efficiency of installed WDDs at other real-world upper catchments.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Engineering
Uncontrolled Keywords: 1) Woody debris dams 2) Leaky barriers 3) Geomorphology 4) Natural flood management 5) Flood modelling 6) Flooding
Date of First Compliant Deposit: 8 March 2024
Last Modified: 08 Mar 2024 10:24

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