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WoodJam—Sediment dynamics of instream wood jams and managed installations

Periodic Reporting for period 1 - WoodJam (WoodJam—Sediment dynamics of instream wood jams and managed installations)

Reporting period: 2018-08-27 to 2020-08-26

To combat expected increases in the frequency and severity of drought, floods, unpredictable rainfall and other extreme weather conditions, the Intergovernmental Panel on Climate Change and Paris Agreement signatories have called for substantial investment in adaptation infrastructure required to secure future water supplies, with 2018 green bond investment in water projects totalling approximately £14 billion. The EU Water Framework Directive and the Water Criteria of the Climate Bonds Standard have encouraged the use of nature-based solutions, including instream leaky wood barriers, to improve catchment physical and ecological resilience and reduce the need for hard engineering infrastructure. Instream leaky wood barriers cause a backwater rise behind the structure, enhancing water storage and channel-floodplain connectivity. The contributions of natural instream wood to increased fishery production, improved ecological status, and channel roughness in wooded catchments are well documented. However, little is known about the engineering performance of constructed leaky barriers during extreme events, optimum design to reduce dam collapse impacts, and the effect of leaky barriers on channel incision and water quality. The work carried out investigated the hydraulic and sediment transport effects of instream leaky wood barriers using a combination of hydraulic flume epxeriments, fieldwork, and hydraulic modelling, to improve the representation of leaky barriers in modelling frameworks and the design and assessment of engineered logjams used in natural flood management projects.
Dr Follett conducted hydraulic flume experiments and theoretical analysis to demonstrate a physical analogy between porous canopies and channel-spanning log jams, resulting in a prediction of the extent of backwater rise behind channel-spanning logjams based on flow and jam physical parameters, including a method to assess logjam physical characteristics from field measurements without the requirement of jam disassembly (Follett et al. 2020). This allows representation of wood jams in modelling frameworks including hydraulic flood models, leading to improved design and assessment of natural flood management projects. This result was disseminated through conference attendance (American Physical Society-Division of Fluid Dynamics, 2019), an article in MIT News and on Twitter. In addition, experiments were conducted measuring the bedload scour and deposition formed around model jams that partially spanned the vertical channel, with varying discharge, jam porosity, and lower gap height. A portion of these results were presented in a peer-reviewed conference paper (Follett and Wilson 2020, RiverFlow 2020); a peer-reviewed manuscript is in prep. Further experimental work measured variation in wake hydrodynamics due to jams with different physical designs representative of the range of variation observed at field site (mansucript in prep). A field monitoring program was developed in collaboration with a DEFRA-deisgnated natural flood management site including installation of monitoring equipment measuring multiple aspects of flow and sediment transport associated with to installation of 105 engineered logjams. Research results from this work informed flume experimental work during the project, and a manuscript is in prep. Initial testing and validation of of backwater rise prediction equations (Follett et al. 2020) in hydraulic modelling framework has been conducted and work is ongoing in collaboration with industrial project partners. Project results were communicated to multiple stakeholders at in-person meetings and presentation at the River Restoration Centre conference (2019) and will be further disseminated at 2 upcoming conference presentations (APS-DFD 2020, AGU 2020). Communication activites to members of the public are planned in association with the 2020 European Researchers' Night in Cardiff, UK.
Energy and momentum constraints were combined to predict backwater rise from unit discharge and a dimensionless structural parameter, allowing representation of channel-spanning wood jams in modelling frameworks. This novel approach allows description of intact wood jams with a common metric describing jam flow blockage, which may be deduced from field measurements. This result was published in a peer-reviewed international journal (Follett et al. 2020, Geophysical Research Letters) and established a new, ongoing collaboration between Dr Follett and an EU researcher, Dr Isabella Schalko (ETH Zurich). Improved modelling capability enhances the innovation capacity and strengthens the competitiveness and growth of flood engineering consultant companies due to the increased ability to represent logjams in hydraulic flood models. Dr Follett has conducted initial activities testing implementation of backwater rise prediction equations in hydraulic models and is conducting ongoing work in collaboration with industrial project partners. In addition to the industrial sector, potential end users include government sector partners, who would benefit from research results and improved modelling accuracy to develop design guidance and policy recommendations. Project results have been disseminated to the government sector including Natural Resources Wales and the Environment Agency throughout the project, and successful proposals for further work were developed with both industrial and government partners. This aids EU Water Framework Directive policy objectives which encourage the use of engineered logjams and other natural flood management interventions in order to reduce flood damages and prepare for an expected increase in severe floods due to climate change. Additional end users include residents of flood-prone areas, who benefit from improved mental health due to engineered logjam installations and increased understanding of logjam physical effects. Dr Follett has communicated research results with members of the UK National Flood Forum which has led to the opportunity to communicate research results to end users including local Flood Action Groups composed of affected community members (November 2020), which will acknowledge EU funding, and contribute to community evaluation surveys.