Assessing and improving efficiency of leaky barriers as flood flows reduction features in environmentally sensitive catchments

Overview

Floods are a damaging global problem with multiple interacting drivers making it extremely difficult to mitigate (Figure 1). Traditional, engineered – ‘grey’ – flood management structures (like dams and levees) are costly both in monetary and environmental terms. This has motivated a move away from ‘grey’ structures towards many distributed small-scale interventions, which are cheaper and more environmentally friendly (Lane, 2017).
Leaky Barriers (LBs) in small channels are one such intervention that can have multiple benefits: reducing flooding, diffuse pollution and erosion; and improving biodiversity. LBs allow low flows to pass unimpeded, but cause high flows to back up, providing temporary water storage in and around channels (Figure 2). They are cheap to install and while the effect of each LB is small, their cumulative effect has been suggested to be as effective as much larger engineered structures. The problem though is that this cumulative effect is highly uncertain largely because we don’t yet understand the impact of an individual barrier on flow (Metcalfe et al., 2017). Existing attempts to address this rely on mathematical modelling (e.g. Milledge et al., 2015). But the models are rarely tested against observations, perhaps because such observations are so difficult to collect.
This project aims to assess the efficiency of leaky barriers on the management of flow in environmentally sensitive catchments. This will be achieved by an interdisciplinary approach using field monitoring, and physical and numerical experiments.
The project’s objectives are:
O1: Quantify the three-dimensional morphology of leaky barriers in UK streams
O2: Understand the behaviour of a variety of leaky barriers (LBs) under storm conditions using physical and numerical experiments
O3: Develop a new relation to understand how LB’s modify the flow that can inform improvements in barrier design
O4: model the impact of multiple barriers for a set of intensively monitored South Pennine gullies (Figure 3) to test model performance, quantify intervention impact and help identify management strategies for an environmentally sensitive catchment.

Methodology

Understanding the behaviour of individual barriers is the key challenge in the project and underpins all the objectives. Observations on the hydraulic behaviour of leaky barriers are difficult to collect. Field studies are logistically challenging offer limited control on input discharge and pre-intervention behaviour. Laboratory modelling offers more control and can more rapidly generate large empirical datasets with scope for determining design characteristics (e.g. Leakey et al., 2020). This project overcomes these limitations by using a new large flume that allows experiments at a full scale; and combining these observations with numerical modelling allowing a full understanding of LB hydraulics. This will be achieved by applying 4 approaches:
1. Field reconnaissance using high resolution imagery to identify morphological structure of LBs and geometry of the channels.
2. Flume experiments will be carried out in controlled conditions in a suite of hydraulic flumes at Newcastle University (see link below). You will trial a range of LB designs under a range of flow conditions measuring water surface geometry and flow velocity fields.
3. Numerical modelling through the application of an existing CFD code applying an immersed boundary approach to assess how the water passes through the dam. The data will be analysed to allow the development of a new wear equation for LB’s considering both the permeability of the dam and the back water effect. This new empirical relation will be used in a reduced complexity model.
4. Reduced Complexity (RC) Modelling will be developed to improve process understanding and new wear equation, gained from both the physical and numerical experiments. This model will be developed and applied to understand and manage flow in gullied South Pennine peat catchments.
Partners at University of Manchester will provide: 1) field data collected within the PROTECT project (stage and discharge time-series for individual leaky barriers; rainfall and discharge data before and after installation of tens of barriers within 1 ha catchments; results from a survey of gully block properties); 2) guidance on design and interpretation of model tests against this field data.

Project Timeline

Year 1

Literature review and compilation of published datasets; receive training on novel sample collection methods and familiarizing with laboratory methods and capabilities and selecting a suitable initial experimental design drawing on in house expertise; familiarizing with numerical modelling and CFD; first set of flume experiments.

Year 2

First set of CFD experiments, designed to reproduce and complement experimental results from flume. Second iteration of flume experiments (to examine more complex geometries and/or debris representation).

Year 3

Second set of CFD experiments and development of reduced complexity model for runoff attenuation features. Present results at European Geoscience Union General Assembly.

Year 3.5

Synthesise field and modelling datasets; attend international conferences (including American Geophysical Union fall meeting); publication and thesis writing.

Training
& Skills

On completion of your PhD you will:
1) be an expert in natural flood management a key growth area for science sector with demand from consultants, practitioners and policy makers in the UK and beyond;
2) have developed tools to quantify the impact of leaky barriers that are expected to prompt considerable interest and demand;
3) have developed technical expertise in: a) designing, performing and analysing scaled analogue experiments (specifically flume experiments); b) Computational Fluid Dynamics modelling (as a user); c) reduced complexity hydraulic modelling.
Specialist training for flume experiments will cover design, construction, data collection and analysis for scaled analogue experiments.
The numerical modelling component will make use of existing approaches, and the student will receive training in how to set these experiments up, run them, and process/interpret the data.
The supervisory team has the necessary expertise to train the student in these specialist skills, all supported by a dedicated team of technicians in Newcastle. In addition to receiving regular supervisory meetings and support at Newcastle and Durham, the student will also be enrolled in a graduate training programme at Newcastle University and through IAPETUS-specific training, gaining a range of transferable skills relevant to completion of the PhD and developing a career path, including writing research proposals and giving oral presentations. S/he will attend national and international conferences (e.g. American Geophysical Union, San Francisco / European Geosciences Union, Vienna) as well as networking events and outreach activities, developing an important network for feedback and future employment. The student will attend and contribute to the programme of regular departmental seminars and paper reading groups on a wide range of topics, to support the development of a well-rounded scientist.

References & further reading

Lane SN. (2017). Natural flood management. Wiley Interdisciplinary Reviews: Water, 4(3).
Leakey S, Hewett CJ, Glenis V, Quinn PF. (2020). Modelling the impact of leaky barriers with a 1D Godunov-type scheme for the shallow water equations. Water, 12(2).
Metcalfe P, Beven K, Hankin B, Lamb R. (2017). A modelling framework for evaluation of the hydrological impacts of nature‐based approaches to flood risk management, with application to in‐channel interventions across a 29‐km2 scale catchment in the United Kingdom. Hydrological Processes, 31(9).
Milledge DG, Odoni NA, Allott T, Evans M, Pilkington M, Walker J. (2015). Annex 6. Flood risk modelling. In: Restoration of Blanket bogs; flood risk reduction and other ecosystem benefits: Final report of the Making Space for Water project. https://www.moorsforthefuture.org.uk/__data/assets/pdf_file/0026/87542/Final-Making-Space-for-Water-report.pdf
Video of the Newcastle flume: https://armfield.co.uk/2020/05/newcastle-s60-research-flume/

Further Information

Dr David Milledge: david.milledge@newcastle.ac.uk
Prof Richard Hardy: r.j.hardy@durham.ac.uk

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