Quantifying and simulating the impact of flood mitigation features in a small rural catchment

Abstract

The management of fluvial flood risk in the UK is undergoing a paradigm shift, with a change in emphasis from structural defences to working with natural processes where possible. Natural Flood Management has been advocated by several interest groups as a potential option for providing a low cost, sustainable solution to catchment flooding. An integrated monitoring, field experimentation and modelling campaign has been undertaken to assess the potential of Natural Flood Management (NFM) to reduce flood risk in the rural Belford Burn catchment, Northumberland (5.7km2). The village of Belford failed to satisfy a risk-based cost-benefit criterion for structural defences, despite a number of floods occurring in recent years. The alternative low cost NFM mitigation approach taken in Belford involves the use of soft-engineered Runoff Attenuation Features (RAFs) that intercept or modify hydrological flow pathways. Within the Belford catchment 35 RAFs have been installed to date, including interception bunds, permeable timber barriers, large woody debris and offline storage ponds. The performance of a number of RAFs has been rigorously assessed using a combination of analyses of in situ observed data and modelling techniques. An innovative ‘Pond’ Model has been developed, which uses in situ observational data and physically-based methods, for evaluating the operational performance of the RAFs and assessing their impact on a number of historical flood events and design storms. In addition, the physical functioning and methodological approach of the Pond Model has been evaluated against a peer-reviewed hydraulic model. Also a hydrological modelling package was modified to also demonstrate the impact of RAF attenuation at the catchment scale, with the aim of creating a methodology for transferring the knowledge gained at Belford to other small catchments. This research has quantified the impacts of individual RAFs in the Belford catchment. From analyses of historical events, the Pond Model reveals that a network of attenuating features has the potential to significantly reduce peak flow (by up to 30%). However, for larger return interval design storms (for example 1:100 year return interval 24 hour duration) it is demonstrated that a certain/threshold of RAF attenuation features are required before the aggregate effects cause reduction in peak flow. The potential transferability of the approach and the methods used could have benefits for other similar small catchments (<10km2). An assessment of cost effectiveness is made that includes the comparison between the original iv cost of the proposed Belford flood alleviation scheme using a traditional structural methods and the RAF based scheme.