United Utilities in conjunction with the Environment Agency, the Rivers Trust and Cooperative Insurance were interested in understanding the potential for Natural Flood Management (NFM) to protect communities in the North West, as well as deliver broader environmental benefits. This trial focused on a catchment in Lancashire where downstream communities are regularly flooded by water draining from five river catchments, which together cover 150km2.
In order to understand the potential for NFM to help these communities, Viridian Logic ran their HydroloGIS model (summarised here on Toll Assessor) to prioritise which natural interventions to create across these catchments to most reduce flooding for least cost. The interventions included planting woodland, returning to grassland or creating water retention features. Viridian identified the best 2%, 5% and 10% of solutions for reducing flooding, as well as the best 2%, 5% and 10% of solutions to simultaneously reduce flooding, diffuse pollution and erosion/siltation. Ambiental entered these solutions into ReFH2 and hydraulic models to calculate the reduction in flood heights they would give at individual properties in one of the effected communities, looking mainly at 1 in 50 and 1 in 500 year flood events. Viridian converted the flood data into economic benefit using the avoided costs associated with flood defence at a property scale, or the cost of renovating a house post-flood for those properties that expect a water depth great than 600mm (where entry to the property can no longer be prohibited for fear of structural damage). Finally, Viridian identified the carbon sequestration benefits of the best 2% NFM solutions and APEM quantified the reduction in diffuse nutrient loading in the stream due to the combined solutions, using a mixture of Farmscoper and SIMCAT modelling.
Without NFM, 68 properties were modelled to flood during a 1 in 50 year event, 12 of which were undefendable due to the depth of flooding. The top 2% NFM solutions reduce these numbers to zero. In addition to this, for a 1 in 500 year storm, the same NFM interventions would reduce the number of flooded properties from 90 to 52 and the undefendable properties drop from 50 to zero.
In this location 1 in 50 year events have happened every 5 years, which means the top 2% NFM gives between £2M and £11M cost benefit over 30 years relative to PFR alone, depending on the householders’ will to install property level protection (considering costs of flood damage; PFR installation; habitat creation; and habitat maintenance). Typical levels of uptake for this protection suggest the benefits would be closer to the top of this range.
During the implementation of this modelling project a few challenges were thrown up, particularly in terms of trying to predict benefits across multiple floods, demonstrating an appetite for property-level NFM conversion and in quantifying benefits. To add more detail to the latter point, due to the property-scale focus this project did not have the scope to explore additional benefits of installing NFM, such as the benefits of reducing flooding for infrastructure, communal space, the avoidance of disruption for business and the reduced costs of clean up. Despite these challenges the model outputs clearly show the potentially for carrying out Natural flood management in the Wyre catchment. More social science research may be necessary on the appetite among residents for implementation at this scale, but NFM reduces the necessity of persuading householders to accept PFR measures in their homes.
Accompanying this case study Angus Middleton kindly ran a webinar on the HydroGIS model and this work in the Wyre catchment.
Viridian Logic, FloodRE, United Utilities, Co-Operative Insurance, The Rivers Trusts
Which ecosystem services were focused on?:
- Drinking water
- Water supply
- Carbon sequestration & storage
- Detoxification and purification in air, soils and water
- Erosion control
- Flood control
- Hazard regulation