Onno Bokhove, Mark Kelmanson, Thomas Kent

This paper offers a protocol for conducting a quantified assessment of the relative merits of both existing and proposed methods of Natural Flood Management (NFM). Assessment is based on the rarely used concept of flood-excess volume (FEV), which approximately quantifies the volume of water one wishes to eliminate via flood-mitigation schemes, and is exemplified using publicly available river-gauge data for recent well-known extreme-flood events in Yorkshire, UK. The following question motivates the study: what fraction of the FEV is reduced, and at what cost, by a particular (suite of) flood-mitigation measure(s)? The approach presented admits juxtaposed cost assessments, of disparate and popular NFM measures, that are neither available nor considered in existing flood-mitigation policy. With promulgation of a societally useful protocol in mind, quantification and interpretation of alternative cost scenarios are facilitated using the authors novel visualisation of flood-alleviation basins as partially filled, realistically constructible, two-metre-deep square lakes of side-length approximately one-to-two kilometres.

In the first case study, a hypothetical flood-alleviation scheme for the River Calder at Mytholmroyd, comprising flow-attenuation features, tree planting and peat restoration, and reservoir storage, is critically assessed alongside reasonable cost estimates. The clear quantification and visual representation of the analysis indicate that the fractions of FEV reduced by the NFM measures, while not insignificant, are dwarfed by the careful draw-down of reservoirs. The second case study, of the River Don at Sheffield, extends the analysis via a range of scenarios that attempt to sample realistic seasonal rainfall distributions across a catchment, and in doing so elucidates both the potential and uncertainty of numerous mitigation schemes under different conditions. We corroborate the growing consensus that, while NFM measures can reduce low-level flooding locally, the spatial scale at which they are effective is limited and may not upscale to the catchment level. Our FEV analysis and protocol thus not only offers a concise quantification of the effectiveness of disparate and in-tandem flood mitigation measures but also further highlights the issue of NFM scalability.