Communicating (nature-based) solutions using flood-excess volume, for three extreme UK and French river floods. River Research and Application

Abstract

As interest mounts in natural water-retention measures and alternative Nature-Based Solutions (NBS) as complementary options to civil-engineering measures, catchments and cities face a growing number of possible protection strategies to mitigate flooding. We offer a straightforward and educational protocol, for quantified flood-mitigation assessment of protection strategies, targeted for effectiveness analysis and decision-making involving stakeholder participation. It is based on the concept of flood-excess volume (FEV), i.e., volume exceeding a threshold and generating flood damage, and exemplified for recent extreme-flood events in the UK and France. The central questions are: what fraction of FEV is reduced, and at what cost, by particular flood-mitigation measures? Quantification and interpretation of cost scenarios are facilitated using a novel visualisation of flood-alleviation basins as partially filled, 2m-deep square lakes with side-lengths of ~1-2 km. We consider two 2015 floods of the River Calder in Mytholmroyd (Yorkshire, UK) and the River Brague in Biot (Alpes-Maritimes, France). Each case has different flood-mitigation measures: flow-attenuation features, tree planting and peat restoration, and reservoir storage (Calder), and; retention measures, flood-walls, and bed- widening (Brague). Our straightforward protocol enables fast, quantifiable and easy to understand exploration of protection strategies using multiple measures, and in doing so highlights the issue of NBS scalability.