The importance of retention times in Natural Flood Management interventions

The Nature-based Solution for reducing flood peaks known as Natural Flood Management is an increasingly popular policy option for flood mitigation in the UK and other parts of the world.   It involves a variety of different types of management intervention, including both in-stream and off-line storage elements.   These are often implemented on the basis of convenience in the choice of both sites and materials without any consideration of the importance of unimpeded flow during normal events, and the need for cumulatively large volumes of storage during extremely high flows that could flood downstream communities.   Yet past work (e.g. Metcalfe et al. HESS 2018) has suggested that there may be a rather subtle effect of retention times on peak flows.   If storage elements fill too quickly the effect on downstream hydrograph peaks during events that flood properties will be small.   Equally, if storage elements fill and then drain too slowly, their effectiveness during multi-peaked extreme events will be much reduced. Given the cost of adding sufficient storm-effective storage to a catchment is high, it is important that the interventions are constructed to work for those design events that affect flood risk in the downstream community.   This paper describes a simple tool for evaluating the retention times of in-stream structures of different types that can be used to make natural flood management more effective.