The importance of retention times in Natural Flood Management interventions

Abstract. The starting point for this study is the simulation study of Metcalfe et al. (2018) which suggested that retention times of the order of 10 h are required for natural flood management storage features to have a maximum effect on large flow peaks. A analysis of the celerity characteristics for some log jams at Tebay Gill, Cumbria (upland UK), suggests that the impacts of storage in slowing the celerities is only of the order of minutes. An analysis of storage-discharge dynamics based on observations at 4 jams reveals that the dynamics of storages can be represented with time constants of between 3 and 213 min, still well short of those required to maximise the effects for larger flood flows. That is not to say that there will not be reduction in flood peaks for smaller events, only that for large events the effects will be limited. A spreadsheet tool for retention times has been developed to help in the design of new schemes that is freely available.