Sensitivity maps for impacts of land management on an extreme flood in the Hodder catchment, UK

It is increasingly recognised that the management of land and the management of water are strongly interdependent, and that integrated management approaches are needed. There is evidence that modern land management practices have an effect on runoff generation in rural upland areas, so there is the potential to use land management control as a tool in flood risk mitigation programmes. Flooding from historical extreme rainfall events must be considered when designing mitigation programmes, especially if the designs have to take into account the possibility that such events will become more frequent in the future.The largest 90 min rainfall ever recorded in the UK was 117 mm, recorded in 1967 in the Dunsop tributary (25 km2) of the Hodder catchment, northwest England. Extensive land management changes have recently been made in the catchment, including peat restoration, tree planting and reductions in sheep stocking density, and the analysis of the flooding in 1967 has been undertaken as part of a wider study on the potential impact of the recent changes.A method is demonstrated in which maps of sensitivity are plotted which show how peak flows for extreme events are affected by spatial patterns of changes in runoff generation. This method uses a gridded model for runoff generation containing an embedded hydraulic model of the river network which can calculate sensitivities efficiently and accurately using reverse algorithmic differentiation. The modelling examines the sensitivity of the hydrograph peak at the catchment outlet to changes in flashy runoff generation, which can be affected by land management practices including the blocking of open drainage channels in peat moorland. It is concluded that, as a result of hydraulic and geomorphologic dispersion in the network, changes in land management that affect the flashiness of runoff generation at small scales would probably have had only a relatively minor effect on the flow peaks at the outlet from the catchment during the 1967 extreme event.

► This study evaluates an extreme flood in the Hodder, UK.
► A novel adjoint modelling approach is used to examine cause and effect.
► Maps can be created showing how floods are affected by changes in runoff generation.
► This allows conclusions to be drawn on the impacts of land management change.