Leaky dams augment afforestation to mitigate catchment scale flooding.

Despite calls for large‐scale afforestation to alleviate flooding, the effectiveness of such action remains unclear. Simulations with the SHETRAN hydrological model are therefore carried out for the 335‐km2 Irthing catchment and its 1‐10‐km2 headwater catchments in northwest England to determine: (a) whether forests can reduce flood peak discharges in a large catchment; (b) the proportion of the catchment that requires afforestation to be effective; and (c) the extent to which a combination of afforestation and natural flood management features (leaky dams) improves upon afforestation on its own. Four‐year simulations were run with a range of forest covers and extents of leaky dam installation, the latter modelled as a channel hydraulic resistance. Hydrograph, flood frequency and peak discharge magnitude responses to forest cover simulated (and observed) in the headwater catchments are replicated in simulations at the full scale. Afforestation on its own can reduce the frequency of given flood magnitudes but has a variable impact on individual peak discharge magnitudes. For the Irthing, a 76% forest cover reduces the mean discharge of 20 peaks in a partial duration flood series by 10% relative to the current 21% forest cover but reduces the largest peak by only 2.5%. Accompanying adverse effects include 17.5% reduction in long‐term runoff and loss of agricultural land. By contrast, leaky dams mitigate flood frequencies and peak discharges effectively, over a range of discharge magnitudes, with no reduction of annual runoff. The dam installation required for a 10% mean peak discharge reduction reduces the largest peak by 22%. The flow resistance increases by which the dams are simulated have still to be translated into specific dam designs. Nevertheless, and considering the figures indicatively, dam installation in 20% of the Irthing streams with Strahler orders of 1–3 achieves a 10% peak discharge reduction with only 35% forest cover and 7.5% runoff reduction. The study illustrates the potential for a dense network of leaky dams to augment the impact of afforestation on flood mitigation, especially at the largest discharges, while minimizing adverse impacts on water resources and food security. [ABSTRACT FROM AUTHOR]