Evaluation of nature-based solutions for flood risk management in the Oitavén-Verdugo River Basin (NW Spain)

Floods are recurrent phenomena with significant environmental and socio-economic impacts. The risk of flooding can increase when land use changes. The objective of this research has been to obtain an integrative methodology based on the development of a model in HEC-HMS, calibrated and validated from events between 2018 and 2022, and to apply simulations employing the use of Nature-Based Solutions (NbS) tools. This model has been applied in Verdugo-Oitavén River Basin (NW Spain). Three different scenarios propose (1) the reforestation of 30% of abandoned agricultural land across the basin, (2) reforestation upstream (S-Upstream), and (3) reforestation in two sub-basins (S-Downstream). Upstream afforestation provided a similar reduction to catchment-wide afforestation for both peak discharge and hydrograph volumes. The S-Upstream simulation reaches peak reductions of 8%, but this percentage decreases when precipitation events are long lasting, reaching a reduction of 3.3% for events of 5 days or more. On the other hand, downstream reforestation has minimal effect (1%) in reducing maximum discharge of events. The use of NbS-based strategies would improve integrated watershed management, reduce flood risk, and improve environmental governance.