Assessing the impacts of natural flood management on downstream flood risk: a case study of the River Aire

Natural flood management (NFM) offers a potentially sustainable solution to mitigating flood risks, yet its effectiveness at larger catchment scales and under varying rainfall intensities remains uncertain. This thesis combined a quantitative meta-analysis of 454 NFM studies with hydrological (SD-TOPMODEL) and coupled hydrological-hydrodynamic (SD-TOPMODEL to Flood Modeller 2D) modelling in the River Aire Catchment (81.4 km2), to address research gaps. The meta-analysis revealed a shift in focus over time from localized flood mitigation to catchment-wide strategies, accompanied by increasing emphasis on ecosystem services. Contrary to previous findings, the effectiveness of NFM interventions were not found to diminish with increased catchment size nor rainfall intensity. Using SD-TOPMODEL, this research simulated diverse NFM interventions and their combinations at a high resolution (5m), including woodland planting, soil aeration, and hedgerow planting, achieving 4.2-16.0% flood peak reductions. NFM interventions were found to be most effective during single-peaked storm events and dry antecedent conditions. Combined interventions demonstrated greater resilience and effectiveness than single interventions in the catchment, emphasising the importance of integrated flood management strategies. A coupling of SD-TOPMODEL with Flood Modeller 2D was used to investigate downstream impacts of NFM scenarios (combining leaky dams, riparian buffer strips and other interventions mentioned above) and the maximum catchment woodland scenario, revealing changes in inundation extent and depth. An unintended inundation increases for a large village occurred under a widespread woodland creation scenario highlighting the importance of integrated and targeted NFM planning at the catchment scale. This thesis concludes that storm characteristics and pre-event conditions play a more significant role in determining NFM efficacy than catchment size or rainfall intensity. The findings emphasise the need for strategically designed, catchment-scale intervention networks and advocates for modelling approaches as essential tools for optimizing NFM planning and ensuring its effectiveness in wider flood risk management.