Use of Spatially Distributed TOPMODEL to Assess the Effectiveness of Diverse Natural Flood Management Techniques in a UK Catchment

While natural flood management (NFM) is becoming more widely used, there remains a lack of empirical evidence regarding its effectiveness. The primary uncertainties arise from two key aspects: first, the determination of NFM effectiveness is constrained by the relatively small catchment scales studied to date; second, the combination of multiple NFM interventions within a catchment may lead to flood peak synchronisation. In this study, both instream and terrestrial NFM interventions were modelled using a spatially distributed hydrological model, Spatially Distributed TOPMODEL (SD-TOPMODEL). To demonstrate how the scale and type of interventions interact to influence flood peaks, we integrated various NFM interventions and land cover changes, including woodland planting, soil aeration, floodplain restoration, and hedgerow planting. In comparison to previous versions of SD-TOPMODEL, we improved simulation efficiency to enable grid-based modelling of up to a 200-year return period flood event for an 81.4 km² catchment with 5 m resolution. Following extensive parameter calibration and validation, the model demonstrated stability and provided a reliable fit for flood peaks, achieving a Nash-Sutcliffe Efficiency coefficient of up to 0.93 between modelled and observed discharge. The results highlighted the effectiveness of NFM interventions in reducing flood peaks at the scale studied, particularly during single-peaked storm events and under dry pre-event catchment conditions. Moreover, the combined use of multiple interventions was more effective and resilient than single interventions, with flood peak reductions ranging from 4.2% to 16.0% in the study catchment.