From reach to catchment-scale impacts: High-resolution hydrodynamic modelling of Nature-based solutions in the Cocker Catchment, UK

Cocker River catchment (145 km²), Lake District, Cumbria, United Kingdom. This study investigates the influence that Natural Flood Management (NFM) features have on flood behaviour at the catchment scale using a high-resolution, two-dimensional hydrodynamic modelling approach. A high-performance computing framework, based on the High-Performance Integrated hydrodynamic Modelling System (HiPIMS), was applied to simulate two events; 1) pre-NFM implementation - Storm Desmond flood in 2015, and 2) post-NFM implementation – High rainfall event in 2021. Leaky wooden barriers and other NFM features were explicitly represented using UAV-derived digital terrain data at 2 m and 4 m spatial resolutions. The simulations indicate that the hydrological response to NFM within the Cocker catchment is strongly event dependent. Clearer flow attenuation and hydrograph smoothing were observed during the smaller 2021 event, while impacts during the extreme 2015 event were modest and spatially variable. Localised water retention within the Whinlatter sub-catchment translated into small but measurable downstream changes in flood levels, alongside indications that delayed flows may interact with contributions from other tributaries. These findings highlight the importance of event magnitude, spatial configuration, and flow timing when assessing the role of NFM in catchment-scale flood risk management. • A novel representation of NFM features at the catchment-scale using a 2D hydrodynamic model (HiPIMS). • A single NFM site has a modest yet detectable impact on flood risk at the catchment scale. • Impacts are more pronounced immediately downstream, diminishing with distance downstream. • NFM interventions reduced 2015 peak levels by 0.44–0.70 %, and 2021 peaks by 1.30–3.93 %. • Local flood reductions may not translate to downstream reductions if flood peak synchronisation is not considered.