Towards national-scale hydrodynamic flood modelling: feasibility, resolution sensitivity, and computational trade-offs

Surface water flooding (SWF), when intense rainfall overwhelms drainage systems and inundates streets, homes, and infrastructure, is the most widespread form of flooding in the UK and a rapidly escalating global hazard under climate change and growing urban exposure. In England alone, around 3.2 million properties are at risk. In the extreme cases, SWF can develop rapidly with little or no warning and exhibit highly dynamic, fast-moving flow conditions, behaving like an “inland tsunami”, with debris-laden flood waves overwhelming streets, vehicles, and buildings within minutes. It can paraly transport, disrupting essential services, and, in some cases, cause catastrophic loss of life. Recent events have highlighted the deadly consequences of such flooding, including the July 2021 Zhengzhou (China) flood with nearly 400 fatalities, the 2021 floods in Germany and Belgium with over 200 deaths, the October 2024 flash floods in Valencia, Spain causing 237 fatalities, and widespread cyclone- and monsoon-driven flooding across South and Southeast Asia in 2025 causing more than 1,000 deaths and displacing millions.

At large spatial scales, SWF does not occur as isolated local events. Intense rainfall may occur simultaneously or sequentially over wide areas, and interconnected river networks, drainage systems, and infrastructure can couple multiple local flood processes into a single, spatially extensive flood system. Understanding and predicting such large-scale, interacting flood dynamics is therefore essential for both national-scale risk assessment and real-time forecasting.

Numerical modelling provides an indispensable tool for representing SWF processes. However, due to their highly transient, shock-like behaviour, hydrological or simplified hydraulic approaches are often insufficient. Fully hydrodynamic models solving the two-dimensional shallow water equations with shock-capturing capability are required, but their computational cost has historically limited their application to city or local-catchment scales. Scaling such models to regional or national extents is not a simple domain enlargement problem, but introduces coupled challenges related to computational demand, terrain resolution, and modelling strategy. As a result, fundamental questions remain regarding the feasibility of national-scale hydrodynamic modelling, the computational resources required, the sensitivity of flood hazard metrics to DEM resolution, and the trade-offs between alternative large-scale simulation strategies.

To address these questions, we conduct a national-scale hydrodynamic flood modelling experiment over England using the High-Performance Integrated hydrodynamic Modelling System (HiPIMS) accelerated by multi-GPU computing. Event-based simulations are performed over the England at DEM resolutions of 10 m, 20 m, and 40 m to systematically quantify resolution effects on flood hazard representation and associated computational costs. The experimental design also enables comparison between alternative national-scale modelling strategies, including domain-wide versus partitioned simulations.

The results delineate the practical feasibility limits, resolution sensitivity, and performance trade-offs of national-scale hydrodynamic flood modelling, and provide quantitative guidance on the computational and data requirements for moving towards national-to-street-scale, physics-based surface water flood forecasting and risk assessment.