Super-fast flash flood simulation using steady-state flow solvers

Flash floods are a rapid burst of flood water that can cause extreme damage to populated areas. The European floods in France, Belgium, Germany and the Netherlands in the summer of 2021 featured a wide range of flash floods with a large number of casualties and vast financial damage. Reflection on the risk reduction strategies have reemphasized the need for early warning systems in the upstream catchments of North-Western Europe. For applications such as this, the speed of flow simulations is critical, as the quality of real-time forecasting often depends on the frequency and amount of simulations that can be carried out as new weather forecasts come in. We present a new type of flood hazard model that, in many typical cases, solves flash flood hazard a 100 times faster with similar accuracy. The developed method employs steady-state solvers for diffusive wave water flow equations to skip the dynamical process and directly estimate relevant parameters such as maximum flow height, maximum flow velocity and relative arrival time of the flood water. These paramters are often the most important for warning systems and descision making in risk reduction. Our adapted algorithm improves upon traditional steady-state flow solvers by employing inversed flow accumulation results and compensation for partial steady-state flow. We show the accuracy of the method is similar to full dynamic water flow simulation in many types of events, such as the extreme 2003 floods in the Fella Basin (Italy), Hurricane-induced flooding on Dominica and the flood impact in Limburg in 2021 (The Netherlands). On average, with highly similar accuracy, calculation time was reduced from approximately 6 hours to 2.5 minutes. We further investigate the limits of the developed methods, in particular to practical applications in different type of flood events. While the sensitivity of the model to initial conditions is similar to that of regular flood models, the sensitivity of the hydraulic aspects is lower. Finally, we discuss potential usage for early-warning, spatial descision support systems and serious gaming approaches. While further investigation is required to fully validate the method, a break-through in flood hazard assessment could be on hand.