Wall of Waters: Identification of realistic worst-case scenarios for extreme rainstorms and simulation of hazard attributes of resulting flash floods

Extreme rainfall storms can generate such extreme flash floods that the so-called wall of water phenomenon occurs, which is an immensely fast rise of a wavefront through a street canyon or a natural water course often causing particularly devastating flood damages. This study presents a categorisation method to identify reasonable worst-case rainstorm events from station data that result in catastrophic flash flood events. The identified 24 events were shown to be considerably more extreme than typically employed design storms with a return interval of 100 years. In the selection of the extremes, special care was taken to consider different hazard attributes, which are associated with intrinsically different observed hyetograph shapes, including rectangular, short and long triangle, and double peak shape categories. Hydrodynamic simulations were carried out using the 2D robust shallow water model hms ++ to quantify the flooding impact of the 24 rainfall events in a small urbanised catchment. The simulated hazard attributes showed clear differences as a function of hyetograph shape with strong variations in peak discharges, peak water depths, hazard velocities and flashiness (shortest time to crest). The use of real heavy rainfall series is therefore essential for the generation and further analysis of wall of water events.