Development of surface water flood scenario catalogues for urban flood forecasting: A case study of le Jarret basin, Marseille.

The development of efficient urban surface water flood forecasting systems is particularly challenging with respect to accurately anticipating the space-time location of heavy precipitations, and rapidly evaluating flood probabilities to issue timely alerts. This study presents an approach based on a pre-computed catalogue of flood inundation scenarios. This approach can serve as an intermediate alternative between basic rainfall threshold-based approaches, and computationally intensive real-time hydraulic simulations. The construction of the catalogue of flood scenarios is illustrated using the Jarret River basin in Marseille, France as a case study. The methodology uses a nine-year (2014-2023) radar rainfall reanalysis with 15-minute temporal and 1-kilometer spatial resolution to define a panel of representative rainfall hyetograph shapes for two-hour convective rainfall events of different return periods. A Telemac 2D hydrodynamic model via the CARTINO 2D approach is then used to obtain the flood scenarios related to each hyetograph. Two approaches are developed to build the hyetographs: (1) a temporal pattern analysis resulting in the distinction of three characteristic hyetograph shapes (short triangle, long triangle, rectangular), and (2) a monofrequency method using triangular hyetographs with consistent return periods across 15min to 2h durations, combined with a spatial attenuation according to the drainage areas impacted by each rainfall duration (cf. concentration times). Both approaches are applied for five return periods (5, 10, 20, 50, and 100 years) under three antecedent moisture conditions, to generate flood catalogues including 45 and 15 scenarios respectively. The resulting catalogues demonstrate the significant influence of temporal rainfall variability on inundation patterns over small catchment areas. As a next step, both approaches will be integrated in an experimental forecasting chain, and be evaluated through the reanalysis of past events. These predefined flood catalogues offer a practical framework for rapid flood response in urban areas exposed to high-intensity, short-duration rainfall events.