Simulation of an urban flash flood: the 2017 flood event in Mandra, Attica

This study deals with the 2D flood simulation of an urban flash flood event that took place on November 15, 2017 in Mandra, Attica. The well-known hydrodynamic software HEC-RAS was used for the flood simulation. The model input was an ensemble of 100 hydrographs, derived from the simulation of the rainfall-runoff process of the Agia Aikaterini catchment, which flows into Mandra. The study focuses on a grid search-based calibration of the model parameters, by comparing simulation results to post-flood field data. A Morris-based sensitivity analysis was used in order to reduce the number of parameters to be calibrated and therefore reduce the dimensions of the problem and the computational cost. Five parameters were selected for the sensitivity analysis: a) the Manning coefficient of the city roads; b) the Manning coefficient of urban blocks characterized by low roughness; c) the Manning coefficient of urban blocks characterized by high roughness; d) the confidence interval of the empirical distribution of the 100 hydrographs ensemble; e) the energy slope used for the downstream boundaries. It was found that the parameters with the most significant impact were the input hygrograph’s confidence interval (first) and the Manning coefficient of the city roads (second).