Estimating critical rainfall for flash flood warning systems using integrated hydrologic-hydrodynamic modelling

Flash floods are one of the most severe natural hazards worldwide; they can occur within a few minutes or hours, and can move at high flow velocities, striking with violence and little warning. Early warning of flash floods is extremely important for vital risk mitigation and requires the knowledge of the critical rainfall producing flooding that is typically considered as “warning index”. The small spatial and temporal scales at which flash floods occur make the prediction of critical rainfall challenging, particularly in data-poor environments, where high-resolution weather models and advanced monitoring networks may not be available.

In this research, we present a methodology to estimate the critical rainfall for flash flooding based on an integrated hydrologic-hydrodynamic model. The model is applied in the Lilantas River catchment in Evia, Greece, considering a relatively large number of rainfall and soil moisture conditions scenario combinations in order to (1) determine inflow hydrographs used as boundary conditions for the hydrodynamic model and (2) calculate the distribution of “critical hazard” across the cells of the two-dimensional (2D) computational domain. In the present work, we define critical hazard combining the main hydrodynamic characteristics that are water depth and flow velocity, and we import all calculated “critical hazard” values into a GIS-based database.

Key findings include maximum peak discharges from all simulated scenarios, allowing a sensitivity analysis of varying Curve Number and soil moisture conditions, as well as the effects of rainfall duration and intensity combinations on flood responses. Furthermore, based on the calculated critical hazard, estimates of critical rainfall values for the selected study area are provided, along with an example of the flood warning system’s operation.