Derivation of rainfall thresholds for pluvial flood risk warning in urbanised areas

In the recent past throughout the Mediterranean area, many extreme events such as floods, debris flows and landslides occurred. Mediterranean ephemeral streams have specific features compared to other river systems; their basins are small and highly torrential and may generate flash-floods (Camarasa-Belmonte & Soriano-Garcia, 2012). Moreover, the rapid transformation processes of urban areas induced the increase of catchment imperviousness and the derived increase of surface runoff generated during rainfall events. However, flooding events in urban areas occur quite frequently as a consequence of rain events of lower intensity than the design one, even in case of correct network dimensioning. The use of a reliable flood forecasting model in urban areas can play an important role in managing land and water resources. The purpose of this work is the development of a Decision Support System (DSS) for flash flood warning in an urban area. Usually, flood warning systems are based on on-line hydrological and/or hydraulic models in order to provide forecasts of water stages or discharges at critical river sections (Martina et al., 2006; Diakakis, 2012; Wu et al., 2015). This procedure is inappropriate for flash flood warning in urban areas or in catchments with a small area. According to the approach proposed by [Amadio et al., 2003; Wu et al., 2015], in this study the rainfall threshold has been estimated in an urban area by coupling results of hydro-dynamic model in terms of water stage and flooding area. Particularly, dependency of the antecedent soil moisture conditions has been neglected because urban areas are characterized by imperious surfaces This study proposes a methodology to point out in urban areas rainfall thresholds used in flash flood warning which should be influenced by the uncertainties in the rainfall characteristics, including rainfall duration, depth and storm pattern. Particularly, the methodology here developed has a modular structure consisting of different modules: synthetic hyetographs definition to gain the hydrological input to the hydraulic model; transformation of flood discharge to inundated area through a two-dimensional hydraulic model the FLURB-2D model (Aronica & Lanza, 2005) and, finally, quantification of threshold rainfall associated with specific inundation criteria.