Abstract
Abstract. In Italy, growing urbanization is leading to a higher risk of flooding of small water courses, especially in steep catchments of limited area, where severe flash flood events can occur. The assessment of flash flood hazard requires new modelling tools that can reproduce both the rainfall–runoff processes in the catchment, and the flow processes in the drainage network. In this paper we propose the use of a simple two-dimensional hydraulic model for analysing a flood scenario in a small valley within the urban area of the city of Bologna, Italy. Historically this area has been prone to severe flood events, the most recent of which occurred in 1955 and 1932. Since then there has been a significant increase in urbanization of the lower portion of the catchment, while the natural stream bed has been partially replaced by a culvert. The two-dimensional hydraulic model was therefore applied at catchment scale, in order to simulate the possible effects of historical scenarios in the present catchment configuration. Rainfall and runoff data measured during recent rainfall events were used to calibrate model parameters. Model results show that the current culvert section would be insufficient to drain the runoff produced by intense rainfall events, with potential inundation of surrounding urban areas.
Highlights
The risk of flooding of small water courses is a major problem in several urban areas in Italy
Historical and recent records report that the urban areas of Bologna located downstream of the high and are prone to severe flood events caused by small water courses
Model results suggest that the vulnerability of the urbanized areas to flash flood events has significantly increased during the last 50 years, due to the increased urbanization, and the replacement of the natural stream bed with a culvert
Summary
The constant growth of urbanization, with the consequent decrease of soil permeability and loss of space for river and stream beds, is leading to increased flood hazard and vulnerability (Hapuarachchi et al 2011) In such conditions, severe rainfall events over steep catchments of limited area can produce dramatic consequences, such as occurred in November 2011, when the city of Genoa and several towns and villages in northern Italy were hit by catastrophic flash floods. The risk flooding of small water courses in urban areas is often underestimated and few measures are taken for prevention and mitigation For this purpose, the use of innovative hydrological and hydraulic models may be useful for analysing the complex rainfall– runoff processes in the catchment and the consequent flow processes in the drainage network, and for providing a quantification of the hazard and vulnerability components of flood risk. We propose the use of a simple two-dimensional (2D) hydraulic model to simulate a reference flood scenario in an urbanized small catchment, considering historical intense rainfall events and analysing the possible effects in the present catchment configuration
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