Simulation of flash floods in ungauged basins using post-event surveys and numerical modelling

The hydrological and hydraulic processes of the flash flood that affected Atrani village (Amalfi Coast of Southern Italy) on 9 September 2010 are analysed using post-flood surveys and numerical modelling. Observations in this case are particularly challenging because of the small spatial and temporal scales of the event, relative to the sampling characteristics of conventional rain and discharge measurement networks. Given these aspects, some of the relevant characteristics of the event were deduced by analysing field data that were collected immediately after the event. A width function instantaneous unit hydrograph (WFIUH) rainfall-runoff model is applied to derive a flow hydrograph for the area immediately upstream of the village. This hydrograph is inputted as an upstream boundary condition for the simulation of the propagation processes. A two-dimensional innovative hydraulic model is used to reproduce the flood within the village. The use of an unstructured mesh enabled a detailed simulation of water flow within the narrow and winding alleyways of the village. The results of the simulations indicate agreement with the field observations. The implemented approach is suitable for simulating flash floods in similar contexts and, therefore, may be used to predict potential future events.