A Study Of Flash Flood Occurrences At A Site Versus Over A Forecast Zone PDF Download

In Mediterranean Europe, flash flooding is one of the most devastating hazards in terms of loss of human life and infrastructures. Over the last two decades, flash floods have caused damage costing a billion Euros in France alone. One of the problems of flash floods is that warning times are very short, leaving typically only a few hours for civil protection services to act. This study investigates if operationally available short-range numerical weather forecasts together with a rainfall-runoff model can be used for early indication of the occurrence of flash floods. One of the challenges in flash flood forecasting is that the watersheds are typically small, and good observational networks of both rainfall and discharge are rare. Therefore, hydrological models are difficult to calibrate and the simulated river discharges cannot always be compared with ground measurements. The lack of observations in most flash flood prone basins, therefore, necessitates the development of a method where the excess of the simulated discharge above a critical threshold can provide the forecaster with an indication of potential flood hazard in the area, with lead times of the order of weather forecasts. This report is focused on four case studies in Mediterranean part of Europe: i) The September 2002- flash flood event in the Cévennes-Vivarais region in the Southeast of the Massif Central in France, a region known for devastating flash flood; ii) the August 2003-flash flood event in both Fella subcatchment of Tagliamento watershed and upstream part of Isonzo river basin, iii) the October 2006-flash flood event in Isonzo river basin and iv) the September 2007-flash flood event in Upper Sava river basin in Slovenia. The French case study is described in more detail with the principles and methodologies being explained that are then applied to the remaining three case studies. Also, there were more data available for the 1st case study. The critical aspects of using numerical weather forecasting for flash flood forecasting are being described together with the threshold - exceedance approach previously postulated for the European Flood Alert System (EFAS). The short-range weather forecasts, from the Local model of the German national weather service, are driving the LISFLOOD model, a hybrid between conceptual and physically based rainfall-runoff model. Results of the study indicate that high resolution operational weather forecasting combined with a rainfall-runoff model could be useful to determine flash floods more than 24 hours in advance.