Early Flash Flood Warning: A Feasibility Study with a Distributed Hydrological Model and Threshold Exceedance

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.JRC.H.7-Land management and natural hazard

EFAS Bulletins Yearbook 2007

The EFAS bulletins yearbook 2007 gives an overview for the year 2007 of EFAS events, news and EFAS-performance. It gives an overview of all external EFAS alert reports in 2007 as well as a quick overview of the new EFAS-IS web service. It collects all EFAS bulletins of this year.JRC.H.7-Land management and natural hazard

The Benefit of High-Resolution Operational Weather Forecasts for Flash Flood Warning

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 study is focused on the Cévennes-Vivarais region in the Southeast of the Massif Central in France, a region known for devastating flash floods. This paper describes the main aspects of using numerical weather forecasting for flash flood forecasting, together with a threshold - exceedance. As a case study the severe flash flood event which took place on 8¿9 September 2002 has been chosen. Short-range weather forecasts, from the Lokalmodell of the German national weather service, are used as input for the LISFLOOD model, a hybrid between a 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.JRC.H.7-Land management and natural hazard

The European Flood Alert System EFAS - Part 2: Statistical Skill Assessment of Probabilistic and Deterministic Operational Forecasts

Since 2005 the European Flood Alert System (EFAS) has been producing probabilistic hydrological forecasts in pre-operational mode at the Joint Research Centre (JRC) of the European Commission. EFAS aims at increasing preparedness for floods in trans-national European river basins by providing medium-range deterministic and probabilistic flood forecasting information from 3 to 10 days in advance, to national hydrometeorological services. This paper is Part 2 of a study presenting the development and skill assessment of EFAS. In Part 1, the scientific approach adopted in the development of the system has been presented, as well as its basic principles and forecast products. In the present article, two years of existing operational EFAS forecasts are statistically assessed and the skill of EFAS forecasts is analysed with several skill scores. The analysis is based on the comparison of threshold exceedances between proxy-observed and forecasted discharges. Skill is assessed both with and without taking into account the persistence of the forecasted signal during consecutive forecasts. Skill assessment approaches are mostly adopted from meteorology and the analysis also compares probabilistic and deterministic aspects of EFAS. Furthermore, the utility of different skill scores is discussed and their strengths and shortcomings illustrated. The analysis shows the benefit of incorporating past forecasts in the probability analysis, for medium-range forecasts, which effectively increases the skill of the forecasts.JRC.A.1-Work programme E

The European Flood Alert System - Part I: Concept and Development

Abstract This paper presents the development of the European Flood Alert System (EFAS), which aims at increasing preparedness for floods in trans-national European river basins by providing local water authorities with medium-range and probabilistic flood forecasting information 3 to 10 days in advance. The EFAS research project started in 2003 with the development of a prototype at the European Commission Joint Research Centre (JRC), in close collaboration with the national hydrological and meteorological services. The prototype covers the whole of Europe on a 5 km grid. In parallel, different high-resolution data sets have been collected for the Elbe and Danube river basins, allowing the potential of the system under optimum conditions and on a higher resolution to be assessed. Flood warning lead-times of 3-10 days are achieved through the incorporation of medium-range weather forecasts from the Deutscher Wetterdienst (DWD) and the European Centre for Medium-Range Weather Forecasts (ECMWF), comprising a full set of 51 probabilistic forecasts from the Ensemble Prediction System (EPS) provided by ECMWF. The ensemble of different hydrographs is analysed and combined to produce early flood warning information, which is disseminated to the hydrological services that have agreed to participate in the development of the system. In Part I of this paper, the scientific approach adopted in the development of the system is presented. The rational of the project, the system¿s set-up, its underlying components, basic principles and products are described. In Part II, results of a detailed statistical analysis of the performance of the system are shown, with regard to both probabilistic and deterministic forecasts.JRC.H.7-Land management and natural hazard

The European Flood Alert System - Part 1: Concept and Development

This paper presents the development of the European Flood Alert System (EFAS), which aims at increasing preparedness for floods in trans-national European river basins by providing local water authorities with medium-range and probabilistic flood forecasting information 3 to 10 days in advance. The EFAS research project started in 2003 with the development of a prototype at the European Commission Joint Research Centre (JRC), in close collaboration with the national hydrological and meteorological services. The prototype covers the whole of Europe on a 5 km grid. In parallel, different high-resolution data sets have been collected for the Elbe and Danube river basins, allowing the potential of the system under optimum conditions and on a higher resolution, to be assessed. Flood warning lead-times of 3-10 days are achieved through the incorporation of medium-range weather forecasts from the Deutscher Wetterdienst (DWD) and the European Centre for Medium-Range Weather Forecasts (ECMWF), comprising a full set of 51 probabilistic forecasts from the Ensemble Prediction System (EPS) provided by ECMWF. The ensemble of different hydrographs is analysed and combined to produce early flood warning information, which is disseminated to the hydrological services that have agreed to participate in the development of the system. In Part I of this paper, the scientific approach adopted in development of the system is presented. The rational of the project, the system¿s set-up, its underlying components, basic principles, and products, are described. In Part II, results of a detailed statistical analysis of the performance of the system are shown, with regard to both probabilistic and deterministic forecastsJRC.H.7-Land management and natural hazard

A near real-time procedure for flood hazard mapping and risk assessment in Europe

The availability of a real-time operational system for mapping flood hazard and assessing potential consequences might be extremely useful to help emergency response and management and to mitigate the impact of large flood events. This work describes the development of an experimental procedure for rapid flood risk assessment within the European Flood Awareness System (EFAS), which since 2012 provides operational flood predictions for the major European rivers as part of the Copernicus Emergency Management Services. The hydro-meteorological data set available in EFAS is used to derive long-term streamflow simulations and design flood hydrographs in a wide number of locations, covering all the major European river network. Flood hydrographs are then used as input to a hydrodynamic 2D model to create a high resolution dataset of areas at risk of flooding for different return periods. Whenever a flood event is forecasted in EFAS, the flood maps of the river network sections potentially involved are merged together, based on the estimated magnitude of the event. In order to take into account the different flood forecasts available in EFAS, different combinations of flood hazard maps may be produced, to highlight the possible range of uncertainty in predictions. The merged flood maps can be combined with the available spatial information about land use, population, urban areas and infrastructures, to assess the potential impact of the forecasted flood event in terms of economic damage, affected population, major infrastructures and cities. A preliminary version of the procedure has been successfully tested in reproducing flooded areas and impacts in the recent floods in Bosnia-Herzegovina, Croatia and Serbia. Moreover, the reduced computational times are compatible with near real-time applications, even in case of multiple flood events affecting several countries. Currently, the integration of the procedure within EFAS for operational use is being tested.JRC.H.7-Climate Risk Managemen

Evaluation of the satellite-based Global Flood Detection System for measuring river discharge: Influence of local factors

One of the main challenges for global hydrological modelling is the limited availability of observational data for calibration and model verification. This is particularly the case for real time applications. This problem could potentially be overcome if discharge measurements based on satellite data were sufficiently accurate to substitute for ground-based measurements. The aim of this study is to test the potentials and constraints of the remote sensing signal of the Global Flood Detection System for converting the flood detection signal into river discharge values. The study uses data for 322 river measurement locations in Africa, Asia, Europe, North America and South America. Satellite discharge measurements were calibrated for these sites and a validation analysis with in situ discharge was performed. The locations with very good performance will be used in a future project where satellite discharge measurements are obtained on a daily basis to fill the gaps where real time ground observations are not available. These include several international river locations in Africa: Niger, Volta and Zambezi rivers. Analysis of the potential factors affecting the satellite signal was based on a classification decision tree (Random Forest) and showed that mean discharge, climatic region, land cover and upstream catchment area are the dominant variables which determine good or poor performance of the measurement sites. In general terms, higher skill scores were obtained for locations with one or more of the following characteristics: a river width higher than 1km; a large floodplain area and in flooded forest; with a potential flooded area greater than 40%; sparse vegetation, croplands or grasslands and closed to open and open forest; Leaf Area Index > 2; tropical climatic area; and without hydraulic infrastructures. Also, locations where river ice cover is seasonally present obtained higher skill scores. The work provides guidance on the best locations and limitations for estimating discharge values from these daily satellite signals.JRC.H.7-Climate Risk Managemen

EFAS-Meteo: A European daily high-resolution gridded meteorological data set for 1990 - 2011

Data sets of spatially irregular meteorological observations interpolated to a regular grid are not only important for climate analyses but are also essential in order to derive climatologies for rainfall-runoff models which require meteorological data sets as input forcing. For example, in the European Flood Awareness System long term observed meteorological data are used to drive the hydrological model LISFLOOD to obtain long term time series of simulated discharges at a pan-European scale. Those long term time series of simulated “proxy” discharges can then be used for statistical analysis, e.g., to derive return periods or other time series derivatives. In this report, we present a comprehensive pan European high-resolution gridded daily data set (EFAS-Meteo) of precipitation, surface temperature (mean, minimum and maximum), wind speed, vapour pressure, calculated radiation and evapotranspiration (potential evapotranspiration, bare soil and open water evapotranspiration). The data set was created as part of the development of EFAS and has been continuously updated throughout the last years.JRC.H.7-Climate Risk Managemen