As a joint contribution to the realization of the Flood Risk Management Plan for the International Rhine basin (measure 4.3 (2)), the Flood Forecasting Centres along the Rhine organized on 25 and 26 June 2019 a workshop on Flood Early Warning at the office of the secretariat of the International Commission for the Protection of the Rhine (ICPR) in Koblenz. The workshop was attended by about 20 participants from 13 different forecasting centres in 6 countries of the Rhine basin. The aim of the workshop was to learn from each other through the exchange of information and experience about the national state of affairs and plans for the future on (hydrological) early warning and early warning systems, flash floods and extreme precipitation events.
The representatives of the forecasting centres presented their daily work, ongoing projects and expected developments, but also limitations in early warnings. In addition, some examples of European projects and projects from other river basins were also presented. The purpose was to find common interests and synergies in the daily work and plans for the future of the different forecasting centers and to avoid overlaps.
Local forecast requirements in Switzerland and Austria
The representative from Switzerland presented the efforts in the field of nowcasting for Alpine rivers where a lead time of 6 hours is considered as maximum. A 2 hours lead time would already be off great benefit. At present, the Swiss authorities do not provide a nowcasting product. A prototype of a nowcasting system for 2 or 3 regions is under development.
The situation in the Austrian part of the Rhine basin is similar to the ones in Switzerland; the river basins are small and steep and react extremely fast on precipitation. Therefore, early warning depends highly on precipitation forecasts. However, the forecasting model for the Rhine catchment rivers in Austria is not very good in forecasting flash floods and experiences from other areas are needed.
Pilot studies and new systems in Germany
In Bavaria, a pilot study for early warning in small catchments is currently conducted. The model that is used is the grid based LARSIM model with a 0.7 km resolution. The model is driven by a combination of radar observations, radar tracking and an EPS weather forecast.
In the federal state Rhineland Palatinate, efforts are made to improve early warnings for smaller river by using radar precipitation data. At present new model approaches are tested to simulate infiltration processes during extreme precipitation events on dry soil.
In the federal state of Saxony in Germany extreme precipitation events have caused much damage in the past years. Up to 150 mm of precipitation in a period of 12 hours have been observed during these events. A new forecasting system for catchments smaller than 200 km2 has been developed which provides forecasts with a lead time of 24 hours. It has been concluded that in some extreme cases numerical weather forecasts cannot predict the exact location, time and intensity of an extreme precipitation event. Further, precipitation is in some cases detected by the radar only 15 or 30 minutes before the flood event. Early warning may extend the warning time but also the uncertainty.
Forecasting systems in France
The representative from the French DREAL Grand Est presented the Vigicrues Flash system, a new operational flash flood warning system that was implemented in France in March 2017. The warning system combines radar-gauge rainfall grids with a simplified distributed rainfall-runoff model for flash flood warnings at ungauged locations.
A presentation on early warning on a pan-European scale was given by Eric Sprokkereef from the EFAS Dissemination Centre. An overview of EFAS and GLOFAS focusing on the existing functionality, recent developments and expected innovations was presented. In connection to the subject of the workshop, special attention was given to the flash flood part of EFAS. Participants concluded that flash floods on a European scale are something completely different from flash floods on a regional scale. In the EFAS system we give information for “flash floods” with a lead time up to 3 days ahead, whereas the regional services are very happy when the can provide a forecast for an event with a lead time of 6 to 12 hours. Participants discussed the meaning of the word “Early” in Early Warning and it was concluded that the earliness depends highly on the size and characteristics of the catchment warnings are provided for.