The current extensive floods in western Germany bring us to a logical comparison with floods that hit the Czech Republic in the recent couple of years. Let us make a comparison between the current floods in Germany with the tragic flooding of Novojičínsko in June 2009 and flooding of Liberecko in August 2010.
The first data about precipitation measured during the current catastrophic floods in Germany and other countries allow a limited comparison with data from floods that hit our country in 2009 and 2010. Wikipedia gives a cumulative average of 100 to 150 mm for the most intense precipitation in North-Rhine Westphalia and Rhineland-Palatinate. The Reicherscheid station measured 207 mm over 9 hours and the Cologne station measured 154 mm over 24 hours.
During the tragic June 2009 floods (11 victims) in Novojičínsko, Bělotín and Hodslavice stations measured more than 120 mm/day, of which 110 mm over three hours. This led to a 200 m3/s discharge on the Luha Watercourse from a catchment area of 100 km2, which was approximately three times the 100-year discharge (Q100). The Hejnice station in Liberecko recorded total precipitation of 179 mm/day during the August 2010 floods (5 victims). Similar values were recorded by several other rain gauges in Jizerské Mountains. Maximum discharges for instance in the Jeřice Watercourse in Chrastava reached 271 m3/s from an area of 76 km2, which meant approximately double Q100.
A comparison of precipitation totals from similar territorial morphology (highlands, submontane character) and the observed consequences shows that outflows of German watercourses also reached multiples of 100-year discharges. Such discharges are rather a geological force and it is possible to build neither flood control systems nor bridges and other crossings. Water (or rather a hydro mixture with a higher specific weight) is coming not only through stream beds but sometimes also directly from the terrain. A large quantity of branches and whole trees as well as pushed debris, especially gravel including the coarsest boulders flows in the rivers. The flood has an absolutely unstable cascade character when the river makes dams on bridges and other bottlenecks out of the floating objects. Subsequently, the watercourse can burst them and a new wave proceeds further or the river bypasses the barrier and erodes the terrain depending on its amount and inclination and creates new beds. This is in accordance with experience from floods on our territory as well as with documented consequences from Germany. However, there the precipitation hit a significantly larger area of thousands of km2. The Novojičínsko area that was hit in 2009 had an area of approximately 230 km2 and the affected area in Liberecko in 2010 amounted to approximately 500 km2. Extreme precipitation in western Europe affected a much larger area and hence also larger watercourses and therefore had significantly worse overall consequences.
Authors: Ing. Petr Březina (T. G. Masaryk Water Research Institute)