System Explanation of Floods in Central Europe

As a result of the moderate climate in the middle latitudes in Central Europe floods can occur at all times of the year. Since the Alps with their central location in Europe supply the most important river systems of the neighboring countries (e.g. Rhine, Rhone, Danube, Po), they also play the key role for floods occurring in these rivers. Furthermore, the Alps form a climatic barrier between the temperate climate of Central and Northern Europe and the Mediterranean climate of Southern Europe. Due to this constellation the Alps have a major significance in two respects. Firstly, extreme weather situations arise from the different climates north and south of this mountain range. Secondly, the main river systems are exposed to these extremes. While floods may be caused by storms (storm tides, hurricanes) or seismic activity (tsunami) in coastal regions, in inland waters like rivers or lakes they are mainly caused by unusual precipitation events. When further parameters and processes add to these events (snow melting, frozen or saturated grounds, ice packing on rivers), this will lead to extreme floods. These hydro-meteorologically induced floods can be classified into three categories:

1. Extensive floods caused by persistent precipitation in the context of certain quasi-stationary cyclical weather conditions (e.g. Vb).
2. Extensive floods caused by frequent precipitation over a longer time interval (cyclical weather conditions) in combination with snow melting (winter floods).
3. Locally bounded floods caused by short-term stationary heavy precipitation events (e.g. summer thunderstorms), especially at smaller stream courses.

Most floods in Central Europe occur during the later months of winter, because during this time either the grounds are sealed (frozen or saturated) or large amounts of water are already stored in snow or ice. Moreover, zonal atmospheric conditions (westerly circulation patterns) predominate, which produce an increasing succession of low pressure areas combined with precipitation. This coincidence of various factors for example led to two exceptional large floods of the Rhine River and its tributaries in the 1990's (type 2; e.g. Fink et al., 1996).

Figure1: Typical drift of the cyclones after van Bebber (Figure: ZDF)

Certain general weather situations may be solely responsible for causing large inundations (type 1). Thus, especially regions northeast of the Alps are susceptible to floods due to the so-called Genoa-cyclogenesis (Vb-weather conditions, cf. Figure 1). Common consequences of this weather condition are events of heavy precipitation north of the Alps as moist and warm Mediterranean air meets much cooler air from the northern side of the Alps at this point. In addition to that intensifying effects are triggered by the orography of the Alps, the Carpathian Mountain Range and the northern adjacent highlands. According to this causation the last large flood events in Central Europe, such as the events at the Oder (1997, e.g. Gruenewald et al., 1998), Elbe (2002, e.g. Rudolf and Rapp, 2003; James et al., 2004) and or in the Bavarian lower Alps region including the Danube River (1999/2005, e.g. Rudolf et al., 2005) were induced. This general weather situation occurs especially in early and midsummer as warm Mediterranean air streams to Central Europe. A cyclogenesis very similar to the Genoa-Low (general V-weather conditions, cf. Figure 1) may also lead to disastrous precipitation events on the southern side of the Alps. One such constellation resulted in intense floods and landslips in Piedmont (northeast Italy) in 1994 (Buzzi et al., 1998). The amount of precipitation was to some extent more than 300mm/36h, which inevitably led to high tides of the Po River.

In individual cases convectional rain events can be long lasting and show high intensity resulting in local floods. Such showers especially occur during transition seasons and summer months in combination with elevated cold air advection. The meteorological causes for these locally bounded flood events (type 3) are typically large vertical gradients in temperature and wind shears on the one hand and a quasi-stationary situation of the atmospheric disturbance on the other. As a consequence such extreme showers remain nearly on the spot. Again, the orography represents a considerable factor causing such events to happen mostly in the highlands. Alone for this reason precipitation rates may reach 50mm and more within a few hours. In most cases these rainstorms are combined with heavy thunderstorms and hailstorms.

Besides the development of the precipitation also the hydrological characteristics and actual state of the catchment area are of importance for the intensity of floods. As a general rule, the following rule applies between the river-size respectively the size of the catchment area and the high water drift: the larger the catchment area, the longer the duration of the flood event and the temporal offset between the causative rainfall event and the appearance of the flood. Thus, floods in creeks and small rivers close to the river head (i.e. with a small catchment area) are characterised by faster runoff formation after heavy rainstorms and flood waves generated during or directly after the precipitation event. These flash floods cause the water level to rise drastically within a few hours, whereas the whole event often lasts less than one day. The inundations caused by flash floods are mostly less extensive, however the damages caused by high flow velocity are often very significant. This phenomenon occurred during the floods of the Erz Mountains Rivers in August 2002 (IKSE, 2004). The short early warning time that remains for disaster management authorities to inform the affected population is problematic in such a case.

Figure 2: Normalised hydrograph of selected water level gauges for the rivers Elbe and Mulde during the flood in August 2002.

However, floods occurring within the great lowland rivers are characterised by slow rising peak waves and thus a comprehensive early warning time. This can be explained firstly by the long flow distances from the upper reaches where the heavy precipitation events commonly occur, and secondly by the flat topography and thus the slow runoff formation in the adjacent part of the catchment area. However, the duration of these events is significantly longer compared to upstream sections of the river. Floods at the Lower Rhine or Lower Elbe river may last several weeks depending on duration, intensity and extension of the causing precipitation. Figure 2 illustrates this fact by comparing normalised hydrographs of different gauge stations of Elbe and Mulde River during the flood event in August 2002. The fast rise and descent of the flood wave can be clearly seen at the gauge stations Nossen (Freiberg Mulde) and Golzern (upstream of the Vereinigte Mulde). However, the Elbe gauges Meissen and Magdeburg show a slower rise, but also a considerably longer duration of the event. The long continuance of these events poses a substantial danger to effective flood precaution arrangements, especially to older dykes. Because of the continuous hydraulic stress the dykes can get soaked or washed out and may eventually break. Due to the runoff amounts and the topography flood areas are mostly of a large extension in both cases, for naturally occurring inundations of lowland rivers and for floods caused by dyke damages. This could be observed e.g. at the Oder river event in 1997 (Gruenewald, 1998) or the Elbe flood in 2002 (IKSE, 2004; Munich Re, 2003). The damages caused by extensive floods underlie many influencing factors, which either have a direct effect on the extent of the inundation area (e.g. dyke safety arrangements, dyke heightening) or reduce the vulnerability of the affected areas (e.g. early warning, securing of buildings with sandbags, experience in dealing with floods, technical flood protection in buildings) (Kreibich et al., 2005a, 2005b).

References

• Buzzi, A., N. Tartaglione and P. Malguzzi, (1998): Numerical Simulations of the 1994 Piedmont flood: Role of Orography and Moist processes, Mon. Weath. Rev., 126, 2369-2383.
• Fink, A., U. Ulbrich and H. Engel, (1996): Aspects of the January 1995 flod in Germany, Weather, 51, 34-39.
• Grünewald, U. et al., (1998): Ursachen, Verlauf und Folgen des Sommer-Hochwassers 1997 an der Oder sowie Aussagen zu bestehenden Risikopotentialen, Deutsches IDNDR-Komitee für Katastrophenvorbeugung e.V., Deutsche IDNDR-Reihe 10b, Bonn.
• IKSE (Internationale Kommission zum Schutz der Elbe) (2004) Dokumentation des Hochwassers vom August 2002 im Einzugsgebiet der Elbe. IKSE, Magdeburg.
• James, P., A. Stohl, N. Spichtinger, S. Eckhardt and C. Forster, (2004): Climatological aspects of the extreme European rainfall of August 2002 and a trajectory method for estimating the associated evaporative source regions, NHESS, 4, 733-746.
• Kreibich H, Thieken AH, Müller M, Merz B (2005a) Precautionary measures reduce flood losses of households and companies - Insights from the 2002 flood in Saxony, Germany. In: van Alphen J, van Beek E, Taal M (eds.): Floods, from Defence to Management. Taylor & Francis Group, London, 851-859.
• Kreibich H, Thieken AH, Petrow T, Müller M, Merz B (2005b) Flood loss reduction of private households due to building precautionary measures - Lessons Learned from the Elbe flood in August 2002. NHESS 5: 117-126.
• Rudolf, B. and J. Rapp, (2003): The Century Flood of the River Elbe in August 2002: Synoptic Weather Development and Climatological Aspects, Quarterly Report of the German NWP-System of the Deutscher Wetterdienst, No. 2, Part 1, 8p.
• Munich Re (2003) Topics Annual Review: Natural Catastrophes 2002. Munich Re Group, Munich.
• Rudolf, B., H.Frank, J. Grieser, G. Müller-Westermeier, J. Rapp and W. Trampf, (2005): Hydrometeorologische Aspekte des Hochwassers in Südbayern im August 2005.