WW-I-3: Floodwater

The picture shows an aerial view of a landscape that has been flooded over a large area. The settlement areas are completely under water.Click to enlarge
Floodwater results from heavy or long-term rainfall or snowmelt.
Source: Photograph: © mb67/stock.adobe.com

2019 Monitoring Report on the German Strategy for Adaptation to Climate Change

Table of Contents

 

WW-I-3: Floodwater

The time series for the floodwater regime is characterised by individual repeated floodwater events both in the winter half-year and in the summer half-year. It was not possible to discern any significant trends. Subject to weather constellations, there are spatial focal points where floodwater occurs. However, as a rule several river basins are affected.

The stack column graph shows the mean number of flood days in the hydrological winter half-year (November to April) and in the hydrological summer half-year (May to October) from 1961 onwards, differentiated for the Danube, Rhine, Weser, Elbe and Eider/Schlei river basins. The development in the Danube river basin shows a quadratically decreasing trend in the hydrological winter half-year, all other data series are trend-free. There are strong fluctuations between the years.
WW-I-3: Floodwater

The stack column graph shows the mean number of flood days in the hydrological winter half-year (November to April) and in the hydrological summer half-year (May to October) from 1961 onwards, differentiated for the Danube, Rhine, Weser, Elbe and Eider/Schlei river basins. The development in the Danube river basin shows a quadratically decreasing trend in the hydrological winter half-year, all other data series are trend-free. There are strong fluctuations between the years.

Source: Run-off measurement of federal states
 

Repeated floodwater events

Compared to the fluctuations and changes in mean runoff values, there is greater awareness among the public of floodwater events because they have an immediate impact on human activities as they can cause personal injury and material damage.

The time series starting in 1961 clearly indicates that the floodwater regime varies from year to year. This is true for both the extent of floodwater events and for seasonal distribution. Floodwater days distributed across 79 of Germany’s river basins were subjected to evaluation. Floodwater days are days on which the mean daily run-off value is higher than the mean floodwater run-off (MHQ) calculated for the reference period 1961 to 1990. The MHQ is calculated separately for the hydrological winter half-year (November of the previous year to April) and the summer half-year (May to October) on the basis of the highest run-off values (HQ) respectively.

When averaging the number of floodwater days for all levels observed in a river basin, it becomes clear at which locations the floodwater events were concentrated in which years. Floodwater events can be triggered by regionally limited weather constellations. For the summer season this typically refers to rain that continues for several days and to heavy-rain events which can be very localised. In the winter season floodwater is frequently caused by thaw in connection with rainfall, because under these circumstances run-off from major amounts of meltwater may occur within a few hours.

As far as summer floodwater events are concerned which occurred since the turn of the millennium, the years 2002 and 2013 stand out. In August 2002 floodwater in Germany was particularly prevalent in the Elbe and Danube areas. These events were caused by extreme rainfall continuing for days, which resulted in weeks of emergency operations in order to overcome the ensuing flood disaster. Likewise, floodwater events at the end of May and in early June 2013 were triggered by several days of rainfall. Apart from Germany and Austria, other countries in central and eastern Europe were particularly badly affected. The month of May 2013 is considered to be one of the periods in which precipitation was highest since meteorological records began. In 2017 the low pressure front Alfred led to several days of rain resulting in floodwater in the region of the Harz mountains and foothills. Likewise, the Weser river basin was one of the most heavily affected.

The most recent high floodwater event occurred in January 2011, with focal points again in the Elbe and Main areas, but other river basins were also affected. The floodwater was preceded by a December month with comparatively high precipitation while, at lower geographical level, substantial amounts of snow accumulated. This meant that considerable amounts of water equivalent were stored in the blanket of snow when suddenly a low-pressure area coming in from the Atlantic triggered a strong thaw in the second week of January, which led to rapid melting of the snow cover, even in the mountains. This period of thaw was immediately followed by several rain fronts with prodigious precipitation.

As indicated by the time series so far, the development of floodwater days does not show any significant trends either for the summer or for the winter season. The development of floodwater is always related to specific combinations of weather conditions which have so far not occurred either systematically, regularly or repeatedly. The distribution of floodwater days impacting the hydrological winter and summer seasons has so far not indicated a trend either. The events occur in both halfyear periods, albeit a little more frequently in winter.

Climate change cannot be accounted for by a single floodwater event. Atmospheric conditions and largescale weather patterns which favour the occurrence of floodwater suggest a wide-ranging variability. Warming enables the atmosphere to store fundamentally more water vapour, i.e. absorb moisture, thus increasing the potential for heavy rainfall. Westerly winds might increase in winter; likewise the frequency and intensity of so-called Five B (Vb) weather front trajectories might increase in summer. Under these kind of weather conditions, low-pressure areas shift to Central Europe from the Mediterranean where they are charged with water vapour. Often these low-pressure areas bypass the Alps und then deposit their rain in the eastern uplands and the eastern foothills of the Alps. The weather conditions which cause the Five B trajectories can remain stationary for a long time causing continuous rain and even heatwaves.

Apart from climate change, there are however numerous other developments which affect the phenomenon of floodwater events. Increasing sealing and compaction of the soil in catchment areas, as well as barriers to natural flood plains and embankments result in higher run-off into rivers.

 

Interfaces

VE-I-1: High-water closures to shipping on the Rhine

FiW-I-2: Incidence of storms and floods

 

Objectives

Protection from increasing floodwater risks in riverbasins (DAS, ch. 3.2.14).

Designation of floodwater areas and creation of retention areas (WHG, §§76 (2), 77).