Floods

The discharge of a stream fluctuates naturally over the course of the year. In Germany, natural runoff is mainly determined by rainfall and snowmelt in the Alps in summer. Evaporation also plays a role in summer. On average, high runoff occurs in winter and spring, as the snow falls as rain due to the higher temperatures and the snow melts in the low mountain ranges due to the higher temperatures. In late summer and fall, on the other hand, runoff is lower (UBA 2023; UBA 2011).

Extreme flood events are caused by certain weather constellations. In winter, continuous rainfall lasting several days to weeks, sometimes in combination with thawing and melting snow in the low mountain ranges, is usually responsible for flooding. In summer, it is often a case of rainfall lasting several days, which can occur in combination with heavy rainfall (DKKV-Topic Heavy Rain). In contrast to river floods, floods induced by heavy rainfall cause small rivers and streams in particular to rise quickly and also occur away from bodies of water (UBA 2023). Besides the amount of rainfall, it is crucial how much water reaches the river in an ‘effective runoff’ form. Depending on the vegetation, soil and terrain conditions, water runs off at different rates, is temporarily stored or seeps away. Soil that is already saturated as well as very dry soil can also absorb less water (LfU Bayern).

Over the past centuries, humans have intervened massively in the natural flow patterns of streams and rivers. Natural floodplains close to rivers have been developed for commercial or residential purposes, floodplains have been drained and converted into agricultural land (UBA 2011). Dike construction and river straightening mean that rivers are less branched and water flows away faster, which favors more intense flood waves. At the same time, more damage is caused when a flood wave hits a settlement or industrial site (UBA 2022).

Figure 3: Flood in Passau in June 2013 (Wikipedia)

Climate change has a significant impact on the frequency and intensity of flood events. From 1881 to 2022, the average air temperature in Germany rose by 1.7 °C, which is changing previous precipitation patterns (UBA 2023). While summer rainfall has remained largely unchanged, winter rainfall has increased by around 26 % (UBA 2023, LfU Bayern). Rising temperatures also mean that air can absorb more water vapor – 7% for every 1 °C rise in temperature. In combination with increased evaporation above the sea surface, this can lead to more intensive rainfall (DWD 2024) (DKKV Topic Heavy Rain). Following the summer floods in June 2024, an attribution study by the German Weather Service (DWD) investigated the extent to which the events are linked to climate change. The study shows that the probability of comparable rain events has already increased by a factor of 1.4 compared to a world that is 1.2 °C colder – i.e. 40 % more likely (DWD 2024).

In addition, higher temperatures reduce snow cover in winter, as less precipitation falls as snow and instead as rain, which runs off directly. This increases runoff in winter. At the same time, there is less snowmelt in summer, which reduces runoff in summer and favors low water situations. In general, however, few significant trends can be identified so far, as most situations that trigger floods depend on the respective weather constellation (UBA 2023).

Technical flood protection is an integral part of comprehensive flood risk management in order to protect people and infrastructure from flooding. This includes, for example, dikes, flood protection walls and mobile flood barriers, as well as artificial retention basins, dams and controllable and uncontrollable retention areas. In the city, the proper operation of the sewer network and pumping and sewage treatment plants is also crucial. In Cologne, for example, this is ensured by monitoring in discharge control centers. Furthermore, 900 slide gates can be installed in the canal network to protect it from incoming floodwater from the Rhine (SteB).

However, technical measures alone are not enough to protect against flooding. Settlement along the river and the technical protection structures built centuries ago have had a massive impact on the flow of the river or stream (BMUV 2017). Concrete measures for a nature-based solution include alluvial forests, the creation of grassland, reforestation, unsealing and conservation tillage in agriculture (UBA 2011). Technical measures must be supplemented and in some cases replaced by such nature-based solutions and, above all, precautionary measures (land, building, behavioral and risk prevention) must be implemented and strengthened. In order to keep the damage potential as low as possible, more space should be given to the river and, if possible, no building land should be designated in flood-prone areas (SteB, UBA 2017). Strict regulations are required for flood-adapted construction and use where construction is permitted despite a legally defined floodplain (BezReg Köln). Particularly in the case of headwaters and tributaries, natural retention areas already hold water decentrally within the catchment area. In addition, behavioral and risk prevention are important building blocks for raising the population’s flood awareness and mobilizing them to take precautions themselves (DKKV Topic Risk Communication, Eigenvorsorge gegen Hochwasser). This includes the provision of information on hazards, such as hazard maps and water levels, as well as forecasts and warnings. In addition, there are private property protection measures (DKKV Topic Constructional Civil Protection) and financial instruments such as natural hazard insurance (DKKV Topic Insurance in Risk Management).

1. Pay attention to weather and flood warnings. Use warning apps such as NINA or KATWARN.
2. Follow the instructions of emergency services and pay attention to barriers and warning signs.
3. Do not enter any bank areas. There is a risk of undercutting or breaking off.
4. Avoid driving on flooded roads. Keep roads clear for emergency vehicles.
5. Vehicles should be removed from flood-prone areas in good time. Caution: Underground garages are life-threatening in the event of flooding or heavy rain!
6. Seal windows and doors as well as drainage openings (backflow protection).
7. Avoid cellars and lower-lying rooms.
8. Switch off burners, boilers and electrical appliances in rooms at risk of flooding in time.
9. Hazardous substances such as used oil, chemicals, paints, etc. should not be stored in rooms at risk of flooding and should otherwise be removed in time.
10.  Pay attention to sick people, people in need of care and children and it is best to move them to safety in advance.

General precautionary measures and tips on how to behave in the event of a disaster are compiled by the BBK here: BBK Rategeber für Notfallvorsorge und richtiges Handeln in Notsituation and on the BBK Website.

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