How do people deal with floods
Change of use - landscapes change their face
In the public, the influence of humans is primarily discussed for the development of floods: The following are mentioned:
- the change in land use - especially the sealing and construction of the natural retention areas along the waterways,
- the river expansion - especially the straightening,
- the climate change.
In the past, many areas with favorable storage and seepage properties were lost to natural retention. They were diked, drained and then settled or used for agriculture. This had an unfavorable influence on the development of the floods. Water is drained off quickly and collected in narrow channels. This creates ever larger flood waves, which also run faster.
Today's goals have changed. Today one tries to promote the natural retention in the catchment area of the rivers again. The sealing of surfaces can be mitigated through targeted planning and seepage-promoting measures. It is possible, for example, for rainwater to seep into settlement areas, for soil management adapted to the terrain, and for the preservation of grassland, hedges and fields. The principle behind all of these measures is: water should seep away wherever it occurs. Even if, viewed individually, they cannot prevent floods, they all contribute to dampening flood waves, especially in the case of smaller bodies of water. In the case of large flood discharges, however, the possibilities of human influence become less and less. With a higher-quality use of possible flood areas, the potential for damage caused by humans increases significantly in the event of flooding.
Humans shape their environment by colonizing it and using the soil for food and supply. These artificial changes in nature cannot remain without effects on the runoff of precipitation.
If grassland is converted into arable land or forest into pasture, surface runoff increases because the soil can then store less water. Hardly noticeable in the short term, a significant change in the landscape takes place over decades and centuries. This has a more pronounced effect in small catchment areas than on large rivers.
Hardly any other keyword is mentioned as often in connection with floods and the suspected causes as sealing. In Bavaria, 11.2 percent of the land area is designated as settlement and traffic area (as of 2009). However, only 47% of this area, i.e. around 5% of the country's area, is considered to be completely impermeable to water. This means that 95% of the rain in the large river areas (such as the Main and Danube) falls on unsealed areas. In smaller, densely populated catchment areas, on the other hand, the sealed portion can be higher and thus significantly favor floods.
During extreme rainfall, the soil hardly stores the water. If it is soaked or even frozen, a “natural” sealing occurs and thus the same flood effect as with sealed surfaces.
Rivers and streams are as diverse as the landscape itself. Each body of water has its own dynamics, which are reflected in the material balance, in the habitats for the fauna and flora, in the river bed, in the floodplain and in the runoff behavior. Rivers and streams are not rigid elements; they are constantly evolving. They change their course if they are not held in a rigid corset by structural measures.
When there is high discharge, the bodies of water not only carry water, but also debris, such as sand and gravel, and drifting material, such as pieces of wood up to entire tree trunks and roots. Bed load is deposited in places with a lower flow velocity and changes the flow profile - the flow depth and the width of the water. The changed runoff profiles can lead to floods occurring more frequently and with higher water levels. In areas where embankments can cause damage, it is a task of water maintenance to keep the runoff profiles free of sufficient size.
Propellants floating near the surface of the water can, for example, become wedged in narrow spaces or under bridges and thus quickly lead to high, localized damming. This can also significantly increase the risk of flooding in adjacent areas. Therefore, the possible problem areas must be checked both during and after a flood in order to limit the risk of flooding.
Settlement and traffic routes are growing in the valley areas. As a result, less and less space is made available for the bodies of water to spread out in the floodplain. But the river and floodplain form a unit. If the floodplain is stolen from the river, flood can no longer spread and the runoff is concentrated on the water itself. The consequence is higher water levels during floods and a faster start of flood waves.
Natural climate changes have occurred over and over again over millennia. The massive release of carbon dioxide and other greenhouse gases by humans since the beginning of the industrial revolution in the 18th century is something new in geological history.
All climate researchers agree that the atmosphere is warming. However, it is still unclear how this increase in temperature will affect the water balance in our latitudes. In order to investigate the consequences for the water balance in Bavaria, Baden-Württemberg and Rhineland-Palatinate in more detail, the water management administrations of the three countries are carrying out the cooperation project KLIWA (climate change and water management, www.kliwa.de) together with the German Weather Service.
For southern Germany, the studies since 1931 show an increase in annual mean temperatures between 0.5 and 1.2 degrees, depending on the sub-area. The total annual precipitation has changed little in this period. However, the distribution of precipitation has changed over the course of the year. Precipitation decreases in summer and increases in spring and winter.
In order to be able to estimate future changes in flood discharge, water balance models with different regional climate scenarios have been calculated in recent years. In the EU project ESPACE (European Spatial Planning: Adapting to Climate Events, www.espace-project.org), using the example of the Franconian Saale, suitable procedures were developed how flood protection concepts can be adapted to climate change.
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