As a flood area in the legal sense, it is, according to most water laws, the areas that can be flooded during extreme floods . In Germany, in most of the federal state laws, areas are designated as floodplain that, statistically speaking, can be flooded once every hundred years.
These areas specially designated for this purpose are subject to a high probability of being affected by flooding under appropriate hydrological, climatic or weather conditions. In normal times, however, they can be used as grassland , for forestry or for recreational and sports purposes. Development should be avoided, but not always prevented in areas with high settlement pressure.
Natural, undeveloped rivers and river landscapes have with their floodplains a natural floodplain ( inundation area ) in which the vegetation is prepared for temporarily high water levels. Structural measures (e.g. river straightening , embankments , residential and commercial areas ) can impair the function of these natural floodplain areas or change the flow and drainage behavior of the body of water.
Since a flood volume, which can spread in the flood plain, is delayed and temporarily stored in the floodplain, floodplain areas are always of importance for the attenuation ( retention (water management) ) of flood waves . The pore volume in the soil can also have an impact on the flood through seepage . As a rule, an extreme flood in the upper reaches of the river will be significantly reduced further below.
The lower the gradient in the body of water, the longer the flow path and the wider the floodplain, the greater the retention. Since in the past natural floodplains were separated from the floodplain by river dikes , the natural retention capacity of many Central European rivers has decreased, especially since the middle of the 19th century. The creation of controllable flood polders or, in the case of smaller rivers, flood retention basins can counteract this effect.
In Germany, the Federal Water Management Act (WHG) contains two definitions for flood plains:
- (1) sentence 1 of the WHG defines floodplain areas based on their location: Floodplain areas are areas between surface waters and dykes or high banks and other areas that are flooded or flowed through during floods or that are used for flood relief or retention.
- By para. 2 WHG, the provincial government will be empowered and required to fix flood areas by ordinance. At least the areas in which a flood event is statistically expected once every 100 years and the areas claimed for flood relief and retention are to be defined as flood areas. Floodplains that have not yet been established are to be temporarily secured.
Building bans according to § 78 Paragraph 4, 5 WHG. In addition, the general duty of care according to (2) WHG applies : Every person who may be affected by floods is obliged to take appropriate precautionary measures to protect against adverse flood consequences and to reduce damage, in particular the use, within the framework of what is possible and reasonable of land to the possible negative consequences for people, the environment or property from floods.WHG only provides for specific protective measures for formally established or provisionally secured floodplains, e. B.
Due to the competence to deviate from competing legislation , the federal states have been able to adopt different regulations in their water laws since March 1, 2010. So far only Bavaria has made use of it.
- For example, the Lower Saxony Water Act , § 115
- para. 3 no. 5 GG
- Deviation from § 78 WHG by Art. 46 Para. 4 Bavarian Water Act (BayWG) of February 25, 2010 GVBL p. 66, BayRS 753-1-UG with effect from March 1, 2010 to March 1, 2012 (cf. BGBl. I 2010, 275)