Exceptionally heavy or prolonged rainfall and snowmelt may lead to flooding. Floods are natural phenomena that cannot be prevented. The adverse effects of floods are aggravated both by the growing number of settlement areas and property assets in risk areas as well as the decreased natural water retention capacity of landscape and soil caused by intensified land use.
Floods in Berlin are caused by heavy or persistant rain, for example. Flood waves differ, depending on the nature of the rain. Heavy rain following storm fronts can often be observed during the summer months. It is characterized by the highest rainfall intensities, regionally contained and relatively short in duration. Heavy rain is the main cause of rapidly rising flood waves, e.g. at river Panke. Such floods form a lot faster due to the city’s high degree of impervious soil coverage (cf. Environmental Atlas 01.02). Prolonged rain in larger catchment areas is the main cause of flooding at the Erpe (Neuenhagener Mühlenfließ), Tegeler Fließ and the rivers Spree and Havel. Such flood waves are much shallower but last longer in the waters affected.
No flood protection is perfect. Flood damage may only be reduced or prevented in the long run, however, if a comprehensive system to manage flood risks is in place. The management of flood risks is a communal project for society. It comprises various aspects such as prevention, protection, precaution and restoration / regeneration. The key to keeping flood damage at bay lies in the combination of state prevention and proactive individual citizens. Any person who may be affected by flooding is obliged, within the limits of the person’s possibilities and reason, to take appropriate preventive measures in order to protect against adverse effects of flooding and to mitigate further damage, in particular to adapt the land use to the possible adverse flood effects for humans, the environment or material assets (Section 5 (2) of the Federal Water Act (Wasserhaushaltsgesetz, WHG)). Measures included in the flood risk management plan are conducive to superior flood control, and, beyond that, they contribute to improved flood precautions and flood risk prevention at river Elbe (FRM Plan, 2015).
Identifying flood areas is part of flood prevention and flood control. The Federal Water Act (WHG) stipulates that at least those areas must be defined by ordinance, which are statistically expected to be flooded at least once every 100 years (cf. Section 76 WHG). Defining flood areas serves the protection against the dangers of floods, i.e. by
- preserving natural retention areas,
- controlling flood discharge,
- reducing existing and avoiding new damage potential, and,
- flood-adapted handling of substances hazardous to water.
The use of flood areas must thus be adapted to minimize flood damage and to counteract the loss of water retention capacity. To keep the damage potential from rising, building restrictions have to be observed in flood areas, amongst other things. This prevents future damage. Identifying and publishing flood areas also aims at raising awareness of potential flood risks.
Only flood areas with a substantial risk of flooding are identified in risk areas. The flood risk assessment in accordance with Directive 2007/60/EC of the European Parliament and of the Council on the assessment and management of flood risks (FRMD) revealed that there is a potential significant risk of flooding in the following areas: Tegeler Fließ, Panke, Erpe, Lower Havel / Lower Spree and Müggelspree, including the Gosen waters including Seddin Lake. These were therefore defined as risk areas in accordance with Section 73 WHG (see Figure 1; SenUVK 2018).
