RO-R-6: Settlement use in flood-risk areas – case study | Umweltbundesamt

RO-R-6: Settlement use in flood-risk areas – case study

Along the Bavarian part of the river Main, just under 8 % of the terrain that would be flooded in case of a once-in-ahundred-year flood event, was in use for settlement purposes in 2012. Transport infrastructures take up almost half of those areas, but also trade and industry as well as housing take up a major part of the terrain in flood plains.

Avoiding settlements in terrain vulnerable to climate risks

In connection with climate change, it is one of the essential tasks of spatial planning to make appropriate provision for potential risks. On one hand, spatial planning can help to steer the development of settlements deliberately into directions where risks associated with climate change either do not exist at all or exist only to a manageable degree. On the other hand, spatial planning can ensure that recognisably vulnerable areas are to be kept free, as far as possible, from settlement. Relevant risks include the movement of masses of materials such as rock falls, landslides or sinkholes as well as – in coastal areas and on islands – storm surges. Such hazards can increase regionally as a result of climate change, if in future extreme weather situations and weather patterns increase in both frequency and intensity.

It is also expected that climate change will bring about changes in precipitation regimes as well as hazards resulting from flooding events, as these may occur more frequently and also more violently. Besides, the occurrence of floods and associated damage potential will be influenced considerably also by past and current human activities. For example, in former times natural flood plains in many river valleys were eradicated by measures such as building dykes and river modification. Numerous watercourses were shortened thus increasing their flow rate; in the event of flooding the run-off from countless tributaries accumulates faster in a river bed. Nowadays floodwater waves are often steeper than in former times, and their duration is shorter. Therefore, there is an increased risk of damage from floodwater. Besides, valuable material assets were constructed in the shelter of dykes where the land was formerly available to rivers as flood plains. If the flood protection provided proves inadequate when these settlements or industrial estates are flooded, the damage caused can be considerable.

Nowadays, protection from flood events is covered by the German Water Resources Act in accordance with specifications contained in the FRMD. Furthermore, it is required in Germany in accordance with precautionary land use to stipulate binding specifications for flood plains on the basis of (statistically speaking) once-in-ahundred- years flood events (HQ100); these specifications have to conform to specific protective rules. In such areas, both the specification of urban land use plans for new building locations in the outskirts, and the construction or extension of buildings, are prohibited or permitted only in exceptional cases. Since 2018, it is furthermore mandatory that flood formation areas be identified, and express permission is required for such areas where specific changes of land use such as conversion of grassland to arable land or conversion of alluvial woodland to another type of utilisation are proposed.

However, in the past many forms of construction such as transport infrastructures, industrial and trade buildings as well as residential buildings had already been implemented in numerous flood plains; and these are now at risk from flooding. The example of the evaluation of a flood risk management plan for the Main river basin shows that here just under 8% of the terrain – which would be flooded in case of a once-in-hundred-years flood event – was used for settlement. Transport infrastructures take up almost half of those areas, but also trade and industry as well as housing take up a major part of the terrain in flood plains. According to current legislation, these area percentages are not to be increased in future. Nevertheless, the pressure on these areas remains high, especially in river valleys which are already densely populated and offer very little leeway for development. This pressure must be resisted by provident spatial planning, regional and urban development.

Interfaces

WW-I-3: Floodwater

BAU-R-5: Insurance density of extended natural hazard insurance for residential buildings

RO-R-5: Land used for human settlements and transport infrastructure

Objectives

Reinforced protection against increasing flood risks by means of passive safeguarding measures; safeguarding existing run-off and retention areas; substantial expansion of retention areas by 2020 while making extensive use of all available potentials (DAS, ch. 3.2.14)

Maintaining the function of flood plains as retention areas; as far as possible – restoration of former flood plains suitable for use as retention areas (WHG, § 77)

Safeguarding existing flood plains as retention spaces; restoration of flood plains as retention spaces; risk provisioning in potential flood areas (Handlungskonzept Klimawandel, MKRO 2013, ch. 3.1)

Preventive inland flood protection, especially by means of safeguarding or restoring alluvial meadows, retention areas and flood relief spillways (ROG, § 2 (2) 6)