"There are currently no statutory flood protection regulations in Germany against flash floods in small watershed areas, like those we experienced last week in North Rhine-Westphalia and Rhineland-Palatinate. As a result, very few or no flood protection measures have been implemented in the flooded regions thus far. This needs to change. Germany has extensive flood protection in place, but this is limited to river floods," says Dr.-Ing. Reinhard Hinkelmann, professor of water resources management and modeling of hydrosystems at TU Berlin.
The difference between river floods, such as those along the Elbe in 2002 and 2013, and the recent flash foods, comparable to those in Simbach, Bavaria and Braunsbach, Baden-Württemberg in 2016, is that flash floods are the result of extreme precipitation in small catchment areas. They are characterized by very rapid runoff processes from mountains and hillsides over streams that become rivers and have very short warning times in contrast to the floods of large rivers such as the Elbe or the Rhine, where warning is possible several days in advance. As a result, the lower and middle sections of sloping sites as well as valley locations near streams are particularly at risk," explains Hinkelmann.
Effective protection against such flash floods is not possible without millions of euros of investments and tough measures such as the construction of large drainage systems, somewhat like the city walls of the past, outside of settlements; dams fitted into the landscape; or retractable walls and bedload barriers in the upper and middle reaches of streams - like those in alpine regions - to hold back trees and boulders. Furthermore, it will be important to adapt any protective measures to local conditions and features. However, changes in residential and landscape planning, such as further bans on building in flood-prone areas, should not be off limits either. "We need to consider measures which extend beyond existing actualities and situations. This includes, for instance, whether buildings which have been torn away by flooding should be rebuilt in the same location."
A further consequence of the extreme weather events of the last few days is the need to change the assessment basis for flood protection measures. "Currently, most flood control measures are designed for a river flood that statistically occurs once every 100 years. In this case, increased reference events should be considered, meaning even more extreme weather events should be taken into account, such as those that might only occur every 200 years," continues Hinkelmann.
The flood control system in the Saxonian town of Grimma, which was constructed following a flood catastrophe along the Elbe in 2002, is one example of a successful flood control measure, even if it cannot be used as a blueprint for all affected municipalities, as monument protection takes a central role in Grimma. "Additionally, it took 17 years to implement those flood protection measures. We cannot take another 17 years to implement measures needed to protect settlements on the upper and middle reaches of streams and in valley locations."
According to Hinkelmann's estimates, extreme precipitation is a result of climate change. For flood protection, this means that the impacts of climate change must be more seriously addressed at policy level. "Even in the future, we will not attain 100 percent protection against floods. The level of protection is expected to decrease even after the implementation of many more measures and life will become riskier. This will further increase the pressure on policymakers," concludes Hinkelmann.