Letting things sink in and evaporate

When it comes to rainwater management in the city, there is a more environmentally friendly alternative to drainage into the sewer: it can seep into the ground or be retained on green roofs and evaporate. But does the general population accept such measures for natural rainwater management? A research group led by Lisa Bannert and Prof. Dr. Tim Freytag from the Institute of Environmental Sciences and Geography at the University of Freiburg have examined this issue using the example of the cities of Freiburg, Hannover and Münster. A key finding of the study is that satisfaction with green roofs and infiltration wells is particularly high among people who use or live in the surrounding area. In Freiburg, green roofs are widespread and popular compared to the other two cities - and ecological aspects such as urban climate and environmental protection are more important to respondents than financial benefits.

Natural rainwater management aims at a water balance in residential areas, which comes as close as possible to the undeveloped, natural state. Green roofs, for example, retain a large part of the rainwater and release it through evaporation over time, thereby positively affecting the urban microclimate. In addition, the vegetation of green roofs leads to an increase in biodiversity of flora and fauna. The same applies to infiltration wells, which additionally ensure that more groundwater can reform locally. Most of the pollutants are filtered during passage through the live topsoil and partially decomposed by microorganisms. In addition, they are suitable for other uses, such as rainwater retention basins, recreational and sports field or parks. Conventional drainage, on the other hand, drains rainwater that cannot seep through the sealed surfaces of residential areas into drains and streams and rivers. This requires the complex construction of sewer systems for large amounts of water. The outflows are often left untreated so that pollutants such as engine oil, brake and tire abrasion or heavy metals get into the water. In addition, the flood risk is increased in heavy rain because the sewers can fill to capacity and streams and rivers can overflow the banks.

“One of the key findings is that public and private infiltration wells as well as green roofs have a much higher acceptance rate if an appropriate facility exists,” says Freytag. 97 percent of those who have a green roof rate it positively - compared to 78 percent of households that do not. Compared to Münster and Hanover, green roofs are more widely known and accepted among the people of Freiburg. More than half of the respondents are in favor of multifunctional use of the infiltration wells, which, in turn, increases acceptance: People who use such facilities in their free time rate them extremely positively. An example from Freiburg is located near the old train station in the Wiehre: “The complex offers attractive seating and is used in summer for drinking coffee and sunbathing, while the hollow serves as a sled slope in the wintertime.”

It also shows that the respondents are paying attention to ecological aspects: more than three quarters agree with the statement that rainwater systems are important for the urban climate. Ecological reasons are the most important motivation for the establishment of a facility on their own property in the face of financial benefits. “One reason could also be that the financial incentives are hardly known,” explains Bannert. “Only one-third of those surveyed know the split sewage fee, which can be used to save a portion of the fees for natural rainwater management.”

Background information

In the research project “Water Balance of Settled Waters” (WaSiG) funded by the German Federal Ministry of Education and Research (BMBF), scientists investigated the topic of “natural rainwater management"” using the examples of Freiburg, Münster and Hanover. They collected and evaluated data from 1,613 households in order to determine the acceptance of appropriate measures. The project was managed by the Münster University of Applied Sciences. Teams at the University of Freiburg from human geography and hydrology were also involved. Field partners included the Freiburg Environmental Protection Agency and private engineering firms.

www.fh-Münster.de/forschungskooperationen/wasig/index.php

Original publications

• Freytag T., Bannert L., König F. (2018): Naturnahe Regenwasserbewirtschaftung im Planungsprozess: Hintergründe, Problemfelder und Erfolgsfaktoren. Leitfaden. Freiburg. Download: http://dx.doi.org/10.6094/UNIFR/15714
• Freytag T., Hackenbroch K., König F., Bannert L. (2017): Akzeptanzanalyse von Regenwasserbewirtschaftungsmaßnahmen bei Anwohner*innen: Ergebnisbericht. Freiburg. Download: http://dx.doi.org/10.6094/UNIFR/13497