Drainage in practice: "These are basics that you need to know as a planner."
Heavy rainfall events have been increasing in Germany for years. Rolf Johannsen is a professor of engineering biology in Erfurt and explains where landscape architects still make mistakes in drainage in practice and what potential good planning has.
Garten + Landschaft: Mr. Johannsen, this year southern Germany in particular is struggling with heavy rain and heavy thunderstorms. How do you deal with all the water in public spaces?
Green spaces play an important role here because they offer the necessary infiltration opportunities, especially during heavy rainfall. Of course, this depends on the degree of sealing and the type of soil. However, rainwater can also be drained off superficially, for example via troughs that are only temporarily filled with rainwater. Above-ground bodies of water such as ponds, rivers or retention basins are of course another way of retaining rainwater and can also be used as a design element. Extreme events should be taken into account when designing green spaces. So do not plan high-value uses in swales!
What needs to be considered during planning?
The framework for such projects is provided by the Federal Water Resources Act and the State Water Act of the respective federal state. The approval and supervisory authority is the respective environmental agency, city or district. The systems themselves are planned in accordance with recognized engineering standards, i.e. on the basis of DIN regulations and DBA regulations. The precipitation amounts are obtained from the German weather service. These are basics that planners need to know.
What are some outstanding examples?
In the Emscher area, there was and still is an extensive project in which the river is being transformed from a pure wastewater river into a near-natural body of water. As part of this, Emscher Park was also created and the Federal Garden Show was organized in Dortmund. Very good work has been done there.
Are water and greenery inseparable for you?
Yes, important biotopes for animals and plants are being created here. Bodies of water and alternating wet spots offer interesting locations for plants. In addition, rainwater is well purified when it seeps through living soil.
“Some people actually lack basic hydrotechnical knowledge.”
Planners rarely start from scratchwhen developing drainage systems. Is it also possible to find good solutions in existing buildings?
Absolutely, especially when renovating and converting residential areas. In Dresden, for example, prefabricated buildings in a residential area were demolished and the drainage system was taken out of the ground and transformed into a more natural watercourse. The conversion of old industrial and commercial areas also offers the opportunity to reopen piped watercourses and integrate them into green spaces. The redesign of parks often offers scope for combining aesthetic aspects and the natural course of the terrain in rainwater management.
Where do planners still have some catching up to do?
Experts specify the design events to be complied with. Some planners cling to this and work through it in a bookkeeping manner, without taking into account that there can also be extreme events. Underground facilities such as sewage pipe systems are dimensioned tightly for reasons of economy, but when it comes to the general layout of the site and the distribution of uses, you are of course freer and can plan for something like this.
Is there simply a lack of know-how?
Some people actually lack basic hydrotechnical knowledge. For example, flooding is caused by backwater, erosion by locally concentrated flow energy or erosion in the subsoil.
“Even with comprehensively planned drainage, the risk of flooding can certainly not be completely ruled out.”
That’s a lot of physics. Should universities teach more about this?
In Erfurt, we also focus on engineering biology, near-natural watercourse development and hydrotechnology in our training. But if you’re a landscape architect working in this field, you should have this basic knowledge.
The weather is becoming more and more extreme. What challenges do you see?
The trend is indeed like this. In many areas, there is a slight increase in heavy rainfall and flooding. Bavaria and Baden-Württemberg have already set their reference values for new planning at 20 percent above the statistics. Brandenburg, but also the Halle and Leipzig areas and the Thuringian Basin, on the other hand, are increasingly struggling with drought. Here, it would make sense to increase rainwater infiltration in order to achieve groundwater recharge. When redesigning company green spaces, urban sports facilities and parks, it is now also conceivable to install cisterns. This would move away from the use of groundwater and drinking water for irrigation.
Is that all there is to it?
Even with comprehensively planned drainage, the risk of flooding can certainly not be completely ruled out. You just have to be prepared for this and give nature enough space.
More about heavy rain: The 2021 floods are considered to be the worst natural disaster in Germany since the storm surge in 1962. Read more here: Flood 2021.