Impermeable hard surfaces in cities such as asphalt and roofs increase the amount of stormwater. The solution is found when we learn to trust the power of the earth and vegetation.
Stormwater is rainwater and meltwater that flows in the built environment, such as parks, streets and roofs. As the urban structure becomes denser, there are more and more impermeable surfaces. When, at the same time, climate change increases rainfall, the capacity of pipe networks is simply not enough.
"We should hold the water with the handbrake so that it doesn't go directly into the rainwater well. In the future, sustainable stormwater management will require small, decentralized solutions to support centralized solutions", Häme University of Applied Sciences' research teacher of the built environment Outi Tahvonen says.
Large amounts of stormwater can cause flooding and increase sewage overflows. Nutrients, harmful substances and garbage can be carried into waterways with stormwater. To help, absorbing, evaporating and storing solutions are needed even before the water slips into the centralized stormwater network.
An important role in green areas
In the new residential area of Kangasala near Tampere, the flow of stormwater is slowed down in the local stormwater area. Stormwater collected from small houses, low-rise buildings and streets is first directed to the water treatment area, where the sediments are collected and the flow of water is slowed down.
"We have built absorption prints that imitate the depressions visible in nature as traces of the Ice Age. In Supa, the water reaches the aquifers of the soil and is slowly absorbed in place. The suppa area is small, but quite sufficient. Thanks to that, there is no need to transport water through pipes to centralized systems elsewhere," vice president Henrik Bos VRJ Western Finland describes.
According to Bos, green areas should also be seen as storm water areas. Stormwater layers should be planned for them, where water can be stored and absorbed.
"The fear of water must stop in the construction and infrastructure sector. All cities are built in such a way that you have to get rid of stormwater quickly. This requires new thinking," says Bos.
Vegetation keeps water in the fields
Traditionally, the water has been diverted away from the plots quickly, and new water may be run separately into the yard to water the vegetation.
Outi Tahvonen brought the perspectives together. Land and vegetation are part of the solution.
Vegetation binds nutrients and other substances, and different layers of soil filter out harmful substances from stormwater when the water is absorbed. Plants also evaporate some of the water. Deep-rooted plants create looseness in the soil, allowing water to better absorb into the deeper layers.
"The construction industry plays a key role if we are going to survive climate change and the biodiversity crisis. Plants are a way to control atmospheric carbon dioxide. At the same time, they help prevent cities from flooding and increase comfort," says Tahvonen.
For example, green roof structures slow down the flow of water considerably. Whereas water flows down from a metal or felt roof in seconds, a green roof first retains water inside. Normal rain can fall for hours before it starts coming down the gutters from the green roof.
"Thinking further, roofs that retain water can be built. This would be of great importance in managing rain spikes. The water collected on the roof could then later be used to irrigate the green areas," envisions Bos.
Man can imitate nature
In stormwater management, it is possible to use processes that mimic the natural water cycle: absorption, evaporation, conduction and storage.
In the construction site, you can choose the element that is most naturally suitable for each environment, and plan stormwater management especially based on it. This requires close cooperation between the HVAC designer and the yard designer.
Planning in which stormwater, vegetation and soil properties such as growing media and gravel are considered together is called integrative planning.
"I have previously worked as a garden designer myself in the capital region. On some construction sites, we planned the delay of stormwater on the site with different pressures. When the pictures came back from the HVAC designer, they always had a well placed at the bottom of the prints," says Tahvonen.
"We easily become slaves to our previous education and habits. Sustainable management of stormwater requires active rethinking."
The ball does not roll uphill
Stormwater planning and construction based on decentralized and organic models is already being done. For example, VRJ has created an operating model with its largest partners, in which a green building professional reviews extensive plans before tendering them.
"This is a big, positive change. The biggest construction companies are beginning to understand stormwater management better and better," Henrik Bos reflects.
Cities also have their own guiding solutions for decentralized planning of stormwater, but a large-scale change requires a change in legislation and building criteria.
"If you want a snowball effect, the ball has to start rolling from the top down, be it the European level, the national level or the city level. When a common will is found, the market follows. Nothing steers the market as effectively as legislation. It also guarantees equal competition," says Bos.
According to Bos, you could get good tips from, for example, Germany, the Netherlands or France. Cooperation between the green sector and the construction sector, joint gatherings and networking are also needed. The authors of building guidelines should discuss the future more together.
"Instead of solving today's problems, we should think about where we are going, how to build in the future and how we can get there together. It is important to share know-how."
Text: Hanna Moilanen
Photos: Henrik Bos/VRJ Group, Outi Tahvonen
A concise information package for construction site water management
The Capital Region's construction site water guidelines give builders concrete tips on, among other things, avoiding harm caused by construction site water, monitoring water quality, and creating a construction site water management plan.
"The guide contains its own instructions for house builders and infrastructure builders, and it covers well the issues encountered on the construction site. This should also be looked at elsewhere in Finland, even if the local conditions and requirements are different", expert Jani Kemppainen about RT says.
The instructions include, among other things, a list of auxiliary questions for planning site water. It also has illustrated examples of a well-maintained street construction site, an excavation site and an apartment building site.
Check out the Capital Region's site water guidelines here (hsy.fi)