Stormwater Management

What is stormwater?

Stormwater is rain- and meltwater gathered on developed areas. Foundation drainage water is often treated together with stormwater. The management of stormwater and foundation drainage water is primarily regulated in the Land Use and Building Act, but other legislation also affects water management and structure maintenance.

Goals of stormwater management

The management of stormwater plays a significant role in residential satisfaction, adjusting to climate change and the state of water in local areas.

The first goal of stormwater management is to ensure that nothing gets wet harmfully. This is essential for private property, environmental protection and environmental load. It is not practical to have to dismantle something otherwise usable because something gets wet.

Part of normal amounts of rain and meltwater are absorbed into the ground and the rest are directed forward through drains, sewers and ditches usually into the ocean. Direction routes are planned so that they have enough room for water during rain. The sufficiency of the routes is improved by delaying water as close to the surface where it gathers from. Delaying water means creating a queue in a manner of speaking. Water can enter the sewer or ditch on its own turn, avoiding overcrowding immediately as rain begins. 

Rainwater is reasonably clean. However, when it lands on something, it binds impurities to it. These impurities or nutrients must be removed from stormwater before it can be directed forward to ensure that the waterways stormwater ends up in do not get any more polluted. 

To ensure absorption and due to delaying and treatment, there might be some water in depressions along streets or on greenspaces in parks or among plantings. That water is waiting for its turn to be absorbed or being directed forward. There can be many different structures and direction routes meant for water treatment under different surfaces. According to the city of Oulu’s Stormwater Plan (in Finnish), natural solutions such as stormwater pools, wetlands and biofiltration are emphasized in stormwater treatment. Additionally, the natural solutions can support biodiversity by creating living environments for water birds, aquatic organisms and pollinators. 

Flooding stormwater systems

Now and again, situations arise when water cannot be absorbed or cannot be directed to correct routes to the stormwater system. The system can be full, frozen, or otherwise blocked. Even in such cases, nothing should get wet harmfully. This means creating optional routes that are not always primarily meant for water. Such places or routes are called flood routes. In city planned areas, streets and some parks serve as flood routes. The goal is that non-dangerous amounts of water flow across streets during heavy rain or immediately after it. The planning principle is to ensure that that amount will not exceed 10 cm. A critical limit is 30 cm, but even such amounts are not dangerous if optional routes are offered to people. Thus, it is important to ensure the status of routes and depth of puddles during and after heavy rains, for example, to avoid damage to vehicles.

During heavy rains, water may gather in underpasses for different reasons. Floodwater on streets that cannot enter the stormwater system may end up in underpasses. Underpass pumping stations are planned for large amounts of water, but sometimes their capacity runs out. A general issue is blockages in sewer grates due to trash moving with water.

Construction site waters

Managing water in construction sites is extremely important because they can cause issues in the environment, such as cloudy water, decline in water plant growth, siltation in beaches and bottom of water, as well as suffocation of fish and spawn. Additionally, untreated construction site waters can cause siltation, blockages and corrosion in sewers, wells, pumping stations and other structures. Further information from the city of Oulu’s construction site water instructions (in Finnish).