What is stormwater?
Stormwater is rainwater and snow melt that flows along the ground surface, roofs of buildings and other surfaces. Stormwater regulations are also applied to drainage water from the foundations of buildings. Other types of stormwaters can also end up in the stormwater drainage system, for example waters from construction sites.
More drainage water in the future
It is predicted that due to climate change rains will intensify and dry periods will get increasingly common in Finland. There exists many surfaces on city areas that do not efficiently absorb water, for example asphalt and concrete. These surfaces prevent stormwater from absorbing into the soil. Intensifying rains combined with hard surfaces mean more and more stormwater that needs to be managed and redirected. The increasing amount of runoff water can cause flooding on streets, yards, stormwater systems and outfalls. It is important to manage drainage waters in order to minimize flood damages and improve drainage water quality.
Many different pollutants, for example from traffic and sites, accumulate in stormwaters. The pollutants decrease the quality of the local water bodies and groundwaters. The soil and the groundwaters in Oulu naturally contain a lot of iron. Oftentimes the groundwater surface is lowered in order to ensure proper drainage on construction sites. Groundwater lowering initiates a process that causes orange-brown iron dregs to pile up in stormwater structures and to clog perforated pipes. The widely occurring acid sulfate soils in Oulu also accelerate metals dissolving into the water in the area. Acid discharge washes off the sulfate soils if the soils remain above the groundwater surface.
Stormwater management is a common matter
The purpose of stormwater management is to prevent damage caused by runoff stormwater to properties and the environment. Stormwater runoff is redirected from plots to public area stormwater management systems. These systems include, for example, stormwater sewers, open ditches, hollows and different structures that retain stormwater or delay its flow rate. Usually a stormwater system's outfall locates in its nearest water body, from which the water eventually flows into the Bothnian Bay.
The best method for treating stormwater is infiltrating it into the soil. Infiltrating should be done as close to the water source as possible. Infiltrating is the reason why it is important to conserve pervious soil surface covered in vegetation and to utilize infiltrating stormwater structures. In case stormwater infiltration is not possible for example due to the soil's qualities, runoff water should be delayed to decrease flow rate and quantity of stormwater in management systems. Stormwater can for example be collected for irrigation or redirected to a rain garden. Building a system for infiltrating and/or delaying stormwater on a plot is required for new construction projects. By delaying the stormwater runoff we can minimize the flooding risk of our stormwater systems. It is especially important to manage stormwater on sites because it can cause water clouding, decreasing water plant growth, siltation of beaches and the bottom of water and fish and spawn suffocation. Additionally, untreated runoff water from sites may cause siltation, clogging and corrosion of sewers, wells, pumping stations and different structures. Read more in the City of Oulu's site runoff water instruction manual (in Finnish).
According to the City of Oulu's stormwater program (in Finnish), the city's stormwater treatment emphasizes nature-based solutions. These solutions include using stormwater systems like stormwater ponds, wetlands and biofiltration, all of which aim to delay and purify water. In addition, nature-based solutions support the biodiversity of nature by creating living environments for water birds, organisms and pollinators. Our goal is that stormwater management is considered in all phases of construction, from planning and mapping out local master plan areas to acquiring building permits and maintaining buildings. With good stormwater management we can ensure a healthy and comfortable environment for all residents.