Stormwater and its management

Stormwater refers to the surface runoff of rainwater and meltwater formed in built-up areas. Drainage water from foundations is also counted as storm water. Runoff from undeveloped areas is not stormwater, but runoff from natural areas.

The more compactly the area is built and the larger part of the catchment area's surfaces are impervious to water, the more and faster surface runoff, i.e. stormwater, forms. As the urban structure becomes denser and the plot size decreases, an ever larger part of the ground is covered under buildings and asphalt surface. The proportion of the ground surface that absorbs water decreases and, in addition, the number of trees that evaporate water decreases, so surface runoff increases.

In built-up areas, population, traffic and industrial activity cause emissions that reduce the quality of stormwater. Harmful substances end up in stormwater from, among other things, traffic exhaust gases, road wear, corrosion of vehicles and building materials, animal excrement, sewer overflows, and pesticides.

The previously conventional method of directing stormwater away from built-up areas in sewers as efficiently as possible directly into waterways weakens the quality of receiving waterways, causes erosion and the risk of a decrease in groundwater resources, and changes the hydrology of built-up areas. Today, in stormwater planning, even more attention is paid to the local treatment of stormwater, which aims, among other things, to prevent the resulting pollution of waterways, reduce the load on the stormwater drainage network, improve the conditions for groundwater formation, and prevent erosion in urban streams and other drainage channels.

Stormwater drainage area update

Consultation on changing the stormwater drainage area

The Riihimäki Urban Environment Service Area is preparing a decision pursuant to Section 17 a of the Water Management Act (681/2014) concerning the annex to the stormwater drainage agreement approved by the Riihimäki City Council on 22 August 2016 (§ 90). The annex map defines the stormwater drainage area and the ditches that are part of the maintenance of Riihimäki Water.

The current appendix map was prepared on 1 June 2016 in accordance with the stormwater network and was last updated in 2019. The aim of the update work being started now is to bring the stormwater drainage area up to date by adding areas where the stormwater network has been built since the previous update or for which construction of the network is planned, and to correct the shortcomings identified in the previous demarcations.

The proposals for new stormwater drainage areas and expansion areas are presented on the map attached to this consultation. The map also shows the areas for which stormwater networks are planned, as well as the ditches that are part of the maintenance of Riihimäen Vesi.

Background

According to Section 17a of the Water Management Act, a municipality may, after consultation with the water management company, decide that the water management company shall ensure the drainage of stormwater in the area specified in the decision in accordance with the needs of community development. Stormwater drainage is part of stormwater management referred to in Section 103b of the Land Use Act.

The above decision is conditional on the following:

1. the water supply company is able to provide stormwater drainage economically and appropriately; and

2. The fees charged to cover the costs of sewerage are reasonable and equitable.

In addition, the decision requires that the municipality and the water supply company have agreed on stormwater drainage or, if there is no such agreement, that stormwater is drained in the area referred to in the decision in accordance with the local plan, stormwater plan, street plan or general area plan. The City of Riihimäki and Riihimäen Vesi have signed an agreement on stormwater drainage in 2016.

Planned changes to the stormwater drainage area

The attached map shows how the current stormwater drainage area is proposed to be changed. Expansion areas built since the previous stormwater drainage area update in 2019 have been included in the stormwater drainage area shown on the map.

The new expansion areas are based on the planned stormwater network and the development of land use in the areas.

The expansion areas are presented on the map (pdf) and numbered by region:

1. Patastenmäki loop

2. Mattila industrial area

3. Fork in the road

4. North Juppala

5. Nurmelantie and Suojala area

6. Kumela and Punkantie intersection

7. Räiskylänkatu–Polvikatu

8. Koivistonmäki

9. Räätykänmäki

 

Water supply plant readiness and payments

The board of directors of Riihimäki Water Supply Company has proposed updating the stormwater drainage area in its meeting on 18 December 2024 (§ 79) in accordance with the attached map. Minor corrections have been made to the map since then. Riihimäen Vesi is able to provide stormwater drainage in the area specified in the decision in an economical and appropriate manner as required by the Water Supply Act.

The decision is not considered to create a financially unreasonable situation for Riihimäen Vesi, and the fees charged for stormwater drainage are not considered to be unreasonable for property owners or holders. The decision also does not jeopardize the financial situation of the water supply company.

The stormwater charges charged by Riihimäen Vesi are decided annually by the water utility's board of directors. The charges for 2026 were approved on 24 September 2025 (§40). Current and future price lists are available on Riihimäen Vesi's website: https://www.riihimaenvesi.fi/asiakkaalle/hinnastot/.

The decision on changes to the stormwater drainage area will enter into force when the decision becomes legally binding.

Notification of the consultation letter

The consultation letter will be notified in accordance with sections 55 and 62 of the Administrative Procedure Act (434/2003) as a general notice by publishing a notice on the website of the City of Riihimäki.

The notification is made as a general notification because there are more than 30 interested parties.

Announcement on the website 24.4-25.5.2026

Link to map: Stormwater drainage area 2026 (pdf)

 

Further information: Päivi Sundman, city gardener

Tuuli Tetri, network engineer, Riihimäen Vesi, tel. 050 529 7924, tuuli.tetri(at)riihimaki.fi