Benefit: Flood and flow control

Flood mitigation is the main benefit of water retention. In Finland floods are most common during snow melting season in the spring. Snow melts relatively fast and the volume of water is high. In rivers ice dams increase the flood risk in the upstream.

FLOODS IN INLAND WATERWAYS

Floods occur for example in the rivers of Lapland and Ostrobothnia. In Lapland rivers are large and the volume of melting snow big. Ostrobothnia is low-lying region where a large share of land area is under agriculture. Especially spring floods are common. Mires and lakes that would level the flood are scarce. A lot of mires have been drained for agricultural purposes, and there are only few lakes along the river courses. Some cities, such as Pori in Satakunta, which are located near estuaries suffer from floods.

FLOODS IN THE BALTIC SEA

In the coastal areas of the Baltic Sea occur occasional floods, especially in the southern coast and Oulu region in Northern Ostrobothnia. Sea water level in the Finnish coast is affected by wind, air pressure and the total water volume in the Baltic Sea.

FLASH FLOODS

There is an elevated flood risk during torrential rains especially in population centres. In centres a large share of land is paved and water retention potential is minimal. Sewer network is filled up if the precipitation level exceeds 100 mm. Parks and other unpaved areas in centres reduce the flooding risk.

OTHER BENEFITS OF WATER RETENTION

Water retention affects indirectly soil and water quality. Floods and surface run-offs catch nutrients, minerals and organic compounds amongst other, and carry them into the water system. The soils, especially agricultural lands, suffer from the loss of nutrients. At the same time, the excessive amount of nutrients and organic compounds cause eutrophication in lakes, rivers and the Baltic Sea. Water also transports toxins from terrestrial ecosystems to aquatic ecosystems, and some of them concentrate in organisms, for example fish and crayfish.

Tulvakarttapalvelu. Tulvakeskus. Suomen ympäristökeskus, Elinkeino-, liikenne- ja ympäristökeskus. [In Finnish]