Wetlands are areas that are covered by water for at least most of the year and where vegetation and open water alternate. In wetlands, the flow speed of water is reduced and the sediment carried by water settles to the bottom of the marsh. At the same time, nutrients that are bound to the sediment are removed from the cycle. The wetland’s vegetation uses some soluble phosphorus for growth directly from the water and from the sediment through their roots, thus reducing the total phosphorus content of the water leaving the wetland. The wetland’s vegetation and microorganisms bind nitrogen and release it back into the atmosphere.
Wetlands generally balance out the peak flows that take place in autumn and spring and, as a result, the water surface of the wetland begins to rise as the flow speed is reduced. The importance of wetlands both for flood control and water protection is largely based on their size in relation to the drainage basin and the inflow from it. If the wetland is too small in area in relation to the flow going through it, the flow speed will be too great and consequently the retention period will be too short. When water flows too quickly through the wetland, the sediment doesn’t get a chance to settle to the bottom of the pool and the amount of nutrients used by the plants has no significant effect on the total quantity of nutrients. With right sizing, however, both water protection and flood control benefits can be drawn from the wetland for the water bodies below it. In ecological flood control, it is primarily a matter of retaining the water in the drainage basin.
An Example of the Importance of Wetlands for Flood Control
The Vähä-Komu wetland deals with the water of a 2,200 hectare drainage basin with forest ditches especially in times of flood when the nutrient and sediment loads are at their highest. The wetland is 20 hectares in size, which is about 0.9 per cent of the drainage basin area, and thus its relative size is sufficient for the protection of waters and to balance sudden flood peaks, with the rising water in the wetland acting as a temporary buffer. When the water level rises by 20 centimetres in an area of 20 hectares after thundery rain, this translates as 40,000 cubic metres of water. The filling and emptying of the storage basin balance the flow better than a corresponding rise of water surface in a narrow streambed.
The smaller the drainage basin and the larger the wetland, the more effectively the wetland reduces fleeting flood peaks and acts as a solution for water protection at the same time. The wetlands in Härmälä and Vuorela in Southwest Finland are excellent examples of headwater region wetlands whose size in relation to the drainage basin is great. A wetland of a few hundred square metres is sufficient to handle the runoff waters from a few dozen hectares. In the protection of waters, it is important to look close as well as far and not to forget that brooks and rivers grow out of little rivulets.