Dagvatten i Märsta : förslag till anläggning för ekologisk hantering samt metodöversikt

Abstract: Stormwater in urban areas has been treated as a problem for a long time. The large proportion of impermeable surfaces inhibits the water from infiltrating in the ground to the same extent that it does in nature. This causes intensive surface flow during rain, which in turn causes inconveniences such as decreased accessability, floods and increased risk of desease spreading. Drainage through pipes is the traditional and usually functional metod for storm water disposal. That is as long as water flows are moderate. But precipitation varies naturally in amount and intensity and additional impermeable surfaces make some pipes insufficient for heavy or lasting precipitation. Poor infiltration will in the long run disturb the balance in groundwater levels. In addition to this there is also a need for alternative methods that handle the pollution, a need which has been stressed more and more in the last decades. Older sewer systems combine storm water with wastewater for treatment in purification plants. That way pollution is taken care of; but intensive waterflow decreases the puification effect and pipe overflows can occur in lowland areas. Since the second half of the 20th century, all new stormwater systems are separate from wastewater systems. Stormwater is disposed in nature, which means a risk for species and ecosystems. Today ponds, wetlands and permeable surfaces are often seen in urban environment. These are examples of "ecological stormwater handling", which has had a breakthrough connected to the strife for sustainable development. The idea of ecological stormwater handling is to lead the water back into its natural cycle and let natural processes take care of the purifi cation. Using stormwater in the urban environment also gives other benefi ts. This type of facilities are often designed as nice recreational areas and at the same time the visual water cycle can have an educational function. In some cases it is directly economically beneficial to replace the traditional systems with ecological ones. It is important that the natural systems used for stormwater handling are not at risk of being damaged. Since the 1970's the leading term for alternative methods for stormwater handling has been "local stormwater treatment". Together with the development of techniques and the exchange of ideas, a multitude of expressions have come to be used for description of similar phenomenons. Here "ecological stormwater handling" is used as the main term since it is understandable and quite well established. Within this term "local stormwater treatment" has a more narrow definition. The purpose of the proposal is to show an example of how the methods presented can be taken into practical use. Märsta is a small but expanding town in Sigtuna municipality just north of Stockholm. There are already a number of stormwater treatment facilities and the opportunities of craeating a coherent ecological stormwater system in the urban area. The new facility is to be situated in Steningedalen nature reserve, which streches from central Märsta toward lake Mälaren. The creek of Rosersbergsbäcken has its natural outflow into the stream of Märstaån, which runs through Steningedalen valley. But it is led into a culvert a little further upstream. The idea is to restore the creek and lead it through sedimentation ponds and to merge with Märstaån and at the same time improve the accessability to the area for the inhabitants in Märsta. By linking toghether the new facility with existing ones, a long continuous green strip is created through Märsta and all the way down to lake Mälaren. It will also be a link in a blue-green park strip through Märsta.