Hantering av dagvatten : sambandet mellan dagvattenanläggningens storlek och dess total kostnad

Sammanfattning: A changing climate could result in more intense rainfall events. This might result in an increasedload on stormwater systems in the urban environment. The building or updating of stormwatersystems is relatively expensive. To reduce costs, it is important to build the right type of system.This project aims to examine what type and size of stormwater system is most efficient from aneconomic perspective. Different parameters were analysed to compare alternative types of stormwater systems, includingthe effectiveness in retaining water, the building cost as well as the maintenance cost. In this reportthree alternative solutions were studied: retention tanks, retention ponds and a local solution inthe form of gravel swales. These alternative options were modelled using two different numericalmodels within the software MIKE URBAN. One smaller model over Viksjö and one larger modelover Kungsängen. Retention tanks and retention ponds were split into three systems with a differentnumber of facilities. The systems were examined for ten and one facility in both models, threefacilities in the smaller model and four in the larger. The local solution was modelled by mimickingthe swales spread across all catchment areas. To examine the effectiveness in retaining water all thesolutions and systems had to achieve a set of criteria: a max flow in the end pipe of the system,no flooding, and a rain with recurrence interval of 10 years and duration of 24 hours. From theset of critera a total retention volume was obtained from modelling the different systems in MIKEURBAN. The costs of building and maintaining the systems were calculated from the retentionvolumes. As the models had existing pipe-networks two results were obtained, one including andone excluding the cost of the changes made to the pipe network. The result calculated from modelling retention volumes in MIKE URBAN showed that the systemwith a larger number of facilities required a lower total retention volume than the systemswith fewer facilities. Retention tanks with ten facilities required the smallest total retention volumefollowed by retention ponds, whereas gravel swales required the largest volume. The local solutionrequired the least change of the pipe network followed by systems with ten, three/four and onefacility. The economic calculations showed that the building cost for retention ponds was the lowest followedby gravel swales and retention tanks. The maintenance cost of retention ponds was the highestfollowed by retention tanks and gravel swales. The total cost, pipes excluded, calculated from thederived retention volumes, building cost, and net present value of the maintenance for 40 years showedretention ponds being the best solution from an economic perspective. Retention ponds costaround four times less than retention tanks and gravel swales. Gravel swales and retention tankscost about the same. When the cost of the pipe network was included retention ponds were still the best option, followedby gravel swales and retention tanks. The difference in cost depending on the number of facilitiesincreased in favour of the systems with a larger amount of facilities when the cost of rebuildingparts the pipe network was included