Utvärdering av öppna dikens status och funktion : en förstudie i Västmanlands län

Abstract: Evaluations of open Ditch status and function, with an assessment of suitability of a conversion to a two stage ditch as a low maintenance alternative. To illustrate Swedish drainage problems this work focused on two agricultural drainage associations assessed in detail with a new method called MADRAS (Minnesota Agricultural Ditch Reach Assessment for Stability). With the assessment of the MADRAS method estimates were made to conclude the ditches operating problems. This was made through measurements of sediment depths, mass bank failures etc. in order to quantify ditch problems and propose practical actions for improved ditch stability and water quality. The MADRAS surveys were conducted in two main ditches that drains 2900 hectares of land on the northern shores of Mälaren. In the study measurements of the ditch cross-sections were also conducted to compare actual profiles with the original map profiles. The evaluation showed that parts of the studied ditch sections were operatively poor; main reason was mass bank failure, undercut channel banks, sediment aggregations and narrowed ditches. The profile survey shows that a majority of the profiles 80 % has sediment accumulations there remains only 64 % of the original profile from the map. The survey also shows that 20 % of the profiles have been expanded by an average of 117 %. The study gives support to conduct further detailed investigations in the area and then accordingly determine an appropriate restoration plan. The problems that ditch companies face today are to find means to deal with the higher flows. When this no longer can be overcome with the deepening of ditches, since additional drainage today severely restricted and new rulings from the water council is demanded for even smaller changes within ditch companies, alternative solutions must be proposed. Here proposed is to target especially exposed ditch stretches and reshape them into a two-stage ditch. There are many advantages with this design, but despite the fact that they do not lead to increased drainage they currently fall under the same strict rules and needs court rulings to trial the impact assessment. To have drainage companies to implement two-stage ditch technologies in their ditches a simplified permitting process for construction is necessary. Imposition of these constructions is justified as they have wetlands characteristics of environmental benefits these are a greater immobilization of nitrogen and phosphorus as well as decreased maintenance requirements. The ditch evaluation method MADRAS is here considered to be a practical tool to be able to gather quantifiable measure of the ditch failing features. The evaluation provides a good foundation for finding the types of measures that should be applied to improve the function of the particularly vulnerable ditch stretches. The evaluation showed that parts of the surveyed ditches were operationally poor, primarily because of mass bank failure, undercut channel banks and narrowed ditches. A viable solution in the worst affected ditch sections is to introduce a two-stage ditch. These measures would result in more stable channel banks but also even out peak channel flows that in the long run will improve water quality. MADRAS method is made to measure and estimate the drainage systems operating problems and thereby determine if a maintenance or restoration plan may be developed in the area. The ditch evaluation method MADRAS is a suitable method for ditch evaluation in Swedish conditions for identifying problem affected ditch stretches.