Mänsklig påverkan på mindre vattendrag i skogslandskapet : en inventering av vägtrummor och skyddszoner kring dessa i Uppsala län och norra Västmanlands län

Abstract: Interference in an ecosystem means a disturbance. When harvesting, forest roads have to be build to cope with heavy duty vehicles. Crossing brooks can not always be avoided. In this case bridges or culverts have to be built. There are many different kinds of culverts and depending on what type you choose and how you place the culvert, the watercourse and the water organisms can be harmed in different ways. If you choose the best alternative and place the culvert correctly, migration of aquatic living organisms is possible. Migration barriers can be high velocity in the culvert, large jumping height from the brook or too small water depth close to the culvert. Forestry is carried out as effectively as possible. This can result in damaged protection zones along the brooks. To leave protection zones means incurred costs. Furthermore, it takes time to consider which trees to leave and the harvester may not see the watercourse or may lack knowledge about the value of protection zones along streams. I inventoried 118 culverts. Johan Spens at the Department for Aquaculture at SLU in Umeå has developed a calculating model, the ecohydraulic model, which can predict a culverts function and determine whether it is a possible migration barrier. I tested the model by comparing it with two other models, System Aqua developed by the Swedish Environmental Protection Agency and guidelines developed by the Swedish Fisheries Agency. I could test all three different models at 21 culverts, comparing the results in respect of whether the culverts were migration barriers. The ecohydraulic model calculated that 17 culverts was a migration barrier, the Fishery Agency's guidelines resulted in 18 barriers and System Aqua in 10. In comparison to System Aqua and the Swedish Fisheries Agency guidelines, I conclude that the ecohydraulic model provides a realistic evaluation of the culvert function. It also takes into consideration seasonal variations in water levels, thus providing information on whether the culvert is under-dimensioned. Furthermore, the model is easy to use for persons without aquatic – ecological competence.