Flood mitigation in Trodalen, Øyer, Norway

Øyer municipality started a development project to establish 220 units of family housing, but after the first houses were built the project was put on hold due to potential flooding problems and lack of adequate flood protection. The development area is surrounded by the steep river Søre Brynsåa River on the northwest side, and the closed Trobekken creek on the other side. During heavy precipitation, the river and the creek cause flood problems. These challenges are expected to increase in the future due to climate change. Nature-based Solutions was therefore implemented to deal with problems in the Trobekken Creek.

The lower part the creek was closed for a distance of approximately 120 meters.  Open watercourses are more robust and therefore more inclined to handle floods and will also form a new habitat for species associated with water. The opening of the creek increases its capacity during a flood situation. A small retention dam was established to retain sediments that are transported in the creek during flood situations and to reduce water velocity. This implementation keeps the creek as natural and untouched as possible, and limits the future maintenance to this specific area. It will also decrease the need for extensive measures upstream and limit sediment transport to the blue-green park area (described below) further down. Vegetation measures were implemented along the creek for erosion protection. There were no specific erosion control measurements needed upstream of the reopened creek. A blue-green park was created as buffer zone in the lower part of the housing development area. This zone serves as a retention measure during a flood and as a blue-green nature park for inhabitants for the rest of the time.