Flood mitigation options to reduce flooding risks in Bjørkelangen town

Summary

We assessed three different flood mitigation options, based on simple models, to reduce flooding problems in Bjørkelangen town. The mitigation options i) small debris dams, ii) re-meandering of Lierelva, and iii) restoration of Liermåsan and Bliksrudmåsan were considered for current and future flow scenarios in a climate change context. We evaluated flood reduction potential for peak flow episodes with return periods of 100 years. Restoration of Liermåsan and Bliksrudmåsan may store approximately 4-5% of the total water volume comprising maximum daily peak flow and approximately 2-3% in context of future climate change. Importantly, however, the total water storage capacity increases non-linearly with increasing area of peat land and establishing peat land in 5% of the catchment would create a 10% flooding reduction. Construction of small debris dams in forested areas at approximately 1,000 sites may store approximately 5% of the total water volume comprising the 1-day peak flow and approximately 2-3% in context of future climate change. Re-meandering of Lierelva was not found to have significant effects on flood mitigation in Bjørkelangen. Restoration of Liermåsan and Bliksrudmåsan should receive highest priority due to a combination of highest cost-efficiency and high flood mitigation potential.