CGU HS Committee on River Ice Processes and the Environment 18 th Workshop on the Hydraulics of Ice Covered Rivers Quebec City, QC, Canada, August 18-20, 2015 Flood frequencies in places prone to ice jams, case city of Tornio Lauri Ahopelto 1 , Mikko Huokuna 2 , Juha Aaltonen 3 and Jarkko J. Koskela 4 Finnish Environment Institute, Mechelininkatu 34a, P.O.Box 140, Helsinki, Finland, FI-00251 lauri.ahopelto@ymparisto.fi mikko.huokuna@ymparisto.fi juha.aaltonen@ymparisto.fi jarkko.j.koskela@ymparisto.fi ABSTRACT: The river Tornionjoki and the twin city of Tornio and Haparanda on the border of Finland and Sweden is known for ice jams. There have been multiple ice jam floods along the river during the last century that have caused monetary losses. Since the beginning of 2014 flood damage compensation in Finland is paid by insurance companies instead of the government. Normally damages resulting from a flood that has return period of 50 years or more are compensated. Finnish Environment Institute (SYKE) writes statements for the insurance companies about flood frequencies in unclear cases. This is not an easy task in the case of an ice jam flood because of the lack of observations and the limited resources. There are relatively few studies on flood frequency analysis in places prone to ice jams and they have mainly focused on developing the methodology (e.g. Beltaos, 2012). This study aims to apply such methodologies in places where there is relatively much information on ice jams and ice conditions compared to other sites in Finland. With a such application it is studied, how ice jam monitoring practices should be improved during the ice jam floods and whether general national guidelines for estimating flood frequencies could be improved. River Tornionjoki has an exceptionally long record of ice breakup dates in Finland, over 300 years. Also ice thickness measurements are more plenty than in other Finnish rivers. Analysis shows that for this location the direct method can be used for flood frequency analysis. However, because of the lack of proper data, the direct method is not an option for most of the Finnish rivers. A HEC-RAS model application shows that the location is very complicated and some key information and data is missing to trustworthily estimate frequencies with the