Hydrol. Earth Syst. Sci., 24, 5595–5619, 2020 https://doi.org/10.5194/hess-24-5595-2020 © Author(s) 2020. This work is distributed under the Creative Commons Attribution 4.0 License. New flood frequency estimates for the largest river in Norway based on the combination of short and long time series Kolbjørn Engeland1, Anna Aano1,2, Ida Steffensen3, Eivind Støren3,4, and Øyvind Paasche4,5 1The Norwegian Water Resources and Energy Directorate, Oslo, Norway 2Department of Geosciences, University of Oslo, Oslo, Norway 3Department of Earth Science, University of Bergen, Bergen, Norway 4Bjerknes Centre for Climate Research, Bergen, Norway 5NORCE Climate, Bergen, Norway Correspondence: Kolbjørn Engeland (
[email protected]) Received: 2 June 2020 – Discussion started: 10 June 2020 Revised: 7 October 2020 – Accepted: 13 October 2020 – Published: 24 November 2020 Abstract. The Glomma River is the largest in Norway, with a icant non-stationarities within periods with increased flood catchment area of 154 450 km2. People living near the shores activity, as was the case for the 18th century, including of this river are frequently exposed to destructive floods that the 1789 CE (“Stor-Ofsen”) flood, the largest on record for impair local cities and communities. Unfortunately, design the last 10 300 years at this site. Using the identified non- flood predictions are hampered by uncertainty since the stan- stationarities in the paleoflood record allowed us to estimate dard flood records are much shorter than the requested re- non-stationary design floods. In particular, we found that the turn period and the climate is also expected to change in the design flood was 23 % higher during the 18th century than coming decades.