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Hydrology and Earth System Sciences An interactive open-access journal of the European Geosciences Union
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Preprints
https://doi.org/10.5194/hess-2020-269
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/hess-2020-269
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  10 Jun 2020

10 Jun 2020

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This preprint is currently under review for the journal HESS.

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 Kolbjørn Engeland et al.
  • 1The Norwegian Water Resources and Energy Directorate, Oslo, Norway
  • 2UDepartment of Geosciences, University of Oslo, Norway
  • 3Department of Earth Science, University of Bergen, Norway
  • 4Bjerknes Centre for Climate Research, Bergen, Norway
  • 5NORCE Climate, Bergen, Norway

Abstract. The Glomma river is the largest in Norway with a catchment area of 154 450 km2. People living near the shores of this river are frequently exposed to destructive floods that impair local cities and communities. Unfortunately, design flood predictions are hampered by uncertainty since the standard flood records are much shorter than the requested return period and also the climate is expected to change in the coming decades. Here we combine systematic- historical and paleo-information in an effort to improve flood frequency analysis and better understand potential linkages to both climate and non-climatic forcing. Specifically, we (i) compile historical flood data from the existing literature, (ii) produce high resolution X-ray fluorescence (XRF), Magnetic Susceptibility (MS) and Computed Tomography (CT) scanning data from a sediment core covering the last 10 300 years, and (iii) integrate these data sets in order to better estimate design floods and assess non-stationarities. Based on observations from Lake Flyginnsjøen, receiving sediments from Glomma only when it reaches a certain threshold, we can estimate flood frequency in a moving window of 50 years across millennia revealing that past flood frequency is non-stationary on different time scales. We observe that periods with increased flood activity (4000–2000 years ago and < 1000 years ago) corresponds broadly to intervals with lower than average summer temperatures and glacier growth whereas intervals with higher than average summer temperatures and receding glaciers overlap with periods of reduced number of floods (10 000 to 4000 years ago and 2200 to 1000 years ago). The flood frequency shows significant non-stationarities within periods having increased flood activity as was the case for the 18th century, including the AD 1789 (Stor-Ofsen) flood being the largest on record for the last 10 300 years at this site. Using the identified non-stationarities in the paleoflood record allowed us to estimate non-stationary design floods. In particular, we found that the design flood was 23 % higher during the 18th century than today and that long-term trends in flood variability are intrinsically linked to the availability of snow in late spring linking climate change to adjustments in flood frequency.

Kolbjørn Engeland et al.

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Kolbjørn Engeland et al.

Kolbjørn Engeland et al.

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Short summary
We combine systematic- historical and paleo-information to obtain flood information from the last 10 300 years for the Glomma river in Norway. We identify periods with increased flood activity (4000–2000 years ago and the recent 1000 years) that corresponds broadly to periods with low summer temperatures and glacier growth. The design floods in Glomma were more than 20 % higher during the 18th century than today. We suggest that trends in flood variability are linked to snow in late spring.
We combine systematic- historical and paleo-information to obtain flood information from the...
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