Abstract
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 the climate is also 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 timescales. We observe that periods with increased flood activity (4000–2000 years ago and <1000 years ago) correspond 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 numbers of floods (10 000 to 4000 years ago and 2200 to 1000 years ago). The flood frequency shows significant non-stationarities within periods with increased flood activity, as was the case for the 18th century, including the 1789 CE (“Stor-Ofsen”) flood, 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.
Highlights
Floods are among the most widespread natural hazards on Earth
The results from the X-ray fluorescence (XRF) scan (Ti/total scatter, Ca/total scatter, and K/total scatter) and the greyscale value (BW) from a photo of the core are shown as a function of depth in Fig. 9 together with a photo of FLP213
This gyttja, carrying a low minerogenic content, is referred to here as “the background signal” which is characterized by its dark colour (BW < 30), high loss on ignition (LOI) (30 %–40 %), low dry-bulk density (DBD) (< 0.3 g cm−3), and magnetic susceptibility (MS) with values close to zero (< 5 SI × 10−5)
Summary
Floods are among the most widespread natural hazards on Earth. The impacts, destruction, and costs associated with hazardous floods are increasing in concert with climate change and increase in economic value within areas susceptible to floods, a tendency most likely to strengthen in the decades to come (e.g. Alfieri et al, 2017; Hirabayashi et al, 2013; IPCC, 2012). The impacts, destruction, and costs associated with hazardous floods are increasing in concert with climate change and increase in economic value within areas susceptible to floods, a tendency most likely to strengthen in the decades to come In Europe, spatial flood patterns are changing in terms of both timing and magnitude (Blöschl et al, 2017, 2019), challenging us to examine new ways of interlinking different types of data, and flood information on different timescales. Earlier studies have shown that uncertainties can be reduced if, for instance, historical data are included in estimation of floods with long return periods Engeland et al.: New flood frequency estimates for the largest river in Norway
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