Abstract
Lake sediments can retain imprints of past floods, enabling reconstructions that span well beyond instrumental time series. Time series covering thousands of years can document the natural range of flood variability, which is critical for understanding the potential causality between changing flood patterns and climate. Here, we analyzed sediments from Lake Sandvinvatnet in southwest Norway. Detailed environmental magnetic analyses of an 830cm-long sediment core covering the last 6,500 years captured decadal scale trends in local flood frequency. Magnetic susceptibility (MS) assessments were carried out both on split cores and individual samples to track variability in sedimentary influx; the ratios of MS measured at 77 K and 293 K (MS ratios) provided information on potential changes in source regions. The results suggested that sediments from the Buerdalen valley dominate the signal in the core, and the amount of ferromagnetic (high MS) carriers increases during flood events. These carriers were assumed to be transported from slope deposits in Buerdalen during rainstorm-triggered flood events. The reconstructed flood frequency, based on sediment layers with ferromagnetic carriers, showed high variability over the past 6,500 years, and the finding was validated by overlapping with known historical floods in the area. We observed periods with a high frequency of extreme floods (4100-3140 cal. yr BP) compared with intervals with a few or no extreme floods (6050-4100 cal. yr BP). Floods in this region are commonly a result of intense rain events during fall and snow and glacial melt during late spring and summer. The systematic frequency changes during the past 6,500 years suggest a certain persistency in the processes that cause floods, where mean trends in summer temperature and precipitation may have played a role.
Highlights
In October 2014, several villages and small cities in western Norway were hit by destructive floods [Davies, 2014; The Norwegian Water Resources Energy Directorate (NVE), 2020b]
All samples but one of the rate of change (ROC) of magnetic susceptibility (MS) above the 95th percentile had a ratio lower than two (Figure 4B); this indicates that both high values and extreme changes in MS in SA16/17 are correlated with ferromagnetic properties
Since the climate projections indicate a warmer and wetter climate, the situation in the future may be different from any other time interval seen during the late Holocene
Summary
In October 2014, several villages and small cities in western Norway were hit by destructive floods [Davies, 2014; The Norwegian Water Resources Energy Directorate (NVE), 2020b]. The contribution from snowmelt was negligible, whereas glacier melt constituted a substantial contribution in (partially) glaciated catchments [The Norwegian Water Resources Energy Directorate (NVE), 2020a]. In the small city of Odda, the flood destroyed five houses and critical infrastructure, including roads and bridges (Dannevig et al, 2016). Odda was only one among numerous cities in western Norway that were hit by the flood in 2014 and again in 2015. The tendency of increased flooding due to changing climatic conditions has raised concern among many municipalities and at a federal governmental level (Alnes et al, 2018; Norwegian Climate Service Center, 2020)
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