Abstract
Abstract. A 2600-year long composite palaeoflood record is reconstructed from high-resolution delta plain sediments of the Hasli–Aare floodplain on the northern slope of the Swiss Alps. Natural proxies compiled from sedimentary, geochemical and geomorphological data were calibrated by textual and factual sources and instrumental data. No fewer than 12 of the 14 historically recorded extreme events between 1480 and the termination of the Hasli–Aare river channel correction in 1875 were also identified by coarse-grained flood layers, log(Zr / Ti) peaks and factor 1 anomalies. Geomorphological, historical and instrumental data provide evidence for flood damage intensities and discharge estimations of severe and catastrophic historical floods. Spectral analysis of the geochemical and documentary flood series and several climate proxies (TSI, δ18O, tree-rings, NAO, SNAO) identify similar periodicities of around 60, 80, 100, 120 and 200 years during the last millennia, indicating the influence of the North Atlantic circulation and solar forcing on alpine flood dynamics. The composite floodplain record illustrates that periods of organic soil formation and deposition of phyllosilicates (from the medium high catchment area) match those of total solar irradiance maxima, suggesting reduced flood activity during warmer climate pulses. Aggradation with multiple sets of flood layers with increased contribution of siliciclasts from the highest catchment area (plutonic bedrock) (e.g. 1300–1350, 1420–1480, 1550–1620, 1650–1720 and 1811–1851 cal yr AD) occurred predominantly during periods with reduced solar irradiance, lower δ18O anomalies, cooler summer temperatures and phases of drier spring climate in the Alps. Increased water storage by glaciers, snow cover and snow patches susceptible to melting processes associated with rainfall episodes and abrupt rises in temperature substantially increased surface runoff on slopes and discharges of alpine rivers. This interpretation is in agreement with the findings that the severe and catastrophic historical floods in the Aare since 1670 occurred mostly during positive SNAO (Summer North Atlantic Oscillation) pulses after years or even decades dominated by negative SNAO and cooler annual temperatures.
Highlights
Mountain regions like the Alps cover sensitive and vulnerable ecosystems which are exposed to changes in atmospheric circulation, extreme meteorological events, and the effects of land use
The key-cores retrieved from the Hasli–Aare floodplain (Figs. 1, 2) show alluvial delta plain facies defined by sequences of shallow gravel channel beds, sandy levee and crevasse splay deposits, coarse-grained flood layers within overbank deposits, palustrine fine-grained sediments, organic-rich beds and peat horizons
Our results suggest that composite palaeoflood series can be generated from high-resolution delta plain sediments of the Hasli–Aare, which reproduce the fluvial dynamic and related mechanisms, including trends, clusters and gaps of floods in alpine catchments
Summary
Mountain regions like the Alps cover sensitive and vulnerable ecosystems which are exposed to changes in atmospheric circulation, extreme meteorological events, and the effects of land use. In the context of global climate change, it is striking that the annual mean temperatures in the Alps have increased 3 times more than the global temperature over the last 100 years (Hartmann et al, 2013). The warming trend is not dependent on altitude (Hansen et al, 2010); the strong vertical environmental gradient, with major altitudinal limits such as the treeline, the snowline, and so on, makes the alpine area very sensitive to climate changes. L. Schulte et al.: A 2600-year history of floods in the Bernese Alps, Switzerland
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