Rekonstruksjon av flommer i Glomma gjennom holosen - Effekter av klimaendringer på ulike flomregimer

Abstract

When the Glomma river discharge exceeds 1500m3/s a river birfuaction is activated at lake Vingersjøen in Kongsvinger, Norway. The distributary of the Glomma river allows eroded and transported flood sediments from the Glomma catchment to be deposited in lake Vingersjøen. Studying high-resolution sediment cores from lake Vingersjøen, this study presents an extension of instrumental and historical flood records and produces a continuous record of holosen river-flood activity during the last approximately 7300 years. The sediment cores contain light grey minerogenic layers which are interpreted as flood layers based on statistical analyzes of loss-on ignition (LOI), dry bulk density(DBD), x-ray fluorescence core scanning (XRF) and magnetic susceptibility (MS) measurements. About 200 flood events have been identified and reveal that the frequency of flood has changed significantly during the Holocene, with low activity during the warm periods of the mid-Holocene and the Holocene Thermal Maximum (HTM) (c. 8000-5000 cal. yr BP), and increased activity during the cold periods of the neoglacial (after c.5000 cal. yr BP) and the little ice age (c. 500-100 cal. yr BP). Largest activity occurring from 2500 cal. yr BP until present during cold climate and heavy precipitation. Catchment samples have been collected from lake Vingersjøen´s catchment to separate flood signals from Glomma during colder climate periods and local runoff from rain floods and extreme precipitation events during warmer climate periods. Results suggest snow melting as the dominating flood triggering mechanism, although floods in lake Vingersjøen during the HTM may indicate a shift towards rainstorm-triggered floods in this period.