Identifying the sedimentary imprint of high-frequency Holocene river floods in lake sediments: development and application of a new method

In an effort to improve the understanding of floods and how they relate to climate we have developed a method that objectively identifies the sedimentary imprint of individual river floods in downstream lake basins. The method is applied to a high-resolution lake sediment core retrieved from Southern Norway, resulting in a detailed record of Holocene river-flood activity covering the last 10,000 years, including floods also recorded by instrumental and historical data. The minimum number of individual floods recorded for this period is similar to 100. On multidecadal to centennial timescales significant change in flood frequency is observed that arguably can be attributed to large-scale climatic changes such as the varying amount of winter precipitation and summer rainstorms. Flood frequency was low during the early Holocene (9770-7700 cal yr BP), and even lower for the period that followed, lasting until 5500 cal yr BP For the next 2500 years, a modest increase in flood activity followed. This trend was truncated 2500 cal yr BP by a sudden shift towards increased flooding frequency. With the exception of a short interval around 1000 cal yr BP, when the number of floods again was low, this tendency of increased flood activity prevailed until present; including "Stor-Ofsen", a large flood that occurred in AD 1789, and also three other historically documented river floods. (C) 2010 Elsevier Ltd. All rights reserved.