Palaeoflood Hydrology: Reconstructing Rare Events and Extreme Flood Discharges

The estimation of rare, large magnitude floods is problematic due to short gauging station records and their limited spatial distribution. The instrumental record can be lengthened by hundreds or thousands of years by estimating discharges of past floods using geological and botanical evidence (palaeostage indicators) left by flood waters. In the former, stratigraphic sequences of sand and silt deposited in slackwater and eddy sedimentary environments are described and dated by geochronological methods (radiocarbon and luminescence techniques). In the later, flood impacts on trees producing scars and other damages (e.g., candelabrum trees) are identified and dated using tree-ring counting. These palaeostage indicators enable to calculate flood discharges using hydraulic modelling, and both flood ages and magnitudes are the input data necessary for improving flood frequency analysis. The scientific and technological interest of these studies is evident for design purposes of critical structures (dams, bridges), risk planning, and for understanding the response of flood patterns to climate change.