Hydrological reconstruction of extreme palaeoflood events 9000–8500 a BP in the Danjiang River Valley, tributary of the Danjiangkou Reservoir, China

The Danjiang River, a major tributary of the Danjiangkou Reservoir, is famed for the current national South-to-North Water Diversion project. Palaeohydrological investigations were carried out in the Danjiang River valley. A set of palaeoflood slackwater deposit (SWD) beds was identified embedded in Holocene loess soil sequences within riverbank cliffs based on sedimentary criteria and analytical results. These palaeoflood SWDs were deposited from the suspended sediment load of overbank flooding. These extreme palaeoflood events were dated to between 9000 and 8500 a BP from optically simulated luminescence (OSL) dating and pedostratigraphical correlation. The palaeoflood peak stages were calculated between 404.4 and 412.5 m using the slackwater flow depth method. The palaeoflood peak discharges were calculated between 8570 and 23,350 m3/s using the HEC-RAS model. These results have extended the flood data sequence of the Danjiang River to a 10,000-year time-scale. The ages of the palaeoflood events coincide well with the contemporaneous global climatic events at 8500 a BP, a time that the monsoonal climate shifted from the early Holocene dry conditions to the mid-Holocene climatic optimum. These findings will play an important role in flood risk estimation, the development of water resources, and will be significant in understanding the effects global climatic variations on hydrological systems in catchment of the Danjiangkou Reservoir.