Sedimentological and hydrological studies of the palaeoflood events in the Ankang east section in the upper reaches of the Hanjiang River

Abstract

通过沿汉江上游河谷深入的考察,在安康东段发现了典型的古洪水滞流沉积剖面.通过采集样品、实验分析,确定它们是古洪水在高水位滞流环境当中的悬移质沉积物.根据地层对比、OSL测年和相关文化层年代,确定它们分别记录了发生在BL+AL与YD事件转折阶段(12500 a B.P.)的古洪水事件和发生在1000-900 a B.P.(1000-1100AD),即北宋后期的洪水事件.根据古洪水SWD的高程恢复其洪峰水位,结合相关参数,利用面积比降法计算恢复流量.结果表明在万年尺度,汉江上游古洪水洪峰流量介于35970~47400 m³/s之间.同时,利用2010年大洪水洪痕恢复计算洪峰流量,对古洪水洪峰流量计算结果进行了验证.进而结合历史洪水和观测洪水数据,获得了汉江上游万年尺度洪水洪峰流量与频率关系.这为汉江上游的水利水电和交通工程建设以及沿岸城镇防洪减灾提供了基础性数据.;Through field investigation in the upper reaches of the Hanjiang River,palaeoflood slackwater deposits were found in the Ankang east section.The results of sedimentary analyses indicate that these slackwater deposits are typical suspended sediment load deposits of floodwater.By stratigraphic correlation,OSL dating and archaeological dating,the palaeoflood events represented by SWD1 were dated to be between the palaeo-climatic stage of BL+AL and Younger Dryas Event (YD,12500 a B.P.).During this period,the upper reaches of the Hanjiang River experienced a series of rainstorm flood events.The pollen records from the Southern Lake of Shennongjia area in the upper reaches of the Hanjiang River also suggest that this was the period of high climatic fluctuation frequency.The SWD2 in the east Ankang section appears in the bottom of well-developed modern topsoil layer.At the edges of Ankang-Yunxian river terrace,the set of SWD layers often cover the culture layer which belongs the Northern Song Dynasty when climate condition was very unstable,and serious drought and flood disaster occurred based upon historical records.The palaeoflood events recorded by SWD2 were dated to be AD 1000-1100 (i.e.1000-1100 a).Based on the reconstruction of palaeoflood peak stages and the hydrological parameters for the river section,the palaeoflood peak discharges were between 35970 and 47400 m³/s.These values were checked with the the reconstruction of the flood in 2010 based on the stage indicator identified during the fieldwork,and the results showed the Ankang hydrologic station measured data error is only 4.52%.Thus we believe that the peak flow calculation for the upper reaches of the Hanjiang River canyon ancient flood is reasonably accurate,the calculation results are reliable.Based on our calculation from sedimentological perspective the historical flood investigation results were combined with observed flood series,with a large value of the complete sequence of frequency calculation method,and the upper reaches of Hanjiang River in the eastern part of Ankang section million-years-scale peak flow and frequency relations were established.The results provide data for establishing flood discharge-frequency relationship at a 10000-year time-scale in the upper reaches of the Hanjiang River.This is very critic and will provides key data for hydraulic engineering and flood control in the upper reaches of the Hanjiang River and the flood control and disaster mitigation engineering construction along the waterfront towns in the region,except the scientific value of the Hanjiang River's regional changes in climate and hydrology in response to the global climate changes.