| The Yellow River and Hanjiang River Basin has a very far-reaching impact on China’s ancient civilization in the middle reaches of the Yellow River, the Jin-shaan Gorges of the middle Yellow River and upstream Hanjiang River since ancient times are frequent flood disasters, however flood data only tens of years or even decades, cannot meet the requirement that the frequency of flood, but cannot guarantee the accuracy of both sides the water conservancy project of flood control standard, so the study of Palaeoflood of extreme events is very necessary.We have done some field survey in the Jin-shaan Gorges of the Yellow River and the upper Hanjiang River, and we found two profiles which content several slackwater deposits and palaeostage indicators. They were analyzed in the sedimentological laboratory in Shaanxi Normal University, including particle-size distribution, magnetic susceptibility, percentage composition of calcium carbonate and loss on ignition. To determine the LHK, WCC and CFC section of Palaeoflood events during deposition of the peak water level with relationship between SWDs thickness and sediment paleoflood method, slope area method model estimated maximum peak discharge of LHK and WCC section of Palaeoflood directed by SWDs, We also reconstructed the peak discharge of the modern flood of the river with the stage indicator at the site with the same method. This proved that the reconstruction of the palaeofloods is credible. The main results are as following:(1) Both eyewitness accounts and the analysis of proxy environmental indicators of the LHK, WCC profiles show that all of the deposits in the study area can be identified into the classical slackwater deposits. A set of palaeoflood slackwater deposits (SWDs) were identified interbedded in the Holocene loess-soil sequence within the cliff riverbanks, the profile of WCC in the upper reaches of the Hanjiang River is located in the Yangxian County section of Hanjiang River, the left bank of a terrace road inside the platform. The two palaeoflood SWDs has a wavy horizontal bedding, the level is clear, compact and compact structure compared to other layers in the wild, easy and other layer identification.(2) Magnetic susceptibility, loss-on-ignition, calcium carbonate and grain-size distribution were analyzed in the laboratory. The results show that the LHK and WCC profiles are relatively similar. The magnetic susceptibility and loss-on-ignition values of the four SWDs in LHK profile are low and the calcium carbonate is opposite; the magnetization of SWDs in WCC profile rate and palaeoflood loss are gradually reduced, the content of CaCO3 increases suddenly. The analysis of Magnetic susceptibility, grain-size Sedimentology characteristics indicated that the ingredients of the SWDs of LHK and WCC were mainly silt. This indicates that the grain degree of the Yellow River’s palaeoflood SWDs to rough than the ancient flood SWDs of Hanjiang river. The sorting of the two are good, the natural frequency distribution curve was high and narrow, showing a normal distribution. The analysis of physical and chemical indexes show LHK and WCC these two containing sedimentology are typical of ancient floods SWDs suspended sediment profiles.(3) Through stratigraphic correlation analysis and OSL dating, the 4 layers SWDs of the LHK in the middle Yellow River of Jin-shaan gorges is a reliable record during 3200-2900 a B.P. flood events; the SWD1 layer of WCC profile in the upper Hanjiang River is recorded in the range of 3100～2900 a occurred between B.P., SWD2 layer is the ancient flood 1730～1685 a B.P. (AD220～265). The two section of the palaeoflood events in the global climate change events consistent with. At the end of the mid Holocene Mega thermal period in about 3200～2900 years ago, the climate from warm humid to arid and cold change obvious deterioration stage, lead to extreme climatic events have mutations continue to appear in the national scope, flood disaster is more frequent occurrence, this is our country’s end the early Western Zhou Dynasty, the social unrest and historical turning stage, so the LHK profile of the Yellow River and WCC profile of the upper reaches of the Hanjiang River ancient flood SWDs is the period of climate change recorded in 1800～1700 a B.P. during this period, the climate also is in a stable state, resulting in social unrest, poverty, war continuous, frequent natural disasters, the Hanjiang River in Yangxian County of SWD2 in WCC profile is the palaeoflood period climate change record..(4) From the paleoflood detention sedimentary layer thickness and elevation, the flood peak water level of the 4 paleoflood SWD records were determined. The flood peak flow were supposed to be between 48190-52260 m3/s,18780～19050 m3/s, 18460～19280 m3/s after the reconstruction through the gradient method. Using this method, the calculated flood peak of the modern flood compared with the real data. At the same time, the recovery results flow also in line with Drainage area- big flood peak discharge formula that Baker proposed. This showed that the method of calculation of the Holocene flood peak flow, hydrological parameters and calculation results were reasonable.The results of this study, the success of sedimentology and hydrology reveals the basic features of the Yellow River in the Jin-shaan gorges and upstream Hanjiang River, its LHK and WCC profile character description also provides certain help for the following the river paleoflood study; the Jin-shaan gorges and upstream of Hanjiang River provides more research on hydrology palaeoflood more abundant information, the Yellow River water resources development, flood control and water conservation has laid a scientific foundation, has very important scientific significance of revealing regional climate and hydrological change and global change the response law. |
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