Study On The Holocene Ancient Hydrological And Climatic Events In The Jingyuan Section Of The Yellow River And The Longmen Section Of The Yihe River

Holocene abnormal climatic events have been a hot topic in studies on rapid or sudden global climatic change. Recently, global-scale hydrological models have been undertaken in many different regions. These results show that global climate change may substantially alter river flow regimes. These anomalous patterns led to substantial variability in fluvial systems at certain periods. Especially in monsoonal regions, the fluvial systems have an intensified response to increasing climatic variations. Under the abnormal climatic conditions, cataclysmic hydroclimatic episodes, e.g., serious droughts and extreme flood disasters, occurred frequently in river basins.Holocene paleoflood hydrology plays a significant role in determining how river systems respond to global anomalous climatic events. Our investigations use paleohydrology, sedimentology, Chronology and paleoclimatology in the Jingyuan-Jingtai gorges in the upper Yellow River, and the Longmenxia gorges in the middle Yihe River to identify physical records during the Holocene. The main objectives are to establish the chronology of hydroclimatic events and provide solid evidence that extreme paleofloods and severe droughts are part of climatic declines during global climate change. The findings are listed as follows:(1) During the field investigations in combination of the sedimentology knowledge, the slackwater deposits bedsets (SWDs) in two sites were investigated respectively:the 14 SWDs were deposited on the frontier of the first terrace land and were identified at the Jinpingcun site (JPC) in the Jingyuan-Jingtai river reach in the upper Yellow River gorges; the 5 SWDs were interbedded into the Holocene loess-sol profile (L0/S0) and were identified at the Longmenxia site in the middle Yihe River gorges. The analyses on the sedimentological and physical-chemical properties, the mentioned SWDs above were deposited during the Holocene extreme flood after the flood recession. Therefore they are the physical materials to reconstruct the paleoflood hydrology.(2) In the study, the paleoflood stages are reconstructed by the SWD top elevation methods, end-points methods or the thickness of the SWD and the estimated suspended sediment load during the extreme paleofloods. The paleoflood peak discharge estimations were calculated by the slope-area method or the HEC-RAS model methods.The results are as followings:the peak paleo-stages at the JPC site were 1307.40-1311.40 m and the reconstructed peak discharges were 11900-17700 m3/s respectively; the peak paleo-stages at the LMX site were 156.90-158.80 m and the reconstructed peak discharges were 11000-15800 m3/s respectively. Based on the Pearson-Ⅲ curves and combining with the paleoflood discharge data, historical flood discharge data and the gauged data, the return period were analyzed at the Lanzhou gauge station and Longmen gauge station. The results above mentioned exert significant influence in the improvement for the accuracy of the unknown extreme and rare floods.(3) Ages of the paleoflooding events were determined by the pedo-stratigraphic correlation and optical simulated luminesces method (OSL). The chronologies at the three studied sites were reconstructed:the paleoflood events at the JPC site occurred at 3200-3000 a BP; the paleoflood and the historical events at the LMX site occurred at 3200-3000 a BP,1800-1700,700-550 and 350-250 a BP.(4) Paleo-hydroclimatic correlations are the focus in the paleohydrological study. The results form the paleoflooding events and the Holocene climate change show the well correlations between extreme flooding events and the abnormal climate change. The climatic background of the studied paleoflood events were listed below: the transition from the Mid-Holocene Climatic Optimum to the Late-Holocene, the climatic deteriorations from the East Han to Weijin Dynasty, the transition from the Medieval Warm Period (MWP) to the Ming-Qing Little Ice Age and the Ming-Qing Little Ice Age. The climatic characteristics above mentioned were characterized by the climatic instability and deteriorations. Besides, the results show that both extreme floods and droughts were part of climatic variability during the climate change. (5) The ideal sections used to reconstruct the paleo-discharge are the stable, anti-erosion bedrock gorges, in order to guarantee the accuracy and reliability of the reconstructed results. Besides, from the experience view and generally speaking, the method, in combination with the thickness of the SWD and the estimated suspended sediment load in the flood water to reconstruct the paleo-stages and the HEC-RAS method to reconstruct the paleo-discharges, is the most suitable way to study the paleohydrology. And, the tests on the roughness n are necessary to lower its influence on the the results of the paleo-discharges. At last, we also need to take kinds of measures and ways to calibrate and test the reliability and accuracy of the paleohydrological reconstruction.(6) Based on the sedimentology and the sedimentological characteristics in the fieldwork, we distinguish the sedimentological differences between the paleoflood slackwater deposits and the dammed-lake deposits. And, with the chronology, paleohydrology and the historical documents, we identified the historical floods occurring in the AD223 and AD 1761. The paleo-stages and the paleo-discharges of the two historical floods were reconstructed by the paleohydrological methods. The results suggested that the paleo-stages and paleo-discharge of the AD 1761 flood were higher than those of the AD223 flood.