Holocene Palaeoflood Events Related To Climate Change In The Upper Weihe River And The Middle Yellow River

The Holocene was the rapid expansion period of the mankind's civilization,and was also the vital period of social booming.Meanwhile,the human environment has been evoluted into contemporary aspect.The emergence of agricultural civilization and the improvements of productivity tools play an important function in promoting social progress revealing the intimately relationship between human and nature.The upper and middle reaches of Yellow basin is suffered from frequent flood disaster in the period of the Holocene,where there are many sites of historical and cultural heritage.It has a profound impact on Chinese ancient civilization.Palaeoflood hydrological investigations and textual criticism were implemented to conduct a study in the Tianshui East reach in the upper Weihe River and in the Longmen stream segment of the middle Yellow River.We used the immediate reference and the sedimentological criteria to identify the palaeoflood Slackwater deposits(SWD)along the Weihe and Yellow River.The peak flood stages and discharges of these palaeoflood events were estimated by using the principle of hydraulics and hydrology.The combination of the pedo-stratigraphical correlation and Optically Stimulated Luminescence(OSL)method has been used in dating specific ages of Holocene palaeoflood deposits.The climatic background above the studied palaeoflood events were characterized by the climatic instability and deteriorations.Based on the Pearson-?(P-?)curves and combining with the gauged data,historical flood discharge data and the palaeoflood discharge data,the hydrologic frequency analysis were implemented at the gauge station.(1)Indications of four-layered palaeoflood slackwater deposits of the Holocene and palaeoflood were found interbedded within the cliff riverbanks along the Huanancun(HNC)profile of the Weihe river,and found four-layered palaeoflood slackwater deposits of the Holocene and palaeoflood indicators interbedded within the cliff riverbanks at the Xianghanqiao(XHQ)profile of the Yellow river.Through the integrated estimation of the field investigation,sediment and the physical and chemical analysis,we found these SWD formed through deposition of the suspended sediment load.They represent the geology evidence of palaeoflood events that took place in prehistoric period.(2)We observed the characteristics and hydrological parameters of the sediment profile to provide fundamental data for hydrological recovery.Our methods for the analysis of these sediment samples included particle-size distribution,magnetic susceptibility and loss on ignition.We selected 13 cross sections at the HNC site along a 2-km river and selected 15 cross sections at the study site along a 3-km river reach in XHQ site.Estimated results were obtained by using the slope-area method and the U.S.Army Corps of Engineers River Analysis System(HEC-RAS)model.These indicated that the palaeoflood peak discharges were 11,420 and 20,100 m3/s.and 46,280-48,800 m3/s,respectively.The sensitivity test on the model performed an error between-10.99%and 8.79%and-10.88%and 7.85%using the same hydrological parameters respectively.(3)Based on the gauged flood data,historical floods and Holocene palaeoflood discharge,the flood frequency-peak discharge relationship curves was established.At the Nanhechuan gauge station,the hundred-year flood discharge was estimated to 9430 m3/s,the millennium flood discharge was estimated to 14170 m3/s,and the ten thousand-year flood discharge was estimated to 20100 m3/s;and at the Longmen gauge station,the hundred-year flood discharge was estimated to 28500 m3/s,the millennium flood discharge was estimated to 34800 m3/s,and the ten thousand-year flood discharge was estimated to 48800 m3/s.It has achieved a good result in increasing the reliability and reasonability of the design floods by joining the palaeoflood data.(4)Choosing a good cross section was one of the keys to reconstruct the palaeoflood event.We should choose a graded profile,which the river channel is regular and smooth.The bedrock valley has a strong ability to resist erosion and scouring.In the Loess Plateau of China,there are always coming extraordinary flood with thick silt concentration.It has a very calming effect using a integration of the relationship of the thickness of sediment concentration and HEC-RAS hydrological model for palaeoflood reconstruction.The paper expounds the precaution about scientific and rational selection of river roughness values avoiding disturbing the results.(5)Using a combination of the optically stimulated luminescence(OSL)dating method and stratigraphic correlation and archaeological dating of historical relics from the profiles,the four-layer slackwater deposits in HNC site showed extraordinary floods that occurred between 1800 and 1600 a BP,and these four bed sets of slackwater deposits in XHQ site recorded the extraordinary floods that occurred between 3200 and 3000 a BP.This finding confirms that palaeoflood events and droughts occurred during the Shang dynasty(?3550-3000 a BP)in China,for which severe climatic degeneration has been recorded in remaining documents.High-resolution climatic proxies and these hydro-climate events are interrelated with climatic fluctuation endowed by cooling and drying during the same period.The results confirm that the river response system has adapted to climate change.They also offer a more reliable reference for comprehending the reciprocal actions between regional hydrological systems and climatic variations in the monsoon region.