Study On Hydrological Restoration Of Rare River Flood Events In A Number Of Rivers In The Monsoon Region Of China

Systemic records of gauged floods events are too short to understand long-term hydro-climate change and evaluate the rare flood discharges.Palaeoflood stage identifications(i.e.,slackwater deposits)of geomorphologic and sedimentary evidence were identified using palaeohydrological criteria along the river valley.Well-sorted flood slackwater deposits(SWD)were checked by geophysical and geochemical proxies.Palaeoflood chronology was established by OSL dating technology in combination with archaeological dating of anthropogenic remnants,and with pedo-stratigraphic correlation.In the bedrock area,palaeoflood peak stage was calculated by the elevation of end-point of flood SWD and slackwater flow depth method.Peak discharges were estimated by slope-area method and step back method.Magnitude and frequency of rare flood with the river basin were quantified analysis.Palaeoflood hydrology can provide unique and valuable rare flood information for flood design,flood check and flood risk assessment of major hydraulic engineering.Palaeoflood investigations were carried out along the middle Yellow River and the upper Hanjiang River valley during the 2011-2016.The paper highlights a multidisciplinary approach to analysis for Holocene palaeoflood events in the large rivers of monsoon zone,such as the Jin-Shaan gorges,Yanhe River,Yiluohe River and the upper Hanjiang River.(1)Based on palaeohydrological criteria,grain-size distribution and geochemical elements,in combination with modrnn large flood sediment characters,well-sorted suspend sediments in the back area were identified as palaeoflood SWDs at the Peijiachuankou(PJCK)site in the Jin-shaan gorges,Guojiachuan(GJC)site in the Yanhe River,Longmenxia(LMX)in the Yihe River,Longhutan(LHT)site in the Luohe River,Dujiagou(DJG),Wanchuncun(WCC),and Xintancun(XTC)site in the upper Hanjiang River.The maximum palaeoflood peak discharges were estimated to 36,000 and 15,000 m3/s at the PJCK site of the Jin-Shaan gorges and GJC site of the Yanhe River,respectively.Reconstruction Holocene rare flood maximum peak discharges at the DJG and WCC site of the Yangxian reach,and the XTC site of the Ankang reach along the upper Hanjiang River is 33,000? 53,950 m3/s,respectively.These palaeoflood discharges can improve flood frequency estimates in combination with historic flood and gauged flood series.Holocene rare flood data were used to reexamine to the flood frequency curve and to redefine regional maximum flooding.(2)Palaeoflood chronology was established in the middle Yellow River and upper Hanjiang River.There are six periods of the major rare flood clutters in the major tributaries of the middle Yellow River,such as 9000-8500,8000-7800,6000-5000,4200-4000,3200-2800 and 1900-1700 a B.P.and 4200-14000 a B.P.more often.There are four periods of the major rare flood clutters between Sanmenxia and Huanyuankou reach("Sanhuajian")in the middle Yellow River,such as 7000-6800,4000-3900,2400-1800,600-0 a B.P.and 2400-1800 a B.P.more often.There are two main rare flood events identified at 6000-5000 and 4200-1000 a B.P.in the Weihe River,north of the Qinling Mountains.Whereas there are three main rare flood events identified at 3200-2800,1900-1700,1000-900 a B.P.in the upper Hanjiang River.They are closely related to monsoon climatic frequent shift phase,especially in cooling phase and strong ENSO actives period.(3)Flood slackwater flow depth method was defined in detail.This new method reduces the underestimation of palaeoflood stage in the traditional palaeohydrological approach.Because water depth above flood SWDs is not taken into account in calculation of palaeoflood stage,both the elevation of the tops and the end-point of flood SWDs only provide a minimum estimate.Especially in some cases,the end-point of flood SWD was not preserved and the elevation of top of flood SWD is obvious low,this approach can be applied to estimate the flood stage.Reconstruction flood discharges using this method are larger than that of the elevation of flood SWD top surface and the end-point of flood SWD.Meanwhile,the major uncertainty of flood slackwater flow depth method is how to accurately assess the bulk of suspended sediment load in floodwater(p).A careful assessment of the parameter p is essential.On the one hand,the relationships between the thicknesses of modern flood SWD,the elevation of flood stage,flood waterdepth can be investigated at the multiple sites,which are useful for assessing and minimizing the uncertainties of the parameter p.On the other hand,sediment concentration for modern large floods at the hydrological station can be statistical analyzed,a range and/or average value of p for modern large flood were selected.(4)Magnitude of Holocene rare flood were statistical analyzed in the middle Yellow River and the upper Hanjiang River.The maximum palaeoflood peak discharges were calculated to be 50,000-55,000 m3/s at the Wubao and Longmen hydrological station along the Jin-Shaan Gorges,which is 2-2.5 times than the maximum gauging flood data.The relationship between palaeoflood peak discharges and drainage areas can be expressed by Q=340D0.39 in the major tributaries of the middle Yellow River,which provides an upper limit on the magnitude of Holocene rare flood and also falls in the global maximum curves.The relationship between palaeoflood peak discharges and drainage areas can be expressed by Q=540D0.35 within the middle Yellow River basin.The maximum palaeoflood peak discharges were calculated to be 33,000 m3/s,55,000 m3/s,65,000 m3/s at the Yangxian,Ankang and Baihe hydrological station along the upper Hanjiang River,which is about twice than the maximum gauging flood data.Moreover,the magnitude of Holocene rare flood in the upper Hanjiang River is much bigger than that of the Weihe River,and is approximately similar to the magnitude of the middle Yellow River.(5)It is essential to distinguish between rare flood deposits in the large rivers and flash flood deposits in the gullies.Loess rapture fissures were filled with fresh red clay clasts and matrix on the trench walls in the south of the Machangyuan Ruins.These rapture fissures extend in two intersecting regular directions,including N30°-40°W and N40°-50°E.The palaeo-earthquake was associated with regional tectonic structures including a NNW-trending left-lateral strike-slip fault(Lajishan fault)and a NE-trending local fault(Bazhougou fault).The colourful red mudflow is easily distinguished over the dull yellowish loess landscape.Comparative analysis of grain-size distributions and geochemical elements of local outcrops indicates that the red mudflows were sourced from the gullies on the valley side where unconsolidated Neogene red clay formation was exposed.These red mudflow deposits and flash flood deposits is obvious different from palaeoflood SWDs in the large river.This case study provides insights into the Holocene tectonic activities,climate variability and prehistoric human activities in environmentally sensitive zones over the world.