无定河下游全新世古洪水研究

对无定河流域野外考察,在其下游苏家圪坨(SJGT)地点发现夹有古洪水滞流沉积物(SWD)的全新世剖面。古洪水SWD厚度30 cm,具有平行微薄层理特征,且直接覆盖在东汉文化层之上。室内分析结果表明古洪水SWD有着与2012年洪水SWD相似的沉积学特征,以粗粉沙为主,分选良好,磁化率和烧失量较小,说明它记录了一次无定河下游的大洪水事件。文化层光释光(OSL)测年结果显示该次古洪水事件发生在1 900～1 700 a BP。古洪水洪峰水位和洪峰流量分别为765.9 m和10 530 m³·s^-1。同时利用2012年洪水洪痕水位验证,在同一断面用相同参数恢复洪峰流量为1 030 m³·s^-1,实测洪峰流量为1 000 m³·s^-1,误差为3.09%,说明古洪水洪峰流量恢复结果合理可靠。研究结果延长了无定河下游洪水数据序列,对水资源、水能源的开发利用提供了科学依据。

Palaeoflood events in the lower reaches of theWudinghe River

In recent years,palaeoflood has been widely studied inthe middle reaches of the Yellow River and its tributaries.However,WudingheRiver,Shaanxi Province,China,which is the largest tributary between Hekou andLongmen section of the Yellow River,has not been reported in these studies.AHolocene profile with palaeoflood slack water sediments was found at theSujiagetuo (SJGT) site in the lower reaches of the Wudinghe River basin throughdetailed field investigation.The lower reaches of the Wuding River are bedrockcanyon sections,which are suitable for palaeoflood hydrology research.The upperand lower boundaries of the palaeoflood SWDare clear with other layers and the thickness of SWD is 30 cm .Thepalaeoflood SWD,characterized by thin bedding,directly covers the Eastern Hancultural layer.Experimental results show that coarse silt is the main grainsize composition of the palaeoflood SWD and 2012 flood SWD and accounted for59.58% of the palaeoflood SWD and 46.52% of 2012 flood SWD.The standarddeviation (σ) and separation coefficient (S₀) of palaeoflood SWD are smaller,i.e.,1.51 and0.79,respectively,which are close to the standard deviation of 2012 flood SWD(0.33 and 0.65 respectively).The smaller values of deviation (σ) and separationcoefficient show an effective sampling and sampling pretreatmentprocedure,indicating that they are well sorted during the sedimentationprocess.The magnetic susceptibility of the palaeoflood SWD and 2012 flood SWDare extremely low (37.8×10^-8 m³·kg^-1 and 28.3×10^-8 m³·kg^-1),indicatingthat they are both fresh sediments and are hardly affected by weathering andsoil formation.Palaeoflood SWD and 2012 flood SWD are also characterized by lowignition loss and high calcium carbonate content.These experimental dataindicate that the sedimentological characteristics of the palaeoflood SWD andthat of the flood SWD in 2012 are similar,which indicates that it recorded alarge flood event in the lower reaches of Wudinghe River.OSL dating of thecultural layer shows that the palaeoflood event occurred in 1 900-1 700 aBP.According to the 22% bulk sediment of the Baijiachuan Hydrological Stationin the Wudinghe River,the slack water flow depth method of palaeoflood SWD isused to restore the palaeoflood stage to 765.9 m.Based on the hydraulicparameters such as the gradient of 0.002 5 and roughness coefficient of0.035,the peak discharge of palaeoflood was reconstructed to be 10 530 m³·s^-1 by the slope-areamethod.Using flood mark water level in 2012,the calculated value of peakdischarge is 1 030 m³·s^-1 at the same section and parameters,and the error between thecalculated peak discharge and the measured value is 3.09%,which shows that itis reasonable and reliable to restore the peak discharge of palaeoflood usingthe slope area method in the lower reaches of theWudinghe River.Results extend the flood data series of the lower reaches of theWudinghe River and provide a scientific basis for the development andutilization of water resources and energy resources.