Abstract:

In recent years,palaeoflood has been widely studied in the middle reaches of the Yellow River and its tributaries.However,Wudinghe River,Shaanxi Province,China,which is the largest tributary between Hekou and Longmen section of the Yellow River,has not been reported in these studies.A Holocene profile with palaeoflood slack water sediments was found at the Sujiagetuo (SJGT) site in the lower reaches of the Wudinghe River basin through detailed field investigation.The lower reaches of the Wuding River are bedrock canyon sections,which are suitable for palaeoflood hydrology research.The upper and lower boundaries of the palaeoflood SWD  are clear with other layers and the thickness of SWD is 30 cm .The palaeoflood SWD,characterized by thin bedding,directly covers the Eastern Han cultural layer.Experimental results show that coarse silt is the main grain size composition of the palaeoflood SWD and 2012 flood SWD and accounted for 59.58% of the palaeoflood SWD and 46.52% of 2012 flood SWD.The standard deviation (σ) and separation coefficient (S₀) of palaeoflood SWD are smaller,i.e.,1.51 and 0.79,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 separation coefficient show an effective sampling and sampling pretreatment procedure,indicating that they are well sorted during the sedimentation process.The magnetic susceptibility of the palaeoflood SWD and 2012 flood SWD are extremely low (37.8×10^-8 m³·kg^-1 and 28.3×10^-8 m³·kg^-1),indicating that they are both fresh sediments and are hardly affected by weathering and soil formation.Palaeoflood SWD and 2012 flood SWD are also characterized by low ignition loss and high calcium carbonate content.These experimental data indicate that the sedimentological characteristics of the palaeoflood SWD and that of the flood SWD in 2012 are similar,which indicates that it recorded a large flood event in the lower reaches of Wudinghe River.OSL dating of the cultural layer shows that the palaeoflood event occurred in 1 900-1 700 a BP.According to the 22% bulk sediment of the Baijiachuan Hydrological Station in the Wudinghe River,the slack water flow depth method of palaeoflood SWD is used to restore the palaeoflood stage to 765.9 m.Based on the hydraulic parameters such as the gradient of 0.002 5 and roughness coefficient of 0.035,the peak discharge of palaeoflood was reconstructed to be 10 530 m³·s^-1 by the slope-area method.Using flood mark water level in 2012,the calculated value of peak discharge is 1 030 m³·s^-1 at the same section and parameters,and the error between the calculated peak discharge and the measured value is 3.09%,which shows that it is reasonable and reliable to restore the peak discharge of palaeoflood using the slopearea method in the lower reaches of the Wudinghe River.Results extend the flood data series of the lower reaches of the Wudinghe River and provide a scientific basis for the development and utilization of water resources and energy resources.

Key words: palaeoflood hydrology')">

palaeoflood hydrology, slackwater deposits, Holocene; Wudinghe River