Research On The Influence Of Characteristics Of Water And Sediment Transport In Heihe River

The development of hydropower cascade cuts off the river and blocks the natural water sediment transport of the river,so that some of the sediment is deposited in the reservoir area.Not only caused significant changes in river water and sediment relations,but also had extremely far-reaching effects on the particle size characteristics of river sediments,such as changing the composition and distribution of river sediments,result in the change of sediment particle size of the river etc.Heihe River Basin is the second largest inland river basin in China,so far,seven cascade hydropower stations have been developed in the upstream of Heihe River.To a large extent,it has been changed the relationship of the water and sand,which has a series of influences on the evolution of the river channel and the changes of the river ecosystem.Therefore,it is of great significance to carry out the study on the relationship between the spatial distribution of particle size characteristics of river sediments and the changes of river flow and sediments,and to the ecological restoration of the Heihe River Basin and the rational operation of reservoirs.There are the main conclusions from the study:(1)The distribution of water and sand in the upper reaches of the Heihe River is uneven,with large interannual variations.The flood season from May to October is the concentration period of precipitation in the upper reaches of the Heihe River and also the concentration period of runoff and sediment yield in Heihe River Basin.The amount of sediment transported upstream of the mainstream of the Heihe River originates mainly from the deep mountainous areas of the Qilian Mountains,where the amount of sediment transported by the control section of the Zhamashk Hydrological Station and the Qilian Hydrological Station accounts for 62.2%of the sediment discharge of the Heihe River,respectively.The analysis of Mann-Kendall non-parametric test indicated that the annual runoff of the Zhamashike and Qilian hydrological stations in the upper reaches of the Heihe River had a significant increase,and the annual sediment transport volume showed a slowly increasing trend.The runoff and sediment in the two stations are closely related,indicating that the runoff and sediment have the same source.While the Yingluoxia hydrological station is affected by the runoff and sediment production from the Huangzangsi to the Yingluoxia and the sediment interception by the reservoir,the annual runoff is completely opposite to the annual sediment transport trend.The correlation between runoff and sediment is extremely poor,indicating that the runoff and sediment comes from different sources,and the abrupt change in sediment discharge at the Luoxiaxia hydrological station occurred in 2001.(2)Precipitation is the main source of recharge in the upper reaches of Heihe River,the contribution rate of precipitation in the Qilian and Yingluoxia hydrological stations is over30%,and increased precipitation and rising temperatures are the main reasons affecting the increase in upstream runoff;The changes of sediment discharge from the Huangzangsi to the Yingluoxia hydrological station are closely related to human activities,the contribution rate of reservoir construction and soil erosion to the reduction of sediment transport at the Yingluoxia hydrological station is 101.5%,the main reason for the significant reduction in sediment discharge is the silt trapping of reservoir at the Yingluoxia hydrological station,the average annual siltation sediment is 90.65×10~4 t,and soil erosion is also an important reason for the increase of sediment discharge in the upper reaches of the Heihe River before the 1990s.(3)On the spatial scale,the average grain size in the upper and middle reaches of the Heihe River is significantly different.And the average grain size is represented by:the lower reaches of the Baoping Reservoir to the Yingluoxia river section(0.68mm)>middle reaches(0.42mm)>Yeniugou(0.30mm),it shows that sediments are affected by the hydropower cascade development.In the reach of cascade reservoirs,the particle size is significantly larger than that of natural rivers.Due to the influence of cascade reservoirs,sediments in the reservoir area is layered and silted,sediments with larger grain diameters are deposited first,and sediments with finer grain sizes are deposited in the upper layer.Compared with the natural reach,the kurtosis is not obvious and the broad peaks,the median diameter and the average particle size are 0.04mm and 0.07mm,respectively,and it is the smallest among all sampled river sections,and the range of particle size groups also becomes narrower.The mean particle size and average particle size of the sampling point in the backwater area of the reservoir were 0.29mm and 0.44mm,respectively,which was significantly larger than that of the natural reach.The sorting coefficient was less than 0.35.The sorting was very good,the kurtosis coefficient was the largest,and the peak type was the highest.Sharp.The reducing reach downstream of the reservoir are 0.073mm and 1.16mm,respectively,and it is the largest among all sampled river sections.The sorting coefficient is0.94.The sortability is the worst among all sampled reaches and the peaks is sharp.The influence of cascade dams makes the increase of bottom sand particle size distribution curve in the reservoir area become relatively flat and the level of the gradation curve terminology does not have a strike level,while the downstream of the dam shows a continuous and slowly rising phenomenon.Hydropower cascade development also have an impact on the median grain size and average grain size of the sediment.This effect is mainly manifested in the reservoir area,which reduces its value and increases it downstream of the reservoir dam.