Study On Paleoflood And Relationship Between Water And Sediment At Jinghong-Lancang River

Lancang-Mekong River is a famous international river.The Yunnan section of Lancang-mekong River is an important section for hydropower development in China,especially in the middle and lower reaches.With the development of hydropower development technology,in order to ensure the safety of the reservoir,it is necessary to extend the flood sequence as far as possible to improve the design flood accuracy and the prediction of sediment erosion and deposition in the reservoir.The paleoflood study added the ultra-long time scale flood information into the existing flood sequence,extended the flood sequence,and had important significance for improving the accuracy of design flood.It is of great significance to deduce the relationship between water and sediment from paleoflood in order to understand the environmental conditions during flood discharge and improve the design accuracy of reservoir sediment discharge system.However,there are still few studies on paleoflood and water-sediment relationship in Lancang River basin at present.Therefore,hydrological information in the period without measured records can be used to make up for the shortcoming of hydrological data and measured data in southwest China,and provide a scientific references for eco-environment control and cascade's operation mangement within the basin.In this paper,section(LCJ-MT)of fluvial sediments in jinghong dock of Lancang River was selected for research.The flood events in the shang dynasty were recorded by field observation of river sediments,particle-size,magnetic susceptibility analysis and carbon 14(AMS 14C)dating by accelerator mass spectrometry.Using sedimentology and hydrology to restore the paleoflood hydrology,and jinghong section Wannian scale sequence,the flood of Lancang River hydrologic frequency and return period,according to the paleoflood peak flow,using the Lancang River jinghong history period of natural rivers,the relationship between flow and sediment concentration back stepping paleoflood flood discharge of sediment concentration,the following conclusions:(1)According to the field survey,jinghong section of Lancang River is an anti-erosion section.The LCJ-MT section is located in a region with gentle slope and few human activities.The sediment particles are relatively fine,mainly fine sand.The color is turbidity yellow-gray,presenting a horizontal slightly undulating bedding,which conforms to the characteristics of the field of paleoflood detention sediments.The carbon 14 method of accelerator mass spectrometry(AMS14C)was used to obtain the record of the flood events in the shang dynasty of Lancang River.(2)As for the physical and chemical properties of the stagnant sediments,the particle-sizes of paleoflood deposits are fine,mainly composed of clay and silt,and well sorted.In the analysis of the magnetization rate,the frequency magnetization rate of the paleoflood stagnated sedimentary layer appears the lowest value,which indicates that the river stagnated sediment in the deposition process,the deposition rate is faster,the ferromagnetic mineral content is relatively less,proving that the sedimentary layer is suspended sediment of the river.(3)The water level and flood peak flow were recovered by using the determined paleoflood slackwater deposit thickness and sediment concentration method and specific area method,and the results were as follows:the water level of paleoflood recorded in paleoflood slackwater deposit layer was about 541.1 meters,and the flood peak flow was about 12773 m3/s.Through the measured data and the relationship between peak flow and area,it is proved that the restoration result is reasonable.(4)According to the paleoflood peak flow stagnation sediment recovery data,historical investigation flood data in jinghong station measured flood data,it is concluded that the once ten thousand flood at jinghong section was about 24816 m3/s,the coming of the flood peak flow of about 20381 m3/s,a once-in-a-century flood peak flow of about 16146 m3/s.(5)Selected the hydrological data such as the measured section area,water level,flow rate and sediment concentration in eight years from 1963 to 1980s in jinghong hydrological station,and obtained the water-flow and flow-sediment concentration relationship of the ingghong section of Lancang River as the natural river.The section of jinghong hydrological station was stable in 1963-1980s,and the balance of scouring and silting was maintained,eliminating the influence of the change of section on the water level and flow rate.In addition,there is a significant positive correlation between runoff and sediment concentration in jinghong hydrological station during flood season.(6)By calculating the paleoflood water level through the water-flow correlation equation,the relative error between the paleoflood water level and the paleoflood water level recovered by sediment thickness and sediment concentration method is 1.64%,which proves that it is reasonable to use the water-sediment relationship of historical flood to deduce the water-sediment relationship.According to the correlation equation of discharge and sediment concentration obtained by statistics,the sediment concentration in paleoflood was about 5 kg/m3.