| Flood sediment is the most important carrier of flood information,and the in-depth study of its characteristics is the basis of digging flood hydrological information research,and is also an important means of river palaeoflood reconstruction,so it has always been a hot topic in the field of geoscience and hydrology.On November 13,2018,another landslide occurred near the village of Baige on the Jinsha River.The landslide blocked all the main stream of the Jinsha River and formed the barrier lake.By the time of manual intervention on November 13,the reservoir capacity had reached 578 million m3.In order to reduce the damage caused by the quake lake outburst to the downstream,the relevant departments decided to carry out artificial intervention to release the flood,the diversion channel was formally opened on the 13th,forming the maximum peak flow of 31,000 m3/s,is a once-in-a-million flood event.In the process of flooding along the Jinsha River,the Jinsha River valley area about700 km below Baige was submerged or silted,which washed out the houses,water conservancy facilities,municipal facilities,farmland and other areas along the river,causing huge economic losses.In addition,there are a lot of flood tracks,erosion and accumulation landforms and sediment deposition on the banks of Jinsha River and in the valley basin.After the occurrence of the flood,field investigations were conducted on the Jinsha River flood sediments in the reach of Binducun and Shigu Town in Yulong County,Deqin County,Yunnan Province for two times successively.Sediment thickness,longitude and latitude coordinates,flood mark,river bed gradient,flood mark boundary,river level,river section,flood erosion and accumulation landform were accurately recorded and measured,and the flood sediment samples were systematically collected along the river.This paper uses remote sensing image interpretation and GIS technology to extract flood inundation range and sediment deposition,the particle size magnetic susceptibility of heavy mineral quartz sand was tested on the surface micro-state characteristics of flood sediments.The relationship between sediment thickness and sedimentary water depth was analyzed,and the genetic mechanism causing the sediment characteristics was discussed,and the following conclusions were drawn:(1)The flood caused a total of 12973.21×104 m2 of inundation from Baige Yan Sehu to Shigu Town,of which the floodplain inundated area of about6464.00×104 m2;The total amount of sediment deposition is estimated to be about 1006.03×104 t.(2)Sediment characteristics are:Sediment grain size:The grain size components of flood sediments can be roughly divided into three types:sandy silt,silt and silt,among which the main type is sandy silt.The second is silty sand and silty sand.Along the river,the median particle size and average particle size decreased with the increase of the distance from the barrier lake.The grain size distribution curve was dominated by unimodal and bimodal,and the main peak was high and narrow.The skewness is negative skewness,the sorting overall difference.By comparing the characteristics of sediment in the flood peak and the retreating stage,the interim grain size,mean grain size,skewness and kurtosis of the flood peak are all higher than that of the retreating stage,while the sorting is poor.The magnetic susceptibility of the sediments varied from 2.62×10-8 m3/kg to 5.30 10-8×m3/kg,with an average of 3.82×10-8 m3/kg.The magnetic susceptibility along the River showed a gradually increasing trend,and the magnetic materials were mainly ferromagnetic minerals,and the sediments were fresh sediments,and no loamforming occurred.In addition,taking Yangu village section of Judian town as an example,the magnetic susceptibility is analyzed,and the results show that the magnetic susceptibility of different landforms is different.Sediment heavy minerals:the combination form is magnetite-ilmenite-effluodrite-zircon-pseudotitanite-hornblende,among which the stable minerals magnetite and ilmenite are the dominant minerals,and the contents of other minerals are different at each sampling point.Secondly,the heavy mineral ATI index proves that the apatite in the sediments has not weathered.The stability index and ZTR index increased with the increase of the distance from the sampling point to the barrier Lake.The composition of the exposed strata of the barrier accumulation body and the heavy mineral assemblage were basically the same,indicating the same source of sediments.The surface micro state of the sediment quartz sand:the particle morphology is mainly subangular and subcircular,and the roundness gradually increases from upstream to downstream;the mechanical characteristics such as adhesion fragments and mechanical V-shaped pits on the surface of the particles indicate that the hydrodynamic action of the particles is complex and intense;the polished surface and scratches indicate that the particles are affected by high water level and fast flow rate.(3)The correlation analysis between measured flood sediment thickness and flood depth data shows that there is no correlation between them;At the same time,the error range between the simulated flood water level and the measured flood section water level is 3.59-179.67m,which proves that the flood peak water level restored by using the relationship between the two may be much larger than the actual value in the palaeofflood research.(4)The shear forces of 15 river sections from Maoding Village to Shigu Town range from 26.95-193.51 N/m2,by comparing this value with the sediment characteristics,it is found that the main reason for the change of sediment characteristics in the flood is the change of river bed shear force caused by the change of longitudinal slope of river bed;In addition,sediment characteristics are also affected by topography,human activities,vegetation cover,flood level and other factors,which together act on the transport and accumulation of flood sediments. |
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