| China's Loess plateau region is the largest concentrated loess deposition area in the world,and the study of loess in this region can well invert the global environmental change since quaternary.Natural or artificial accumulation of loess records the changes of the surrounding environment during the deposition process,so it can be used for the restoration and deduction of the ecological environment in historical periods and engaged in the research of the reconstruction of the ancient environment.The loess depression in northern Shaanxi has a long history,and it has a lot of information in the process of sedimentation.the simulation study of the loess depression accumulation can effectively reconstruct the ancient environment in the loess hilly-gully region.Based on the physical process of erosion and sediment production in the watershed,this study analyzed the sediment characteristics,topographic features and regional rainfall characteristics of warping dams,built the accumulation model and preliminarily delineated the accumulation types.At the same time,the reconstruction of hydrodynamic and geographical environment of warping dams is discussed,and sediment morphology simulation and event analysis of surface erosion are carried out by taking the heavy rain and flood disaster in Yulin on July 26th as the breakthrough point.The main conclusions are as follows:(1)the basis of classification of sedimentary types of accumulation-accumulation model is established.Revise the sub-rainfall erosion and sediment transport model established in Chabagou watershed,establish the sub-rainfall erosion and sediment production model based on topography and erosion power in Huangtuwa basin,and conduct numerical conversion based on average bulk density,and finally build the sediment accumulation model based on sediment erosion and runoff.Among them,the box dimension method was used for the first time to calculate the terrain fractal dimension of loess depression basin,and its fractal dimension value was 2.0,compared with the fractal dimension value of northern Shaanxi,the result was reliable and could be used for related research.(2)The types of accumulation are classified.The standard of erosive rainfall in Huangtuwa basin was revised to 9.2 mm by empirical frequency method,and with 53.3 mm,the average annual maximum rainfall from 1953 to 2017,which were used as the parameters to divide the"natural dam","non-natural dam" and"non-dam".The accumulation in Huangtuwa basin is simulated by using the truncated cone formula,and the sedimentary types are systematically divided according to this formula.the results show that the "natural dam" deposit area is from dam site to 1058.18 m height,the"non-natural dam" deposit area is from 1058.18 m height to 1058.48 m height,and the"non-dam" deposit area is from 1058.48 m height to gully head.(3)The particle size characteristics of 1400 soil samples at 4 profile points from dam to Yingguagou were analyzed.Particle fraction gives that the average content of clay and fine silt decreases and the coarse grain has increasing trend,from dam to gully head,which shows that the farther from erosion source,and fine particulate matter in sediments is easier to transport,combined with edging effect make the grain more fine;at the same time,the high correlation between grade variation coefficient and maximum grade shows that the grade is more coarse,the sorting features worse and starting from the sand,the kinetic energy is more and more big,and kinetic energy reduce lead to the sediment worse sorting features attributed to the reduced longitudinal slope of dam and sediment resistance,finally,simple mixed sediments extremely deposit inside the dam.The results of grain size parameter analysis shows that the coarser the average particle size of the sediment layer from the dam to gully head,the wore sorting features,the more symmetrical the frequency distribution curve and the more moderate the curve shape.The grain size frequency curve shows the characteristics of single peak and very few double peaks,indicating that Huangtuwa basin is dominated by erosion deposition,and the influence of wind is very small.;The frequency curve shows that the proportion of sediments,mainly silty sand,is larger and larger,indicating that the main particle size of Yinguagou is growing bigger and bigger.(4)The sediment model was verified by using the grain size cycle law of sediment.Taking the minimum average particle size as the symbol of the end of the deposition cycle layer,21 deposition cycle layers are divided in Yinguagou.The change of" binary deposition" law of sedimentary cycle layer in spatial distribution shows that the farther away from dam site,the weaker the law.According to the obvious degree of law,it is concluded that QA and QB section points are approximately the boundary between"natural dam" and“non-natural dam" sedimentary areas,and QB and QC section points are approximately the boundary between "non-natural dam"and"non-dam" sedimentary areas.By comparing the sedimentary classification and demarcation results of sedimentary model,it is considered that the characteristics of surface soil particle size distribution have well verified the delineation results of mathematical model method.(5)Revealed the difference law of hydrodynamic force and dam operation mechanism in silt dam deposition area.The hydrodynamic differences of warping dams are as follows:in the deposition area of warping dams,"natural dam" is located in front of the dam,and the hydrodynamic force is weak,mainly deposition."non-natural dam"is located in the river storage area,with strong hydrodynamic force and delta sedimentation."non-dam" is located in the channel,dominated by erosion,and the sedimentation is very weak.Silting dam has its own characteristics of accumulation and sedimentation classification in different operating States.From the establishment of silting dam,the range of "natural dam" area has gradually decreased,and the"non-natural dam" area has gradually increased.As a result,the sand blocking and erosion reducing function of warping dams gradually weakens.When the dam reservoir is full of silt,the warping dam is approximately the "non-dam" of the channel,but compared with the longitudinal gradient before dam construction,the protective effect of soil and water loss is enhanced.(6)Soil erosion characteristics of "7.26" Yulin heavy rain and flood disaster were analyzed.The simulation analysis of sedimentary morphology of "7.26" event showed that the height of mud water produced by heavy rain was 1058.76 m,72.95 cm higher than the dam site,which belonged to "non-dam" sedimentary area.The analysis of the surface erosion of this heavy rain shows that the smaller the bulk density,the stronger the erosion on the slope surface with the increase of the elevation under the condition of consistent other conditions.The surface bulk density and average particle size increase or decrease synchronously along the dam site to the gully head in the channel,and the bulk density is gradually increased.There is no obvious trend in grain size,particle size parameters and bulk density distribution from the middle to both sides of the ditch,and its main grain size statistics shows a decreasing trend of "M" type,which indicates that the grain size from the middle of the ditch to both sides has a decreasing trend. |
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