| As the important engineering measures to prevent soil erosion,check dam has the benefit on sediment reducyion,and at rhe same time,the sediment deposition in the dam with large erosion and environmental information has important theoretical and practical significance for finding out the source of sediment,revealing the Loess Plateau soil erosion rules,and making soil and water conservation measures and other issues.Huangtuwa natural formed dam located in the Loess Plateau hilly and gully region of Northern Shaanxi.It is a full-silt type natural dam formed by the local nine cattle mountain collapse,and it is currently known the longest running dam in China,and it is also an ideal place to mining sediment deposit information.This research takes the Huangtuwa region as the research object,and analysis grainsize characteristics of the area through the experiment of particle size of soil collection at the lowest in natural formed dam,and divided the profile into different sedimentary cycles according to the median particle size variation.The formation time of the sedimentary cycle is determined by the 137Cs content and the rainstorm data.It calculated the sediment deposition in the region by building the capacity curve equation.This research determines the optimal factor combination of fingerprint identification in the study area,and the sediment contribution rate is calculated for different sediment source using multivariate mixture model.And combined with the amount of sediment deposition,the sediment contribution amount of different sediment source is calculated.The main results are as follows:(1)The sediment is mainly composed of fine silt and coarse silt in Huangtuwa natural formed dam,and the whole grain is fine.The content of clay,very fine sand,and fine sand,medium and coarse sand is 0 at the several depths.The change trends between clay and fine silt,coarse silt and very fine sand are the same,the same increase and decrease,and the two groups have the opposite trend.In the 350 soil samples,only one soil sample size frequency curves showed double peak shape,and the others were single peak,indicating that the sedimentary environment was relatively stable.(2)According to the variation of the mean particle size,the 7m deep profile AI is divided into the 44 sedimentary cycles.Based on the determination of 137Cs content in profile C and the data of rainstorm data,the sediment deposition time of each cycle in profile C is determined.Based on the one-to-one sedimentary cycle correspondence between the profile AI and the profile C,the formation time of partial sedimentary cycle is inferred in profile AI.The sediment layer Ⅰ to Ⅶ are formed in 1994-1995,1991,1985,1983,1982,1961 and 1959 respectively.(3)According to the DEM data in Huangtuwa area,the capacity curve equation is fitted,and the fitting effect is good.Combined with the average soil bulk density of 1.25g/cm3 in natural formed dam,the sediment accumulation on the elevation of 1051.7-1058.7m in Huangtuwa natural formed dam is calculated,which is a total of 341.285×104t.After 1959,it deposits sediment 153.770×104t.Among them,in 1959 the amount of deposition sediment is 15.41 ×104t;in 1961 the amount of deposition sediment is 11.629×104t;in 1982 the amount of deposition sediment is 9.523×104t;in 1983 the amount of deposition sediment is 7.461 ×104t;in 1985 the amount of deposition sediment is 9.784×104t;in 1991 the amount of deposition sediment is 11.523×104t;in 1994-1995 the amount of deposition sediment is 12.200×104t.The average soil erosion modulus in 1959-2016 is 11046.70 t/km2.a,which belongs to the extreme strength erosion.The soil erosion intensity has experienced the trend of the first increase and then decrease.(4)According to the element content in soil samples of four potential source of sediment erosion(agricultural cultivated land,woodland,grassland and gully wall)in Huangtuwa dam control small watershed,and using the non-parametric Kruskal-Wallis H-test test method and using stepwise discriminant analysis method,it determines the optimal factor combination,including Pb,MgO,Hf,Al2O3,P,S,and it can be able to classify correctly 92.4%surface soil samples.(5)The sediment source contribution rate of different sedimentary cycles and the whole profiles is calculated by multivariate mixing model.For the whole 7m deep profile,the sediment from the farming land accounts for 84.68%,the contribution rate of the forest land accounts for 1.15%,the contribution rate of grassland is about 0,the contribution rate of gully wall accounts for 14.17%.Due to the influence of different factors,the contribution rate of sediment is not constant in different sedimentary cycles and in different time periods.Among them,in 1959-1981,the sediment contributions of farm land,woodland and grassland,gully wall are 83.20%,0.66%,0,16.06%respectively;in 1982-1990,the sediment contribution ratio are 91.06%,1.91%,0,7.04%respectively;in 1991-2016,the sediment contribution ratio are 73.60%,5.56%,0,20.84%respectively;and the sediment contribution rates before 1959 are 83.08%,2.13%,0,14.79%respectively.Regardless of the time period,the agricultural land is the main source of sediment erosion in the area. |
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