| The most serious area of soil erosion is on the Loess Plateau in China. It is need to study how to control the soil erosion. However, it is difficult to find a satisfactory information carrier because of the limitation of the deposition time.Natural dam in Huangtuwa (Pangjiagou basin in the Zizhou country in Northern Shaanxi) is a good source of information. It is the longest natural dam and whole silt type which has 440 years old. The study on the information of Huangtuwa natural dam sediment deposition and the changing of erosion can reveal and in-depth study the erosion evolution law nearly 500 years on the Loess Plateau in northern Shaanxi. Establishing a long time erosion changes and sediment yield sequence, scientific understanding the influence of human activities on the erosion of the loess plateau, can provide based data on the control of soil erosion on the loess plateau.This study takes Huangtuwa natural dam as the research object. Taking use of the hand drill to collection 350 samples in the depth of 7m in the middle of Huyuanze ditch and analyzing these grain size characteristics, we understand the information of size characteristics and deposition; on the basis of these, according to the division principle of sedimentary layer to divide the sedimentary cycles; using 137Cs and flood data to establish time coordinates and calculate the deposition rate; combined with the measured data through GPS and DEM data to fitting the altitude and cumulative area?altitude and the cumulative capacity curve equation; According to the cumulative capacity curve^ bulk density?sedimentary cycle to calculate sediment yield, and according to the time coordinates to calculate annual sediment yield, finally calculate the multi-year average soil erosion modulus.Main content of the conclusion is as follows:(1)Comparing to the sediment in front of dam, the sediment grain size of Huangtuwa natural dam gully sediment is slightly bold; but comparing to the primary loess, it is still relatively small. From the grain size composition analysis, the content of coarse silt is the highest, followed by very fine sand, fine sand and coarse sand content is few, the particle size distribution is distributed at different depths and frequent changes in the maximum and minimum value, reflecting the historical period sedimentary environment changes frequently. The Huangtuwa area is dominated by a single peak, only with a small amount of twin peak, which shows that the sedimentary environment is relatively stable, and the source of sediment is single. From the profile parameters of particle size analysis, average particle diameter change shows "fine-coarse-fine ". The sediment bedding change above 408cm is significantly and the dynamic environment is complicated; sediment sorting is poor, frequency curve positively skewed, grain diameter distribution in the thick end, sample kurtosis have medium and a very narrow two kinds.(2)According to the content change of colloidal particles, clay particle and fine powder sand, the Huangtuwa channel development history is divided into three stages: The first stage is the depth of 408cm-700cm, the content change of colloidal particles, clay particle and fine powder sand is relatively slow, and the sedimentary layer thickness is thick; The second stage is the depth of 266cm-408cm, the content change of colloidal particles, clay particle and fine powder sand is severe; The third stage is the depth of 0cm-266cm, the content change of colloidal particles, clay particle and fine powder sand is again slowly. Characteristic parameters of the distribution trend is basically similar, and the change trend is contrary to the distribution trend of colloidal particles, clay particle and fine powder sand in depth, in consistent with the change of very fine sand and fine sand. From the>100um distribution law analysis, the section corresponds to the strong rainfall in depth of 36cm,58cm,86cm,182cm,292cm, the depth of 86cm corresponds to the history of the strongest rainfall.(3)According to the difference of grain size composition, sedimentary layer thickness and colors, and the clear boundaries, through the analysis of fine particles (< 0.005mm), median particle size and coarse particle (>100um), taking the "fine-coarse-fine" regulation as the classification standard, the depth of 7 meters is divided into 60 sedimentary cycles. The 0-30cm is farming layer. In the rest of layers, there are39 layers which the median particle size has only one peak and it is a single sedimentary cycle; other median particle sizes and the average particle sizes have at least two peaks and correspond at least two o erosive rainfall. The whole profile corresponds to at least 79 sedimentary cycles. Using the content of 137Cs can figure out the sediment which formed in 1964,1986 years, and using the flood data to correct the time. First confirm the sediment which is formed in the years of 1959,1972,1977-1978,1994, then according to the data of rainstorm and heavy rain to confirm the time of other Sedimentary cycles.(4) According to the established time coordinate, to calculate the deposition rate in 1959-2015 nearly 55 years in Huangtuwa region. The value is 6.33cm/a. The rate in 1959-1984 is greater than the average deposition rate, while the rate in 1985-2015 is less than the average deposition rate; maximum deposition rate is 10.8cm/a, occurring in the two stages of 1973-1978 and 1959-1963; minimum deposition rate is 3.6cm/a, occurring in 1994-2003; the deposition rate has a great relationship with the number of heavy rain and rainstorm.(5)According to DEM, extracting the contour information and fitting the altitude and cumulative area, altitude and the cumulative capacity curve equation, calculate the amount of deposition is 132.947 million tons in the depth of 7 meters. Due to the limitation of rainfall data, the earliest sediment determined is formed in 1959, corresponding the depth of 0-348cm, and the amount of sediment is 45.655 million tons. Before the years of 1959, the total sediment is 87.292 million tons and thickness is 3.52 meters. Although the deposit thickness is little difference, the amount of sediment is great difference. It reflects that the soil erosion situation is improvement after 1959. The average value of soil erosion modulus in Huangtuwa region is 8150t/km2*a, which belongs to the extreme intensity erosion area; According to the soil erosion modulus, the time (1959-2015) could be divided into two stages, and the boundary is the five years of 1979-1983, the soil erosion modulus in 1959-1978 is greater than the average, which belongs to the extreme intensity erosion. After 1980s, the soil erosion modulus is less than the average, which is moderate, intensity erosion. |
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