Study On The Law And Generation Of The Size Of The Flood Sediment In The Loess Plateau

China is one of the most serious soil erosion countries in the world, with the most serious soil erosion in the Loess Plateau. Serious soil erosion, not only damage to the environment, but also threaten food security, and also restrict the sustainable development of the society. In terms of Loess Plateau, it is composed of numerous small basins, each of them is a separate small watershed sediment yield unit. And the research for the small watershed sediment yield laws for promoting the process of soil erosion in the Loess Plateau in-depth study, understanding of human activities on the impact of increased soil erosion in the Loess Plateau, provides basic data and scientific basis to select soil Erosion control measuresIn this study, we choose the small watershed-wide depression punch of the Loess Plateau whole silt type of "puzzle" dam as the research object. Through the collection of 1953-2013 annual rainfall data and field data in Huangtuwa areas, based on the study of the terrain, we use a hand drill getting four soil profiles by the lateral drilling, and analyze the law of sedimentation spin meter loop check dam. Through the137Cs characteristic time scale, combined with collected rain data, we establish the time coordinate sedimentary section, analyze particle gradation of dam silt sediment. Based on the different sedimentary section in the particle size distribution of the components at the same depth, the mode characteristics of sediment deposition, erosional rain and mud deposited layer cycles response to the relationship of sand silt deposition rate information and the relationship of between deposition rate and rainfall erosivity, we could estimate the erosion amount of typical sedimentary cycles deposition levels by rainfall, and preliminary analyze the sources of check dam sediment, reveal the rule of soil erosion and sediment yield based on the spatial and temporal variation, and carry out the research of silt Loess depression natural mechanism in Huantuwa regions. The main conclusions are as follows:(1) We analyze the sedimentation law of six typical sedimentary layer cycles (Ⅰ, Ⅱ, Ⅲ, Ⅳ, Ⅴ, Ⅵ). We find the major sediments isby natural formation below300cm, and the impact of human activities on small deposits is small (sedimentary layer cycles Ⅳ, Ⅴ, Ⅵ). Butit get more impact from human activities above 300cm (sedimentary layer cycles Ⅰ, Ⅱ, Ⅲ) and less natural factors. From the early deposition to now, human activities on the loess silt dam is more and more.(2) Based on the 137Cs and rainfall data, we could determine that the typical sedimentary layer cycles Ⅰ, Ⅱ, Ⅲ formation time were 2001,1964 and 1959, the depth of 48-64cm,98-118cm,120-148cm range of times typical time deposits (in depth profiles of representative R) was formed in 1994,1978 and 1977. Heavy rains make the maximum contribution to sedimentary layer. By the correlation analysis of heavy rains and a half months of rainfall’s erosivity and sedimentary layer sediment particle, we find that heavy rains and a half months of rainfall erosivity keeps a highly significant correlation to a maximum particle size of the sediment, it indicates the sediment particle size there corresponds heavy rainfall (Storm) process.(3) In the two years- period 1959-1964,1964-1977, the rate of deposition is relatively large;It reached the maximum deposition rate in 1977 during 55 years, and in that year, the deposition rate of ditch slope profile R and S is larger than profile P and Q; Before 1977, the deposition rate of the four slope profile generally is larger than the time after1977. The average deposition rate of profile R of Huangtuwa natural loess silt dam is 5.36 cm/a during 55 years (1959-2013 years).and the rate of P deposition profile is 5.43 cm/a, profile Q is 5.29 cm/a, profile S is 5.18 cm/a. The deposition rate profile loess sediments shows the "first increased and then decreased" trend, which has a certain relationship with decreases of the average annual rainfall and erosion erosivity during the 40 years (1960-1999). During the six years of 2001,1994,1978,1977,1964,1959 and the three years of the formation of the sedimentary layers(Ⅳ、Ⅴ、Ⅵ),the amount of erosion of Huangtuwa areas is 938595.9 tons.The average annual deposition is 104288.4 tons of the nine larger depositions. The rainfall erosion modulus is between 18293.45～42152.32 t/km2·a-1 and the average is 33107.44 t/km2·a-1.(4) In this paper, we analyze nine sedimentary layers and find slope erosion is the mainly source of sediment to eight sedimentary layers in Huangtuwa areas. Slope erosion is the main erosion in Huangtuwa areas Small Watershed, and channel erosion relatively is weaker that make less contribution to sedimentary layer cycles. The content of coarse sand sediment (>0.1mm) is proportional to the flood sediment concentration (single rainfall deposition amount), but the maximum deposition sediment grain size of sedimentary layer cycles is not proportional to necessarily the size of > coarse sand sediment content of 0.1mm. The content of coarse sand sediment (>0.1mm) in Huangtuwa dam.