The Morphology And Controlling Factors Of Rills On Steep Slope In Yuanmou Dry-Hot Valley

The problem of soil erosion has been a concern of Jinshajiang dry-hot valley for a long time, Yuanmou is one of the most typical places in this area, the special soil types and climatic conditions in this area lead to an extensive development of gully erosion and erosion landforms, it has caused a great impact on the regional economic development and people’s daily production and life. Rill erosion is the main form of slope erosion, the forking, merging and connectivity of rills on the slope promote the development of rill erosion, and then formed a complex form of erosion. This paper selects three typical steep slopes of different soil properties and morphological development in Yuanmou dry-hot valley, the method of photogrammetry and 3D laser scanning was used to obtain dense point cloud of each steep slope, and the software of Agisoft Photoscan was used to establish the 3D model of steep slope. The dense point cloud data and micro-topographic profiler was used to measure the detailed parameters of cross-sections of each rill. The main contents and conclusions are as follows:(1) There is a great difference between rill morphological development of steep slope and gentle slope in Yuanmou dry hot valley, the gradient of steep slope is generally between 40° ~80°. Through the observation of 3 research places around Xiaoleizai Village, the rill of steep slope can be roughly divided into 3 types according to the shape, such as the braided shape, the parallel shape and the skirt shape. Most of the rills are parallel shaped and has a distribution of cluster grouping, each of the rill is basically formed and has an independent channel, morphological rules and intensive development resulting in strong erosion on the slope.(2) The braided shape of steep slope can be divided into 3 layers according to the soil properties, the development of the upper layer of rills are most intensive, branches of rill was more developed and the cross-sectional morphology showed characteristics of small width and greater depth. Twelve characteristic cross-sections were selected on the slope. The characteristic cross-sections width of middle upper layer of rills are between 13.44~53.07 cm, the depth are between 8.32~64.70 cm, and rill width-depth ratio are between 0.44~2.95. The characteristic cross-sections width of middle upper layer of rills are between 12.41~54.62 cm, the depth are between 5.85~57.77 cm, and rill width-depth ratio are between 0.45~3.05. The characteristic cross-sections width of middle layer of rills are between 10.92~45.39 cm, the depth are between 6.27~59.80 cm, and rill width-depth ratio are between 0.54~3.65. The cross-sectional morphology of lower layer of rills showed characteristics of wide width and small depth. The characteristic cross-section width of lower layer of rills are between 25.01~84.79 cm, the depth are between 7.98~43.54 cm, and rill width-depth ratio are between 1.58~4.10. The rill inclination of the 3 layers is 5.38°, 8.13° and 11.31°, respectively. The rill density is 2.29m/m2; the complexity is between 1.33~5.43, the average value is 2.65.(3) The overall morphology of the parallel shape of rill in steep slopes is similar to the upper layer of the braided shape of steep slope, but the rills are relatively smaller, there are many branches of rill in the middle and lower layers, and showed a "bamboo shoot" shape of erosion morphology. The rill width decreased, while the depth has increased from the top down. Four characteristic cross-sections were selected on the slope of the upper and lower two layers. The characteristic cross-sections width of upper layer of rills are between 16.18~41.19 cm, the depth are between 5.83~35.04 cm, and rill width-depth ratio are between 0.99~3.07. The characteristic cross-sections width of lower layer of rills are between 10.22~40.27 cm, the depth are between 9.33~40.63 cm cm, and rill width-depth ratio are between 0.42~2.00. The rill inclination of the 3 layers is 8.97°, 6.89° and 9.87°, respectively. The rill density is 3.69 m/m2; the complexity is between 1.00~2.93, the average value is 1.71.(4) There are obvious differences between the rills morphology of skirt shape of steep slope and other steep slopes. From top to bottom, the rills showed a trend of convergence from more to less. The bottom longitudinal section is smooth and there is no obvious washing hole, but there are traces of collapse in the lower layer. Six characteristic cross-sections were selected on the slope of the upper and lower two layers. The characteristic cross-sections width of upper layer of rills are between 15.30~44.70 cm, the depth are between 8.76~26.58 cm, and rill width-depth ratio are between 0.97~2.49. The characteristic cross-sections width of lower layer of rills are between 9.52~45.71 cm, the depth are between 6.47~25.41 cm, and rill width-depth ratio are between 0.94~3.93. The rill inclination of the 3 layers is 6.39°, 7.03° and 7.56°, respectively. The rill density is 0.82 m/m2; the complexity is between 1.00~5.01, the average value is 2.34.(5) Under the same conditions, such as rainfall, vegetation cover and human disturbance, etc., the development of rills on steep slope is mainly controlled by two factors of topography and soil properties. Slope is the deciding factor. Through the comparison of several parameters of rills on steep slope, the result showed that the slope and rill depth was negatively correlated. In addition, the slope length mainly affects the plane shape of rills on steep slope. The larger the length of the slope is, the more frequent appearance of the divergent rills and the rill convergence we will fin d. Soil property is the material basis for the formation of rills on steep slope, moreover, the situation of soil crust and anti-erosion ability has a significant influence on the size and morphology of rill sections.