| The morphology of rill cross-section is often asymmetric,and topography is the important factor affecting the development and evolution of the asymmetric morphology of rill cross-section.This article choose dry-hot valley in Yuanmou as the study area,took out field investigation and mapping of cross-sectional shape and topographic conditions of rills in this area,extracted more than 712 groups cross-sectional data by using ArcGIS10.2,and calculated the rill cross-sectional relevant parameters,and divided the cross-sectional form of rills accordingly,and then used mathematical software such as Excel and Microsoft Office 2007 to count and analysis the topographic factor parameters.Finally,the morphological parameters of the rill cross-section and the parameters of the topographic factors ware fitted separately,and the correlation between the topographic factors and the asymmetry of the rill cross-section was obtained.The research provided theoretical guidance for soil and water conservation.The main conclusions are as follows:(1)the cross-sectional asymmetry characteristics of rills in the dry-hot valley of yuanmou were studied by selecting such parameters as the width asymmetry rate(Ak),area asymmetry rate(As),erosion line asymmetry rate(Ax)and erosion surface asymmetry rate(Ap).The results shows that in that area the average value of width asymmetry rate is 1.4137,the maximum value of width asymmetry rate is 15.0342 and the minimum value of width asymmetry rate is 0.0091,and there are 69 groups of quasi-symmetric cross-sections and 643 groups of asymmetric cross-sections.In terms of subdivision,the proportion of asymmetric cross-sections of Shadi village is 85.71%,and that of tho village,bald spine son,yuanmu man site,banzaoli village and gantang is 90.38%,83.33%,89.95%,84.62% and 92.35%,respectively.The average value of area asymmetry rate is 1.4945,the maximum is 15.8976,and the minimum is 0.0195.In terms of subdivision,The ratio of asymmetric cross-section to total cross-section of area in each subarea is 85.71%,96.15%,93.75%,91.39%,84.62% and 92.35%,respectively.The average value of the erosion line asymmetry rate is 1.2948,the maximum is 12.2348 and the minimum is 0.0623.The ratio of asymmetric cross-section to total cross-section of erosion line in each subarea are 85.71%,86.54%,77.08%,87.08%,76.92% and 90.52%,respectively.The average value of erosion surface asymmetry rate is 1.4378,the maximum is 13.0445,and the minimum is 0.0504.The ratio of asymmetric cross section of erosion surface to total section is 88.89%,92.31%,91.67%,90.91%,76.92% and 92.05%,respectively.In conclusion,the values of the four asymmetric parameters and the proportion of the asymmetric sections both indicate that the cross section of rills in dry-hot valley of yuanmou is asymmetric.In addition,the fitting relationship between four parameters--the asymmetric rate of groove width,the asymmetric rate of erosion line,the asymmetric rate of area and the asymmetric rate of erosion surface is positively correlated with each other.(2)as the trench slope increases,the value of the asymmetry rate deviating from the quasi-symmetric interval decreases gradually.Besides,the deviation values of the area asymmetry rate and erosion line asymmetry rate decrease monotonously,and the deviation values of the groove width asymmetry rate and erosion surface asymmetry rate fluctuate slightly.This indicates that the larger the slope is,the more inclined the rill cross-section is to quasi-symmetrical shape.In addition,the larger the slope difference on both sides of the cross section is,the larger the value of the asymmetry rate deviating from the quasi-symmetric interval is.The rill bending coefficient divides into intervals of [1,1.05],(1.05,1.1],(1.1,1.15],(1.15,1.2],(1.2,1.3],(1.3,+?),and the deviation values of asymmetric rate of groove width from the quasi-symmetric interval are 0.5220,0.7507,0.7257,0.8506,0.8857 and 1.3008,respectively.With the increase of bending coefficient,the deviation value increases gradually.,and the deviation value of area asymmetry rate,erosion line asymmetry rate and erosion surface asymmetry rate have this rule,too.This indicates that the cross section of rill has asymmetrical shape with the increase of bending coefficient.The catchment area mainly includes the catchment area of gully head and the catchment area of section position.The influence of catchment area on the asymmetric shape of rill cross section is negligible.while the catchment area at the location of the cross section has a great influence on the shape of the cross section of the rill.Establishing the relationship between the catchment area ratio and the asymmetry ratio on both sides of the section location.the results shows that the minimum values of channel width asymmetry rate,area asymmetry rate,erosion line asymmetry rate and erosion surface asymmetry rate deviating from the quasi-symmetric interval all appears when the catchment area ratio is(1.0,1.5],and the values are 0.4094,0.7288,0.5317,and 0.5994,respectively.When the catchment area ratio was less than or greater than that of this area,the deviation values are all larger,which indicates that the closer the catchment area ratio is to 1,that is to say,the closer the catchment area on both sides of the section is,the quasi-symmetrical shape of the cross-section is,and the larger the difference of catchment area is,and the large difference in catchment area means that the cross section has asymmetric shape.With the increase of the rill turning Angle,the proportion of the number of asymmetric groups in the cross-section increases gradually,and the value of the asymmetry rate deviating from the quasi-symmetric interval increases,and the cross-section becomes more asymmetric with the increase of the turning Angle.(3)the development process of rill morphology is affected by a variety of topographic factors,and the cross-sectional morphology is usually asymmetric.The slope is large,and the rill is straight and short,the rill profile shape is "v-shaped",and the rill cross section has quasi-symmetric characteristics.In addition,the greater the gradient difference between the two sides,the more obvious the asymmetry is.The straight slope is smooth,and rills on the slope are affected uniformly by external forces such as slope material and rill flow,tending to be more quasi-symmetric.Convex slope and concave slope are affected unevenly by external forces,which is more conducive to water and sediment collection.Sediment on slope is easy to accumulate at the bottom of gully,especially on concave slope,and both of the rill cross-section developed under these two kinds of slope have asymmetric form.The bending coefficient is an important parameter to describe the rill morphology.The bending coefficient of rill is large,and the flow at the bottom of rill is mainly eroded by inertia,and the erosion on both sides of the rill wall is uneven,and the rill cross section is asymmetric.Catchment area mainly includes catchment area at ditch head and catchment area at both sides of section position.The difference of catchment area on both sides of section position is the most obvious to the asymmetry of rill cross section,the manifestation is uneven catchment area on two sides: the large side has more catchment and sediment collection,more sediment accumulation at gully bottom and the trench wall is relatively slow,while the other side is just on the opposite.This difference results in the asymmetric shape of the section.Other topographic factors,such as slope,slope direction and slope difference,are important factors affecting the development of asymmetric shape of rill cross-section,as well. |
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