| Soil erosion is one of the main factors which results in land function degradation and ecological environment deterioration.Because of the unique climate and soil types in the Jinsha River Dry-hot Valley which located in the upstream of Yangtze River,there are geomorphological landscape of ravines,serious soil erosion and various soil types.As the initial form of gully erosion,the formation and development of rill erosion not only contributes to the process of soil erosion,but also make immeasurable effects on people's life,production and economic sustainable development.So it's more urgent to study the evolution characteristics and laws of rill morphology.Taking the Yuanmou Dry-hot Valley as study object,based on artificial scouring experiment,analyze the evolution characteristics of rill cross-section under the same slope,flow rate and the different soil properties conditions,using the methods of high-precision three-dimensional laser scanner and related technical softwares.The main contents and conclusions are as follows:(1)In the process of rill evolution,the scale of each cross-section varies widely,and the rill morphology in No.2 and No.3 plot also shows significant differences.In the process of scouring experiment,the depth of left side of rill cross-sections in No.2 plot are between 1.56-14.05 cm,the depth of right side are between 2.47-13.89 cm,the top width are between 18.97-48.98 cm,and the erosional area of cross-sections are between 12.40-201.20cm2.The depth of left side of rill cross-sections in No.3 plot are between 1.09-16.06 cm,the depth of right side are between 1.18-10.53 cm,the top width are between 44.96-71.93 cm,and the erosional area of cross-sections are between 8.80-298.24cm2.In general,the rill scale in No.2 plot is smaller than No.3 plot's.The ratio of depth to width of rill in No.2 plot are between 1.46-10.44,and the average value is 5.25,while in No.3 plot are between 1.46-19.91,whose average value is 6.99.It shows that the width of rill cross-sections in both No.2 and No.3 plot are all greater than the depth.In the process of rill evolution,the scale and morphology changes of rill cross-sections in No.3 plot is more complicated than No.2's.The ratio of shape factor of rill cross-sections in No.2 and No.3 plot are 1.28-3.26 and 1.49-3.03 respectively,and the ratio of morphology of cross-sections are 0.35-0.54 and 0.33-0.67 respectively.Different rill cross-sections show various morphology characteristics in the process of scouring experiment.As for the asymmetry ratio of the width of rill,the values in No.2 plot are between 0.26-1.20,while in No.3 plot are between 0.18-5.30.The degree of left erosion and right erosion is different in the process of rill formation.The range change in No.3 plot is wider than No.2's,which shows that the degree of lateral erosion is greater than No.2's,and there is great uncertainty and variability on the direction of lateral erosion.Finally,most cross-sections in No.2 plot form the left-deflection-type,while in No.3 plot form the right-deflection-type.(2)In the process of scouring experiment,the morphological parameters of rill cross-sections in No.2 and No.3 plot show remarkable law.First,the depth and erosional area of rill cross-sections in both two plots present increasing trend as time passed by except several sections.The depth of most sections and scouring times are best fitted with power functions.There is great uncertainty in the change of rill width with scouring times in No.2 plot,which reflects the variability of flow path,scale and morphology in the process of rill formation.The width of cross-sections in No.3 plot has regularity of monotonically increasing.Secondly,the ratio of width to depth,the ratio of shape factor,the ratio of morphology and Asymmetry ratio of width in both two plots present unremarkable law with scouring times,and the determination coefficients of most sections are extremely low.The ratio of width to depth of several sections that have great fitting relations present decreasing trend with the increase of scouring times,and the decreased rate gradually slower.It shows that lateral erosion is predominant in the early stage of rill evolution,then changes into incision erosion in the latter stage.According to the ratio of shape factor,the shape of sections in both two plots is extremely dramatic in the early and middle stage of rill evolution,and they finally all change into "triangle".The ratio of morphology of sections in No.2 show decreasing trend with the increase of scouring times,which reflects the morphology of rill cross-sections develops from "wide shallow shape" to "narrow deep shape".All sections in No.3 are no obviously regularity,and there are different shapes in the process of scouring.The change laws between the asymmetry ratio of width of sections in both two plots and time are increasing or decreasing,and the fitting degree of them are high or low,the uncertainty and complexity of the lateral erosion direction in the process of rill erosion are also shown.(3)According to the changing regulation of the morphology of rill cross-section,the rill depth and the erosional area along the thalweg show the trend of increasing first and then decreasing in No.2 plot,and the depth of the upper and lower parts of the rill is smaller than the middle part of the rill.The width of the rill shows the alternating distribution of "wide-narrow-wide".This is due to the continuous change of water flowing in the circulation function process of erosion-handling-accumulation.In each scouring experiment,change of the rill depth,width and erosional area in No.3 plot have no uniform regularity,but overall,the local ditch segment still presents the fluctuation trend of trade-off.The ratio of width to depth generally shows some oscillation and alternating,and upper part of the slope has greater oscillation in No.2 plot.In the early stage of rill evolution,the ratio of width to depth of each section changes greatly,and presents a decreasing trend from rill upstream to downstream in No.3 plot.In the latter stage of rill evolution,the ratio of width to depth of each section changes very little,and presents a slowly increasing trend from rill head to mouth.Furthermore,in the early and middle stage of rill evolution,change of the ratio of shape factor of rill cross-sections in both No.2 and No.3 plot is more complex,which is roughly the wave shape,while the latter is a gentle curve,the final section shape is close to the "triangle",only there are some differences in scale.Change of the ratio of morphology of cross-sections generally shows the trend of decreasing and a alternating distribution with increasing and decreasing.Change of the asymmetry ratio of width of rill in No.2 plot presents the regularity of increasing first and then decreasing,in the middle part of the slope tends to smooth,and then increase to decrease.Change of the asymmetry ratio of width of rill in No.3 plot is extremely complex,and the change in direction and degree of the left erosion and the right erosion are very frequent.(4)Under the same condition as water flow,slope,slope length and vegetation coverage,different soil types have important influence on the morphological evolution characteristics of rill cross-section.The clay and silt content of vertisol is fairly high,and its texture is sticky and heavy.The ability of permeability is poor,so there is high moisture content.The soil compaction is higher in the depth of 40 cm,and the soil erosion resistance is strong.The silt content of dry red soil is higher,and its texture is light.The ability of permeability is strong,so there is lower moisture content.The soil compaction in the same soil depth is lower,and the soil erosion resistance is weaker.All these result in the scale of rill in No.2 plot is smaller than No.3's.In addition,because the soil texture of vertisol is homogeneous and the content of organic matter is low,it is easy to form "V-shape" cross-section whose rill wall is smooth in vertisol region.In the process of rill evolution in No.3 plot,the scale and morphology of rill has great changes,and the evolutionary process is extremely complex,which are due to the dry red soil develops vertical joints and mixes with root,clod and gravel.At the same time,the scale and morphology of rill will change greatly because of the distributary and confluence along the thalweg in the process of rill evolution. |
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