Asymmetry Morphology Characteristics Of Gully Cross-sections In Yuanmou Dry-hot Valley

Gully is a channel scoured by temporary surface water.Gully morphology,including the planar,cross-sectional,and longitudinal profiles,can act as a link between past,present,and future gully erosion.The shape of gully cross sections(GCs)can not only be used to estimate the gully volume and erosion rate,but also to illustrate the evolutionary process induced by incision and lateral erosion.In this study,we measured 456 gully cross-sections(GCs)with the Leica DistoTM D8 laser in the Yuanmou Dry-hot Valley of China,mapped them with CASS software,and calculated the morphologic parameters.The results show that:(1)Select four GCs asymmetry indexes(e.g.Asymmetry ratio of width(al),Asymmetry ratio of area(aa),asymmetry ratio of erosion line(aw),Asymmetry ratio of erosion area(ar))to evaluate the asymmetry degree of GCs in Yuanmou Dry-hot valleys.In different areas,use al,aa,and aw as reference indicators,the GCs of Maoyi village are mainly left-deflection,the other four areas GCs are mainly right-skewed.However,use ar as reference index,the GCs of Julin village are mainly right-skewed,the other four are mainly left-deflection GCs.At different locations,GCs at the gully body and gully mouth are mainly right-skewed.But there is a little different that GCs of gully mouth are mainly right-skewed when use al and aa as the reference indexes,and use aw and ar as the reference indexes,GCs of gully mouth are mainly left-deflection.In short,GCs are mainly right-skewed,followed by the left-deflection and quasi-symmetrical.(2)The number of cross section of left-deflection,quasi-symmetrical and right-skewed GCs are in a decreasing trend with the depth increasing,and the depth of three types are concentrating in 0-8m.When the depth is between 0-4m,left-deflection,quasi-symmetrical and right-skewed GCs are 44.22%,43.36%,and 48.98% respectively.When the depth is between 4-8m,left-deflection,quasi-symmetrical and right-skewed GCs are 41.50%,44.25%,and 37.24% respectively.Judging from the width of different types,the number of three kinds of GCs showed an increase-decrease trend with the increase of its width.When the depth of GCs is between 8-16 m,the number of left-deflection,quasi-symmetrical and right-skewed GCs increase to maximize,and left-deflection,quasi-symmetrical and right-skewed GCs are 54.42%,59.26%,and 47.45%,respectively.After the number of GCs become less gradually with the increase of width.The number of left-deflection,quasi-symmetrical and right-skewed GCs are in a decreasing trend with increasing erosion area,and the erosion area of three types are concentrating in 0-50m2.Here,left-deflection,quasi-symmetrical and right-skewed GCs are 53.74%,50.44%,and 54.08%,respectively.Judging from the width to depth ratio of different types,the number of three kinds of GCs present a trend of decreased sharply after a sudden increase with the increase of its width to depth ratio.The number of GCs increase to the peak,left-deflection,quasi-symmetrical and right-skewed GCs are 70.07%,74.34%,and 58.16%,respectively.Finally,the number of GCs decreases rapidly with the increase of the width to depth ratio.In a word,judging from four indicators of the depth,width,erosion area and width to depth ratio,there is no obvious difference between left-deflection,quasi-symmetrical and right-skewed GCs.(3)The Yuanmou Dry-hot Vally has unique soils and climatic condition,which result serious gully erosion problem.Previous studies have shown that asymmetry morphology characteristic of GCs is determined by soil properties,vegetation coverage,flow direction,and piping landform.In this region,there were obvious differences between the top and down solum.The topsoil is dry red soil that has good cementing and unusually hard,however the subsoil is powder mass of sedimentary rocks those property is very soft.Residual weathering crust on top of gully has been a great protection to subsoil,it reliability has become the key factor in the study about cross-sections.Effect of asymmetrical scours along both rivers has been made a difference that due to centrifugal force when the flow passes a curve of the river,and it is a real performance on the effect of asymmetry.Moreover,the holding function of vegetation roots will result difference between right and left erosion.Piping landform(e.g.vertical hole,sink hole,and the soil bridge)have accelerated the development of furrow bank and collapse,and have an effect on morphological characteristics to a great extent.