Study On Morphological Evolution Of Rill Based On Three-dimensional Laser Scanner

Soil erosion not only destroys the ecosystem,but affects human production and life.Special climatic conditions and soil types make the rill erosion serious and diverse in the dry-hot valley of Jinsha River.The soil and water loss is particularly sever.Rill erosion is the initial period of gully erosion.The formation and development of rill caused serious soil erosion,and seriously affected human activities.The differences in the development stage of rill erosion are mainly reflected in the morphology characteristics.At present,the research on rill erosion is mainly based on the ideal slope surface under artificial intervention.The results of researching rill erosion have not been found on natural slope.It is meaningful to study the morphological evolution characteristics and laws of the natural slope in the wild.In this paper,a micro-watershed of natural conditions is selected as the research object in Shadi Village Yuanmou Dry-hot valley.High-precision three-dimensional laser scanner and total station were used to scan and measure in January 2016,January 2017 and August 2017.In combination with related technical software in the room,the morphological evolution of natural slopes under the same soil properties,different slopes,slope lengths,and slope conditions is analyzed from the single-channel and the gully scales.The main content and conclusions are as follows:(1)During the development of rills,the size of the morphological changes are extremely complex,and the changes of parameter value are significant.During the three monitoring sessions,the width of rill section increased continuously,but increase range is different.Except for individual sections,with the passage of time,the depth and section area greatly increase.When the No.1rill is first monitored,most cross sections are more than 17cm in width,depth is greater than 7cm,and cross section area is more than 60cm~2.Most of the cross section width is greater than 25cm,depth is greater than 10cm,and cross section area is greater than 80cm~2 for No.2 rill.When the No.1 rill was monitored for the second time,most cross sections are more than 28cm in width,depth is greater than 10cm,and cross section area is more than 150cm~2.Furthermore,during the second monitoring of No.2 rill,most of the cross section width is greater than40cm,depth is greater than 20cm,and cross section area is greater than 500cm~2.During the third monitoring of No.1 rill,most of the cross section width is greater than 40cm,depth is greater than 12cm,and cross section area is greater than200cm~2.During the third monitoring of No.2 rill,most of the cross section width is greater than 60cm,depth is greater than 26cm,and cross section area is greater than 700cm~2.In general,the shape and size of the two rills have become larger over time,but the degree of change has been different.Due to the continuous dispersion and integration of runoff along the road,the size of the two rills varies greatly.Except for the width-depth ratio of individual sections less than 1,the width-depth ratio of the two rills is greater than 1.In the two monitoring periods,most of the cross sections are mainly lateral erosion,supplemented by downward erosion.The dominant position is occupied by lateral erosion in the evolution process of two rills.In three monitorings,the width-depth ratio of No.1 rill is shown to gradually decrease from the upstream to downstream,while No.2 rill is opposite.From the view of shape index?vaules,there is no obvious laws of the cross-section shape of the two rills in the three monitoring.The shape of the rills varies greatly during rill development.It indicates that the rill lateral and downward erosion formed under the natural conditions in the field are uncertain and complex.The rill bottom curve length and the ditch bottom line of the two rills have first increased and then decreased.During the two monitoring periods,the average trench width and area show an increasing trend,which is related to the enhancement of rill erosion ability.The perimeter of No.2 rill is monotonous growth.The perimeter of No.2 rill presents the regularity of increase first and then decrease.The reason for this difference is that the of ability headward erosion of No.1 rill is stronger than No.2 rill.The variation of the bending coefficient of the two rills shows a trend of increase first and then decrease.This change trend is consistent with the length of the groove curve and the length of the straight line.The gradient of the two rills shows a trend of decrease first and then increase.(2)There are obvious differences between the parameters of morphological evolution of rill systems.The total length,density and complexity of the two slope rill systems show a tendency from increase to decrease,among which the total length of the three monitoring of No.1 rill systems is between 32.486-51.139m.the density is between 0.760-1.196m/m~2 and complexity is between 1.128-1.407.The total length of No.2 rill systems is between 24.276-24.723m,the density is between 1.588-1.617m/m~2 and the complexity is between 1.287-1.279.It indicates that the development of No.2 rill systems is more than No.1 rill systems.Overall,the number of nodes and the length ratio of the two rill slope systems are increasing,the bifurcation ratio of No.1 rill slope system is monotonically increasing,but No.2 rill slope systems is monotonically decreasing.The values of the fractal dimension of the boxes in the two rill slope systems vary from increase to decrease,while the fractal dimension values calculated by Horton's theorem greatly vary,and there is no obvious regularity.The fractal dimension of No.1 rill slope systems presents reduce first and then increase to decrease,and the No.2 rill slope systems is gradually decrease.Because the calculation of the fractal dimension of the boxes depends on the length of the rill systems,the calculation by Horton's theorem depends on the internal topology of the rill systems.The erosion area,accumulation volume,erosion volume and net erosion amount of the two rill slope systems increase over time.Except for the accumulation volume of the No.2 rill slope systems is smaller than the No.1 rill slope systems for the first monitoring,the other parameter values are larger than the No.1 rill slope systems.In the second and third times,the erosion parameters of the No.1 slope rill systems are larger than the No.2.This is due to the the fact that area of No.1 rill slope systems is larger than the No.2 rill slope systems,and the rill network is more denser.The increasing erosion parameters reflect the increasing erosion of the slope surface,and the increase of the size and shape of the rill.The information entropy and integral value of erosion indicate the intensity of the slope erosion.The larger the value,the worse the erosion appear,the rill morphology and size are also quite different.The information entropy and integral value of erosion show a monotonically increasing trend,and information entropy and integral value of erosion of the three monitoring in the No.2 rill systems is larger than No.1 rill slope systems.This shows that the slope erosion has become more and more severe,and the rill morphology and size have also increased with the passage of time.The No.2 rill slope systems is more developed than No.1 rill slope systems,the erosion intensity more greater than No.1 rill slope systems.(3)From the evolution of morphological characteristics of the single-channel and rill systems,the evolution of the rill morphogenesis formed under natural conditions in the field is uncertain and complex,the overall scale of the morphological evolution becomes larger over time.In the term of morphological parameters quantified on a single-channel scale,the change of width and depth of most cross sections present increasing trend over time.Individual cross section depth has decreased.The cross-section area,width-depth ratio and shape index?values are derived from the width and depth.There is an increase trend for most cross-section area in the two rills,which reflect the expansion of rill scale and describe the process of rill development.Width and depth ratio and shape index?value have large fluctuations,which reveal the complexity of rill morphological during the development of two rills.From the perspective of morphological parameters quantified by the ditch-scale,the length of the ditch is used to describe the regularity of the linear elements in the one-dimensional space of the ditch,which shows the law of increasing first and then decreasing.The density,complexity and the fractal dimension of the boxes also show similar pattern of variation.Length ratios and bifurcation derived from linear features reflect topology structure of rill systems also reflect the changes of length and local adjustment of distribution in rill systems.The description of the scale parameters in the two-dimensional and three-dimensional space of the rill systems reflect that the amount of erosion is increasing,and the scale of the rill morphology is also becoming larger.