| Soil erosion is a major environmental problem what we have been facing,and it is related to agricultural production,economic development and people’s lives.River and soil erosion is the most important environmental issue in dry-hot valley of Jinsha River in upper reaches of the Yangtze River.Gully erosion is a major source of sediment in the lower reaches of the Jinsha River.Yuanmou dry-hot valley is one of the most typical area in the dry-hot valley area of Jinsha River.Special climatic conditions and soil type caused of the ravines and gullies criss-cross,and gully landforms is widely developed in the place.Because of the severe erosion of soil,the lives of local residents is seriously affected and the development of local economy is restricted.Gully erosion is important part of slope surface soil erosion.Complete sequence of gully erosion is general including ripple,rills,shallow groove,dissected gulch and gully.The formation of rill is the early stages and necessary steps of gully erosion.Step-pool is one of the mechanism that rill to ephemeral gully,it is also necessary for formation and development of rills,and an important part of bed of micro-landform.This study is accomplished basis on the scouring experiment in Julin experimental bases in Yuanmou dry-hot valley of Yunnan province.We selected 3 communities to do the water erosion experiment in the experimental bases.Using the three dimensional laser scanner gets the dense point cloud data of the slope and micro-landform of rill’s bed before the experiment begins and after each washing.Using the software RiSCAN_PRO and ArcGIS processing dense cloud data,extraction the vertical-section of step-pool and cross-section of trench,then using Auto CAD software to extract detailed parameters of the step-pool.The research contents and major conclusions in this paper are as follows:(1)In the whole process of water erosion experiment,there are ripples begins to development in slope after the first water erosion.Many different sizes of wide and shallow step-pools are appeared in the overflowing road in the early period of the experiment.After each water erosion,the location and scale of step-pool in rill bed would different degrees of change.Scour process is also accompanied by the development and dying of step-pool.So the number of step-pool is also changing after every water erosion.As a whole,the number of step-pool in rill is increasing along with the times of scouring,spreading anywhere in the channel irregular,but mainly of them from the ditch within the 2~19.5m.The scale of step-pool in rill along with the water flow erosion presented the development tendency that to get bigger and then smaller,and even collapse and demise finally.This tendency is consistent with the general law of development of things in nature.There is also some correlation between the slope,height,length of step and the length,weight,depth of the pool.(2)In the process of the evolution of step-pool in rill,it has various morphological characteristics in different development stages,and the parameters also varied too.In this experiment,the 95 groups parameters of step-pool were compared and analyzed to identify their morphological characteristics.In this paper,the azimuth of step-pool is mainly to the west due to the influence of the aspect of the slope.The gradient of step-pool is between 15°and 62°,and the gradient in 23°~31°group has 36,accounting for 37.895% of the total,and accounted of absolute advantage among the slope of the step.The skewness values of the gradient is 0.778,and the kurtosis is 0.110,it shows that the gradient of step is relatively flat in the process of experimental.The height of step distribution in the range of 3.5cm and 31 cm,the height <9cm for a total of 9,accounting for 44.21% of the total.According the skewness value and kurtosis of the height can be seen that the height of step mainly concentrated in low value area,and height distribution around the average.The width of step is larger,between 4cm and 40 cm,and the width of step in the group 18cm~25cm up to 44,accounting for 46.316% of the total.The length of the step is between 12cm~50cm,there are 30 in the 28 cm to 36 cm group,accounting for 31.519%,the advantage is not obvious.The minimum value of the pool length is 12 cm,and the maximum value is 50 cm,and the dominant group is 20cm~28cm group,a total of 33 which accounting for about 34.737 of the total.The width of the pool is between 8cm~28cm,of which is more concentrated on the width of 12cm~20cm,accounting 61.052% of the total.The depth of the pool is small,although the depth range of 1cm~25cm,but there are more than half(57)of the depth of the step-pool less than 5cm,the depth of 5cm~10cm is only 30.(3)The length,width and depth of the pool are positively correlated with the height,length and slope of the step,in the influence of the height,length and slope on the size of the pool.The influence of the height of the step on the pool scale is significantly greater than the impact of the slope and length of the step on the pool size,and the height of the step has the greatest impact on the length of the pool,the minimum impact on the depth of the pool.However,the influence of the gradient of the step on the pool depth is the most significant,and the impact on the length of the pool is the weakest.The change in the length of the pool is uncertain,due to the combined effects of soil properties,herbaceous root,and rill boundary conditions and other factors.So there is a negative correlation between the changes in the length of the pool and the gradient of the step.But the trend is not obvious.(4)The planar characteristics of the pool is significantly difference.By calculating the oblateness,the shape of the step-pool is majority extremely oval and oval which developed in the runoff scouring effects in this paper.And it is very similar to the fluvial lake cave of sandstone and mud rock in hilly area of the central Sichuan Basin and the planar features of the sinkhole in the period of middle and advanced stage which developed in the gully bed of Yuanmou dry-hot valley.According to statistical analysis,most of the pools are deep pools in this study,shallow pools also have a place,but the extremely deep and extremely shallow pool is very rare.The ration “r” of the width to depth is mainly concentrated in 1~3,a total of 50 which accounting for 52.63% of the total,and followed by 3<r≤5 accounted for 41.06%.However,the group of 0<r≤1 and r>5 is accounted for 1.05% and 5.26% separately.(5)The evolution period of the step-pool can be divided into four stages: budding stage,development stage,stable stage and the period of extinction.The shape of longitudinal profile showing the evolutionary process of step-pool is that wide-shallow step-pool,saucer shaped step-pool,colander-like step-pool and then dying.Basing on the time of step-pool development and duration,the step-pool which has complete longitudinal profile can be divided into 3 types: intact shape,occasionally disappear and present casually.Completing type step-pool has overall evolutional process,and it through budding period-development period-stable period-dying period from it growth to extinct.The present time of occasionally disappear step-pool is more than or equal to three,but there is one time of which has not been observed.The appear time is less than or equal to 2 times of present casually step-pool. |
