| III Subsurface erosion is one of the forms of hydraulic erosion and it is also aunique method of soil erosion.The Yuanmou Dry-hot Valley is the most representative of the Dry-hot valleys in Yunnan Province and is located in the ecological barrier area of upper and middle reaches of Yangtze River.Under the special stratum environment,the latent erosion landforms are widespread in the valley and become one of the major geological disasters in the area.Taking the sink hole of Shadi Village as the research object,after obtaining the basic morphological characteristics data and distribution data of the 252 sink holes,the spatial distribution characteristics of the sink holes are analyzed quantitatively from the angle of classical statistics and spatial statistics,and the relationship between the factors of vegetation,soil,slope and joint fissure and the traps are discussed.The results are helpful to the understanding of the spatial distribution characteristics and developmental mechanism of the sink hole in Shadi village.The main conclusions are as follows:(1)Three methods(average nearest neighbor,Ripley's “K” function,Spatial autocorrelation)areused to calculate and analyze the overall aggregation degreeof sink holes in Shadi village.The results show that,on the whole,the sink hole in Shadi village show a state of agglomeration distribution with a certain scale.This scale is a circle formed with a radius of 234 meters.Through the calculation of the geographical concentration index and the disequilibrium index,it is reflected that the distribution of the sink holes is not balanced in different regions.Therefore,Shadi village is divided into four zones for processing.The results of zone 1 is showed that sink holes are aggregated in the range with a radius of 0-47.55 meters.The sink holes of zone 1reach the maximum space aggregation state while the radius is about 9.55 meters.When the radius is greater than 47.55 meters,the sink holes showa diffuse distribution.The results of zone 2 indicate that in the position of 17.55 meters,the sink holes in zone 2 reach the maximum spatial aggregation state.The difference from zone 1 is that,as a whole,the sink holes are in an aggregate state in zone 2.The results of zone3 show that,the sink holes with a radius in the range of 0-67.71 meters are aggregated.When the radius is 14.71 meters,the zone 3 sink holes reach the maximum spatial aggregation state.Sink holes with a radius greater than 67.71 meters are discretely distributed in the study area.Zone 4 results show that the sink holes with a radius inthe range of 0 to 69.4 meters are aggregated.In the position of about 10 meters,the sink holes reach the maximum space aggregation state in the Zone 4.In the range of radius greater than 69.4 m,the sink holes in the zone 4 are discretely distributed within the study area.Whether it is the overall sink holes distribution or the four sink holes distributions,the K curves in the K function all show the trend of increasing first and then decreasing as the distance changes.To a certain extent,it reveals the spatial aggregation of the sink holes in Shadi village.As the distance increases in study area,the degree of aggregation of the sink holes gathers at first and then spreads.In addition to the discussion of the spatial distribution of the overall and subarea sink holes,the nuclear density tool are used to research the specific aggregating conditions of the sink holes in Shadi village.The results show that there are three major distribution areas of sink holes.And it is located in zone 1,zone 2,zone 3 in study area.(2)Beaded sink holes are a special combination of sink holes.The beaded sink holes in Shadi Village are mainly devided into following types: 3 strings,4 strings,5strings,6 strings,9 strings,10 strings,11 strings,14 strings and 15 strings.Each type has a certain amount and accounts for a certain proportion of the total number of holes.Statistical beaded sink holes account for 58.7 % of the total number of sink holes.Among them,three strings,four strings,and five strings were the main beaded groups,which accounted for 74% of the beaded group,and accounted for 52% of the total number of beaded sink holes.The clusters of 3 and 4 series of beads are mainly distributed in block 4,and there are 5 clusters of beads in a few branches.The gullies in this area are mostly stable,and the cross section of the gully is mostly shallow "U".the ones which have at least 5 sink holes are mainly distributed in block 1 and block 2.The gullies with sink holes distribution in these two regions are mostly active.The vegetation coverage in the trenches is low,and the erosion at the bottom of the trenches is strong.The cross section of the gullies is mostly deep “V”.(3)Not only from previous articles,but in the process of field investigation,it can be also found that there is a close connection between the gully and the sink hole.The gullies of Shadi Village are dominated by young age and adolescent gully,accounting for 65.52% of the surveyed gullies.The number of sink holes distributed in the gullies of young and adolescents is also the largest,and the sum of the numberof sink holes accounts for 67.16% of the total number.The number of sink holes distributed in the gullies of the prime and the old age accounts for the total number of sink holes.32.84%.By statistics,it can also be concluded that the beaded sag is mainly distributed in the young and adolescent gullies.The total amount of beaded sag that falls into the gulch is the total amount of beaded sag located in the development stage of these two gullies.73.08% of the population,while the beaded depressions located in the gullies of the prime and old ages accounted for only26.92%.From the above data analysis,it can be seen that the sink hole is closely related to the development stage of the gully,and the two complement each other.The development of the sink hole promotes the development of the gully,and the development of the gully provides good conditions for the development of sink hole.(4)It is found that the development of the sink hole is mainly influenced by the two major factors of nature and humanity.Natural factors include vegetation,soil,hydrology,joints and slope.The effect of vegetation on the pit is mainly reflected in the expansion of the root system to the soil and the effect of the length of the root system on the depth of the sink hole.The cracks formed by the root system in the soil provide a channel for the infiltration of surface water to some extent,which is similar to the influence of the existence of the joints in the soil.Furthermore,the existence of joint fissures also plays a controlling role in the strike of sinkholes.The characteristics of soil collapsibility and disintegration make the erosion resistance of soil lower and become the foundation of cave development.Differentiation of the erosion resistance or permeability of soil profile or formation is the key to the formation of depression.Under the suitable condition,water becomes the dynamic condition for the development of the sink hole.The high intensity precipitation in the rainy season of Yuanmou can not only promote the development of the sink hole but also destroy the preservation environment of the sink holes.The relationship between the slope of the terrain and the development degree of the sink hole is considered as follows:The slope of the left and right bank of the gully with hole distribution is between 20 °and 50 °,the slope of the upper part of the gully is between 10 °and 40 °,the slope of the lower reaches is between 2 °and 10 °,and the slope of the uplift is between 10 °and 40 °.The topographic gradient condition that is the most likely to occur in the sink hole. |
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