Morphological Changes And Hydrodynamic Characteristics During Development Process From Rill To The Niche-like Ephemeral Gully On Granite Red Soil Slope

The red soil area where the granite parent material developed in southern my country has serious erosion of the Benggang erosion,and its origin and development process need to be further revealed.The niche-like ephemeral gully developed from the rill in the granite red soil slope has different morphological characteristics from the general erosion ditch,and may be the initial form of the collapsed ridge.It is speculated that the soil configuration of"upper red soil(sticky)lower sand soil"in the red soil profile of granite is the material basis for the development of the niche-like ephemeral gully,but the process mechanism of the development of the niche-like ephemeral gully is not clear.In this paper,the characteristics of micro-collapses and the hydraulic conditions for their occurrence during the development of rills were studied,in order to explore the formation and development mechanism of the niche-like ephemeral gully.Indoor scour tests were carried out on the slopes of granite parent red soil in the Yanglong small watershed in Tongcheng,Hubei Province,and the morphological characteristics of the eroded ditch around the sampling site were investigated.The indoor soil tank scour test was designed with 2 slopes(20°in the middle slope and 40°in the steep slope)and 2 scour runoffs(3 L/min low and 6 L/min high),respectively,in3 different soil configurations(single Laterite layer H,single sandy soil layer S,upper red and lower sandy layered soil layer HS),carry out rill scour test to monitor channel shape,soil loss and runoff hydraulic characteristics during the development of rill,the occurrence time of micro-collapse,Location,collapse weight,and hydrodynamic properties of micro-collapses.The following main results are obtained:(1)The laterite layer of granite red soil is loam,and the sandy soil layer is sandy loam.Compared with the sandy soil layer,the laterite layer has a finer texture and higher erosion resistance;the saturated hydraulic conductivity of the laterite layer(0.98cm/d)is less than the sand layer(3.78 cm/d);the laterite layer has a strong water holding capacity,and the water content is 0.03 cm~3/cm~3 higher than that of the sand layer at a higher matrix suction(100 k Pa);under the same water content,the laterite layer Has high shear strength.It shows that the laterite layer can protect the sand below to a certain extent and reduce slope erosion.(2)Field investigation found that there are obvious differences between rill and the niche-like ephemeral gully in ditch depth and upper and lower width of the ditch.The rill groove on the granite slope is in the shape of"V"or"U".The niche-like ephemeral gully is the result of the further development of the rill in the local area.The maximum groove depth/upper width ratio of the rill was 0.40-1.43,while that of the niche-like ephemeral gully was 3.75-10.67.The maximum lower width/upper width ratio of the rill is 0.75-1.00,and the niche-like ephemeral groove is 1.30-3.50.It is preliminarily proposed that the maximum depth/upper width of the trench is 3.75 and the maximum lower width/upper width is 1.30 as the boundaries of rill and the niche-like ephemeral gully.(3)In the scouring test,the HS soil configuration was more likely to develop the niche-like ephemeral gully and the niche-like ephemeral gully existed for a longer time.Under the high runoff flow on the steep slope,the maximum cross-sectional areas at the end of the niche-like ephemeral gully of HS and S were 1102.5 cm~2 and 247 cm~2,respectively.The high runoff flow on the steep slope was more conducive to the formation and development of the niche-like ephemeral gully in the HS.Under different soil configurations,the soil loss in the steep slope with high runoff is greater than that in the middle slope with low runoff;but in the middle slope treatment,the total soil loss of HS is less than S,but it is the opposite in the steep slope.The results show that the upper layer of laterite of HS can protect the lower layer of sandy soil in the case of small slope,and promote the erosion of the whole soil through the formation of the niche-like ephemeral gully in the case of steep slope.(4)The soil configuration has an impact on the runoff Reynolds number required to generate the niche-like ephemeral gully.The Reynolds numbers of the niche-like ephemeral gully produced by S,H,and HS were 1200-1500,greater than 2000,and1000-2100,respectively,indicating that HS was more favorable for the development of the niche-like ephemeral gully.The frequent micro-collapses during the development of the niche-like ephemeral gully caused runoff disturbances.There is no obvious correlation between the runoff shear force and the Fried number and the formation of the niche-like ephemeral gully.(5)The difference of the HS and S micro-collapse resulted in the difference of soil loss between the two.In the HS treatment,the micro-slumps with a single collapse amount greater than 1 kg were the most,and the upper laterite layer enhanced the overall shear strength of the ditch wall,so that the soil block had enough dead weight to collapse.The critical angle(vertical angle of the trench wall)of micro-collapse is greater than 90°,and has a significant negative correlation with the critical height,and the average value of shear strength is H layer of HS>H>S>S layer of HS,indicating that gravity Instability is a major cause of micro-collapse.The average value of pore water pressure in micro-collapse is H>S>HS,indicating that micro-collapse is more likely to occur in the HS soil configuration.The results of this study show that the soil structure of the upper and lower sands of granite red soil is a favorable factor for the occurrence of niches during the development of slope rills.Collapse develops the niche-like ephemeral gully.