Experimental Study On Sediment Yield Process Of Ephemeral Gully Erosion In Granite Hilly Area Of South China

Ephemeral gully erosion is the main source of sediment yield on slopes,which is common in the red soil area of granite region in South China.Ephemeral gully continues to develop will cause serious soil erosion and gradually develop into permanent gullies,forming irreversible of erosion badlands.Moreover,the fertility of soil will be reduced and rivers,lakes and reservoirs will be silted up,eventually leading to ecological degradation and affecting human life and production.Therefore,exploring the erosion regulation of phemeral gullies is of great significance to manage soil erosion and restore ecology in the south.We choose different soil configurations(laterite layer,transitional layer and sandy layer)in the granitic red soil hilly area as the object of this study.The objectives of this study were(i)to examine the ephemeral gully erosion process,(ii)to reveal the role of hydrodynamic parameters of erosion process and basic soil properties on the regulation and morphological evolution of sediment production in ephemeral gullies,and(iii)to provide a theoretical basis for the prevention and management of soil erosion in granite hilly areas of South China.The results are as follows:(1)Obvious variations in physicochemical and mechanical properties were found in different soil layers of granite.The bulk density,saturated water content,capillary porosity,clay content,organic matter,shear strength and index of antiscourability of different soil layers showed that laterite layer > transition layer >sandy layer.The saturated hydraulic conductivity,non-capillary porosity,sand content and disintegration rates of different soil layers showed that sandy soil layer > transition layer > laterite layer.The disintegration rate of sandy soil layer could reach more than 80%,while the disintegration index was the opposite.Correlation analysis showed that the correlation between disintegration characteristics and shear strength of soil mechanical properties and ephemeral gully erosion sediment amount was the most obvious,however,the clay content and sand content were the determining factors affecting the mechanical properties.(2)The sediment yield and eroded sediment sorting characteristics of ephemeral gully erosion are closely related to the structure and properties of different soil layers,and soil properties is the main factor determining the sediment yield way.The sediment yield of each soil layer increases with the increase of runoff rate,in which the sediment yield of different soil layers showed that sand layer > transition layer > laterite layer.The regularities of sediment yield at different runoff volumes were all shown as follows: the maximum sediment yield at the beginning of erosion,exponentially decreasing in the before 10 min,and fluctuating and stable after 10 min.The grain level content of eroded sediment in the laterite and transitional layers was in descending order: 1-2 mm,>2 mm,0.5-1 mm,0.25-0.5 mm,0.1-0.25 mm,and <0.1 mm,while the content of grain level 0.5-1 mm was higher than that of >2 mm in the sandy layer.Therefore,the >2mm and 1-2 mm grain classes were the main sources of eroded sediment in each soil layer,and these two grain classes undergo disruption and dispersion during erosion because their enrichment ratios were less than 1.However,the rest of the grain classes undergo different degrees of enrichment because their enrichment ratios were greater than 1.(3)The hydrodynamic parameters of ephemeral gully erosion processes are important factors in changing the erosion way of slopes.The velocity,Reynolds number,Froude number and unit runoff power showed that the laterite layer >transition layer > sandy layer,however the resistance coefficient,Manning’s roughness coefficient,runoff shear and runoff power showed that the sandy layer >transition layer > laterite layer.During erosion,flow velocity,Froude number and unit runoff power decreased with increasing erosion time,and Reynolds number,runoff shear,runoff power,drag coefficient and Manning roughness coefficient increased with increasing erosion time.Correlation analysis showed that the sediment yields positively correlated with runoff velocity,Froude number and unit runoff power(p<0.01),negative correlations with drag coefficient and Manning roughness coefficient(p <0.01).The path analysis showed that Froude number and flow rate were the decisive factors for sediment yield,and sand content,disintegration rate and disintegration index were the decisive factors for sediment transport rate.The structural equation modeling revealed that the clay content and sand content acted on the sediment transport rate by influencing the soil cohesion and disintegration rate,where the clay content had a negative effect on the sediment transport rate,while the sand content had a positive effect.(4)The physicochemical properties and hydrodynamic parameters of different soil layers are the main factors affecting the development of erosion gully morphology in ephemeral gullies.The degree of downcutting and lateral erosion in ditches of different soil layers both increased with runoff volumes and erosion time.Both downcutting and lateral cutting erosion in the laterite and transition layers were small,while downcutting erosion in the sand layer was more obvious,but lateral cutting was also not obvious.Therefore,the gully width-depth ratio of soil layer increases with the increased runoff volume,and the width-depth ratio in the same runoff rate showed as follows: laterite layer > transition layer >sandy layer.The stepwise regression analysis showed that the main hydrodynamic parameters affecting the width-to-depth ratio of ephemeral gully were runoff power,Reynolds number and velocity,of which velocity was the decisive factor.The results of the study showed that the laterite layer is a protective barrier for granite soil erosion,therefore,the primary task should be to protect the laterite layer,prevention of sandy soil layer exposure and control the slope runoff volume and velocity.