Does joint structure promote the development of gully erosion?

Abstract

Gully erosion is a growing environmental problem around the world, and limits ecological and economic development. Collapse gullies in the granite mountainous region of southern China are unique gully erosion occurrences known locally as Benggang, and its erosion gully is made up of a main gully and multiple branch gullies that form a runoff and sediment drainage channel system. Joint structure is the product of geological movement. To investigate the internal relationship between the morphological development of gullies and joint structure, we measured and recorded morphological parameters, including the area, strike, quantity and depth of gullies and the strike, dip angle, density and wall distance of joints, in four typical granite gully erosion regions. Our findings are as follows: (i) as a unique gully erosion phenomenon, the development intensity of Benggang increases from north to south, and erosion becomes more severe; (ii) the average joint area densities range from 19.41/m2 to 21.85/m2, while the average joint ratios range from 7.40 to 10.32. Overall, joint development gradually increases from north to south, characterized by a high density, clear dominant direction, high dip angle (mostly ranging from 50 to 80°), and high inheritance and considerable influence on the resulting soil structure; (iii) correlation analysis reveals that the erosion gully development direction and dominant joint strike basically coincide, and are significantly positively correlated (P < 0.001); (iv) the average depths of the main gullies in Tongcheng (TC), Ganxian (GX), Changting (CT) and Wuhua (WH) along the dominant joint strike are greater than those along other joint strikes, and deeper main gully depths correspond to more high-angle joints along the same strike. The dip angles of joints along the strike of the branch gully with the maximum depth are greater than 60°, and some branch gullies follow the dominant joint strike. This study concludes that the joint strike controls the development direction of erosion gully and that the depth of erosion gully is greater than that of other strikes in the dominant strike direction where the number of joints is concentrated. Joints aggravate soil erosion and promote the development of gully erosion.