Morphology evolution of vegetation-covered loess gully heads induced by hydraulic and gravitational erosion

Abstract

Gully heads suffer hydraulic and gravitational erosion triggered by rainstorms. However, how hydraulic and gravitational erosion affect the morphology evolution of the vegetation-covered loess gully heads is unclear. Field flow scouring and separation experiments were conducted to simulate the runoff and erosion processes of vegetation-covered gully heads subjected to concentrated flow on the Loess Plateau. The results show that the gully heads experienced early simplex hydraulic erosion and subsequent compound hydraulic and gravitational erosion under concentrated flow, resulting in the initiation, development, shrinkage, and reformation of the scour hole and plunge pool. In the early period, the rates of the scour hole widening and deepening had significant linear relationships with the on-wall flow rate (P < 0.01); moreover, among the hydraulic parameters of jet flow, the cumulative width and stable depth of the plunge pool had the highest significant correlations with the kinetic energy of the jet flow into the plunge pool (P < 0.01). Gravitational erosion contributed 26–50% and 0–26% to the maximum width and depth of the scour hole, respectively, and hydraulic erosion played a dominant role in scour hole development. Nevertheless, an overhanging mass collapse could reduce the depth and width of the plunge pool by 56–87% and 77–93%, respectively. The gully head retreated as a cyclic process of scour hole development, scour hole collapse (scour hole enlargement), overhanging mass collapse (scour hole shrinkage), and scour hole redevelopment. The mutual promotion of hydraulic erosion and gravitational erosion resulted in the sustained retreat of the vegetation-covered gully heads.