Depositional morphology of debris flow in the junction with mainstream channel

Abstract

Debris flow carries a large amount of sediment into the main river quickly, which causes river-blocking or diversion and leads to considerable losses in alpine regions. The research on the deposition characteristics of debris flow at the confluence with the mainstream channel is significant to understanding the riverbed evolution. However, the deposition mechanisms and morphology of debris flow at the confluence with the mainstream channel are not clear. This paper analyzes 80 debris flow cases at the Minjiang basin confluence in the Wenchuan earthquake area according to this phenomenon. It is found that the frequency of debris flows increases sharply after the earthquake. Moreover, the debris flow has the characteristics of sudden and mass occurrence. The deposition morphology of debris flow is also discussed with satellite images. Compared with the surface debris flow, the depositional fan at the confluence is mainly diffused along the flow direction of the main river and often presents an asymmetric broom shape because of the erosion of the main river. The relationship between the possibility of river-blocking and the properties of debris flows is also discussed. Results reveal that the river-blocking is closely related to the discharge and topography of the confluence. Furthermore, debris flow with high discharge (>200 m3/s), large density (>1.8 t/m3), and high velocity (>10 m3/s) easily trigger river-blocking. On this basis, a dimensionless parameter representing the characteristics of debris flow and mainstream hydrodynamic conditions is established, which can be used to measure the characteristics of regional river-blocking. Thus, the discriminant formula of the runout distance and the deposition morphology of debris flows are obtained. This paper studies the interaction between mainstream and debris flow and provides scientific methods for preventing and treating debris flow.