Features and Formation Mechanism of Debris Flow in Dabaiyang Gully in Upper Reaches of Minjiang River

The Dabaiyang gully is located at the Jiaochang epsilon-type structure in the north of Diexi town,Maoxian County,Sichuan Province,which is in upper reaches of Minjiang River.Two large-scale flow debris events occurred in the gully in 1935 and 1991,and the flow debris in 1935 was caused by the 1933 Diexi Earthquake(Ms=7.5) which is huge with peak flow of 557 m3/s.After the analysis of the landform,available loose materials and water conditions of two tributaries(Dagou and Xiaogou gullies) in Dabaiyang gully,it was obtained that there are sufficient conditions for the formation of tributary debris flow,which explains the intense activity of debris flow in the Xiaogou gully:(1)the average longitudinal gradient is 275‰ in the Dagou gully and 398‰ in the Xiaogou gully,and the relative altitude is 2,023 m;(2)the total amount of loose materials is 292,100 m3 in the Dagou gully and 420,100 m3 in the Xiaogou gully,and the source of materials is abundant.The formation mechanism of debris flow was also analyzed.It reveals that the formation of debris flow caused by the fire hose effect in the Dagou gully,and by the fire hose effect and the burst of barrage body in the Xiaogou gully.The flux of historical debris flow was calculated by mud trace investigation method,and the flux of future debris flow was appropriately forecasted,which provides valuable data for preventing debris flow disaster in Dabaiyang gully.