Characteristics of low frequency debris flow and risk analysis in Beijing mountainous region.

By investigating the Huanglianggenqiao Gully in Baimaguan River Basin of the upper stream of Miyun Reservoir in Beijing in March 2015, we analyzed the provenance characteristics of the gully and rainfall excitation conditions. Referring to other debris flow gullies in this area, we calculated bulk density, flow velocity, flow rate and debris flow scale of the studied gully. Finally, we analyzed the potential risk of debris flow in this gully and proposed countermeasures for reducing disasters, which would provide references to the prevention, control and forecast of debris flow in this gully and other low-frequency gullies in this area. The results showed that the total provenance in Huanglianggenqiao Gully was about 17.99×104 m3, which was mainly piled up at the bottom of slope and thereby seriously squeezed the main channel. Short-duration rainfall was the main excitation factor for the debris flow of this gully or the whole area. When rainfall intensity ranged between 31.79 and 40.20 mm/h, debris flow was more likely to break out. The gully belongs to diluent debris gully and the flow velocity is close to 5.0 m/s, affected by provenance reserves. Under the condition of 20-year rainstorm, the peak flow was 187.41 m3/s and the debris flow solid scale was 6.82×104 m3, which were beyond the surplus storage capacity.