Post-earthquake changes and prediction of debris flow scales in Subao River Valley, Beichuan County, Sichuan Province, China

Subao River lies along the Beichuan-Yingxiu fault in Beichuan County, which has been heavily impacted by the Wenchuan earthquake on 12 May 2008 and has become sources of many geo-hazards. On 24 September 2008, a rainstorm triggered a large debris flow in the catchment, causing several deaths and significant damages. A case study on changes of the debris flow was conducted in the river. The peak discharges were calculated in the Guanmenzi, Huangnidi, and Daanshan gullies. Results indicated that the peak discharges corresponded to various return periods in different gullies: 200 years in Daanshan, 100 years in Huangnidi, and 50 years in Guanmenzi. However, the triggering precipitation in these three gullies was only of a 20-year return period. The debris flows had undergone significant changes. Analysis indicated that the changes should be ascribed to the flow characteristics, initiation conditions, and the channel blockage impacted by the rapid accumulation of loose material. Channel blockage was the principal factor increasing the scale of the debris flow. The values on the blocking coefficient were presented based on density, height, and other characteristics of dams. Finally, all of the peak debris flow discharges of the Subao River Valley for a 20-year return period were calculated using the recommended blocking coefficient values.