Abstract: At 20:00 on July 4, 2013, large-scale co-seismic debris flows broke out in Shimian County, Sichuan Province. Affected by the earthquake which hit Lushan County in April, the region is stricken with abundant debris flow sources, of which the sources in debris flow gullies takes up over 64% of the total. Investigating the erosion characteristics in debris flow gullies is of crucial and imperative significance to understanding the development of debris flows in the study area. According to field investigation of the cross sections of three typical co-seismic debris flow gullies in the study area, namely, Hougou gully, Majingzi gully, and Xiongjia gully, we examined the relationship among flow depth, erosion thickness, relative height difference between profiles, and original gully width. For the first time we present the concept of flow direction difference based on curvature radius, and obtained the coefficient of average linear function between erosion thickness and flow direction difference as 0.06, and the coefficient of average linear function between erosion thickness and profile height difference as 0.09. The relationship between erosion and flow depth is h=5Δh+1.798×10^-15, while thickness erosion volume per unit distance and gully width is ΔV=6.845D^0.625. The research findings offer basis for understanding the erosion characteristics of debris flow gullies in the earthquake-stricken area.

Key words: debris flow, gully erosion, erosion thickness, flow direction difference, field investigation, Shimian County