## 5 Discussion
### Debris Flow Development in the Earthquake Triggered Hazard Areas  
After the Wenchuan earthquake, [PERSON] _et al._[44] and [PERSON] _et al._[30] intensively studied the characteristics of the debris flows in the Wenchuan area and found that during the first two years after the earthquake, the debris flow activity increased rapidly because of the heavy precipitation, abundant source material, and blocked valleys. Subsequently, activity of debris flows diminished as a result of the decrease in the amount of loosened soil. The authors predicted the active period would last fifteen years. Similarly, [PERSON] _et al._[50] studied the relationship between earthquakes and debris flows, finding that land forms, precipitation, and source material are the three key factors for debris flow initiation in the earthquake area. Based on historical observations at other earthquake areas, they predicted that the impact of the Wenchuan earthquake on local debris flows would be significant in the next five to six years and would have a lasting impact for twenty years, and that the debris flow system would eventually reach a relatively stable stage. These two conclusions effectively explain the debris flow development results in the present study. During the first year after the earthquake, the geologic hazards triggered by the earthquake, such as collapses and landslides, brought abundant loose solid masses. The surface disturbance and vegetation destruction triggered by aftershocks changed the surface water infiltration and runoff processes, and facilitated the formation of soil erosion and flooding. Meanwhile, heavy rainfall flushed the slope, carried the loose mass, and eventually formed debris flows. For example, heavy rainfalls on 24 September 2008, in Beichuan County, 13 August 2010, in Qingping Town, and 14 August 2010, in Yingxiu Town induced catastrophic debris flows. During the first year after the earthquake, debris flows occurred frequently in the earthquake-affected region  
Figure 9: (**a**) Hazard prevention project in Yingxiu Town, Wenchuan County (site No. 7); (**b**) Human activity in Wenjiagou, Qingping Town (site No. 1); (**c**) Reclamation on the landslide mass in Chenjiaba, Mianzhu City (site No. 4); (**d**) Natural vegetation recovery in Chenjiaba, Mianzhu City (site No. 4). with a development rate of 10.2%. After two or three years, because the loose soil source decreased, along with decreased aftershocks, the intensity of the debris flows decreased, although the debris flow activity was still in progress. The average rate of debris flow development between 2008 and 2011 was 3%. Lithology plays a dominant role in the formation of debris flows in the study sites. The lithology data in the hazard region (Table 6) were interpreted from the China geological map and classified into three cases. The first case is glutenite and limestone, and included the No. 1, No. 2, No. 6, No. 10 and No. 11 sites. These metamorphic rocks represent good soil-forming conditions, and facilitate the formation of debris flow and vegetation recovery. The second case was for sites composed of granite and diorite with little soil development and large broken boulders after the earthquake. These conditions resulted in limited debris flow and vegetation recovery, and included the No. 3, No. 7, No. 9, No. 12 and No. 14 sites. Moreover, this lithology has a dominant proportion in the lithology of stable region. The third case was for sites composed of limestone splint rock, a sedimentary rock with moderate soil-forming conditions with less soil and small broken rock particles than glutenite. Sites No. 4, No. 5, No. 8, No. 13 and No. 15 were classified as limestone splint rock cases and had moderate debris flow and vegetation recovery. The debris flow development was also analyzed according to elevation and slope data (Figure 10). The newly developed debris flows mainly occurred in elevations below 2,000 m, accounting for 92.02% and 88.23% of the total debris flow area in 2008 to 2009 and 2009 to 2011, respectively.