Control Effect of Vegetation on Debris Flows: A Case Study in Batang County, Southwest China

Abstract

Batang County is located in Sichuan Province, southwest China, on the southeast margins of the Qing-Tibet plateau. A wide range of vegetation is distributed from the high altitude mountains to the deep gorges. Batang County is prone to debris flow, of which 151 debris flows were identified by remote sensing techniques (SPOT5) and field surveying in 2009. Based on geomorphology conditions, debris flows can be classified into 33 slope debris flows (SDFs) and 118 gully debris flows (GDFs). The SDFs vegetation coverage was significantly lower than the GDFs vegetation coverage. 72 % of the SDFs have a vegetation coverage below 50 %, and 70 % of the SDFs have a forest coverage below 10 %. However, more than 80 % of the GDFs have a vegetation coverage over 40 %, and only 35 % of GDFs have a forest coverage below 10 %. There is a negative correlation between the SDFs and vegetation coverage. This means that the lower the vegetation coverage, especially forest coverage, the more frequently SDFs occur. This correspondence relationship does not exist in the GDFs. Forest coverage excludes the impacts of shrubland and meadows, thus more accurately reflects the constraining effects of vegetation on debris flow. Vegetation controls debris flow in two ways. (1) Vegetation intercepts rainfall to reduce the volume and velocity of surface runoff, which decreases surface soil erosion. (2) Vegetation roots reinforce the soil, and increase soil shear strength. The primary cause of SDF is surface erosion. The roots of herbs, bushes and trees entwine together to form a dimensional anchorage net which reinforces soil integrity, increases resistance to erosion, and restrains movement by soil particles. The primary cause of GDF is landslides. When a landslide slide face is deeper than 3 m, vegetation roots are sparse, have no consolidation capabilities, and vegetation is ineffective at constraining SDF. However, GDF has multiple sources, and happens where the depth of soil is greater than 3 m. In this case, vegetation roots cannot fix the deeper soil body, thus GDF cannot be restrained.