崩岗地貌发育的土体物理性质及其土壤侵蚀意义——以广东五华县莲塘岗崩岗为例

崩岗集中发育在我国广东、福建等东南7省(自治区),面积约5万km2,是华南地区土壤侵蚀最严重的区域.崩岗是水力—重力复合侵蚀交替作用的产物,也是沟谷侵蚀发展的结果.崩岗主要发育在花岗岩厚层风化壳上,崩岗土体以高黏粒、低砾石含量的粗砂土为基本特征.崩岗崩积锥土体粒径自坡顶至坡脚由粗变细,反映出坡面流水的侵蚀和搬运过程.崩岗土体可蚀性强,可蚀性因子K值平均为0.26,比花岗岩红壤地区的平均K值高0.03 ～ 0.05.崩积锥坡脚K值大于坡顶,即坡脚可蚀性大于坡顶.崩岗崩壁和崩积锥土体的平均黏粒含量为10.13％,大于5％这一泥石流形成的必要条件.崩岗流域地形陡峻,一旦遭遇强降雨,有条件转化成“泥石流”.崩岗形成的“泥石流”平均中值粒径仅为常规泥石流的1/12,砾石含量仅为1/4.因此,崩岗型泥石流(即由崩岗转化成的“泥石流”)并不是通常意义上的泥石流,是广义泥石流大类中的一个新种——泥砂流.

Soil Physical Properties of Collapsing Hill and Gully and Their Indications for Soil Erosion: An Example of Liantanggang Collapsing Hill and Gully in Wuhua County of Guangdong

Collapsing hills and gullies concentrating in 7 provinces (autonomous regions) of southeast China, mainly in Guangdong and Fujian with a total area of approximate 50 000 km², are the most serious area of soil erosion in southeast China. Collapsing hill and gully is resulted from the hydraulic-gravity compound erosion, and is the result of gully erosion. In southeast China, Collapsing hills and gullies mainly generate on the thick layer of weathering crust of granite. High viscous clay and low coarse gravel are the main feature of the collapsing soil. The average median grain size of the collapsing soilsfrom top to toe shows a change from coarseness to fine, reflecting the erosion and transportation processes of running water on slope. The soil of collapsing hill and gully is easily erodible. The erodible factor K value averagely is about 0.26, more than 0.03 to 0.05 compared with the K value of red soil in southeast China. The K value of the collapsing soils is greater in slope foot than that on the top, indicating the erosion at the slope toe is greater than that on the top, which provides a new interpretation for the mechanism of the collapse hill and gully processes. The collapsing wall and colluvial deposits have 10.53% clay content, more than 5% of the necessary requirement for debris flow initiation. When steep collapsing hill and gully terrains are coupled with the appropriate rainfalls, may be transformed into “debris flow”, but the debris flow’s grain size is much smaller (only about 1/12) than that of the conventional debris flow, and gravel content is only a quarter of the debris flow. Therefore, the collapsing hill and gully transformed intodebris flow (namely the collapsing hill transformed into debris flow) is not the conventional sense of debris flow, and it is a new debris flow subtype: Clay sand flow.