Stability assessment of the restored Benggang units in a weathered granite crust region of South China

Benggang, a unique and severe form of gully erosion, is widely distributed in weatherd granite crust regions of South China and causes great harms to ecosystem and human. In the recent decades, large investments had been to restore the collapsing Benggang units. In the comprehensive benefit assessment protocol for soil and water conservation effectiveness of the Benggang restoration, stability assessment of the restored Benggang units lacks. This study developed a protocol to assess the stability of the restored Benggang units in a weathered granite crust region via diagnosing sedimentary layers in a soil profile (a meter away from its debris dam). Characteristic sedimentary layers formed by collapse, or strong or weak surface erosion were defined by biplotting ratios of coarse particles (> 1 mm) (PR) and quartz proportion (QR) in each sedimentary layer to those in the collapsing wall (original soil). Among thirty restored Benggang units investigated after 2-10 years of restoration in Anxi and Changting counties in Fujian province, 27 units were still collapsing or collapsed, consisting of 12 very active ones, while only 3 units were fully stabilized. Rainfall intensity threshold causing collapse (RITC) of the units was identified via matching local rainfall events with the characteristic sedimentary layers of collapsion in the dam-front soil profile, significantly higher RITCs in the stable or collapsing-to-stable units than the continuously or late collapsing units (p < 0.05). Based on the estimated RITCs, no significant differences in stability of the restored units were observed between Changting strategy (ecological restoration only with vegetation) and Anxi strategy (restoration with cashcrop plantation). The findings of this study suggested that the characteristic sedimentary layers in the dam-front soil profile determined by the PR-QR biplotwas feasible for stability assessment of the restored Benggang units and provided a quantitative RITC estimation.