Quantifying provenance of soil originated from mass movement on soil reservoir-bank using rare earth elements

Abstract：The reservoir-bank collapse has caused soil erosion and bank expansion in the lower Yellow River, which seriously affects the ecological environment and agricultural development. Understanding the processes of mass movement occurred on reservoir-bank is important to predict and control bank expansion. However, little research exists concerning how to accurately quantify the process of bank collapse and trace the source of sediment. In this study, a reservoir-bank model with the gentle slope of 3°, steep slope of 40° and abrupt slope of 70°, was constructed according to geomorphological characteristics of the soil reservoir-bank in Xiaolangdi Reservoir, in which rare earth elements were used to trace the provenance of sediment originated from mass movement on reservoir-bank under different rainfall conditions, and quantify the soil loss from the bank contributed to sediment deposition in reservoir. The results show that the sediment in reservoir mainly comes from steep slope, and the percentage contribution of abrupt slope to the total soil loss increases significantly in rainstorms with the precipitation larger than 60 mm. Under the rainstorms, the contributions of the gentle slope, steep slope and abrupt slope to soil loss were 10%, 55% and 35%, respectively. Without rainstorm, the contributions of the gentle slope, steep slope, and abrupt slope to soil loss were 4%, 72% and 24%, respectively. Meanwhile, sediment deposition in reservoir also mainly derived from steep slope and abrupt slope. The contribution of steep slope and abrupt slope to sediment deposition were 49% and 40% under the rainstorms, and the contribution of steep slope and abrupt slope to sediment deposition without rainfall were 67% and 28%, respectively. In addition, most of the sediment generated from the lower abrupt slope accumulates near the reservoir-bank, while the sediment generated from the steep slope accumulates at a distance from the reservoir-bank. Under the rainstorms, the contribution of upper steep slope to sediment deposition was 54% at 240 cm from the reservoir-bank, while the contribution of lower steep slope to sediment deposition without rainfall was 70% at 180 cm from the reservoir-bank. Whether with or without rainfall, the contribution of lower abrupt slope to sediment deposition was all about 54% at 40 cm from the reservoir-bank. Thus, in the near future, engineering measures such as grid protected slope may be used in the reservoir area to protect the steep slope of reservoir-bank, which can effectively reduce soil erosion and bank expansion in the reservoir area.

Keywords: Bank collapse; Mass movement; Xiaolangdi Reservoir; REEs; Rainstorm