Study on soil erosion characteristics of water level fluctuating zone in the Nuozhadu Reservoir, Lancang River

The cascade hydropower development on the main stream of the Lancang River forms a large-scale of riparian zone (also called water level fluctuation zone, WLFZ). The comprehensive action of periodical submerged-exposed and storm runoff causes severe soil erosion in the WLFZ, which threatens the safe operation of Nuozhadu Reservoir (NZDR) as well as the safety of lives and property around the reservoir. In this study, the area of WLFZ in NZDR involving different counties, elevations and slopes was investigated based on the ASTERG DEM V2 30 m data, together with GF-1, the Google Earth image, and erosion needle technology, so as to observe the soil erosion intensity in the riparian zone under different underlying surface conditions in NZDR. The results show that: ① The total area of WLFZ in NZDR is 129.95 km², with 57.29 km² in the Lancang River and 72.66 km² in its tributaries. The area of WLFZ gradually decreases along with elevation reduction. The distribution areas of WLFZ in different slopes are not uniform, with the widest area on the slopes of 15°～25°, while the WLFZ areas of steep slopes greater than 35° are very few. ② The driving factors of soil erosion in WLFZ include rainfall, runoff, wind wave, ship traveling wave, gravity and so on. And the most common erosion types are surge erosion and collapse. ③ Soil erosion monitoring from 2016 to 2018 showed that the average soil erosion intensity in the mainstream of the Lancang River was 38.5 mm/a, while in its tributaries, it was 17.00 mm/a. Besides, the depth of soil erosion in the main stream fluctuating zone was 2.3 times that of the reservoir bay. ④ The changes of slopes, vegetation, and soil moisture in the WLFZ have significant effects on soil erosion. Furthermore, the slope has a significant linear correlation with the annual average erosion depth, and vegetation can significantly reduce soil erosion in WLFZ. Additionally, the alternation of drying and wetting has a significant effect on the soil water content in WLFZ, which in turn changes the soil shear strength. The present study indicates that the soil shear strength decreases significantly with the increasing of soil moisture content.