Landform evolution and failure mode of bank collapses controlled by active fault zones and huge water level fluctuations in the Heishui tributary of the Baihetan Reservoir

Abstract

The Baihetan Reservoir filling began on April 15, 2021, triggering extensive landslides and the reactivation of previously deposited slope material after two periods of 825 m trial impoundments. Reactivation events are distinct in the Heishui tributary. It is crossed by the Zemuhe Fault Zone, with broken rock soil mass inducing bank collapse processes associated with long-term tectonic activity. Following two impoundment cycles, three disastrous slope failures occurred in the Heishui tributary, posing significant threats to road and building safety. We aimed to qualitatively and quantitatively evaluate the transient evolution of bank collapse in the Heishui tributary. Terrain-following photogrammetry and innovative dual-controller cooperative UAV flight campaigns will be conducted in 2022 and 2023. Then, digital orthophoto maps (DOMs) and digital surface models (DSMs) were created to establish and analyze comprehensive bank collapse inventory and distribution laws. Next, a geomorphic change detection (GCD) method was used to calculate the erosion caused by catastrophic bank collapses. The results indicated that 80 bank collapses occurred during the initial impoundment, which increased to 90 after the second impoundment. The collapses mainly occurred at the intersection of the faults and bank slope. Catastrophic bank collapses exhibit precursor signs, with the front edge experiencing failure in areas ranging from 7 % to 46 % during the initial impoundment and destruction occurring during periods of high water levels. In addition, the bank slope increases by 2° to 5°. Although some bank collapses became part of the fluctuating zone after impoundment, the collapse process remained prevalent in the Baihetan Reservoir. This case study focuses on bank collapse geomorphological characteristics, distribution laws, and transient evolution, which can help enhance the understanding of disaster prevention in reservoir impoundment regions.