Karst leakage and its sealing at Wanzi reservoir in Luoping county on the plateau of eastern Yunnan

Built on the Dagan river-a three-tier tributary of the Nanpan river from the Zhujiang river system, Wanzi reservoir is located at Wanzi village on the edge of Luoping basin 13 km from the southwest of Luoping county of Yunan Province. It is a typically karst-damaged reservoir in eastern Yunnan, with its lateral leakage toward lower adjacent valley. The landform of this reservoir belongs to erosion-corrosion alpin and medium-height mountains of eastern Yunan structure. The control drainage area of Wanzi reservoir is 104 km² with the average runoff of 8,843×10⁴ m³. This medium-sized reservoir covers a designed irrigation area of 2,133×10⁴ m² with its designed height of 23.5 m and normal capacity of 1,046×10⁴ m³. It has experienced four times of leakage sealing since its construction in 1959, but the leakage reaches more than 0.5 m³·s⁻¹ at medium water level with water supply of only 141.7×10⁴ m³.In order to provide the theoretical foundation for further successful sealing and even the expanding of reservoir capacity, we investigate the main causes of the failure of leakage sealing. Meanwhile, we also profoundly analyze the factors of leakage according to the previous studies as well as the data about Wanzi reservoir such as stratum lithology, geological structure, geological conditions of hydrogeological engineering, characteristics of karst development, conditions of flow concentration, the failure of sealing, etc. Results show that because the study area is located on the strong karst development zone controlled by stratum lithology and structure, the vacuum suction erosion and the groundwater erosion caused by frequent and substantial water-level fluctuation result in corrosion fissures and the scouring of holes. The additional stress, composed of the water storage pressure and the negative pressure caused by the vacuum due to the coverage and enclosure of reservoir bottom, is larger than the shear strength, or larger than the strength bonding karst cave fillings and cave walls. Consequently, there exist failures of leakage sealing, sinkholes and the repeated karst collapse in the same leakage area, all of which are the main causes of leakage.In spite of several relatively separate leakage belts inside the reservoir area, all the leakage belts outside converge into the karst pipelines on both sides of the reservoir. These karst pipelines characterized by bedded development are narrow due to the holdup of clastic rocks. The leakage sealing spots scattering over a large area bringing huge engineering work and the failure of leakage protection. However, the water-tight curtain properly laid out outside the reservoir area can not only eradicate the leakage, but it can also effectively raise the groundwater level to improve storage conditions. In the way of laying out water-tight curtain, the reservoir capacity can be expanded more than 3 times as large as the previous one, equal to 3 other medium-sized reservoirs.