Application of isotope tracer techniques for assessing the seepage of the hydropower dam at Tri An, South Vietnam

Abstract

Some symptoms of excessive seepage of water were observed at the earthfill dam of the 400 MW Tri An hydropower plant in South Viet Nam. To provide experimental information for assessing the situation, isotope tracer experiments were performed using 24 existing piezometric boreholes encompassing a 1100 m long middle section of the dam. Filtration velocities measured were in the ranges (0.2–30) cm d⁻¹ in the dam body and (0.5–75) cm d⁻¹ in subsurface strata. No abnormally permeable horizons were detected. Permeability coefficients were calculated and compared with that obtained in pre-project hydrogeological surveys and by field tests during dam construction. Tracer experiments yielded permeability of the same range for the subsurface aquifer but from one to two orders of magnitude higher for the dam body. Such a dramatic change of permeability of the dam body me dia is interpreted as a result of interaction with seeping water after reservoir impoundment. A transient erosion process took place during several years. The losses of water by seepage through the dam body and infiltration to underground water were estimated as 1100 m³ d⁻¹ and 15,000 m³ d⁻¹ respectively, less than 1/2000 of the quantity of water consumed for power generation. A multi-well interconnection experiment was performed to determine the velocity and direction of subsurface flows.