Detection of leakage in the plunge pool area at Xiluodu arch dam with an integrated approach

The Xiluodu arch dam is one of the highest dams in the world. A noticeable leakage having a maximum rate of 192 L/s and accounting for 46 similar to 75 % of the total discharge at the site has been observed in the plunge pool area since impoundment. For long-term safety of the dam, an integrated approach of groundwater temperature analysis, hydrochemical data clustering, cross-correlation function and inverse modelling is presented to detect the sources and flow paths of the leakage. It is found that the leakage is originated from three major sources, i.e., the confined limestone aquifer (underlain by the unconfined basalt aquifer), the reservoir and the downstream river. The vertical leakage from the limestone aquifer occurs through unplugged boreholes drilled during site investigation. Inverse modelling shows that induced by particle filling, clogging and rock deformation, the effective permeability of the unplugged boreholes follows the same exponential decay with the near-field basalt rocks upstream the impervious system. Consequently, the amount of the vertical leakage decreases annually, which leads to an annual decrease of the discharge in the plunge pool area. The leakage from the reservoir and the downstream river is dynamically stable in magnitude, which occurs through the basalt aquifer by bypassing the grout curtains in the dam and subsidiary dam foundations, respectively. During the past seven years of operation, the proportions of leakage from the above three sources account for, on average, 44 %, 31 % and 25 %, respectively.