Hydrogeological uncertainties in delineation of leakage at karst dam sites, the Zagros Region, Iran

Leakage from dam reservoirs has been reported in different karst regions of the world. Water leakage occurs through the karst features directly or indirectly. The estimation of leakage locations, path(s), and quantity are subject to error due to uncertainties in the non-homogenous nature of a karst formation, method of study, and limited investigation due to time and cost factors. The conventional approaches for study on the karst development are local boring at the dam site and geological mapping. In this paper, uncertainties associated with conventional hydrogeological approaches are addressed from both qualitative and quantitative points of view. No major solution cavities were observed in boreholes and galleries of some dam sites in the Zagros Region, Iran, but huge karst conduits were discovered during the drilling of a diversion tunnel. This inconsistency is due to the point character of boreholes and the inherent non-homogeneity of karst. The results of dye tracing tests in boreholes may be significantly affected by location of the injection and sampling points, as tests executed at the Saymareh and Tangab Dam sites in the Zagros Region, Iran show. The quantitative uncertainty of leakage is analyzed for diffuse and conduit flow systems for cases with and without any grout curtain, under the combined effect of input uncertainties at the Tangab Dam site, southern Iran. Assuming a diffuse flow system, the mean leakage at 95% confidence interval for both strategies is estimated at less than 5% of the mean annual discharge of the river. Accordingly, the dam can be constructed without the necessity of a grout curtain. However, assuming a conduit flow system, the results reveal a significant uncertainty. A small diameter conduit can convey significant amounts of water under high reservoir pressure heads. The leakage of a 4 m diameter conduit (cross section area of 12.5 m(2)) is 163 times more than the leakage of 0.5 in diameter conduit (cross sectional area of 0.2 m(2)) while the cross sectional area ratio is 60. The uncertainty may be decreased if a detailed study is carried out on the stratigraphic and tectonic settings, karst hydrogeology, geomorphology, speleogenesis, and by performing several dye tracing tests, especially outside the proposed grout curtain area.