Evaluating temporal changes in hydraulic conductivities near karst-terrain dams: Dokan Dam (Kurdistan-Iraq)
Introduction
A large number of dams have been built on karstic limestones and dolomites all over the world (Ford and Williams, 2007, Milanović, 2011). Examples include the Shahid Abbaspour Dam, Iran (Ghobadi et al., 2005); Kalecik Dam, Turkey (Turkmen et al., 2002); Chapar Abad Dam, Iran (Uromeihy and Barzegari, 2007); Salman Farsi Dam, Iran (Fazeli, 2007), and Đale HEPP, Croatia (Bonacci and Roje-Bonacci, 2012). At many sites, excessive seepage issues have been encountered (Dreybrodt et al., 2002, Turkmen et al., 2002, Mohammadi et al., 2007, Bonacci and Roje-Bonacci, 2008, Bonacci and Rubinic, 2009). In extreme cases, operational costs were increased, and dams have been abandoned.
As the water level in the upstream reservoir rises, excessive seepage downstream is expected due to the hydraulic gradient in the surroundings rocks. For example, Turkmen et al. (2002) described a 15-fold increase in the cumulative discharge at three groups of springs (K-1 to K-3; from 15 l/s to 225 l/s), and the occurrence of a new group (K-4), as a result of a 25 m increase in reservoir level. In karst regions, it was suggested that seepage could occur as (1) water flowing through previously abandoned karst conduits; (2) water dissolving and expanding existing conduits (Dreybrodt et al., 2002, Hiller et al., 2011); or (3) water transporting fine materials previously filling fractures, bedding planes, vugs and openings. The latter is sometimes referred to as ‘washing’ or ‘flushing’ (Turkmen et al., 2002, Ghobadi et al., 2005).
The Dokan Dam was built over the karstic terrain of the Zagros Mountains, Iraq (Fig. 1). Its construction included an extensive vertical grout curtain constructed to reduce downstream seepage. Hydrological data (piezometric heads, seepage flux) has been regularly collected from the site since approximately 30 years ago. This study analyses the collected data with the aim of identifying and quantifying temporal changes in the hydraulic conductivity of the rocks and the engineered structures. Using a numerical model of the Darcian flow and comparing it with the collected data, the share of discrete karst conduit seepage is also estimated, and its significance in this dam is quantified.
Section snippets
Study area
The Dokan Dam is a multi-purpose concrete arch dam located in NE Iraq (35°57′N, 44°57′E; Fig. 1). It was built in 1959, across a narrow gorge of the Lesser-Zab River (Fig. 2), and catches an area of 11,690 km2 (SMEC, 2006). A vast reservoir lake (270 km2) was developed at its upstream side. In the downstream section (the ‘tailwater valley’), the river continues to the southeast. A narrow ridge bounds the reservoir bank and the tailwater valley, hereafter referred to as the ’left abutment’.
Hydrological data
A large database, collected by the Dokan Dam office, served as the basis for the analytical and the numerical analyses. This includes records of the reservoir elevation (hr), the head difference across the curtain’s piezometers (Δhc; note that the piezometer readings were not available for this study), and the total discharge at the v-notch weirs located inside the gallery and along the tailwater valley (Table 1). The raw database was refined by a rigorous sorting process (removing wrong dates,
Numerical analysis
To assess the implicit temporal changes in the hydraulic conductivities of the rocks, grout curtain, and shortcrete lining of the gallery, we need to obtain estimates of the hydraulic conductivity values at different times. Noting that we are not interested in the evolution of the system, and considering the scale of this study, an equivalent continuum approach (or equivalent porous medium) (Scanlon et al., 2003, Ford and Williams, 2007, Dafny et al., 2010) can be used to obtain a homogenized
Instantaneous response of the flow system
As illustrated by the above linear trends (Sections 3.2 Changes in the effectiveness of the grout curtain as a hydraulic barrier, 3.3 Changes in the seepage discharge with respect to reservoir level), the hydraulic gradient and the seepage flow are changing in different proportions in response to rising reservoir levels. At any given time, a rise in the reservoir level from +480 m asl to +510 m asl is accompanied by a 50% increase in the head differences across the curtain. The same rise in the
Summary
The high hydraulic gradient induced by the construction of the Dokan Dam, Iraq, has caused not only an induced seepage at the tailwater valley but also temporal changes of the bounding rocks and the grout curtain hydraulic conductivities. In a period of 20 years, changes of +5%, −34%, and −21% are evaluated for effective hydraulic conductivities of the rock, grout curtain, and gallery lining, respectively, in the left abutment. A mechanism of fluctuation and transportation of fine materials from
Acknowledgements
The authors thank the office of Dokan Dam for supplying the hydrological data for this study and Prof. Kamal Haji Karim (University of Sulaimani, Iraq) for clarification regarding the geology of the Kurdistan region. Many thanks to Prof Francisco Gutiérrez (University of Zaragoza) and three anonymous reviewers for their constructive comments, which led to the improvement of this manuscript.
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