بررسي اثر مقياس در برآورد مقدار نشت آب از مخزن و تكيه‌گاه سد بهشت‌آباد

Investigation of scale effect in the amount of water leakage from dam reservoir and abutments (case study: Beheshtabad Dam)

ساخت سدها بر روي سازندهاي كربناته كارستي عمدتاً با مشكل نشت از تكيه‌گاه و مخزن مواجه است. احداث سد بر روي اين سازندها باعث نشت از مخزن سد به سمت پايين دست مي‌گردد. سد بهشت‌آباد در حال حاظر در مرحله مطالعات قرار دارد و جناح راست مخزن اين سد به‌طور مستقيم با سازند دلوميتي-آهكي جهرم در تماس مي‌باشد. با توجه به ناهمگني كارست مقدار هدايت‌ هيدروليكي با مقياس رابطه مستقيم دارد به همين دليل براي تعيين ميزان نشت از مخزن سد روش‌هاي مختلفي با مقياسهاي متفاوت براي تعيين ميزان هدايت هيدروليكي اين سازند استفاده گرديد كه شامل آزمايش‌هاي لوژن، اسلاگ، دارسي، منحني فرود و استفاده از دبي چشمه و سطح آب گمانه مجاور مي‌باشد. بر اساس روش‌هاي مختلف طيفي از مقادير هدايت ‌هيدروليكي معادل توده بين 6-10×1/2 متر بر ثانيه در مقياس كوچك (Sub-local scale) تا 4-10×6/1 متر بر ثانيه در مقياس ناحيه‌اي (Regional scale) با اختلاف 75 برابر بدست آمده است. با توجه به نتايج مطالعات توصيه مي‌گردد از روشهاي مقياس ناحيه‌اي براي بررسي نشت اين سد بهره گرفته شود كه با در نظر گرفتن سطح مقطع تماس آهك با مخزن در حالت بدون ايجاد پرده تزريق، مقدار نشت حدود 4/5 تا 1/7 متر مكعب بر ثانيه پيش‌بيني مي‌شود.

1- Introduction For a dam project in the karstic areas, the most important parameter is the hydraulic conductivity values which are necessary for determining the amounts of leakage from its reservoir and abutments However, secondary porosity and flow networks can cause heterogeneity and anisotropy in karst fields, leading to changes in hydraulic conductivity values and then significant differences in leak calculation values due to scale changes. Limestone formation in a regional scale is generally heterogeneous, and the heterogeneity of hydraulic conductivity (Karami 2002) usually specify the value of heterogeneity in karstic aquifers Kiraly (1975) investigated karstic aquifers and fractures in the Jura Mountains in Switzerland. He reported that the sub-local scale to the well-scale and the great permeabilities on the regional scale are related to karstic conduits and increase hydraulic conductivity. Rovey and Cherkauer (1995) measured the hydraulic conductivities of five hydrostratigraphic carbonate units at different scales and reported that the values of hydraulic conductivities show direct proportionality with the measurement scales. Sauter (2005) studied the various methods, sub-local to regional scales, for determining the permeability in a karstic environment. He mentioned that the various hydraulic conductivities in different scales could be due to the spatial organization and the degree of networking of the drainage system (Sauter 2005). Other hydrological and geological studies have investigated the relationship between scale with values of hydraulic conductivities in fractured rocks (Illman 2007) and sedimentary formations (Chapuis 2010, Galvão et al. 2016). The main objective of this study is an investigation of the scale effect on the amount of water leakage from the reservoir and the dam abutments in the karstic area. We selected the Beheshtabad for the case study of this issue. 2- Methodology Beheshtabad dam is double-arch dam with a height of 180 meters and the reservoir volume of 1050Í106 m3. The dam is situated on the Beheshtabad River with an approximately 33 m3/s flowrate. The right side of the reservoir is in contact with karstic limestone-dolomite of the Sangvil anticline named Jahrom- Asmari Formation with a thickness of about 700 meters. Determining the amount of leakage from the reservoir requires an accurate estimation of the hydraulic conductivity according to the contact scale with the karstic aquifer. The values of hydraulic conductivities have been measured in Jahrom-Asmari Formation of the right-side reservoir using various methods in three scales, sub-local, local and regional scales. We used the Slug and lugeon tests in sub-local scales and conducted simultaneous measurements of spring discharge and boreholes water levels such as pumping wells in the local scale. Also, hydraulic conductivity in the regional scale determined by Recession curve and Darcy method. Finally, the amount of leakage was calculated in different scales and compared based on the hydraulic conductivity values of different scales. 3- Findings The hydraulic conductivity has been calculated on a different scale with a related test for limestone aquifer on the reservoir’s right-side. The values of hydraulic conductivities are not the same in different methods, and it is in the range of 2.1×10-6 to 1.6×10-4 m/s (Table 1). The lowest hydraulic conductivity belongs to a slug test conducted on a sub-local scale, and the highest hydraulic conductivity is for recession curve and Darcy method in regional scale. Also, the spring discharge method and the water levels of boreholes as a pumping well in the local scale estimates the values of hydraulic conductivities. The calculated hydraulic conductivities on the regional scale are about 70 times higher than those for the sub-local scale, which is related to the effect of scale in the karstic environments. According to the results, experiments with a radius of greater than 500 meters will determine the equivalent volume of hydraulic conductivity to the karstic mass region. Table 1. Calculated hydraulic conductivity value using different methods