آناليز و تعيين ميزان تراوش در كانال خاكي آبياري با استفاده از مدل عددي مطالعۀ موردي: كانال اصلي شبكۀ آبياري دشت مغان

Analysis and Determine of Seepage Rate in Irrigation Earthen Canal Using Numerical Model (Case Study: Main Canal of Moghan Plain Irrigation Network)

بخش عمده‌اي از تلفات در امتداد كانال‌هاي انتقال و توزيع آب در شبكه‌هاي آبياري، تلفات ناشي از تراوش است. تخمين دقيق ميزان اين تلفات كمك شاياني به بهبود بهره­وري آب در شبكه­هاي آبياري مي­كند. در اين پژوهش، ميزان تراوش در كانال اصلي خاكي شبكۀ آبياري مغان با استفاده از مدل SEEP/W ارزيابي شده است. نتايج مدلسازي نشان داد كه انحراف مقادير شبيه‌سازي شده تراوش مدل از مقادير اندازه‌گيري شده (CRM) كمتر از 1/0 واحد است. به منظور بررسي بيشتر و پيش‌بيني تغيير خصوصيات هيدروليكي خاك در دراز مدت، با توجه به احتمال وجود شرايط ناهمگن و غير ايزوتروپ در محيط خاك، آناليزهاي مستقلي روي ضريب نفوذپذيري صورت پذيرفت. نتايج اين آناليزها حاكي از آن است كه با توجه به شدت وقوع پديده‌هاي مختلف مانند رسوب‌گذاري و فرسايش در دراز مدت، مي­توان حداكثر ميزان تراوش تا 103× 6/20838 ليتر بر روز و حداقل ميزانتراوش تا 103× 4/245 ليتر بر روزرا پيش‌بيني كرد. نتايج حاصل از اين مدل پيرامون توزيع ساير پارامترها نشان داد كه حداكثر مقدار گراديان هيدروليكي به ميزان 4/1 تا 6/1 در كف و نيز در محل اتصال آن به جداره‌هاي كناري كانال و حداكثر مقدار فشار آب حفره‌اي نيز در فاصله‌اي به ميزان 3 تا 4 برابر عمق آب موجود در كانال، نسبت به كف كانال و در امتداد قائم پروفيل خاك، به­دست مي­آيد.

Introduction Seepage losses are a significant portion of losses along the main earthen canals in irrigation networks. The correct estimation of these losses will greatly help improve water productivity in the irrigation network. Due to the high capability of numerical models such as SEEP/W to estimate seepage rates in the canals, this method has been used as one of the most widely used in various researches (Barkhordari et al., 2019; Mohammad Rezapour Tabari & Mazak Mari, 2016). Despite various investigations in this regard, the seepage rate was not evaluated for the presence of heterogeneous and non-isotropic conditions in the canal bed. Therefore, in the first step of this study, the Seepage rate in the earthen canal of Moghan irrigation network was evaluated using the SEEP/W model. In the second step, in order to further investigate and predict the possibility of changing the hydraulic properties of the soil in the long-term, a comprehensive assessment was carried out on the permeability coefficient. Methodology After collecting field data and measurements based on the existing and governing conditions (hydraulic, geometrical data, and soil geotechnical) along the Moghan canal, and also based on changes in some of these parameters, the problem is defined in the SEEP/W model based on 7 reaches. In order to calibrate the model, evaluation of simulated values with measured values of seepage using the CRM statistical coefficient was considered. Then, in order to further investigate and predict the possibility of heterogeneous and non-isotropic conditions in the soil system, a separate seepage analysis was conducted in the Moghan canal by changing the values of kx and ky kx-1. To achieve this objective, a reasonable range of 5×10-8 to 8×10-7 m/s was determined for kx based on the canal bottom and side walls materials. The range of 0.1 to unity was selected for ky kx-1. Results and Discussion The results of the main simulated showed that the deviation of the simulated values by model seepage from the measured values (CRM) is less than 0.1. Considering the distribution of other parameters such as the hydraulic gradient and pore-water pressure, concluded that due to the higher hydraulic gradient in the canal bottom than its side walls (equal to 1.6 and 1.2 at reaches 1, 2 and 3, and also 1.4 and 1 at reaches 4, 5, 6 and 7, on the bottom and its junction with the side walls in the canal, respectively), the water outlet potential is greater in this area. Therefore, the uniform lining of the bottom and side walls in water canal reaches can be considered as a successful solution that is based only on modelling results. Also, the maximum amount of pore-water pressure is at a distance of 3-4 times of the water depth in the canal, relative to the bottom of the canal and along the vertical profile of the soil. Further, the results of seepage analysis indicate that considering the severity of each of the long-term sedimentation and erosion conditions, it is possible to estimate a maximum seepage rate to 20838.6×103 L/day and a minimum to 245.4×103 L/day as an initial estimate, without the need for modelling in this canal, is predictable. Conclusions In this study, seepage rate in the earthen canal of Moghan irrigation network was evaluated using the SEEP/W model. The modelling results showed that the deviation of the simulated values by model seepage from the measured values (CRM) is less than 0.1. Then, in order to further investigate and predict the possibility of changing the hydraulic properties of the soil in the long-term, due to the possibility of heterogeneous and non-isotropic conditions in the soil system, a comprehensive assessment was carried out on the permeability coefficient. The results of this analysis indicate that considering the severity of each of the long-term sedimentation and erosion conditions, it is possible to estimate a maximum seepage rate to 20838.6×103 L/day and a minimum to 245.4×103 L/day as an initial estimate, without the need for modelling in this canal, is predictable. Also, the results of this model regarding the distribution of other parameters such as hydraulic gradient and pore-water pressure, showed that the maximum amount of hydraulic gradient was 1.4 to 1.6 in the bottom and also at the junction with the side walls of the canal. Also, the maximum amount of pore-water pressure is at a distance of 3-4 times of the water depth in the canal, relative to the bottom of the canal and along with the vertical profile of the soil.