نشریه محیط زیست طبیعی

شبیه‌سازی تغییرات کیفی رودخانه در شرایط حذف سد وشمگیر-گرگانرود

نوع مقاله : مقاله پژوهشی

نویسندگان

گروه مهندسی آبیاری و آبادانی، دانشکده کشاورزی، دانشگاه تهران، کرج، ایران

10.22059/jne.2023.352315.2506

چکیده

حذف سد از روش‌های احیاء زیست‌بوم رودخانه است. در پژوهش حاضر، حذف سد وشمگیر بر روی رودخانة گرگانرود در استان گلستان، به‌دلیل حجم بالای رسوبات درون مخزن و پایان عمر مفید آن مورد مطالعه قرار گرفت. سه سناریو جهت حذف سد وشمگیر انتخاب شد: 1- حذف کامل (مشابه شکست ناگهانی سد در روز آفتابی با مخزن پر از آب)، 2- حذف پله‌ای سرریز و 3- حذف با رسوبات پایدار. در این پژوهش، از مدل دوبعدی CE-QUAL-W2 برای مدل‌سازی هیدرودینامیک و کیفیت آب رودخانة گرگانرود برای شرایط موجود و در سه سناریو حذف سد، در بازة 128 کیلومتری پایین­دست سد وشمگیر تا دهانة دریای خزر استفاده گردید. مدل کیفی با داده­ های شاهد (تراز سطح آب و فاکتور­های کیفی) در دو ایستگاه هیدرومتری آق ­قلا (68 کیلومتر پایین­دست سد وشمگیر) و بصیرآباد (112 کیلومتر پایین­دست سد وشمگیر) برای شرایط موجود گرگانرود واسنجی و تأیید شد. نتایج ارزیابی در بازة زمانی تغییرات سریع رودخانه پس از حذف سد (تا 60 روز) ارائه شده ­است. نتایج سه سناریوی حذف سد نشان داد که میزان اکسیژن محلول (DO) در دو مقطع رودخانه (آق­ قلا و بصیرآباد) افزایش می ­یابد. میزان اکسیژن مورد نیاز زیستی (BOD) در سناریوی اول (حذف کامل) و دوم (حذف پله ­ای) به میزان اندکی (کمتر از 10 درصد نسبت به شرایط رودخانه قبل از حذف) افزایش خواهد داشت. در سناریوی سوم (حذف با رسوب پایدار)، تغییر چشم­گیری در میزان BOD رخ نمی ­دهد. در سه سناریو و در هر دو مقطع کنترل، میزان pH در دو مقطع زمانی قبل و بعد از حذف سد، تغییر اندکی خواهد داشت. میزان کل مواد جامد محلول (TDS) در روزهای اولیه برابر یا بالاتر از میزان آن در قبل از حذف است، ولی با گذشت زمان از میزان آن کاسته می ­شود. نتایج مدل‌سازی نشان می­ دهد که سناریو دوم (حذف پله‌ای سد) گزینه برتر است و سبب بهبود شرایط کیفی رودخانه و بازتوانی محیط زیستابی گرگانرود می ­شود.

کلیدواژه‌ها

عنوان مقاله [English]

Modeling of river water qualitative changes under Voshmgir dam removal condition, Gorganrud River

نویسندگان [English]

  • Javad Masoomifar
  • Mehdi Yasi
  • Javad Farhoudi

Department of Irrigation and Reclamation Engineering, Faculty of Agriculture, University of Tehran, Karaj, Iran

چکیده [English]

Dam removal is one of the methods of restoring the river ecosystem. The decision to remove a dam is based on the multidisciplinary indicators and the type of dam body and its location, as well as the downstream conditions and the type of dam removal method. In the present study, the removal of the Voshmgir Dam on the Gorganrud River, Iran, was considered, because of the high volume of sediment deposition in the reservoir and the loss of its useful life. Three scenarios were selected for the removal of the dam: 1- complete removal; 2- stepped removal of the spillway; and 3- removal with stable sediments. In the first step, numerical modeling for river morphological changes was performed using the HEC-RAS model for unsteady flows with sediment transport. The erosion and sedimentation processes were simulated in six reaches (from the Voshmgir Dam to the Caspian Sea), with total length of 128 km. The results indicated that the second scenario (stepped removal of the spillway) is the best alternative due to the gradual processes of river-bed changes. In the present research, the two-dimensional hydrodynamic and water quality model CE-QUAL-W2 is used for qualitative modeling of Gorganrud River in three scenarios of dam removal, within 128 km downstream of Vashmgir dam to the Caspian Sea Estuary. Qualitative modeling was performed for the existing conditions of Gorganrud (without removing the Vashmgir dam) at two hydrometric stations of Aqqala (68 km downstream of the Vashmgir dam) and and Basirabad (112 km downstream of Vashmgir Dam), where a set of water level and water-quality data was available for model calibration. Model results are presented in the period of rapid river changes after dam removal up to 60 days. Based on the results of qualitative modeling, the amount of dissolved oxygen (DO) has increased in three scenarios of dam removal in two sections of the river (Aqqala and Basirabad). The amount of biochemical oxygen demand (BOD) in the first scenario (complete removal) and the second (stepping removal) has increased slightly (less than 10% of the river conditions before removal). In the third scenario (removal with stable deposition), no significant change in BOD occurred. In three scenarios and in both study periods, the pH changed very little in the two periods, before and after the removal of the dam. The total amount of soluble solids (TDS) in the early days of the simulation is equal to or greater than that before removal. The TDS rate is to be decreased by time. The overall results indicated that the second scenario of the dam removal (i.e. stepped removal of the spillway) is the best alternative due to the less impacts on the river-bed changes, and the gradual processes of river-ecosystem rehabilitation.

کلیدواژه‌ها [English]

  • Dam removal
  • River Ecosystem
  • Environmental modeling
  • Gorganrud River
  • Vashmgir Dam
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نشریه محیط زیست طبیعی
دوره 75، ویژه نامه محیط زیست ساحلی و دریایی
اسفند 1401
صفحه 227-242