تأثیر نشت آب از بستر رودخانه در کاهش حجم آب شور در آبخوان‌های فراساحلی

نوع مقاله : مقاله پژوهشی

نویسندگان

1 کارشناس ارشد مهندسی آب و سازه های هیدرولیکی، گروه مهندسی عمران، دانشکده فنی و مهندسی، دانشگاه آزاد اسلامی، واحد تهران مرکز، تهران

2 استادیار دانشگاه آزاد اسلامی- واحد تهران مرکزی، دانشکده مهندسی عمران

چکیده

بررسی وضعیت آب شیرین موجود در زیر دریا در آبخوان­های فراساحلی (امتداد بخشی از آبخوان تا زیر دریا) می‌تواند برای تأمین نیازهای اکولوژی سواحل در آینده مفید باشد. در این پژوهش به منظور آگاهی از میزان تأثیرپذیری آبخوان‌های فراساحلی از نشت آب شیرین از رودخانه، ابتدا سناریوهای مختلفی با تغییر در ابعاد هندسی آبخوان فراساحلی و مقادیر هدایت هیدرولیکی لایه نیمه ­تراوا و ضریب آبگذری رودخانه تعریف شد. در ادامه با مدل‌سازی پسروی شوری با استفاده از کد عددی SEAWAT، مشخصات گوه شوری شامل موقعیت پنجه شوری، ضخامت ناحیه انتقالی و حجم آب شور در زمان برآورد شد. نتایج نشان داد که مقدار ضریب آبگذری بستر رودخانه در میزان حجم آب شیرین زیر دریا تأثیرگذار است. زمانی که ضریب آبگذری رودخانه کاهش محسوس می‌یابد (به طور مثال m2/d 0025/0 در این مطالعه)، تأثیر آن بر مشخصات شوری و کاهش میزان حجم آب شور بسیار اندک بوده اما افزایش این ضریب تا میزان مشخص می­تواند بیشترین تأثیر (تا 60 درصد کاهش حجم آب شور) را بر مشخصات شوری بگذارد. با این وجود با افزایش قابل ملاحظه این ضریب (به طور مثال m2/d 375 در این مطالعه)، میزان تأثیر آن بر مشخصات شوری کمتر خواهد شد. علت این امر در سرعت بالای جریان ورودی و زمان اندرکنش محدود با گوه شوری دیده شده است. میزان نفوذپذیری لایه نیمه تراوا و طول گسترش آن نیز بر نتایج تأثیر خواهد گذاشت؛ به گونه‌ای که با امتداد لایه نیمه‌ تراوا تا مرز خشکی، از میزان آب شیرین نشت­کرده کم شده و در نتیجه از اثرات آن بر گوه شوری در نواحی نزدیک به بستر آبخوان کاسته خواهد شد.

کلیدواژه‌ها


عنوان مقاله [English]

Impact of River Water Seepage on Reduction of Saltwater Volume in Offshore Coastal Aquifers

نویسندگان [English]

  • Elham Panjnoosh 1
  • S. Sadjad Mehdizadeh 2
1 M.Sc. Graduate of Water and Hydraulic Structures Engineering, Department of Civil Engineering, Central Tehran Branch, Islamic Azad University, Tehran, Iran.
2 Department of Civil Engineering, Central Tehran Branch, Islamic Azad University, Tehran, Iran
چکیده [English]

Investigation on available freshwater below the sea in offshore aquifers (i.e. part of the coastal aquifer is stretched beneath the sea) can provide a resource for ecological demand in the future. In this study, to understand the response of intruded seawater to freshwater recharge that is leaked from the riverbed, various scenarios with different aquitard length and permeability or river conductance have been introduced. SEAWAT dispersive code is used for seawater intrusion simulation and saltwater wedge toe position, mixing zone thickness and saltwater volume were estimated. The results showed that the river conductance influences the volume of freshwater in offshore aquifers significantly. Thereby, small conductance value (i.e. 0.0025 m2/d in this study) leads to a negligible impact on saltwater characteristics but as this coefficient grows up to a specific amount, more freshwater is released from the riverbed which could significantly affect saltwater characteristics (60% reduction in brackish water volume). Nonetheless, extra-large conductance (i.e. 375 m2/d in this study) cannot sharply affect seawater recession due to high-velocity magnitude of recharged freshwater and limited contact time with the saltwater wedge. Hydraulic conductivity and length of aquitard also affect the results; so that the extension of aquitard length to the dry land boundary will cause reduction in the recharged freshwater and consequently reduce the intruded saltwater especially in the vicinity of aquifer bed.

کلیدواژه‌ها [English]

  • Offshore aquifer
  • Toe position
  • Conductance
  • Aquitard
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