فهرست مطالب

  • سال چهاردهم شماره 4 (زمستان 1398)
  • تاریخ انتشار: 1398/11/29
  • تعداد عناوین: 10
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  • زهره امامقلی، مهدی یاسی* صفحات 1-17

    رودخانه‏‏ ها عموما پذیرنده پساب های کشاورزی، صنعتی و شهری هستند. حفاظت و بهبود کیفیت آب رودخانه ها ضروری است. پژوهش حاضر، به بررسی مقایسه ای کیفیت جریان آب در شرایط موجود و در شرایط تغییرات در هندسه آبراهه اصلی یک رودخانه می پردازد. در این بررسی، رودخانه قزل‏‏اوزن در بازه دیواندره‏‏- بیجار به طول 51 کیلومتر انتخاب گردید. براساس داده‏‏های موجود هندسی- هیدرولیکی و کیفیت آب، مدل‏سازی کیفیت آب رودخانه انجام گردید. برای شبیه سازی شرایط موجود رودخانه از دو مدل ریاضی کیفیت آب QUAL2Kw و WASP استفاده شد. پارامترهای مختلف کیفی و هیدرولیکی آب (نظیر: DO، BOD، COD، Norg،NH3، Q،h ، V، Tو (pH مورد نظر قرار گرفتند. نتایج نشان داد که هر دو مدل از توانایی مناسبی در شبیه سازی پارامترهای کیفی آب برخوردار هستند. با استفاده از مدل QUAL2Kw پنج راهکار تغییر هندسی رودخانه (1- عرض رودخانه، 2- شیب جانبی، 3- شیب طولی، 4- زبری و 5- عرض و شیب طولی رودخانه) شبیه‏ سازی گردید. نتایج نشان داد که در صورت کمبود غلظت اکسیژن محلول، راهکار کاهش عرض بیش ترین تاثیر را دارد. در شرایطی که نیاز به کاهش غلظت BOD و COD باشد، ولی منجر به کاهش DO نگردد، مناسب ترین راهکار تغییر همزمان کاهش شیب و افزایش عرض می باشد. تغییرات نیترات در تمامی راهکارها اندک بوده، که نشان دهنده تاثیر پذریری کم این پارامتر است. در شرایطی که غلظت نیترات مشکل آفرین باشد، راهکار افزایش عرض همزمان با کاهش شیب تاثیر بیشتری در مقایسه با سایر راهکارها دارد.

    کلیدواژگان: ساماندهی رودخانه، کیفیت آب، مدل QUAL2kw، مدل WASP، رودخانه قزل‏‏ اوزن
  • مهین اسحاقیان، سعید گوهری*، سید سعید اخروی صفحات 19-33
    آبشستگی پایه پل یکی از مهم ترین مسایل در ایمنی پل ها می باشد. با توجه به فیزیک طبیعی مصالح رودخانه ای (رسوبات غیریکنواخت) و اثرگذاری بالای آن بر ابعاد و تکامل زمانی گودال آبشستگی، بررسی برهم کنش جریان و سازه با بستر رسوبی غیریکنواخت به دلیل تشکیل لایه سپر از اهمیت بالایی برخوردار است. بدین منظور، بررسی توسعه زمانی آبشستگی در اطراف تک پایه دایره ای در بسترهای رسوبی با شرایط دانه بندی متفاوت مورد آزمایش قرار گرفت. لایه سپر توسعه یافته از انتقال انتخابی ذرات ریزتر در رسوبات غیریکنواخت سبب پیچیدگی در تخمین عمق آبشستگی می شود. در این پژوهش، 15 آزمایش در قالب 5 بستر رسوبی مختلف، یکنواخت و غیریکنواخت، در دو جریان ماندگار (20 و 35 لیتر بر ثانیه) و یک جریان غیرماندگار صورت گرفته است. نتایج نشان داد که با افزایش دبی جریان (افزایش عمق جریان، اثر h/b، در شدت جریان مشابه، u/uc) در بسترهای رسوبی یکسان، نه تنها عمق آبشستگی در حدود 27 درصد افزایش یافته است، بلکه لایه سپر درشت تری در سطح بستر ایجاد می شود. مقایسه آزمایش های متناظر در بسترهای یکنواخت و غیریکنواخت نشانگر کاهش قابل ملاحظه (تا 70 درصد) عمق حداکثر آبشستگی با افزایش میزان غیریکنواختی ذرات است. اگرچه میزان کاهش قابل توجهی در عمق آبشستگی با افزایش غیریکنواختی ذرات در بین دو بستر غیریکنواخت مشاهده نشد. کاهش قطر میانه ذرات (افزایش نسبت b/d50) در بسترهای غیریکنواخت با درنظرگیری انحراف معیار هندسی یکسان، سبب افزایش میزان عمق آبشستگی شده است.
    کلیدواژگان: آبشستگی، الگوی جریان، پایه پل، لایه سپر، رسوبات غیریکنواخت
  • زهرا طاهری، کاظم اسماعیلی*، حسین صمدی بروجنی، سعید رضا خداشناس صفحات 35-49

    در حال حاضر بررسی میزان فرسایش رسوبات چسبنده یک بخش مهم در سازه های انتقال آب، با استفاده از پیش بینی و تعیین رفتار رسوبات می باشد. از طرفی در مناطق خشک منابع آب تجدیدپذیر نظیر پساب تصفیه شده شهری به عنوان یک ذخیره آبی مطمین در برنامه ریزی منابع آب به منظور تامین نیاز بخش صنعت و کشاورزی مورد توجه قرار می گیرد. خصوصیات شیمیایی پساب موجب تغییر رفتار رسوبات چسبنده در کانال های انتقال آب می شود. از این رو در این تحقیق با استفاده از یک فلوم دایره ای به بررسی نرخ فرسایش رسوبات چسبنده در دوره های تحکیم مختلف پرداخته شد. در این راستا آزمایش ها در سه حالت آب خالص، سیال حاوی 30 و 60 درصد پساب انجام و دوره تحکیم رسوبات در بستر فلوم 1، 3، 14 و 28 روز در نظر گرفته شد. نتایج نشان می دهد در تمام دوره های تحکیم در ابتدای زمان اعمال تنش برشی، نرخ فرسایش به دلیل حضور رسوبات سطحی ابتدا افزایش و سپس کاهش می یابد. بیشترین نرخ فرسایش در سیال حاوی 60 درصد پساب و در تنش برشی 35/2 نیوتن بر مترمربع رخ داد. همچنین با افزایش دوره تحکیم اختلاف نرخ فرسایش در پساب 30 درصد، با آب خالص و 60 درصد پساب افزایش می یابد که این امر به دلیل وجود بالاتر بودن شاخص ESP بیشتر در شرایط 30 درصد پساب است. در دوره تحکیم های یکسان بیشتر بودن شاخص ESP در سیال حاوی 30 درصد پساب که منجر به پخشیدگی بیشتر ذرات، عمق ته نشینی بیشتر و پوکی کمتر گردید.

    کلیدواژگان: پساب شهری، ESP، نرخ فرسایش، رسوبات چسبنده
  • مجتبی کریمایی طبرستانی* صفحات 51-68

    شکست و خسارت وارد بر سازه های محافظت از آبشستگی نظیر سنگچین بسیار پیچیده بوده و موارد عدم قطعیت مختلفی در برآورد پارامترهای موثر در تعیین اندازه پایدار آن وجود دارند. در نتیجه استفاده از تحلیل قابلیت اطمینان جهت کمی سازی این عدم قطعیت ها و تعیین احتمال پایداری لایه سنگچین بسیار مفید خواهد بود. در تحقیق حاضر با استفاده از دو روش تحلیل قابلیت اطمینان مرتبه اول و روش شبیه-سازی مونت کارلو، اقدام به بررسی پایداری لایه محافظ سنگچین در اطراف پایه پل شد. یک مثال کاربردی با دو پایه مستقر در کانال اصلی و دشت سیلابی مورد بررسی قرار گرفت. نتایج تحلیل نشان داد که عدم-قطعیت های موجود در پارامترهای موثر نظیر سرعت و عمق جریان می تواند قابلیت اطمینان لایه سنگچین را به شدت کاهش دهد به گونه ای که احتمال شکست لایه سنگچین در حالتی که از روش قطعی برای طراحی اندازه سنگ استفاده شد، در حدود 44 درصد برای پایه مستقر در کانال اصلی و 28 درصد برای پایه مستقر در دشت سیلابی می باشند. در ادامه بررسی ارتباط مابین ضریب ایمنی در طراحی کلاسیک قطعی و ضریب قابلیت اطمینان نشان داد که یک رابطه خطی مستقیم بین آنها برقرار است. همچنین تحلیل حساسیت پارامترهای موثر شامل مشخصات جریان، پایه پل و بستر نشان داد که مهمترین پارامتر در قابلیت اطمینان لایه سنگچین، سرعت متوسط جریان می باشد که نیاز به تمهیدات ویژه برای برآورد دقیق تر آن وجود دارد.

    کلیدواژگان: پایه پل، لایه سنگچین محافظ، تحلیل قابلیت اطمینان مرتبه اول، شبیه سازی مونت کارلو، ضریب قابلیت اطمینان، تحلیل حساسیت
  • عزیر عابسی*، علی رحمانی فیروزجائی، مهدی حمیدی، محمد امین بصام، زهرا خدابخشی صفحات 69-81

    مدلسازی و مشاهده پدیده های هیدرولیکی در آزمایشگاه را شاید بتوان اولین گام در راستای شناخت رفتار پیچیده فرایندهای طبیعی در مکانیک سیالات دانست. در این زمینه از زمانهای دور، روش های مختلفی برای اندازه گیری پارامترهای جریان توسعه داده شده است. این روش ها عمدتا مبتنی برآشکارسازی و ثبت متغیرهای جریان در شرایط مختلف حرکت می باشند. با توسعه تکنولوژیهای دیجیتال در سالهای اخیر، روش های متنوعی برای ثبت میدان غلظت و سرعت جریان بدون ایجاد تداخل در محیط توسعه داده شده است. این روش ها بیشتر مبتنی بر شبیه سازی جریان در محفظه های شیشه ای و شفاف، آشکارسازی جریان با کمک لیزر، فلورسنت و ذرات ریز و ثبت تصاویر با دقت و فرکانس بالا جهت پردازش ثانویه است. در این تحقیق، قابلیتهای سامانه اسکن لیزری سه بعدی که جهت آشکار سازی جریان آغشته به فلورسنت در مطالعات هیدرولیک محیط زیست برای اولین بار در ایران و در دانشگاه صنعتی نوشیروانی بابل توسعه داده شده است، معرفی می گردد. در این سامانه با تابانیدن نور لیزر به جت حاوی فلورسنت و تحریک آن، نسبت به آشکارسازی جریان و ثبت تصاویر آزمایش در طول موج خاصی از نور نارنجی اقدام می گردد. علاوه بر تصویر سازی دو بعدی، این سیستم به نحو خاصی طراحی شده است که امکان ثبت و تصویرسازی سه بعدی آزمایشات را نیز داشته باشد. به کمک این سامانه همچنین با تصویربرداری سریع با نرخ HZ 100 از آزمایشات، تحلیل فرکانسی جریانهای آشفته، توسعه طیف انرژی آشفتگی و محاسبه پروفیل شدت و قدرت آشفتگی امکان پذیر می باشد

    کلیدواژگان: لیزر، فلورسنت، اسکن سه بعدی، هیدرولیک محیط زیست، آشفتگی
  • هانیه کاهید باصیری، رضا بابایی، علیرضا فلاح، احسان جباری* صفحات 83-98
    در این پژوهش، روش بدون شبکه چندربعی برای حل مسیله شکست سد با هدف رفع برخی از نقاط ضعف روش های معمول با شبکه توسعه یافت. به این منظور، دستگاه معادلات دیفرانسیل غیرخطی و غیرپایای حاکم با استفاده از تابع چندربعی بازتولید و روند حل آن بیان گردید. همچنین برای گسسته سازی جمله های زمانی از روش اختلاف محدود پیشرو استفاده گردید و نشان داده شد که برای گسسته سازی ترم های مکانی روش ضمنی مناسب است. برای تعیین مهم ترین عامل در دقت و سرعت روش چندربعی، یعنی پارامتر شکل بهینه، ضمن اثبات ناکارآمدی برخی از الگوریتم های پرکاربرد پیشین در حل مسیله شکست سد، یک ایده جدید برای آن ارایه گردید. در این ایده، شرایط اولیه مسیله با تابع چندربعی درون یابی و پارامتر شکلی که دقیق ترین تخمین از آن را داشته باشد، به عنوان پارامتر شکل بهینه انتخاب می گردد. پارامتر شکل به دست آمده برای آنالیز مسیله در گذر زمان نیز بهینه می باشد و برای هر سه مجهول مسیله یکسان است. لذا حجم محاسبات روش چندربعی به شدت کاهش می یابد. همچنین همگرایی و دقت قابل قبول رویکرد پیشنهادی طی حل چند مثال مختلف در مقایسه با سایر روش های عددی و تحلیلی نشان داده شده است.
    کلیدواژگان: روش های عددی بدون شبکه، روش چند ربعی، روش مالتی کوادریک، شکست سد، پارامتر شکل
  • دامون محمدعلی نژادیان، حسین حمیدی فر* صفحات 99-110

    پدیده آبشستگی پایه پل، یکی از موضوعات مهم در زمینه هیدرولیک پل ها و مهندسی رودخانه می باشد. به همین دلیل، بررسی این پدیده تحت شرایط مختلف به منظور تخمین حداکثر عمق فرسایش پایه و کاهش خطر آبشستگی و تخریب پل، امری ضروری می باشد. هدف این تحقیق بررسی اثر همزمان حضور شکاف و اجسام شناور بر آبشستگی اطراف پایه پل است. بر این اساس، نتایج مربوط به آزمایش های مختلف پایه پل در شرایط آب زلال ارایه گردیده است. نتایج نشان داد که هر چند استفاده از شکاف می تواند به تنهایی باعث کاهش 20% عمق آبشستگی شود اما تجمع اجسام شناور این مقدار را تا 15% کاهش می دهد. همچنین اجسام شناور باعث می شوند در حالت بدون شکاف عمق آبشستگی تا 6/17% در مقایسه با حالت بدون تجمع اجسام شناور افزایش یابد. این نتایج نشان می دهد که تجمع اجسام شناور می تواند تاثیر شکاف در کاهش آبشستگی اطراف پایه را خنثی نموده که نیازمند توجه جدی در مرحله طراحی و همچنین انجام آزمایش های بیشتر به منظور بررسی این مساله تحت شرایط مختلف می باشد.

    کلیدواژگان: رسوب گذاری، مهندسی رودخانه، تخریب پل ها، فرسایش
  • راضیه کاوند، مهدی قمشی*، مهدی دریائی صفحات 111-122

    اغلب سدهای مخزنی روی رودخانه های کوهستانی ساخته می-شود؛ لذا مطالعه قوس و زبری بستر روی مشخصات سیلاب ناشی از شکست سد ضرورت می-یابد. بدین منظور آزمایش هایی در فلومی با قوس 90 درجه در آزمایشگاه مدل های هیدرولیکی دانشکده علوم و مهندسی آب واقع در دانشگاه شهید چمران اهواز انجام شد. سرعت موج در زاویه 45 درجه قوس بیشینه است. همچنین با کارگذاری زبری در طول قوس سرعت کاهش پیدا می کند. به طوریکه که در ارتفاع آب بالا دست 35 سانتی متر سرعت موج در زاویه 45 و 90 درجه نسبت به ابتدای قوس در بستر صاف 25 و 18 درصد افزایش و در بستر با زبری 16 میلی متر 18 و 8 درصد افزایش می یابد. ارتفاع موج در طول قوس به علت افت انرژی جریان کاهش می یابد. کارگذاری زبری باعث افزایش ارتفاع موج می شود. در آزمایشات با بستر صاف ارتفاع موج در زاویه 45 و 90 درجه نسبت به ابتدای قوس 19 و 38 درصد کاهش داشته است. درحالیکه در آزمایشات با اندازه متوسط زبری 20 میلی متر، ارتفاع موج در زاویه 45 و 90 درجه نسبت به ابتدای قوس 33 و 44 درصد کاهش داشته است.

    کلیدواژگان: آنالیز ابعادی، روندیابی سیل، قوس 90 درجه، سرعت موج، ارتفاع موج
  • الهام بهمن، عبدالرضا کبیری سامانی*، محمدنوید مقیم صفحات 123-136

    هدف از پژوهش حاضر مطالعه عددی و آزمایشگاهی جریان بر روی سرریزهای هیدروفویل نامتقارن (دارای خمیدگی) است. در این پژوهش به منظور مدل سازی عددی از نرم افزار متن باز OpenFoam v. 4.0.1 استفاده شد. نتایج حاصل از مدل سازی های عددی با استفاده از نتایج آزمایشگاهی مطالعات پیشین و مدل های آزمایشگاهی سرریز هیدروفویل طراحی شده با استفاده از تابع تبدیل ژوکوفسکی در پژوهش حاضر، مورد ارزیابی قرار گرفتند. بر اساس توصیه های مطالعات پیشین، مدل آشفتگی SST k-ω برای شبیه سازی جریان عبوری از روی سرریزهای هیدروفویل استفاده شد. نتایج شبیه سازی های عددی به ازای ویژگی های هندسی متفاوت، نشان داد که استفاده از سرریز هیدروفویل دارای خمیدگی، می تواند احتمال وقوع کاویتاسیون و دامنه فشارهای مثبت در پایین دست سرریز را نسبت به سرریز تاج دایره ای کاهش دهد؛ بدون آن که ارتفاع سرریز کاهش یابد. هم چنین نتایج نشان دادند که در سرریزهای هیدروفویل دارای خمیدگی، بیشترین تنش های برشی بستر و نیروهای فشاری در پایین دست سرریز هیدروفویل با خمیدگی بیشتر رخ می دهند و در نتیجه احتمال فرسایش در پایین دست این از نوع از سرریزها بیشتر است. علاوه بر این، احتمال وقوع فرسایش در پایین دست سرریز تاج دایره ای و سرریز هیدروفیل دارای خمیدگی هم ارتفاع با آن یکسان است؛ اما محدوده وقوع فرسایش در سرریزهای تاج دایره ای بزرگتر از سرریز هیدروفیل دارای خمیدگی هم ارتفاع آن است. بر اساس نتایج حاصل، استفاده از سرریز هیدروفویل دارای خمیدگی، می تواند مشکلات یاد شده در سرریزهای تاج دایره ای را مرتفع سازد، بدون آن که ارتفاع سرریز کاهش یابد.

    کلیدواژگان: تبدیل ژوکوفسکی، نرم افزار OpenFoam، نیم رخ سرعت، توزیع فشار، تنش برشی بستر
  • میلاد صابر، مسعود قدسیان* صفحات 137-148
    حوضچه آرامش نوع 6 معمولا در خروجی لوله ها و مجاری تحت فشار، جهت استهلاک انرژی جریان تعبیه می شود. آزمایشاتی بر روی آبشستگی در مدلی از این حوضچه در آزمایشگاه هیدرولیک دانشگاه تربیت مدرس انجام گردید. پارامترهای متغیر در آزمایشات شامل عدد فرود جریان، قطر لوله ورودی و چهار مدل آبپایه انتهایی (دوپله ای، سه پله ای ، چهار پله ای و پنج پله ای) بودند، که مجموعا در غالب 32 آزمایش طراحی و انجام شد. نتایج نشان داد افزایش عدد فرود باعث کاهش شاخص آبشستگی و عملکرد موثرتر آبپایه می-شود. درمحدوده عدد فرود 99/1 تا 07/4، آبپایه 4 پله ای عملکرد مناسب تری را نسبت به سایر آبپایه ها دارد. در محدوده عدد فرود 27/9 تا 91/13، آبپایه 4 پله ای عمق آبشستگی کمتری ایجاد می کند. حجم رسوب خارج شده از حفره آبشستگی در آبپایه 4 پله ای نیز کمتر از آبپایه های دیگر است. به منظور بررسی تقارن توپوگرافی بستر، شاخص تقارن تعریف شد. توپوگرافی بستر در پایین دست آبپایه پنج پله ای در محدود عدد فرود 1 تا 4، و در پایین دست آبپایه چهارپله ای در محدوده عدد فرود 4 تا 9/13، شاخص تقارن کمتر دارد. تغییرات زمانی عمق آبشستگی نشان داد حدود 65 تا 70 درصد عمق آبشستگی در یک ساعت اولیه اتفاق می افتد. همچنین معادله ای برای پروفیل های طولی حفره آبشستگی ارایه شد.
    کلیدواژگان: حوضچه آرامش نوع 6، آبپایه انتهایی، آبشستگی، شاخص آبشستگی و شاخص تقارن
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  • Zohre Emamgholi, Mehdi Yasi * Pages 1-17
    Introduction

    Rivers are generally receiving wastewater from agriculture, industrial and urban areas. Population growth, urban development and human activities have always been a threat to the quantity and quality of river water flows. In general the purer the water, the more valuable and useful it is for riverine ecology and for abstractions to meet human demands such as irrigation, drinking and industry. Conversely, the more polluted the water, the more expensive it is to treat to satisfactory levels. This leads to disruption of natural food chains and the loss of riverine lives. Protecting and improving the quality of river flows is a priority. The changing hydrological regime associated with the developing water demand schemes may alter the capacity of the environment to assimilate water soluble pollution. In particular, reductions in low flows result in increased pollutant concentrations already discharged into the water course either from point sources, such as industry, irrigation drains and urban areas, or from non-point sources, such as agrochemicals leaking into groundwater and soil erosion. Reduced flood flows may remove beneficial flushing, and reservoirs may cause further concentration of pollutants. Monitoring of water quality in different reaches of rivers depends on the purposes of water uses and requires a long-time and high-cost planning. Numerical simulator models are useful tools for a rapid and low-cost assessment and prediction of water quality in the present and in the future conditions of the rivers reaches. Different scenarios can be tested for determining and evaluating the effects of point and non-points sources of pollutants discharging into the river, and for predicting the effectiveness of alternative restoration plans in the management of water-based lives instream and in riverine riparian areas. In the present study, the effects of discharging pollutants on water quality in a long river reach have been investigated under the present condition and in different scenarios of river training schemes.

    Methodology

    In this study, the 51-km Diwandra-Bijar Reach of the Ghezel Ozan River was selected. Modeling of the existing conditions of river quality was performed using existing geometric-hydraulic and river water quality data. Two mathematical models QUAL2KW and WASP were used to simulate the water quality. Simulation of different parameters (such as: DO, BOD, COD, Norg, NH3, Q, h, V, T and pH) were considered. In order to calibrate these models, RMSE and MAPE statistical indices were used. Using the QUAL2KW model, five river training schemes (variation of 1- river width, 2- side slope, 3- longitudinal slope, 4-coefficient of roughness; and 5- width and longitudinal slope of the river) were considered.

    Results and discussion

    Comparison of river conditions simulation with two models of QUAL2KW and WASP with observational data showed that both models have the proper ability to simulate water quality. The study of river conditions showed that the river flow increased during the study area due to the entry of the sub branch. Due to changes in geometry and river flow, depth and flow velocity are changing along the path. Changes in river water temperature to 35 km are decreasing and then rising. The concentration of dissolved oxygen from the upstream to downstream of the river is decreasing. BOD concentration is rising from kilometer 19. The concentration of nitrate in kilometer 32 has increased due to the arrival of the Cham Zard River. The concentration of Norg has increased from Kilometer 19. This is due to changes in the river section and a decrease in sedimentation due to the increase of flow and entry of pollutants into the river. Ammonia concentration also increased at Kilometer 19 with the arrival of the Cham Zard River, and finally decreased by the arrival of the Yol Gashti river. Investigating scenarios showed that, in decreasing river width, flow velocity increased, resulting in an increase in the concentration of dissolved oxygen that increased the amount of river self-purification capacity. The concentration of NO3, BOD and COD parameters also increased slightly in high Discharge. The effect of the scenario of the Side slope on the water quality and hydraulic performance of the river is very small and has the least impact on the water quality of the river. By reducing the slope of the river bed, the flow rate is reduced, so the dissolved oxygen decreases. And the concentration of BOD and COD parameters has increased and the concentration of nitrate has decreased. This scenario is appropriate for the condition where the river needs to reduce the BOD. By the roughness increases, the flow velocity decreases. Consequently, the concentration of quality parameters (such as: BOD, DO and COD) are decreased.

    Conclusion

    The results indicated that both models are capable of simulating the qualitative status of the river reach. The results of the five river training scenarios prove that wherever the dissolved oxygen (DO) is insufficient in the flowing water, the decrease in the channel width has the greatest effect. Implementation of both the decrease in channel slope and the increase in the channel width is effective in the reduction of BOD and COD, while does not result in a significant reduction in DO. Nitrate variations are almost negligible in all scenarios, indicating a low susceptibility of this parameter to the changes the channel geometry. However, wherever the concentration of Nitrate is a major treat, the increase in the channel width together with the decrease in the channel slope would be an alternative training solution. Keywords: River training, Water quality, QUAL2KW model, WASP model, Ghezel Ozan River.

    Keywords: river training, Water Quality, QUAL2Kw Model, WASP Model, Ghezel Ozan River
  • Mahin Eshaghian, Saeed Gohari *, Seyyed Saeid Okhravi Pages 19-33
    Introduction
    Bridge scour is one of the most important challenges in river engineering. Research into local scour has primarily focused on investigating the impact of different hydrodynamic conditions on scour in a bed of uniformly graded material. However, local scour investigations in a bed of uniformly well-graded material provided knowledge of the underlying processes, field sediment beds are much more complex consisting of non-uniform sediment mixtures ("σ" _"g" ">1.4" ). In the case of complex sediment beds, selective transport of the finer particles due to unequal mobility can make the bed surface to be armored. There have been relatively few studies reported in the literature relating to scour in complex sediment beds, and most of these relate to quite specific situations. With regards to natural river materials (non-uniform sediments) and its great effects on the dimension and the time evolution of scour hole, the interaction of flow-structures with non-uniform sediments is very crucial due to armor layer development. The aim of this study is to improve understanding of scour development and armoring evolution in non-uniform sediment beds for estimating the scour depth in more realistic field conditions. Therefore, the rate of variation of erosion over time around single cylindrical pier is investigated in different bed sediment types.
    Methodology
    The armored layer due to selective transport of the finer particles in non-uniform sediments causes complexity for predicting equilibrium scour depth. The present experiments on local bridge scour were conducted in hydraulic laboratory of the Bu-Ali Sina University (Hamadan, Iran). The pier model with a diameter of 4 cm was put inside a 0.5 m wide, 10.5 m long and 0.5 m depth rectangular tilting flume. In this study, the number of 15 experiments were organized at five different sediment beds, uniform and non-uniform in two steady flow condition (20 and 35 l/s with the same flow intensity of u/uc~0.9) along with an unsteady flow. The duration of tests was fixed at 8 hours in all runs based on the empirical method given by Ettema (1980).
    Results and Discussion
    The experimental results revealed that with increasing flow rate from 20 to 35 l/s (increasing follow shallowness, h/b) at the same sediment bed, not only larger scour depths were recorded, but also the armor layer became coarser. The comparison between the bed configurations with uniform and non-uniform sediments represented dramatical reduction of the scour depth regards with increasing sediment non-uniformity. The effect of non-uniform sediments on scour in a current clear water conditions showed that maximum scour depth was less than scour depth in a uniform sand with the same d50 value. The comparison between these two mentioned bed configurations showed that the change in geometric standard deviation ("σ" _"g" ) from 1.4 to 2 (altering the uniform bed to non-uniform), decreased the maximum depth of scour by 70% and 60% in two corresponding experiments. As the armor layer coarser grains remains at upstream flow bed and at the vicinity of scour hole in the same flow intensity, the scour depth was decreased. Otherwise, there was not remarkable decrease on the scour depth by increasing non-uniformity index, since two sediment beds types were non-uniform. Also, a slight increase on scour depth has been observed by reduction of median grain size in the beds with non-uniform sediments at the same geometric standard deviation. By taking into account of the grain size of the armor layer and ice cover roughness, Wu et al. (2014) analyzed the dimensionless maximum scour depth and they found out that with an increase in grain size of the armor layer, the dimensionless maximum scour depth decreases. Singh et al. (2018) investigated the incipient motion for gravel particles in cohesionless sediment mixtures having silt and sand. The visual observations of the channel bed after the end of incipient motion indicated appearance of gravel particles at the top surface of the sediment bed. The critical shear stress for the gravel particles was found to be lower in the presence of silt. Presence of silt in the mixture affects the critical shear stress for gravel particles. They concluded from the present study that high silt content in the mixture leads to the higher deviation of critical shear stress from the revised Shields curve. They proposed an equation for the determination of critical shear stress of gravel particles in the non-uniform sediment mixture.
    Conclusion
    The results showed that scour depths were reduced dramatically as sediment non-uniformity index ("σ" _"g" ) increase in clear water conditions. The larger particles form an armor layer protecting the bed from eroding. Also, the observation indicated that with increasing flow depth, the armor layer coarsens, and larger scour depths were recorded. However, scour depth increase rate was very different for the various bed sediment types.
    Keywords: Scour, Flow Pattern, bridge pier, Armor Layer, Non-Uniform Sediment
  • Zahra Taheri, Kazem Esmaili *, Hosein Samadi, Saeed Reza KHODASHENAS Pages 35-49
    Introduction

    One of the important influence factors on the critical shear stress of erosion is consolidation. Fluid mud consolidates gradually as a result of self-weight. Many factors affect the consolidation process, including initial concentration, water temperature, hydrodynamic conditions, particle characteristics, and salinity. The critical shear stress for the erosion of cohesive sediment is found to increase with consolidation pressure as well. Bed age can influence the transport characteristics of the deposited sediment by altering the critical shear stress for erosion through compaction. A major number of investigators used circular flume for studying the transport of cohesive sediments. These flumes essentially consists of two main components: a circular channel and an annular cover plate that fits inside the channel. The goal of this study is to better understand the wastewater effects on erosion and self-weight consolidation of cohesive sediment. Also, some experiments were carried out in annular flume using a mixture of cohesive sediment and water with a combination of three concentration of wastewater and four times of consolidation for evaluating their effects on the erosion of cohesive sediments.

    Methodology

    In this work, three sets of experiments with three different levels of wastewater (0%, 30%, and 60%) were conducted in an annular flume. The optimal speed ratio between the ring and the flume to be 1.1 for a water depth of 0.2 m. For the plane bed experiment, the water depth above the bed was maintained at 0.2 m. The sediment was sampled from Pribalut dam reservoir, located in the northern Karun basin, Iran. These fine sediments contain 76% silt and 24% clay. Wastewater used in the experiments was taken from Shahrekord wastewater treatment plant outlet, which is located east of Shahrekord city, Iran. In order to test self-weight consolidation of the cohesive sediment, the mixture of three levels of wastewater (0%, 30%, and 60%) with three different initial concentrations of cohesive sediment (200, 300 and 400 g/l) were poured into nine settling columns. By using the linear interpolation method, in order to have 10 cm height of deposited sediment in the annular flume, the initial concentration of the used mixture of fluid and sediment to fill the flume for consolidation times of 1, 3, 14 and 28 days were found 279, 283, 289 and 291 g/l, respectively. To determine the appropriate ratio of the rotational speed of the ring to the flume, velocity and shear stress profiles were measured in the rotating flume, using an Acoustic Doppler Velocimeter (ADV). The flume was operated at a high speed, corresponding to a bed shear stress of 11.2Pa, for 30 minutes to ensure a homogenous suspended sediment mixture. This routine was followed for four consolidation periods of 1, 3, 14 and 28 days in which the bed was left undisturbed. For each erosion experiment in the presence of wastewater, the flume was started from rest and the rotational speed was increased in time increments of 60 min from the first shear stress to forth shear stress and was considered 240 minutes for the last shear stress. Consequently, all experiments were done for a period of 480 min; meanwhile the test samples were collected in a sampling interval of 15 min during the first hour and 30 min thereafter. The samples were taken from depths 15.2, 18.3, 22.2 and 26.8 cm from the bottom.

    Results and discussion

    During the one-day consolidation period, the process of changes in the erosion rates is quite different. In this period, consolidation of the results showed that the presence of wastewater increased the resistance of bed sediment and reduced erosion rate. In this case, due to the results of the sediment consolidation process, the incompleteness of the consolidation process and biota affecting resulted from to the presence of wastewater can be regarded as the main reasons for this difference. During the one-day period of the third shear stress (0.41 N/m2), the erosion rate increased and the highest erosion rate occurred in the fluid containing 60% wastewater and in the fifth stress. It can be inferred that the presence of wastewater in addition to impact on sediment consolidation caused more cohesion of the sediments that withstood the turbulence of the flow until the fourth shear stress and then experienced abrupt erosion at the last stress. This event occurred for pure water and 30% wastewater at the third shear stress. At consolidation periods of 3, 14 and 28 days, the results show that at different concentrations of wastewater, the erosion rate is increased with increase of the shear stress, such that the highest erosion rate occurred at the largest shear stress. ESP also increased with increasing wastewater percentage in the one-day consolidation period. The presence of wastewater also increased the shear stress of the erosion threshold, which is a trend that is unlike the consolidation periods of 3 to 28 days.

    Conclusions

    The results of this study show that the presence of wastewater has a dual role in the sediment consolidation process such that the rate of consolidation for different time periods in the fluid, it is 30% lower in pure water and 60% in fluids containing wastewater. Compared to pure water and 60% wastewater it was observed that sediment consolidation in 60% fluids is more than pure water. Given the similarity of the conditions of the consolidation experiments (time and type of sediments), the dual role of the wastewater in the consolidation process can be attributed to the difference in fluid quality parameters. In calculating the amount of erosion threshold shear stress, the dual impact of the wastewater is also evident, such that at different time periods, the lowest critical shear stress is for sediments in the fluid containing 30% of wastewater and the highest is for fluids containing 60% of wastewater. Accordingly, the ESP values showed a different trend for the 30% wastewater in all consolidation periods except for one day. For this purpose, the mean value of ESP index and relative height at different levels of wastewater showed that the trend of h/h0 changes for different wastewater percentages was the same as the ESP index changes. Accordingly, ESP values indicate that in fluids containing 30% of wastewater, sediments have more diffusion than other wastewater concentrations.

    Keywords: Wastewater, ESP, erosion ratio, Cohesive Sediment
  • Mojtaba Karimaei Tabarestani * Pages 51-68
    Introduction

    The formation of scour hole around bridge pier is recognized as the number one cause of bridge failure. This phenomenon results from a complex flow field with large-scale turbulence structures generated by flow around the pier. On the other hand, predicting the stability of scour countermeasure around bridge pier such as riprap layer is one of the main challenges in hydraulic engineering. However, the damage mechanism of riprap layer is a very complicated phenomena and many uncertainties affect the exact estimation of different effective parameters in riprap stone design. Therefore, quantifying these uncertainties by using reliability analysis to ensure the stability of the bridge is necessary. Recently, reliability analysis methods have been highly considered due to their high ability to optimize engineering designs and improve project exploitation. Reliability analysis provides a systematic and organized framework to minimize model uncertainties. In addition, it allows the designer to determine the role of each of the effective parameters in the uncertainty of model output. This is essential for identifying important parameters in order to pay more attention to them to achieve their exact value and effect, and ultimately reduce the uncertainty of the model output.

    Methodology 

    In the present study, the stability of riprap layer was investigated by using a reliability-based framework. Monte Carlo Simulation Technique (MCST) and First Order Reliability Method (FORM) were established to determine the stability of riprap layer against shear failure. In FORM, the reliability is measured in terms of a reliability index, β, and it is related to the probability of failure or probability of limit state violation for any limit state. In addition, MCST consists of drawing samples of the basic variables according to their probabilistic characteristics and then feeding them into the limit state function and therefore the probability of failure, Pf, can be found. MCST is considered in principle an exact method, and, FORM as an approximate method. However, FORM is computationally fast and inexpensive as compared to MCST. The first stage in reliability analysis or evaluation of the probability of a system failure is to determine a limit state or performance function between the basic random variables. In the present study, the equation presented by Karimaee and Zarrati, (2013) and Karimaee et al. (2015) is used here as limit state equation. The advantage of this equation is that it can be used for unprotected and protected piers with a collar and circular as well as rectangular piers, and aligned or skewed piers corresponding to the flow direction. The application of the present method was illustrated in an applied example which is a bridge with two piers located in the main channel and flood plain. The data for this case studies was extracted from different previous studies.

    Results and discussion

    At first, results showed that β values and Pf obtained using FORM are having close proximity with MCST results. Therefore, it is also appropriate to use FORM for reliability assessment of riprap layer around bridge pier. In addition, it was found that due to existing uncertainties, the stability of designed riprap size which was calculated from deterministic method was low and equal to 44% for the pier placed in the main channel and 28% for the pier in the flood plain. In addition, two equations between safety factor and reliability index was determined for riprap size design placed around bridge pier in the main channel or flood plain. Results showed that by increasing the reliability index parameter, the safety factor should be increased. Using these equations one can find out an appropriate value of safety factor for desired riprap layer reliability. For example, these equations give the safety factor corresponding to target reliability index (βT) = 3 as 2.93 and 1.46 for the riprap layer size placed around bridge pier located in the main and flood channels, respectively. Next, sensitivity analysis was performed to examine the impact of each random variable on the probability of riprap layer stability in the reliability method. Results showed that the most critical parameter affecting the reliability of riprap size was the mean flow velocity, so that by decreasing the amount of this parameter for about 75% form the Mean value, the amount of parameter β increased for more than 16 times. Therefore, this parameter needs to be determined more accurately in riprap design to decrease the failure probability of riprap layer, efficiently. Finally, the effect of parameters’ uncertainties on reliability analysis of riprap layer was investigated. Results showed that for all of the effective parameters by increasing the parameters’ uncertainties, the stability of riprap layer decreases.

    Conclusion 

    Present reliability assessment methodology showed that due to existing uncertainties, the stability of designed riprap size which was calculated from deterministic method was not reliable as desired. In addition, an overall influence of various random variables on riprap layer’s reliability was assessed through sensitivity analysis. It was shown that if through better quality control; regular maintenance and proper care uncertainties can be minimized, reliability of riprap layer could be improved.

    Keywords: bridge pier, Protective riprap layer, Monte Carlo Simulation Technique, First order reliability method, Reliability index parameter, Sensitivity analysis
  • Ozeair Abessi *, Ali Rahmani Firoozjaee, Mehdi Hamidi, Mohammad Amin Bassam, Zahra Khodabakshi Pages 69-81
    Introduction

    The experimental modeling and laboratory observation is probably the first step in the recognition of the flow complicated behavior in fluid mechanics. Since long time ago, various methods have developed for the measurement of the flow parameters. These methods are based on the illumination and inscription of flow variables in different conditions. Facilities and equipment were temperature and conductivity probes for scalar quantities and Hot-wire anemometers and Acoustic Doppler Velocimeters for velocity measurement as a vector variable in each point. Such equipment will cause disturbances in the flow as they are intrusive into the body of the ambient water. The measurements are point-based and data sampling needs too many probes for each test. Therefore, these probes do not appropriate for data sampling in many applications of the experimental fluid mechanics, especially in small scales. With recent progress in digital technologies, there are various methods have newly developed for the inspection of concentration and velocity field that are non-intrusive. These methods are more based on flow simulation in the transparent chamber, flow illumination with laser and fluorescent or small particles and filming the flow with high accuracy for later visual processing.

    Methodology

    In this paper, the capabilities of the three-dimensional laser scanning system are exhibited which is developed for first time in Iran at Babol Noshirvani University of Technology (BNUT). It includes a water tank, pomp, the three-dimensional laser scanning system, high-speed camera, and data processing apparatus that all located in the darkroom. The optical system consists of two fast scanning mirrors that drive the beam from an argon-ion laser through the flow in a programmed pattern. The system is controlled by a computer for overall timing control, and image capture. Having added an infinitesimal quantity of a fluorescent dye (Rhodamine 6G, Sigma-Aldrich, St. Louis, Missouri), the discharged effluent would be fluoresced under the impression of the laser. So due to the function of laser beam, the jet of fluorescent illuminated and recorded in the wavelength of orange light. The orange filter is used to filter out all the scattered lights of the green laser to increase the contrast, and accordingly quality of the images. The apparatus is set in a glass-made tank with length, width, and height, respectively. A charge-coupled device (CCD) camera with the resolution of x pixels was successively capturing the reflected light in the separated illustrations, at approximately 100 frames per second. Each captured illustration had to be modified for laser attenuation and sensor response at each pixel by using clear and dyed water. Having used image processing techniques in a software coded in C#, subsequently, the stream of images for unsteady flow and also time-averaged results were obtained. So the images are processed by a specially written computer program NITLIF, which is a new version of TFLOOK that previously was developed by Tian and Roberts (2003) at Georgia Institute of Technology. This software through lengthy computational procedures that explained by Tian and Roberts (2003) computes concentration pixel-by-pixel after a complicated calibration process. The images then turn into a real scale of position, time and concentration for every single frame in Tecplot. The program eventually can time-averaged the frames and placed them next to each other to form a two dimensional or three-dimensional configuration of flow dynamic. The accuracy of the dilution measurements is computed . It should be noticed that this system had been originally incepted by Tian and Roberts (2003) and the one that developed here is the new version of it that upgraded for temporal analysis and space-time evolution of the concentration field.

    Results and discussion 

    The system is designed in a way that can record and visualize the three-dimensional configuration of the flow. Due to fast recording of the experiments with our high-speed camera with the frequency of 100 Hz, this apparatus is appropriately able to physically analyze the turbulence of the flow, turbulence energy spectrum and intensity and strength profile of the flow. The turbulence is a fluid motion that characterized by chaotic changes in flow variables. Getting 100 Hz data from each point in this system makes us be able for the frequency analysis of flow turbulent properties.

    Conclusion

    As a demonstration, the results of our observation for an inclined dense jet are exhibited. Th temporally-averaged intensity along vertical cut and energy spectrum are plotted at jet maximum height together with the instantaneous and time-averaged 2D and 3D configurations of the flow. Turbulence kinetic energy spectrums are well fitted with the power law of Kolmogorov theory for the inertial subrange. The time-averaged intensity distribution shows that for the location of maximum high, eddies are always present in centerline which shows the dominance of jet-like behavior in this point.

    Keywords: Laser, Fleurcent, 3D scanning, Environmental Hydraulic, Turbulence
  • Hanieh Kahid Basiri, Reza Babaee, Alireza Fallah, Ehsan Jabbari * Pages 83-98
    Introduction
    In order to avoid meshing and its difficulties and costs, Multiquadric Radial Basis Function (MQ-RBF) method has been developed (Kansa 1990) and has been examined for different types of physical phenomena. In this regard, the present study develops this meshless method for analysis of dam break problem. MQ is more convenient and accurate than other RBF methods for solving partial differential equations (Fallah et al. 2019). This meshless method have advantages such as; 1) creating a continuous response function all over the computational domain, 2) no need to discretize the entire domain with optimal usability in large-scale problems, 3) high capability in modelling irregular and complex geometries, 4) high ability to simulate discontinuities of responses, 5) easy generalization to 3D problems, and etc. Both the accuracy and the convergence rate of MQ depend strongly on its shape parameter (Koushki et al. 2019). So far, researchers have been working on many methods for determining the optimal shape parameter but a comprehensive method has not been developed yet (Babaee et al. 2019). In this study, the commonly previous methods have been investigated for determining the optimal shape parameter and a novel idea has been presented for analyzing the flood flow caused by dam break. The efficiency and accuracy of the present approach compared to other solutions have been shown through three examples.
    Methodology
    The governing PDEs of dam break problem consist of the continuity equation and two momentum equations in two dimensions. MQ approximates solution of 2D equations system using an estimation function in which the unknown coefficients have to be determined for each unknown variable of the PDE, i.e. the velocities in two directions and the pressure. In one hand, for definition of the estimation function, the RBF methods need N center points inside the domain or on the boundaries which leads to N unknown coefficients. On the other hand, the governing PDEs and their boundary conditions again have to be satisfied on N collocation points which leads to N algebraic equations to be solved for the mentioned unknown coefficients. A critical parameter, namely, the shape parameter strongly affects the precision of the estimation function which may be considered constant or variable from point to point for each estimation function. Determining the optimal value of the shape parameter has always been a challenge in using MQ and other RBF methods. In this study it has been shown that the shape parameter in all time steps can be equal and a new high-speed idea is proposed to determine its optimal value. In this approach, the initial conditions of the problem will be estimated using MQ function and it has been shown that the optimal value of the shape parameter in the initial conditions is also the optimal value of the shape parameter for the next time steps and there is no need to be optimized for every next time steps. Therefore the computational cost will be considerably reduced. Also for discretizing the time dependent terms, the forward finite difference method is used and it was shown that for discretizing the local terms, the implicit method must be used by substituting MQ function. Consequently, the presented approach becomes unconditionally stable. In order to verify and validate the proposed approach, three 2D numerical examples are presented. In two of the examples with 1D and 2D behaviors, discontinuities in initial conditions and run times are different. Sharp discontinuities highlight the capabilities of the approach while in long run time shows stability. Besides, results of the proposed approach have been compared with those of other numerical and analytical methods. Also, in this research, inefficiency of previously common methods for determining the optimal shape parameter in solving the dam-break problem was shown (Golbabai et al. 2015). In verification, the RMSE error criterion has been considered which results in errors less than 5 percent. In the third example capability of the numerical model has been demonstrated by a two dimensional dam break flow.
    Conclusion
    Using the MQ-RBF, the disadvantages of mesh-based method including; high cost of meshing, need to fundamental solution dependence on the conditions of each problem, singularity, continuous discretization of domain and need to a regular mesh will be eliminated. The benefits of the proposed idea for the optimal value of the shape parameter respect to existing methods include 1) independency of a secondary solution of the problem, 2) solving problem using just one set of computational centers, 3) independency of the geometry and physics of the problems, 4) needless to optimize the shape parameter at every time step, 5) low computational cost and 6) convenient to use. Finally result of several different examples compared to other numerical and analytical methods showed acceptable accuracy of the present approach.
    Keywords: Meshless Methods, Multiquadric method, Dam break problem, Shape parameter
  • Damoon Mohamad Ali Nezhadian, Hossein Hamidifar * Pages 99-110
    Introduction

    One of the significant issues in bridge hydraulics and river engineering is the phenomenon of bridge pier scouring. Scour is a phenomenon caused by the flow of water in rivers and channels. Determining the maximum depth of scouring is essential because it indicates the amount of potential degradation of the flow around the structure and also plays a decisive role in estimating the dimensions and size of structures. In hydraulic structures, this phenomenon can damage the stability of hydraulic structures because water can wash sediments beneath and around hydraulic structures and carry them in the flow direction. Over time, this phenomenon drains around the bridge pier and, eventually leading to the destruction of the bridge. Generally, two fundamental procedures may reduce scour around bridge piers. First includes changing the flow pattern by using a slot through the pier, for example. The second method includes enhancing the ability of the bed material to withstand erosion by placing riprap in front of the pier, for example. In recent years, piercing a slot through the pier is a new method to control the depth of scouring. The mechanism of operation of the slot is reducing the strength of the horseshoe vortex due to the reduction of the effective diameter of the pier. Although many studies have been carried out on the scouring of bridge piers so far, the study on the effect of floating debris on the effectiveness of a slot in protecting the pier against this phenomenon has not been investigated so far. The purpose of this study is to investigate the simultaneous effect of slot and debris on the scour around bridge piers. So the results of different bridge-pier tests in clear water conditions are presented.

    Methodology 

    The experiments were performed in Sediment Hydraulics Laboratory, Water Engineering Department, Shiraz University in a glass-walled flume with a rectangular cross-section of 0.4 m wide and 9 m long with a slope of 0.002. At the downstream end of the flume, a tailgate was installed to adjust the flow depth. Uniformly graded sediment particles with a median diameter of 0.8 mm were used. In each experiment, the discharge rate was determined to maintain the clear water scour conditions according to the ratio of the average velocity to critical velocity (Uc/U=1). The diameter of the bridge pier model used in this study was 40 mm. The width of the slot used was 10 mm corresponds to ¼ of the pier diameter. Also, a 200 mm long and 12 mm diameter cylinder installed on the pier at the water surface was used to simulate the accumulated debris. The first experiment was performed with a bridge pier without slot and debris accumulation (control experiment), and each other test was a combination of the presence or absence of slot and debris.

    Results and discussion 

    The results showed that although the presence of the slot reduces the maximum scour depth, the accumulated debris neutralized this effect considerably. It was found that using the slot alone can reduce the scour depth upto 20%, but the debris accumulation reduces this value to 15%. In the control case, with the code NS-ND-130, the ds/b ratio is 0.85, where ds and b are the maximum scour depth and pier diameter, respectively. It was also observed that in NS-D12-130, i.e., the simple pier with accumulated debris and flow depth of 130 mm, this ratio is 1, which is 17.6% greater compared to the control test. The reason for this increase is the presence of debris. The slot guides the flow in a straight line and prevents the flow separation. Hence, in the S-D12-130 experiment, i.e., a slotted pier with accumulated floating debris, the maximum scour depth decreased by 11% compared to the control, and the ratio (ds/b) indicating maximum scour depth it reached 0.775, indicating the effect of the slot in reducing scour. Besides, it was observed that with increasing the flow depth, the effect of debris would decrease. Since no relationship has been provided so far to calculate the maximum depth of scour around the bridge piers in the presence of slot and floating debris, the ds values in the present study are compared with the results of other researchers with no slots and no floating debris conditions.

    Conclusion 

    Various methods have been proposed by researchers to reduce scour and protect the bridge pier against scour, for example, a vertical slot through the pier. In this study, the effect of accumulated floating debris during floods on the effectiveness of a slot in reducing the scour around a cylindrical bridge pier was investigated. The results showed that although the presence of the slot reduces the scour depth, the accumulated debris neutralized this effect considerably. Finally, although using the slot can reduce the scour depth to 20%, but the debris accumulation reduces this value to 15%.

    Keywords: Scour, River Engineering, bridge pier, Slot, Debris
  • Razieh Kavand, Mehdi Ghomeshi *, Mehdi Daryaee Pages 111-122
    Introduction

    Considering the comparison between the number of dams constructed around the globe and the number of dam failures, the estimated probability of occurrence of failure in each dam is 10-4 dams a year. Although this probability is seemingly low, the experiences of these accidents through the history indicate that it is irrational and unreasonable to overlook the importance of this issue. Despite the extremely high accuracy of calculations and studies conducted for the construction of reservoir dams as well as the comprehensive, complete, and highly accurate research, the probability of dam failure and the need for preparedness for crisis management and floods caused by dam failures cannot be overlooked due to some limitations such as the lack of accurate statistics on the hydrology and flood characteristics on dam construction sites, the complicated geological problems and issues, foundation construction and full waterproofing challenges, flaws in dam construction, and unpredicted problems in the construction or the operation phase. The construction of most reservoir dams in meanders and rocky river beds explains the rational for this research, which was carried out in a flume with a tangent path.

    Methodology

    The present study was conducted at the physical and hydraulic models laboratory of Water Sciences and Engineering Faculty of Shahid Chamran University of Ahvaz. The rectangular flume used in this research consisted of three 90-degree bends including an acute angle bend, a normal angle bend, and a mild angle bend. A gate was installed and the flood wave created by the dam failure was simulated in the flume. The gate installed divided the flume into an upstream section (the dam reservoir) and a downstream section (the river path). A pneumatic jack was installed on the gate. The jack suddenly sent a wave to the flume downstream using a pressure compressor, thereby simulating the wave caused by the dam failure. The experiments in this study were conducted in the normal-angle bend in this flume. The wave speed and the wave front height of the wave originating from the dam failure were calculated with high accuracy using a fast-switching camera.

    Results and Discussion 

    In the dry downstream, there was no opportunity for the formation of the wave body after it flowed beneath the gate due to the considerable depth height difference between the upstream and downstream sections and the enormous energy. Besides, the wave and the wave failed mass formed at the beginning when the wave passed by the gate before it entered the curved path. The wave continued traveling without showing any fungal or dive state. Moreover, the wave height at the bend inlet was at its peak and it continued towards the end of the canal with a considerably decreasing slope. On the water surface with an upstream of 25 cm, the downstream was dry and the bed was smooth. Besides, the wave speed difference from the angle of 0 to the angle of 45 degrees and the angle of 45 to the angle of 90 degrees showed a 30% increase and a 7% decrease as compared to the beginning of the bend, respectively. This trend showed a 29% increase and an 11% decrease with the 10-mm roughness. It also showed a 25% increase and a 5% decrease with the 16-mm roughness and a 13% increase and a 4% decrease with the 20-mm roughness as compared to the beginning of the bend. The aforesaid trend decreased with an increase in the upstream height because the volume of water released from behind the gate increased, the wave did not hit the flume bed, and the wave continued moving. Moreover, the wave height at the bend inlet was at its peak and it continued moving towards the end of the canal with a considerably decreasing slope. On the water surface with an upstream of 45 cm, the downstream was dry and the bed was smooth. Besides, the wave speed difference from the angle of 0 to 45 degrees and the angle of 45 to 90 degrees showed a 23% and a 15% decrease as compared to the beginning of the bend, respectively. This trend showed a 21% and a 23% decrease with the 10-mm roughness. It also showed a 23% and a 13% decrease with the 16-mm roughness, and a 23% and an 8% decrease with the 20-mm roughness as compared to the beginning of the bend.

    Conclusions

    Since the speed increased at the apex of the bend, the speed was higher at the distances following the apex as compared to a direct path, and the construction of pumping stations or recharge points in the second half of the meander was feasible, it is recommended to take protective measures or cut the meander and turn it into a direct path to control and reduce the loss caused by the failure-induced flood.

    Keywords: Dimensional Analysis, routing flood, 90-degree bend, Wave velocity, wave height
  • Elham Bahman, Abdorreza Kabiri Samani *, Mohammad Navid Moghim Pages 123-136
    Introduction

    Weirs are one of the most common hydraulic structures and are used to regulate the upstream approach flow depth, measure the flow discharge, and evacuate the excess flow discharge in dams, irrigation and drainage networks. Based on the ratio of the total head of the upstream approach flow to the length of the weir, weirs of finite crest length are categorized into four main groups, namely sharp-crested, short-crested, broad-crested, and long-crested type weirs. The thickness of the crest results in different velocity and pressure profiles over the weir crest and consequently tends to various flow behaviors. The short-crested weirs are categorized as three different types, including ogee, circular-crested, and hydrofoil weirs. The hydrofoil weirs are a type of short-crested weirs that are designed on the basis of airfoil theory. This kind of weirs has some merits compared to the other types, such as high discharge coefficient, stability and submergence limit, and low fluctuations of pressure and water free-surface profile. Despite the extensive studies have been carried out on the hydraulic characteristics of the ogee and circular-crested weirs, there is a lack of comprehensive studies on the hydrofoil weirs, and therefore the flow characteristics over the hydrofoil weirs are still unknown.

    Methodology

    A hydrofoil weir is designed, on the basis of the Joukowsky transformation function to the equation of a reference circle on the source coordinate plane. The weir pattern generated on the destination coordinate plane is a function of the radius and the coordinate of the center of the circle on the source coordinate plane. If the center of a circle in the source coordinate plane is offset just on the horizontal axis, the Joukowsky transformation yields a symmetric hydrofoil. In this situation, if the center of a circle in the source coordinate plane is offset as large as the radius of the reference circle, the Joukowsky transformation yields a circular-crested weir. On the other hand, if the center of the circle in the source coordinate plane is offset on both the horizontal- and vertical axis, the Joukowsky transformation yields an asymmetric hydrofoil. So far, only three published studies have investigated the flow characteristics over symmetrical hydrofoil weirs. In symmetric hydrofoil weirs, the height of the weir is small, therefore these weirs have received less attention by the researchers till now. Whereas, by applying the asymmetric hydrofoil weirs instead of the symmetric ones, the weir height increases to be used for practical purposes. The present research subjects to study the flow behavior over the asymmetric hydrofoil weirs using experimental and numerical models. An experimental and numerical investigation was conducted, applying three and five models of the asymmetric hydrofoil weirs, respectively, designed on the basis of the Joukowsky transform function. Numerical simulations were performed using open source, OpenFoam v.4.0.1, CFD software. The interFoam solver and the VOF (volume of fluid) method is used to achieve the water free surface profiles and the other hydrodynamic characteristics of the flow field. The PIMPLE (pressure implicit method for pressure linked equations) algorithm was applied to couple the pressure and velocity equations in two-phase flows. In the present study, structured meshes with hexahedral elements were created by the blockMesh utility of OpenFOAM software. To generate a finer grid mesh close to the weir body and along the water free surface, snappyHexMesh utility was applied. To validate the numerical results, former experimental results and the present experimental data of different hydrofoil weirs were applied. Based on the recommendations of former studies, the k-ω SST turbulence model was used for the determination of flow characteristics over the hydrofoil weirs.

    Results and discussion

    The results of the numerical simulations including different geometrical characteristics, showed that the asymmetric hydrofoil weirs decrease the possibility of cavitation and the range of positive pressure downstream of the weir compared to those of circular-crested weirs, without decreasing the weir height. Also, in the asymmetric hydrofoil weirs, the results demonstrated that the greatest bed shear stresses and the compressive forces occur at the downstream end of the hydrofoil weir with a more camber, therefore, the downstream zone of these weirs is responsible for large values of bed erosion. Furthermore, the possibility of the downstream bed erosion is the same for the circular-crested weirs and the asymmetric hydrofoil weirs, having equal height.

    Conclusion 

    Finally, by applying asymmetric hydrofoil weirs instead of circular-crested weirs, unfavorable flow conditions would be removed, leading to a more safe and economic hydraulic structures, without decreasing the weir structural height. Keywords: Bed shear stress, Joukowsky transform function, OpenFoam software, Pressure distribution, Velocity profile.

    Keywords: Bed Shear Stress, Joukowsky transform function, OpenFoam software, Pressure distribution, Velocity profile
  • Milad Saber Pages 137-148
    Experimental investigation on scour downstream of USBRIV stilling basins M. Saber1, M. Ghodsian 2* 1- Ex.MSc student of Hydraulic Structures, Faculty of Civil Engineering, Tarbiat Modares University, Tehran, Iran 2- Prof. of Hydraulics, Faculty of Civil and Environmental Engineering and Water Engineering Research Institute,, Tarbiat Modares University, Tehran, Iran * ghods@modares.ac.ir Introduction Energy dissipaters are usually installed at the outlets of canals, outlets of chutes and outlets of culverts and other hydraulic structures. They are used to dissipate the excess energy of the outlet flow from the hydraulic structures. For dissipation of excess energy at the outlets, usually stilling basins are used. There are different types of stilling basins introduced and used in different parts of word. The USBR type VI stilling basin was first introduced by Bradely and Peterka (1995) and later on modified by Biechley (1978). In this paper results of experiments on the scour at the downstream of the USBR type VI stilling basin are reported. Experiments Experiments were conducted in a 0.8 m wide, 0.6 m height, and 0.58 m length rectangular channel. The bed and sides of the channel were made of still and glass respectively, and supported by metal frames. Uniform sediment with mean diameter of 1.64 mm were used as the bed material. The sediment layer with thickness of 0.4 m and length of 2.75 m were prepared at the downstream of stilling basin. Experiments were conducted for duration of six hours, at which almost equilibrium scour was reached. The bed topography and depth of scour were measured by a digital point gauge with the accuracy of ± 0.01 mm. The dimensions of different parts of stilling basing was selected following the method introduced by Biechley (1978). Total of 32 experiments were conducted for different Froude number (1.99, 3.03, 4.06, 4.07, 5.97, 8.11, 9.27 and 13.91) and four different stepped end sills (with one step, with two steps, with three steps, with four steps and with five steps). The width, height and length of all the end sills were kept equal in all the experiments. Results It was found that by increasing the Froude number, the length of scour hole increases for all the experiments. By increasing the Froude number from 2 to 4, the relative depth of scour initially increases, reach to a maximum value at Froude number equal to about 3 and then after decreases. The variations of the relative depth of scour for the range of Froude number from 4 t 6 is marginal. But the amount of the relative depth of scour for Froude number greater that about 6 significantly decreases. By increasing the Froude number (from 1.99 to 3.03), the scouring potential of the outflow from the stilling basin increase. As a result relative depth of scour increases. But for higher values of the Froude number, the weep out of the flow from the stilling basin occurs. In this case dissipating potential of the out flow jet from the stilling basin reduces, as a result the relative depth of scour reduces. This decreasing trend is more significant at Froude number greater than about 6. By increasing the Froude number, scour index (2ds/Lt) increases and enhances the performance of the stilling basin. Here ds is maximum depth of scour and Lt is distance of location of maximum scour depth from the stilling basin. The performance of the end sill with four steps was better for the Froude number in the range of 1.99 to 4.07. The amount of scour depth at the downstream of the end sill with four steps was minimum for the Froude numbers in the ranges of 9.27 to 13.91. The volume of scour hole at the downstream of the end sill with four steps was less than that due to other end sills. An asymmetry index was defined to see the symmetry/asymmetry of the bed topography after the experiments. The values of asymmetry indices for the end sills with five steps under Froude number in the range of 1 to 4, and for the end sills with four steps under Froude number in the range of 4 to 13.9 were minimum. Time variations of the scour showed that about 65 to 70 percent of the scour occurs during 60 minutes from onset of experiments. An equation was obtained for the dimensionless longitudinal profile of scour profile. Time variations of the scour showed that about 65 to 70 percent of the scour occurs during 60 minutes from onset of experiments. An equation was obtained for the dimensionless longitudinal profile of scour profile.
    Keywords: Stilling basin type USBRVI, End sill, scour index, scour asymmetry