Iranian Jornal of Watershed Management Science&Engineering

fa مطالعه آزمایشگاهی آبشستگی موضعی در اطراف بازشدگی بندهای اصلاحی شکاف‌دار در جریان‌های واریزه‌ای Experimental Study of Local Scour Around Opening of Slit Check Dam under Debris Flow تخصصي Special پژوهشي Research بندهای اصلاحی شکافدار، سازه&lrm;هایی برای کاهش خطرات جریان&lrm;های واریزه&lrm;ای، کنترل بار بستر، تله‌اندازی بخشی از رسوبات، کاهش انرژی جنبشی سیلاب‌های آواری-واریزهای و تثبیت رودخانه‌ها هستند. پایداری بندهای اصلاحی شکاف&lrm;دار به شدت تحت تأثیر پدیده آبشستگی موضعی پاییندست این سازه‌ها است. در این تحقیق به بررسی آزمایشگاهی تأثیر خصوصیات نسبی هندسی واریزه&lrm;ها شامل طول، عرض، ضخامت و فرم واریزه&lrm;ها، عرض شکاف بند و عدد فرود جریان بر آبشستگی موضعی پاییندست بند اصلاحی باز شکافدار پرداخته شده است. آزمایش&lrm;ها در فلومی به عرض 80 و ارتفاع 60 سانتی&lrm;متر و طول هشت متر با رسوبات غیرچسبنده یکنواخت با اندازه متوسط  d50 = 0.83 میلی‌متر انجام شده است. از واریزه&lrm;هایی با فرم‌های مستطیلی، مثلثی و نیمه&lrm;استوانه&lrm;ای به عنوان مدل واریزه&lrm;ها در شرایط آب زلال استفاده و برنامه‌ریزی آزمایشات براساس آنالیز ابعادی انجام شد. حضور واریزه‌ها نسبت به آزمایش شاهد موجب افزایش آبشستگی شد و از بین فرم‌های مختلف مورد بررسی تجمع واریزه‌ها در این تحقیق، فرم مثلثی بیشترین تأثیر را نسبت به دیگر فرم واریزه‌ها داشته است. نتایج نشان داد که افزایش ضخامت نسبی واریزه‌ها و افزایش عدد فرود جریان موجب افزایش عمق آبشستگی شده و با افزایش عرض نسبی شکاف و طول نسبی واریزه‌ها عمق آشستگی کاهش داشته است. تغییرات عرض نسبی واریزهها، تأثیری متفاوت داشته است طوریکه افزایش آن تا حدی موجب افزایش آبشستگی و سپس روندی معکوس داشته است. Extended Abstract Introduction During the last decades, various kinds of check dams have been extensively used worldwide to trap and control bed load transport, coarse sediment, river stabilization, and the detrimental effects of sediment transport and debris flow during extreme floods. Debris flows, common in mountainous streams and rivers, are multiphase flows consisting of water, mud, coarse sediment, trees, and large boulders that threaten thousands of citizens and endanger buildings, highways, bridges, and infrastructures worldwide. Check dams represent one of the most effective countermeasures for debris flow prevention and control. These structures are categorized into solid-body dams and open check dams. Open check dams can be divided into two main groups: slit dams and sectional dams. Slit check dams are characterized by a slit opening in the central part of the dam structure and should be designed to allow sediment trapping during major floods. Field observations and reviews of reports have shown that downstream of sediment traps, flows often consist of clear water with high erosive power. Therefore, erosive stream power at the toe of slit dams can be extremely high, posing a major threat to these structures. Consequently, it is important to simulate and predict the maximum local scour depth at the toe of these structures to protect them against the consequences of channel destabilization. Based on published literature, experimental studies on scour around slit dam openings are limited. Therefore, in the present research, experiments were conducted to study the effects of three different debris shapes, geometric characteristics, slit width, and flow Froude number on the size of the scour hole formed at the toe of slit dams. Materials and method The experiments were carried out at the Hydraulic and Water Structures Laboratory, Department of Water and Science Engineering, Shahid Bahonar University, Kerman, Iran. The experimental flume had a rectangular cross section with 8 m length, 80 cm width and 60 cm depth. The variables considered in each experiment were debris shape (rectangular, semi-cylindrical and triangular), relative geometric characteristics of debris (three relative thicknesses, four relative widths and three relative lengths), three relative slit widths, and three flow Froude numbers. Prior to each test, a slit dam model was installed, and the flume bottom was covered with a 14 cm deep layer of non-cohesive uniform sediment with D50=0.83 mm. The debris models were made of wooden networks. The slit dam was manufactured from steel with an adjustable opening (slit width) and fixed tightly at a given distance from the flume entrance. Bed-level measurements in the working section or mobile bed and flow characteristics were carried out using different instruments located on the flume. Each experiment was based on a general configuration rather than specific field data. Therefore, no exact scaling was performed, and only a few dimensionless parameters were controlled. Dimensional analysis was used to group the most effective variables governing the scour volume at the toe of the slit dam. At the end of each test, the dimensions and total volume of scour were measured in detail to assess the effects of different parameters on local scour at the toe of the slit dam under the tested laboratory conditions. Results and discussion In this study, it was observed that the scour hole reached equilibrium after 6 h from the commencement of the reference test; therefore, all tests were conducted for more than 6 h. The experimental results showed that scour volume and relative maximum scour depth mainly depended on debris characteristics. The relative maximum scour depth in the presence of debris was, on average, 25% greater than that of the reference test, with the greatest relative scour depth occurring for triangular debris. Relative maximum scour depth was negatively related to relative slit width. Changing the relative thickness of debris also had a significant effect on scour volume. Increasing relative debris thickness led, on average, to a 10% increase in maximum scour depth. Increasing the relative width of debris produced a different effect on scour behavior: relative maximum scour depth initially increased and then decreased. In all tests, relative maximum scour depth decreased with increasing relative debris length. As debris length increased, the vortices formed around the slit became weaker, resulting in a smaller scour hole. In addition, the experimental results revealed that scour volume was significantly affected by relative slit width, as slit width influenced the strength of the generated vortices. Therefore, when relative slit width increased from 0.0375 to 0.0625, relative maximum scour depth decreased by an average of 25%. Furthermore, when flow rate or Froude number increased from 0.19 to 0.24, scour depth increased by 15% for all investigated debris shapes. Conclusion In last decades construction of slit dam is progressively in debris flow prone areas. The present study contributes to investigate experimentally local scour around the slit of open check dam under debris flow may be used to complementary study for slit dam stability. The physical model well presents the scour development process under steady, subcritical, debris flow with uniform sediment and mild slope. According to the dimensional analysis and experimental set up, flume test was carried out for three different shapes of debris with different relative geometric characteristics (three length, three width and four thickness), three different Froude number of flow and three different widths of slit. Presences of debris in flow increase the relative scour depth rather than clear flow. Debris change the flow pattern and increase the scour volume and scour depth by excessive shear stress and vortices around slit of check dam. Also, the scour volume and depth resulted in a higher value with Froude number increasing and slit width reduction. So, the findings of this study shows that the stability of slit check dams under debris flow in addition to other parameters, some extends is depends on extra local scour due to debris. Further physically or numerically studies are recommended to study the local scour process concerning the nonuniform sediment, supercritical flow, steep slope and another kind of open check dams.       بندهای اصلاحی, آبشستگی موضعی, مهندسی رودخانه, سیلاب واریزه‌ای, مدل آزمایشگاهی. Check dams, Local scour, River engineering, Debris flow, Physical model 20 0 http://jwmsei.ir/browse.php?a_code=A-10-1773-1&slc_lang=fa&sid=1 kourosh qaderi کورش قادری kouroshqaderi@uk.ac.ir 100319475328460019275 100319475328460019275 Yes دانشگاه شهید باهنر، mohammad mahdi ahmadi محمد مهدی احمدی mmahmadi@uk.ac.ir 100319475328460019276 100319475328460019276 No دانشگاه شهید باهنر، amir hossein karimzade karnama امیرحسین کریم زاده کارنما amirhossein.karnama@gmail.com 100319475328460019277 0009-0008-1103-3330 No دانشگاه شهید باهنر، elahe alizadeh الهه علیزاده alizadee91@gmail.com 100319475328460019278 100319475328460019278 No دانشگاه شهید باهنر،