نوع مقاله : مقاله پژوهشی

نویسندگان

• محمد رستمی ¹
• امیرحسین رحیمی ²

¹ عضو هیات علمی پژوهشکده حفاظت خاک و آبخیزداری، سازمان تحقیقات، آموزش و ترویج کشاورزی، تهران، ایران.

² دانش آموخته سازه های هیدرولیکی، دانشکده مهندسی عمران، دانشگاه علم و فرهنگ، تهران، ایران.

چکیده

تجمع واریزه‌های چوبی در دهانۀ پل‌ها یکی از عوامل اصلی کاهش ظرفیت عبور سیلاب و افزایش خطر آب‌گرفتگی در رودخانه‌های کوهستانی جنگلی است. در این پژوهش، عملکرد تیرک‌های فولادی هدایت‌کننده در کاهش احتمال گرفتگی دهانۀ پل‌ها با استفاده از نرم‌افزار FLOW-3D و روش دینامیک سیالات محاسباتی (CFD) بررسی گردید. تأثیر آرایش تیرک‌ها و شرایط جریان بر نحوۀ حرکت و عبور واریزه‌های چوبی در سناریوهای مختلف شبیه‌سازی شد. نتایج نشان داد که در شرایط بدون استفاده از تیرک‌های هدایت‌کننده، به‌طور متوسط حدود 49 درصد از الوارها در بالادست پل تجمع می­کنند و موجب کاهش ظرفیت عبور جریان می‌شوند. در آرایش پیشنهادی مطالعۀ پیشین، میزان تجمع واریزه‌ها به حدود 23 درصد کاهش یافت، در حالی‌که آرایش پیشنهادی این پژوهش با هدایت موفق بیش از 94 درصد واریزه‌ها از دهانۀ پل و کاهش تجمع به کمتر از 5 درصد، بالاترین کارایی را در جلوگیری از انسداد دهانه‌ها نشان داد. اعتبارسنجی مدل عددی با استفاده از داده‌های آزمایشگاهی نشان داد که میانگین خطای نسبی پیش‌بینی‌ها حدود 18 درصد است که بیانگر دقت قابل قبول مدل در بازتولید فرآیند انتقال و تجمع واریزه‌های چوبی است. بر این اساس، استفاده از تیرک‌های هدایت‌کننده با آرایش پیشنهادی می‌تواند راهکاری مؤثر برای کاهش خطر گرفتگی پل‌ها و افزایش ایمنی عبور سیلاب در رودخانه‌های جنگلی باشد.

کلیدواژه‌ها

• خطر سیلاب
• آبگذری پل
• واریزه‌های چوبی
• تیرک‌ هدایت کننده
• مدل FLOW-3D

موضوعات

• مدل های ریاضی و بهینه سازی

عنوان مقاله [English]

Numerical Evaluation of the Performance of Protective Steel Beams in Reducing Bridge Opening Blockage Risk Caused by Floating Woody Debris in Mountainous Forested Rivers

نویسندگان [English]

• Mohammad Rostami ¹
• Amir Hossein Rahimi ²

¹ Faculty Member, Soil Conservation and Watershed Management Research Institute, Agricultural Research, Education and Extension Organization, Tehran.Iran.

² Graduate of Hydraulic Structures, Faculty of Civil Engineering, University of Science and Culture, Tehran, Iran.

چکیده [English]

Extended Abstract
Introduction
       Rivers in mountainous forested regions transport not only water and sediment during flood events, but also large amounts of woody debris, including tree trunks, branches, and vegetation residues. These debris materials accumulate around hydraulic crossing structures such as bridges, reducing flow conveyance capacity, increasing water levels, intensifying local scour, and consequently increasing the risk of flooding. In recent years, steel guiding beams installed upstream of bridges have been considered as an effective countermeasure to modify the movement path of woody debris and reduce bridge blockage probability. These beams alter the orientation of floating logs and facilitate their passage through bridge openings.
Despite the promising results of laboratory investigations, uncertainties still remain regarding the optimal arrangement of guiding beams and their performance under different hydraulic conditions. Furthermore, the high cost and limitations of experimental studies have highlighted the necessity of numerical modeling approaches. Therefore, in this study, the performance of steel guiding beams in reducing bridge blockage probability was investigated using the FLOW-3D software and Computational Fluid Dynamics (CFD) techniques. The effects of beam arrangement and flow discharge on the movement and passage behavior of woody debris under flood conditions were evaluated.
Methodology
       In this study, numerical simulations of woody debris behavior and the performance of steel guiding beams were carried out using FLOW-3D software. Model calibration, validation, and scenario analyses were performed based on the laboratory experiments of Harada et al., (2018). First, the geometry of the channel, bridge, and steel beam arrangements were reconstructed according to the experimental setup. The model consisted of an open channel with a specified slope, bridge piers, and different arrangements of steel guiding beams installed upstream of the bridge.
       Three scenarios were simulated: (1) bridge condition without guiding beams, (2) bridge condition with the beam arrangement proposed by Harada et al., (2018) (Type B), and (3) bridge condition with the new arrangement proposed in the present study (Type C). In addition, the effects of woody debris introduction mode (individual and simultaneous entry) and the number of logs on bridge blockage probability were investigated.
            Woody debris pieces were modeled as floating rigid bodies with identical geometric and physical properties, including length, diameter, and density. The logs entered the flow from upstream, and their movement, rotation, collision with steel beams, passage through bridge openings, and accumulation around bridge piers were analyzed. The interaction between flow, woody debris, and steel beams was simulated using the moving rigid body capability of FLOW-3D.
       In each simulation, parameters such as the number and percentage of logs passing through the bridge openings were extracted and compared with laboratory data. While the reference experiments were conducted within a discharge range of 1 to 2.3 L/s, the discharge range in this study was extended to 0.5–3.5 L/s. Finally, the numerical simulation results were compared with experimental observations and previous studies to evaluate the accuracy and capability of FLOW-3D in simulating woody debris behavior and assessing the performance of steel guiding beams.
Results and Discussion
       The numerical simulation results showed that the use of steel guiding beams upstream of the bridge effectively modified the movement path of woody debris and reduced bridge blockage probability. In the absence of guiding beams, debris accumulation upstream of the bridge caused opening blockage, flow backwater effects, and reduced flood conveyance capacity. In contrast, the guiding beams changed the orientation of the logs and facilitated their passage through the bridge openings.
       For the simultaneous entry of 50 logs, the passage percentage without guiding beams ranged from 0 to 60%, whereas it increased to 20–75% in the presence of Type B guiding beams. In the case of individual log entry, the passage efficiency increased by an average of 35% with Type B beams. To improve system performance, a new beam arrangement (Type C) was proposed and evaluated. Under a discharge of 2.3 L/s and simultaneous entry of 20 logs, the passage percentage was 78% for Type B and 94% for Type C. Furthermore, when the number of logs increased from 1 to 20, the average passage percentages for the no-beam, Type B, and Type C conditions were 51%, 87%, and 95%, respectively. The results demonstrated that the proposed Type C arrangement can effectively reduce bridge blockage risk and improve flood conveyance safety in mountainous forested rivers.
Conclusion
       The numerical simulation results demonstrated that the accumulation of woody debris at bridge openings can significantly reduce bridge conveyance capacity and increase the risk of flooding and associated damages in forested mountainous rivers. In the absence of guide piles, a considerable portion of the logs accumulated upstream of the bridge, causing blockage of the bridge opening, flow backwater effects, and a reduction in flood conveyance efficiency. This phenomenon became more pronounced under conditions of higher flow discharge and the simultaneous introduction of a large number of woody debris elements.The results further indicated that the installation of steel guide piles upstream of the bridge effectively improved the movement path of woody debris, reduced direct impacts on bridge piers, and increased the probability of debris passing through the bridge opening. Comparison of different pile configurations revealed that the Type B arrangement proposed by Harada et al., (2018) performed better than the condition without guide piles; however, approximately 13% of the woody debris still accumulated around the bridge piers. In contrast, the Type C arrangement proposed in the present study provided more efficient debris guidance by improving the approach angle of the debris toward the bridge opening. As a result, only about 5% of the woody debris failed to pass through the bridge opening, while the average passage efficiency reached approximately 95%.The analysis also showed that increasing the number of woody debris elements entering the flow substantially increased the likelihood of bridge blockage under conditions without guide piles. However, the use of steel guide piles, particularly the Type C configuration, maintained a high debris passage efficiency even under conditions of high debris concentration. Furthermore, the validation results confirmed that FLOW-3D is capable of accurately simulating the hydrodynamic behavior of woody debris, its interaction with flow, and the performance of debris-guiding structures. Therefore, the model can serve as an effective tool for the design, assessment, and optimization of mitigation measures aimed at reducing bridge blockage hazards in mountainous river systems.

کلیدواژه‌ها [English]

• Flood hazard
• Bridge conveyance capacity
• Woody debris
• Guide beem
• FLOW-3D model