نوع مقاله : مقاله پژوهشی

نویسندگان

• مرضیه بادزن چین ¹
• محمد بهرامی یاراحمدی ²
• محمود شفاعی بجستان ³

¹ دانشجوی کارشناسی ارشد مهندسی عمران-آب و سازه های هیدرولیکی دانشکده مهندسی آب و محیط زیست دانشگاه شهید چمران اهواز، اهواز، ایران.

² استادیار گروه سازه های آبی دانشکده مهندسی آب و محیط زیست دانشگاه شهید چمران اهواز، اهواز، ایران

³ استاد گروه سازه های آبی دانشکده مهندسی آب و محیط زیست دانشگاه شهید چمران اهواز، اهواز، ایران.

چکیده

معادلۀ مانینگ معروفترین معادلۀ تعیین مقاومت جریان در جریان­های ماندگار و یکنواخت است. میزان مقاومت جریان در مجاری باز علاوه بر اندازۀ ذرات رسوبی به نوع فرم بستر و خصوصیات هندسی آن بستگی دارد. روابط متعددی برای تعیین زبری اندازۀ ذره وجود دارد ولی تعیین میزان زبری فرم بستر کمتر بررسی شده است. مروری بر تحقیقات نشان می ­دهد که در زمینۀ اثر ارتفاع فرم بستر دیون بر ضریب زبری مانینگ تاکنون مطالعه­ نشده است. در تحقیق حاضر به بررسی اثر فرم ­های با ارتفاع­ 1، 2، 3 و 4 سانتی­متر و طول ثابت 25 سانتی­متر بر ضریب زبری مانینگ در یک فلوم مستقیم با 5 دبی­ و 5 شیب­ بستر پرداخته شده است. نتایج بررسی­ ها نشان داد که با افزایش ارتفاع فرم بستر و پارامتر استغراق نسبی ( Δ/λ)، ضریب زبری مانینگ افزایش می ­یابد. ضریب زبری مانینگ دیون­ های با ارتفاع­ 2، 3 و 4 سانتی­متر به­طور متوسط به ترتیب 16، 31 و 55 درصد بیش از ضریب زبری مانینگ فرم بستر با ارتفاع 1 سانتی­متر است.

کلیدواژه‌ها

• ارتفاع فرم بستر
• دیون
• ضریب زبری مانینگ
• مقاومت جریان
• رودخانه آبرفتی

موضوعات

• مهندسی رودخانه

عنوان مقاله [English]

Experimental Study of the Effect of the Dune Bed Form Height on Manning's Roughness Coefficient

نویسندگان [English]

• Marzieh Badzanchin ¹
• mohammad bahrami yarahmadi ²
• Mahmood Shafai Bajestan ³

¹ M.Sc. Student in Civil Engineering (Water & Hydraulic Structures), Faculty of Water and Environmental Engineering, Shahid Chamran University of Ahvaz, Ahvaz, Iran.

² Assistant Professor, Faculty of Water and Environmental Engineering, Shahid Chamran University of Ahvaz, Ahvaz, Iran

³ Professor, Faculty of Water and Environmental Engineering, Shahid Chamran University of Ahvaz, Ahvaz, Iran.

چکیده [English]

Extended Abstract

Introduction

Manning's equation is the most popular equation to determine the flow resistance in the steady and uniform flows. The amount of flow resistance in alluvial rivers depends on the type of bed form and its geometrical characteristics. In ripple and dune bed forms, which are formed at Froude numbers less than one (lower flow regime), the flow separation from their crest is the main factor of flow resistance (Shafai Bajestan, 2008; Julien, 2010). The effect of bed form on flow resistance have been studied by few researchers such as: Talebbeydokhti et al. (2006), Omid et al. (2010), Nasiri Dehsorkhi et al. (2011), Chegini and Pender (2012), Kabiri et al. (2014), Samadi-Boroujeni et al. (2014), Kwoll et al. (2016), and Heydari (2020). However, no research has been observed regarding the effect of the dune bed form height on Manning's roughness coefficient. Therefore, the main goal of the present study is to investigate the effect of dunes with different height of 1, 2, 3, and 4 cm on Manning's roughness coefficient in a straight flume under different discharges and bed slopes.

Methodology

The experiments were performed in a straight flume of 12 m in length and 0.30 m wide (Figure 1). In this study, different flow discharge of 10, 15, 20, 25, and 30 l/s and different bed slopes of 0.00001, 0.0001, 0.0005, 0.001, and 0.0015 were tested. Each dune was made in an asymmetric triangular shape with cement-sand mortar. The dune's height was equal to 1, 2, 3, and 4 cm, and the its length was selected as 25 cm. After each dune was built, the sediment with average size (d50) of 0.45 mm was glued to its surface. The total number of experiments in the present study was 100. Dune bed form dimensions were in lower flow regime. All the experiments were carried out at lower flow regime with, the Froude number values ranged from 0.44 to 0.7.

Results and discussion

Figure 4 shows the variation of the Manning's roughness coefficient against relative submergence (y/Δ) for dunes with a height of 1 cm. This figure shows that, with increasing relative submergence and bed slope, Manning's roughness coefficient decreased.

Figure 5 shows the changes in the Manning's roughness coefficient against Froude number for dunes with different heights (slope of 0.0001). It can be seen that as the Froude number increased, the Manning's roughness coefficient decreased.

Figure 7 illustrates the effect of Δ/λ on the Manning's roughness coefficient. This figure shows that the Manning's roughness coefficient increased with increasing Δ/λ. Calculations showed that the Manning's roughness coefficient in dunes with Δ/λ = 0.08, 0.12, and 0.16 was, on average, 17, 30, and 55% more than dune with Δ/λ = 0.04, respectively.

Figure 8 shows the effect of the dune bed form height on the Manning's roughness coefficient. By increasing bed form height, Manning's roughness coefficient increased. Calculations showed that the Manning's roughness coefficient in dunes with height of 2, 3, and 4 cm was, on average, 16, 31, and 55% more than dune with a height of 1 cm, respectively.

Conclusions

The results of this study shows that with increasing the Froude number and the relative submergence (y/Δ), the flow resistance or Manning's roughness coefficient decreases. Moreover, the increase in the Δ/λ and the bed form height leads to an increase in Manning's roughness coefficient. Calculations showed that the Manning's roughness coefficient in dunes with height of 2, 3, and 4 cm was, on average, 16, 31, and 55% more than dune with a height of 1 cm, respectively. In addition, the form Manning's roughness coefficient (nb'') for dunes with heights of 1, 2, 3, and 4 cm is, on average, 30.7, 40.2, 46. 5, and 53.4% of the total Manning's roughness coefficient (nb), respectively.

کلیدواژه‌ها [English]

• Bed form height
• Dune
• Manning'
• s roughness coefficient
• Flow resistance
• Alluvial river