بررسی میدان جریان نزدیک شونده به عرشه پل در مقاطع مرکب با پوشش گیاهی

در زمان بروز سیلاب هایی با دبی های بیشتر از  دبی طرح در رودخانه‌ها و افزایش تراز سطح آب، احتمال این که عرشه پل‌ به زیر آب رود زیاد است. در این شرایط، جریان عبوری از زیر عرشه به صورت تحت فشار رفتار خواهد کرد. با توجه به شکل طبیعی رودخانه ها که بیشتر به صورت مقطع مرکب است، پیش بینی میدان جریان در کانال های مرکب به علت ماهیت سه بُعدی جریان برای مهندسان هیدرولیک دشوار است. شرایط تحت فشار و اختلاف زبری بین کانال اصلی و سیلاب دشت ها از عوامل پیچیده تر شدن تحلیل پارامترهای هیدرولیکی جریان در کانال های مرکب است. در این پژوهش، آزمایش‌ها با پوشش گیاهی صُلب غیرمستغرق و با سه عرض مختلف سیلابدشت و چهار تراکم مختلف پوشش گیاهی با سه عمق نسبی متفاوت به اجرا در آمده اند. نتایج پژوهش نشان داد افزایش تراکم پوشش گیاهی باعث افزایش سرعت طولی در کانال اصلی و کاهش آن در سیلاب‌دشت می‌شود و افزایش تراکم پوشش گیاهی، به‌طور متوسط باعث کاهش 17 درصد مشارکت سیلاب‌دشت در عبور دبی عبوری از مقطع کانال مرکب خواهد شد. وجود عرشۀ پل و شرایط جریان تحت فشار باعث پس زدگی جریان می شود که به کاهش سرعت جریان و دبی جریان عبوری از زیر عرشه پل می انجامد.

investigating the flow field approaching the bridge deck in compound open channels with vegetation

extended abstractintroduction with the occurrence of flood, the velocity and depth of the flow in the river increases and the flow enters the flood plains. the velocity difference between the deeper section and the shallow area causes the transfer of momentum between these areas and complicates the flow structure. the formation process of secondary flows and its pattern in compound channels have been investigated by researchers such as:tominaga & nezu, 1991. the presence of vegetation on flood plains causes complexity in the analysis of hydraulic problems of compound channels. for example, hamidifar et al. (2012, 2014), using laboratory measurements, showed that the presence of vegetation reduces the flow through the cross section by about 30%. at the same time as the water level rises during the flood, the deck of the bridges will go under water and the current passing under it will be pressurized. in this condition, the flow field is affected by the presence of vegetation, compound channel and pressurized flow. in this research, the laboratory investigation of these complex conditions has been done.methodologythe experiments of this research were done with 3 geometric ratios of the compound cross-section, 3 relative depths, 3 vegetation densities, and control experiments in a compound channel with a length of 10 meters and a width of 1.5 meters. the measurement of the flow velocity parameter, the scouring rate of the bridge pier in the conditions of pressurized flow has been done according to the variables mentioned above. results and discussioncomparison of depth velocity and logarithmic velocity distribution in the condition without vegetation on the flood plain, the sign shows that in all sections, the distance between the channel bed and the water surface, the difference between the measured velocity values with the logarithmic distribution of the velocity increases. this difference is due to the presence of the bridge deck and the flow retardation.   also, vegetation causes the depth distribution profile of flow velocity to deviate from the curve of logarithmic flow velocity, and the biggest difference will occur in the upstream area between the interface of main channel and flood plain. this phenomenon increases the amount of apparent shear stress between the main channel and the floodplain.with the increase in the density of vegetation, the percentage of floodplain participation in the total discharge is reduced by 20%. the highest participation percentage of floodplain is about 30% in the state without vegetation. in different densities of vegetation with an increase in relative depth from 0.3 to 0.5, the percentage of floodplain participation in the total discharge is less than 10%. with the increase in the density of vegetation, the difference between the percentage of floodplain participation in different cross section widths has decreased.conclusionsthe findings of recent research to check hydraulic parameters can be summarized as follows:- increasing the density of vegetation increases the longitudinal velocity in the main channel and decreases it in the floodplain.- longitudinal velocity and average depth of flow in the main channel in the case without vegetation is lower than the case with vegetation.-increasing the relative depth increases the percentage of floodplain participation by an average of 5%, and the increase in vegetation density decreases the floodplain participation by 17% on average.- with the increase in the vegetation density of the floodplain, the speed changes in the floodplain decrease compared to the main channel.-the increase in relative depth and density of vegetation increases the non-uniformity of flow lines and finally increases the correction coefficients of kinetic energy and movement size.-in high-density vegetation, the interference effect of eddies prevents the formation of large secondary currents on the floodplain.