مطالعه آزمایشگاهی پروفیل عمقی سرعت در کانال مرکب با پوشش گیاهی در سیلابدشت

رودخانه‌ها غالبا به شکل یک کانال مرکب عمل می‌کنند، به دلیل شرایط مساعد رطوبتی، سیلابدشت‌ها معمولا زیستگاه گیاهان مختلف با چیدمان و تراکم‌های متفاوت می‌باشند. درک بهتر نقش پوشش‌گیاهی در انتقال جریان، رسوب و آلاینده‌ها، نیازمند بررسی دقیق ساختار جریان در حضور پوشش‌گیاهی است. بنابراین در این مطالعه آزمایشگاهی، تاثیر وجود پوشش‌گیاهی صلب در سیلابدشت در دو چیدمان خطی و شطرنجی، سه تراکم %6.04، %1.64 و %0.41و پنج عمق نسبی 0.29، 0.45، 0.5، 0.67 و 0.75 بر پروفیل عمقی سرعت بررسی شده است. در این مطالعه، سرعت نقطه‌ای جریان توسط سرعت‌سنج ‌پره‌ای در دو حالت پوشش‌گیاهی مستغرق و غیرمستغرق اندازه‌گیری شده است. نتایج نشان می‌دهد که پروفیل عمقی سرعت در شرایط بدون پوشش‌گیاهی در تمام عرض کانال مرکب به جز حدفاصل کانال اصلی و سیلابدشت از پروفیل لگاریتمی تبعیت می‌کند. در هنگام وجود پوشش‌گیاهی، پروفیل عمقی سرعت در میانه کانال اصلی مشابه الگوی رایج پروفیل سرعت در کانال‌های بدون پوشش‌گیاهی است و با دور شدن از میانه کانال اصلی و نزدیک‌شدن به حدفاصل کانال اصلی و سیلابدشت، از شکل لگاریتمی خارج شده و s شکل می‌شود. با این حال، در کانال اصلی پروفیل عمقی سرعت در ارتفاعی بالاتر از عمق لبریزی به شکل لگاریتمی می‌باشد. بررسی تاثیر تراکم بر پروفیل عمقی سرعت نشان می‌دهد که با افزایش تراکم، سرعت در کانال اصلی افزایش و در سیلابدشت کاهش می‌یابد. همچنین در جریان با پوشش‌گیاهی کاملا مستغرق، پروفیل عمقی سرعت را می‌توان به سه ناحیه دنباله، اختلاط و ناحیه لگاریتمی تقسیم نمود.

Experimental Study of Vertical velocity profiles in compound channels with vegetation on floodplains

Introduction: Vegetation has traditionally been viewed as a nuisance and obstruction to channel flow by increasing flow resistance and water depth. However, in recent years, vegetation has become a major component of erosion control and stream restoration. Most of research efforts focus on describing vegetation roughness , determining drag coefficients and empirical formulas for resistance under various vegetation configurations. While the development of experimental solutions for vegetative resistance is important, it is also important to understand the detailed characteristics of flow through vegetation. Yang et al.(2007) conducted flume experiments with different types of vegetation, and found that, in the cases of non vegetated floodplains, all measured streamwise velocity distributions followed the logarithmic distribution, but for vegetated floodplains, they followed an Sshaped profile Nezu and Sanju(2008) have investigated turbulence structures and coherent motion in vegetated canopy openchannel flows. They divided the whole flow region into three subzones, i.e., the emergent zone, the mixinglayer zone and the loglaw zone. In the present study, some experiments were undertaken herein under different conditions to elucidate the flow structure. The main focus is to examine how the vertical velocities, are affected by simulated vegetation arranged in emergent and submerged conditions. In addition, the effect of dowel density, configuration, and relative depth are examined. Methodology: The experiments were conducted in a fixed bed rectangular flume, 9 m long and 0.6 m high and 0.8 m wide. The slope of bed flume was 12 ×105. The main channel and floodplain had widths of 24 and 28 cm, respectively, and the main channel had a side slope, s, of 0. The bankfull height, h, was 6 cm. Vegetation were simulated by wooden dowels. The wooden dowels were 140 mm tall and 7 mm in diameter. The dowels were attached to a PVC sheet bolted to the bottom of the flood plain in linear and staggered arrangement. The spacing of the dowels varies from 2.510 cm in both lateral and streamwise directions forming stem density of 0.41, 1.64%, 6.04%. The flume was operated under a uniform flow condition, and measurements of discharge, point velocity and flow depth were taken. Flow depths were measured by means of a pointer gauge, discharges were measured by a digital flowmeter, installed upstream of the channel, and a micro propeller current meter were used to velocity measurements. Within the measurement cross section, located at 5.6 m, the authors arranged ten verticals, where the lateral values of y from the first vertical to the last were 0, 4, 8, 12, 12.2, 26 and 34 cm. When the vertical distance from the measurement point to the bed was less than 175 mm, the measurement interval was 10 mm and 5mm in the main channel and floodplain, respectively. Results and discussion: The experimental results are presented in three parts, flow through nonvegetated floodplain first, flow through emergent vegetation second, followed by the submerged case. The effects of density and dowel configuration are included in each of the sections. Each section ends with a discussion on the effects of rigid dowels on logarithmic profile. In the cases of nonvegetated floodplains, all measured streamwise velocity distributions followed the logarithmic distribution, but for vegetated floodplains, they followed an Sshaped profile. It is seen that after implanting the vegetation over the floodplain, the velocity over the floodplain decreases whereas it increases in the main channel. Also, as the vegetation density, λ, increases, velocity increases in the main channel and decreases in the floodplain. In the presence of emergent vegetation on floodplain, logarithmic profile does not exist even in the main channel, however it seems that the formation of the Sshaped profile in the main channel is under the bankfull height and above the bankfull height the vertical velocity profile takes on a logarithmic profile again. Under Submerged flow conditions, the velocity characteristics in all locations above the dowel array are well illustrated by the semilogarithmic expression that has a slip velocity initially near the inflection point. On the basis of the present experimental results, the whole flow region is divided into the following three subzones: (1) Emergent zone (0 ≤ z ≤ hp), (2) Mixinglayer zone (hp < z ≤ hlog), (3) Loglaw zone (hlog < z ≤ H). In the present study, hp was equal to 0.2 H and hlog was equal to 0.5 H. In the emergent zone (0 ≤ z ≤ hp) the velocity is almost constant due to strong wake effects of vegetation stems although it may behave slightly in a countergradient fashion. In the second zone (hp ≤ z ≤ hlog), the vertical velocity profile are similar in both submerged and emergent conditions, and the effect of bed roughness is completely eliminated and the velocity gradients are reduced and almost fixed. The velocity in the third zone (hlog < z ≤ H) is significantly higher than the velocity in the second zone. In the loglaw zone (hlog < z ≤ H), the loglaw of velocity distribution for rough beds is reasonably applied even to vegetated flows. Comparison the longitudinal velocity profiles for linear and staggered dowel arrangements indicates an increase in the resistance due to the linear arrangement compared to the staggered arrangement. Conclusion: In the cases of non vegetated floodplains, all measured streamwise velocity distributions followed the logarithmic distribution, but for vegetated floodplains, they followed an Sshaped profile. However, in the main channel, higher than the bankfull height the velocity profile is logarithmic. The results shows that as the vegetation density, λ, increases, the velocity increases in the main channel and decreases in the floodplain. Linear arrangement resulted higher resistance compared to staggered vegetation arrangement. The velocity profile at all locations above the dowel array are very well represented by the following semi logarithmic expression. In fully submerged vegetation, the whole flow region was divided into three subzones, i.e., the emergent zone, (0≤z≤hp) the mixinglayer zone (hp < z≤hlog), and the loglaw zone(hlog