Research Article Vertical Velocity Distribution in Open-Channel Flow with Rigid Vegetation

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1 e Scientific Worl Jornal, Article ID 89, pages Research Article Vertical Velocity Distribtion in Open-Channel Flow with Rigi Vegetation Changjn Zh, Wenlong Hao, an Xiangping Chang College of Urban Constrction, Hebei University of Engineering, Hanan 8, China Corresponence shol be aresse to Changjn Zh; Receive 7 December ; Accepte 9 Febrary ; Pblishe April Acaemic Eitors: K. Nemeth an G. Ventra Copyright Changjn Zh et al. This is an open access article istribte ner the Creative Commons Attribtion License, which permits nrestricte se, istribtion, an reproction in any meim, provie the original work is properly cite. In orer to experimentally investigate the effects of rigi vegetation on the characteristics of flow, the vegetations were moele by rigi cylinrical ro. Flow fiel is measre ner the conitions of sbmerge rigi ro in flme with single layer an oble layer vegetations. Experiments were performe for varios spacings of the rigi ros. The vegetation moels were aligne with the approaching flow in a rectanglar channel. Vertical istribtions of time-average velocity at varios streamwise istances were evalate sing an acostic Doppler velocimeter (ADV). The reslts inicate that, in sbmerge conitions, it is ifficlt to escribe velocity istribtion along the entire epth sing nifie fnction. The characteristic of vertical istribtion of longitinal velocity is the presence of inflection. Uner the inflection, the line is convex an groove above inflection. The interaction of high an low momentm flis cases the flow to fol an creates strong vortices within each mixing layer. Unerstaning the flow phenomena in the area srroning the tall vegetation, especially in the ownstream region, is very important when moeling or stying the riparian environment. ADV measres of rigi vegetation istribtion of the flow velocity fiel can give people a new nerstaning.. Introction Vegetation plays an important role in altering flow characteristics (sch as velocity istribtion, Reynols nmber, an Manning coefficient) compare with nonvegetate conitions in rivers []. Generally, the vertical velocity istribtion isrelateirectlytothebeshearstressfornonvegetation flow, while, for vegetate flow, it is mainly ecie by the vegetation rag since the vegetation roghness is mch larger than river be roghness []. The inflence mechanism of aqatic plant on flow is very complicate, which is epenent not only on the cross-sectional shape of river, water epth, ischarge bt also on the species, bening rigiity, istribtion, shape of vegetation, an whether it is sbmerge []. The mechanical effects of rivers with vegetation have been of primary interest for ecaes, becase of their role in environmental fli mechanics. A nmber of sties on theflowantrblencecharacteristicsthroghemergent vegetation are available [ ]. Flow phenomena become more complicate when the flow epth excees the height of vegetation. Laboratory sties on flly evelope flows with sbmerge vegetation have emonstrate rece velocities within the vegetation zone [7, 8]. A series of laboratory experiments were performe to investigate the connection between sbmerge vegetation an the key geomorphology aron the vegetation. The sbmerge vegetation was moele as bnle plastic fibers with a variation in vegetation ensity. The objective of this work is to escribe the etaile characteristics of flow throgh a simlate array of rigi vegetation by examining its effects on the velocity an observing the inflence of vegetation ensity an heights. Measrements of velocity are taken along verticals at mltiple locations at ifferent sections to aeqately represent the conitions everywhere within the flow an to captre the flow response as it moves throgh the vegetation array. The experiments incle single layer oble layer experiments. An the reslts are compare.

2 The Scientific Worl Jornal Eqalizing gri Flviograph Tail gate Pmp Fixe hinge Vegetation zone Lifting screw Unergron tank Recycling tank Figre : The experimental recirclating open-channel rectangle flme setp. Section Section Section Section Section Section Section Section Section (c) Section Section () Figre : Cylinrical ro configrations an measmental locations.

3 The Scientific Worl Jornal Location Location Location Location Location (c) Location Figre : Velocity profiles of pstream, vegetation, an (c) ownstream in the configration a.. Experimental Conitions The experimental systems (Figre ) were compose of two pmps to force water throgh the system an maintain recirclation, an inlet section with trning valves at the pstreamentocontroltheflowischargeangenerate flly evelope trblent flow, an an otlet section with a trianglar ajstable weir at the ownstream en to control the water level. The flme was 7. m long,. m wie, an m high with glass-siewalls an a concrete bottom, so that the interactions betweenthevegetationanflowcolbeobserveclearly. The water levels were constant at some flow rate. The vertical velocitiesoffllyevelopeflowwithrigivegetationwere measre. The height of the rigi ro se in the arrangements an was 8 mm, an the heights in arrangements (c) an () were mm an 8 mm arrange in staggere configrations (Figre ). These experiments focse on the effects of ensity an height of rigi ro on the flow velocity. Measrement locations, shown in Figre, were chosen ownstream an pstream of rigi ro. Flow characteristics were measre at three sections, nmbere Sections to in Figre. Sectionsan,locateotsieofthevegetation area, were intene for stying the effect of rising an lowering of water level on flow characteristics. Section was intene for investigating the inflence of rigi cyclical ro on flow characteristics. At each section, velocity was measreatsixpointsstartingfrombottomofthebetothe water srface. However, e to ADV limitations an flme channel strctre, the ADV sensor sspene at cm above the flme be still measres the flow characteristics at the near be. The measrement was one at. cm increments from the be towars the free water srface.. Reslts.. Velocity Profile. The vertical istribtion of velocity with rigi vegetation col be ivie into two layers: the pper

4 The Scientific Worl Jornal Location Location Location Location Location (c) Location () Location Figre : Velocity profiles of pstream, vegetation, an (c) ownstream in the configration b. nonvegetate layer (>. Hv) an the lower rigi vegetate layer (-. Hv). The pper nonvegetate layer is from the rigi ro top (. Hv) to the water srface. The lower layer is from the be to water srface. Velocity profiles with the rigi vegetation in the first arrangement sbmerge are shown in Figres (c). Velocity profiles with the sbmerge single layer are shown in Figres (c). The velocity in vegetations is ifferent from the velocities pstream an ownstream of vegetation which is complie with the semilogarithmic formla. As seen from Figre in vegetation region, in the back of the rigi ro, the velocity is slower (location an location ) an the slowest velocities approximately are at V =.. This is similar to the velocity profile immeiately behin a owel for sbmerge vegetation flow shown at location an location in Figre an that of other researchers (e.g., Shimiz an Tsjimoto, 99; Lopez an Garcia, ; an Righetti an Armanini, ). Velocityprofileswiththesbmergesinglelayerare shown in Figres (). This is similar to the velocity in configration a (Figre ). As seen from Figre in vegetation region, in the back of the rigi ro, the velocity is slower (location, location, an location ), the slowest velocities approximately are V =., an the shape of the velocity is S, while, in the pstream an the ownstream, the velocity complies with the semilogarithmic formla. Bt becase of the inflence of the vegetation, in the ownstream region, the lowest velocity is in the mile of the flme (location ); the fastest velocity is in location an location, which can be seen in Figre (). Velocity profiles with the short rigi ro sbmerge are shown in Figre an Figre. AsseenfromFigre, in

5 The Scientific Worl Jornal Location Location Location Location Figre : Velocity profiles of pstream an vegetation in the configration c Location Location Location Location Figre : Velocity profiles of pstream an vegetation in the configration. oble layer flows, the fastest velocities are in the lower free stream region (locations ) an the slowest velocities are in the region immeiately ownstream of a tall rigi ro (locations ). The velocities jst besie a rigi ro (locations,, an ) are approximately miway between the fastest an slowest velocities. These reslts are consistent with observations mae from the sbmerge single layer experiment (Figre an Figre).. Conclsions In this paper, flows throgh oble layer vegetation an single layer vegetation are simlate in flme. The flows in oble layer vegetation are more complex than in the single layer case. Some conclsions can be rawn as follows. () The existence of sbmerge plants increases the flow graient,antheroghnessofthebesrfacebecomes larger, which plays the role of spoiler, an with increasing plant ensity is more obvios. () With the increasing of plant ensity, backwater phenomenon exists an is significant. Bt with the increasing of water epth, the phenomenon is weakene. Therefore, the calclation of flow over the river containing plant mst consier the impact of plant resistance. () In sbmerge conitions, it is ifficlt to escribe velocity istribtion along the entire epth sing nifie fnction. The characteristic of vertical istribtion of longitinal velocity is the presence of inflection. Uner the inflection, the line is convex an groove above inflection. () The interaction of high an low momentm flis cases the flow to fol an creates strong vortices within each

6 The Scientific Worl Jornal mixing layer. Unerstaning the flow phenomena in the area srroning the tall vegetation, especially in the ownstream region, is very important when moeling or stying the riparian environment. Conflict of Interests The athors eclare that there is no conflict of interests regaring the pblication of this paper. Acknowlegments This sty was spporte by the Natral Science Fonation of Hebei Province (E), Open Fonation of State Key Laboratory of Hyrology-Water Resorces an Hyralic Engineering (9), an the Program for Hanan Science an Technology Research an Development (9-). References [] Y. Li, Y. Wang, D. O. Anim et al., Flow characteristics in ifferent ensities of sbmerge flexible vegetation from an open-channel flme sty of artificial plants, Geomorphology, vol., pp.,. [] W.Hai,Y.H,Y.Zeng,anJ.Han, Velocityistribtionfor open channel flows with sspene vegetation, Avances in Water Resorces,vol.9,pp.,. [] L.-H. W an X.-L. Yang, Factors inflencing bening rigiity of sbmerge vegetation, Hyroynamics, vol., no., pp. 7 79,. [] T.Fischer-Antze,T.Stoesser,anN.R.B.Olsen, Dnmerical moelling of open-channel flow with sbmerge vegetation, Hyralic Research,vol.9,no.,pp.,. [] J.M.Le,H.C.Chan,Y.Jia,Z.He,anS.S.Y.Wang, Ctting management of riparian vegetation by sing hyroynamic moel simlations, Avances in Water Resorces,vol.,no., pp.99 8,8. [] D. Naot, I. Nez, an H. Nakagawa, Hyroynamic behavior of partly vegetate open channels, Hyralic Engineering,vol.,no.,pp.,99. [7] M. Ghisalberti an H. M. Nepf, Mixing layers an coherent strctres in vegetate aqatic flows, Geophysical Research: Oceans, vol. 7, no., pp. - -,. [8] M. Ghisalberti an H. M. Nepf, The limite growth of vegetate shear-layers, Water Resorces Research, vol., no. 7, Article ID W7,.

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