بررسي تاثير پوشش گياهي در كف و ديوارهاي كانال بر نيمرخ سرعت جريان و برآورد ضريب زبري

چكيده انگليسي :

Investigation of flow velocity profile and friction factor in channel with vegetated bed and walls Razieh Moghbel r moghbel@ag iut ac ir March 7 2009 Department of Water Engineering Isfahan University of Technology Isfahan 84156 83111 IranDegree M Sc Language FarsiHosein Afzalimehr hafzali@cc iut ac irAbstractEvaluating flow resistance in a straight vegetated channel needs to take into account both the effects ofthe hydraulic cross section reduction and the dissipative effects due to the presence of the roughnesselements Plants over the bed and along the banks of rivers can increase the effective flow resistance Also these canopies have a positive influence on water quality sediment transport and habitat diversity This study presents the results of a laboratory flume experimental study on the interaction ofunsubmerged riparian vegetation and vegetation over the bed on the flow velocity Reynolds stressdistributions resistance to flow and turbulent intensity The laboratory experiments were accomplished in a rectangular 0 6 m wide and 20 m long channel Allthe experiments were accomplished in uniform condition and for four states of channel covering A gravel bed with glass walls B gravel bed with vegetated walls C grassy bed with vegetated wallsD grassy bed with glass walls Measuring were carried out in three sections in every run and for threedifferent distances from the wall Totally 90 velocity profiles in 10 runs were measured with vectrino This instrument measures water speed using the Doppler effect by transmitting short pairs of soundpulses listening to their echoes and ultimately measuring the change in pitch or frequency of thereturned sound In this research the log law fits reasonably for the data near the gravel bed and deviates in outer region ofthe flow Because the equivalent sand roughness ks represents a hydraulic roughness scale incorporatingall influences of a rough bed on the flow we expect that for vegetated bed the amount of that is differentfrom gravel bed In this study the equation presented by Stephan 2002 was used for logarithmic lawover vegetated bed The results indicate that the location of the maximal velocity Umax has more distance from the freesurface in the presence of vegetation in walls and the difference between the umax and the velocity at thewater surface is considerable in particular in distance 5cm from the wall So for uniform flow over gravelbed and near the vegetated cover the location of Umax depends on the distance from the wall In flow overgrass vegetation cover with vegetated wall the difference between the umax and the velocity at the watersurface is smaller than in flow over gravel bed with vegetated walls The results of this study shows that in presence of vegetation over the bed and along the walls theReynolds stress distribution isn t linear and follows a parabolic shape in the most profiles Turbulence ofthe flow and the presence of secondary currents in the channel also contribute to the non uniformity of theshear stress distribution For flow over vegetated bed the Reynolds stress reaches to its maximum valueover the deflected plant height in converse the past researches In this section the results of this studyhave more similarity with Wilson s researches The additional surface area of the fronds in Wilson s studyshifted the peak Reynolds stress to a higher level in the flow but in this study the high accumulation ofwheat stem shifts the maximum Reynolds stress to a level above the canopy The estimation of friction factors is an important aspect of work dealing with conservation of land andhydraulic structures on banks and bay shores and also the calibration and validation of river hydraulicmodels In this study we obtained manning friction factor from every velocity profile for comparisonwith amounts suggested with Chow 1959 The similarity in the range of the values obtained in this studyand Chow 1959 was observed Also vegetated bed and walls had the most resistance to flow and gr

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بررسي تاثير پوشش گياهي در كف و ديوارهاي كانال بر نيمرخ سرعت جريان و برآورد ضريب زبري

رودخانه , مقاومت جريان , تنش رينولدز