Hydraulic Characteristics For Open Channel Flow With Suspended Vegetation

Floating vegetation appears in various circumstances,for example in artificial floating islands,wetlands,river courses and lakes,acting as an important part in river landscape forming and ecological restoration.Proper setting of floating vegetation can improve the water quality,biological diversity,living environment of human beings and so on.Compared with submerged and emergent vegetation,floating vegetation mainly acts at the water surface and different boundary conditions leading to complex flow characteristics.Flow in different regions may show different features similar to porous media,open channel,pipe and boundary layer flow.Previous studies adopted rigid cylinder,protruding some distance into the flow,to simulate floating vegetation and investigate flow characteristics experimentally,while research on flow under continuous carpet of floating vegetation,with undeveloped roots,is insufficient.In this dissertation,artificial lotus leaves are used to simulate such irregular vegetation patches or floating vegetation in the flume experiments.Combining the experimental results with theoretical analysis,systematic researches have been conducted to investigate the hydrodynamic characteristics of flow under interactions between floating vegetation,surface wind and channel bed roughness.For open channel flow covered by floating vegetation,the profile of velocity distribution and momentum transfer mechanism varies obviously from the free flow,and then exerts influence on the pollutant and sediment transportation.The effects of floating vegetation on shear and momentum transmission characteristics under different Reynolds number,width-depth ratio and vegetation density,etc.,are studied via flume experiments respectively.Due to the roughness of vegetation,the maximum streamwise velocity appears in the middle of the water depth,and the maximum values of Reynolds stress and turbulence intensity are near the vegetation canopy.In addition,flow structure is affected by vegetation pattern and arrangement besides the density of vegetation coverage.The flow in vegetation-dominant region presents the characteristics of "inward interaction",favoring the settlement of particles,while the flow in bed-dominant region presents the characteristics of "ejection",favoring the incipient motion of sediment.It reveals the mechanism of vegetation effects on pollutant transport and morphology.In closed water bodies such as lakes and reservoir where flow velocity is relatively small,wind-induced flow is significant.Floating vegetation can inhibit such wind-induced flow and wind-induced wave,thus affecting the transportation and spread of the sediment and pollutant.In this dissertation,the structure of wind-induced flow under floating vegetation is studied by experiments.The result shows that the vertical distribution of velocity is varied under different wind shear stresses.First,the wind-induced flow circulation is observed,and the phenomenon is coincident with former researches;surface waves appear under high wind speed and the energy of high-frequency eddies increases.Then,the effect of floating vegetation on wind-induced flow structure varies according to its density.Covered by floating vegetation,the longitudinal velocity decreases and surface wave is restrained.Only when the wind speed is relatively high or the vegetation density is relatively low will the direction of surface velocity be the same as the wind.The greater the wind speed,the larger the dominant frequency of wind-induced wave;the greater the vegetation density,the smaller the frequency and the faster the degradation of energy along the depth.In addition,when flow goes past the vegetation patches,the undulation is obvious in the front edge under the obstruction of vegetation;surface velocity plunges in the middle and stern of the patch,so as the wave fluctuation.The streamwise velocity at the back of patch even reversed,and the wake vortex is gradually generated.The wake vortex weakens and the velocity structure gradually reconverts at some location downstream the vegetation patch,which is about the same distance as the diameter of the patch.In natural lakes and river-course reservoirs,flow is driven not only by wind but also by morphology and gravity,which means the water is not still even without the wind.The hydraulic characteristics of open-channel flow affected by wind and of flow affected by both wind and floating vegetation are also investigated here.The experimental results show that flow with low velocity is more susceptible to wind effects.The velocity profile along the water depth is not the simple overlaying of that in steady flow to that in wind-induced flow,and the interactions between them should be taken into consideration.The dominant frequency of wind-induced wave in open-channel flow is greater than that in closed water body.The floating vegetation constrains the water surface wave.The characteristics of velocity,Reynolds stress,turbulence intensity and the result of quadrant analysis are thus similar to that in open channels without wind.The sensitivity degree of flow under floating vegetation to wind speed,vegetation density and pattern varies with different water depth or velocity.The energy distribution of open-channel flow under both wind and floating vegetation is distinguished from that without wind.Large-scale and small-frequency eddies contribute mainly to the kinetic energy of water.Due to surface waves,the cumulative-energy curve exists a secondary growth phenomenon in the high-frequency section.The achievements in the dissertation can provide suggestions on parameters-selection in mathematical simulations of interactions between ordinary stream flow and wind-induced flow.It also lays the theoretical foundation of future studies about the transport of pollutant,sediment,algae and eutrophication of water.The drag force exerted on floating vegetation alters the hydraulic characteristics of open channel flow.In this study the electronic stress meter was adopted to measure the floating vegetation drag force,and the boundary layer theory was used to calculate the drag force caused by floating canopy,then the measured and predicted shear stresses on the vegetation-water interface were compared and analyzed.The results indicate that it is feasible to use boundary layer theory to calculate the drag force of floating vegetation,especially to fully developed turbulent boundary layer;the frictional resistance increases with the square of depth-averaged streamwise velocity,but the pressure resistance decreases with the square of depth-averaged streamwise velocity;and the drag coefficient varies with both the length Reynolds number(based on the length of floating vegetation patch)and the height Reynolds number(based on the vertical height of the patch).In addition,the cause of resistance from vegetation cover and ice cover was analyzed comparatively,and the computational formula used to calculate the composite Manning roughness of non-fully ice-covered rivers was applied into that of floating-vegetation covered flow.The vertical distribution of streamwise velocity is the basis of the study of bed erosion and pollutant transport,which can be predicted rapidly and accurately based on the analytical solution.In this dissertation,two-power law expression was adopted to solve the vertical profile of streamwise velocity.The parameters about the roughness of vegetation and bed in analytical-solution model were discussed.The influence of the canopy gap was also discussed.In addition,the vertical distribution of shear stress was found to be linear in fully developed flow and a linear equation was established.The calculated values by analytical-solution model were in coincidence with the measured values in the experiments.Results indicate that the roughnesses caused by both vegetation and bed are interacted with each other,which cannot be simply separated apart.Vegetation canopy gap triggers obvious disturbance on flow and imposes great effect on turbulent structure near surface.