Experiment Of Bed Resistance And Turbulent Characteristics Above Biofilm-Coated Bed

Water eutrophication in China has caused severe environmental problems and has been given more and more attention in recent years,which provides adequate nutrients for the reproduction and growth of the microorganisms in the water column.In natural environment,vast majority of microorganisms are living as aggregates,called biofilms,forming on the solid-water surfaces in marine,rivers,lakes and wetlands.Sediments consisted of the boundary of rivers and lakes provide excellent substrata for the attachment and growth of microorganisms.Biofilms growing on the sediment bed will change the geometrical,physical and bio-chemical properties of the sediment mixture and single particles,such as the wet and dry bulk density,the microtopography of the particle surface and the adsorption characteristics.Furthermore,the characteritics of the sediment erosion modes,erosion threshod and settling will be changed under a given flow condition.Thus the characteristics of sediment transport and its environment effect will be quite different from that of sediment bed without biofilm.Therefore the study on the bio-sediment under the current environment situation will not only expand the research of traditional sediment theory,but enhance the interdisciplinary study of hydraulic engineering and environmental science.In this study,flume experiments were conducted in the laboratory to study(1)the bedform characteristics and bed resistance of the bio-sediment bed and(2)turbulent characteristics of the flow on the bio-sediment bed and the effect of the changing turbulent structure,comparing with the identical flow condtition on the sediment bed without biofilm,on the sediment concentration near the bed and suspended sediment distribution along water depth.Aiming the first problem,experiments were done to obtain the bedforms,its dimenions and the corresponding flow conditions.Then the method from the tradition sediment study for noncohesive sediment was adopted and classified the bedforms of bio-sediment bed as dunes.The relationship between the dimension of the bedform and flow condition has been established empirically according to the experiment data.Moreover,the relationship between the hydraulic roughness and dimensions of the bedform has been proposed,and the further proposed was the method to predict the bed resistance.Results show that dimensions of the bedform and bed resistance are reduced by the biofilm,comparing with that of nocohesive sediment bed without biofilm under an identical flow condition.Aiming the second problem,experiments were conducted to compare flow properties on bio-sediment bed with that of clean sediment bed,which including the distribution of time average velocity,Reynolds shear stress,turbulent intensity and turbulent kinetic energy flux,the balance of the turbulent energy and the change of turbulent bursting events.Results show that the time average velocity increases as the biofilms grow on the bed and the change of turbulent chacteristics inhibit the sediment entrainment and suspension of the sediment particles or flocs.Futher anlysis on the distribution of suspended sediment concentration,entrainment of the sediment flux and vertical diffusion coefficient indicates that,after biofilms growing on the bed biosediment particles or flocs assemble towards the bottom and thus increasing the sediment concentration near the bed,entrainment flux for bio-sediment is less than clean sediment when the flow power is low,however up a certain level the entrainment flux for bio-sediment increases to be towfold that of clean sediment.Finally,the change of vertical diffusion coefficient enhances assemble of bio-sediment towards the bottom.