| We have conducted study on developing water quality simulation model for an oxbow lake close to large city. Pollution load have been flown into the oxbow. So the sediment containing much nutrient and organic matter has been accumulated on the bed. It is well known that resuspension of sediment is a major resource of phosphorus、 which once caused algal bloom in this oxbow. Therefore、 resuspension rate and sedimentation rate are important parameters for improving water quality simulation accuracy. We tried to quantify these parameters. We had conducted field observation and laboratory test to understand mechanism and process of resuspension and sedimentation. Mass of resuspension was examined on some observation sites by collecting falling particles into sediment traps. Correlation between resuspension rate and wind velocity was investigated. Correlation between resuspension rate and wind velocity is confirmed in upstream site. However、 it was not confirmed in downstream site. Sedimentation rate was investigated in two different methods: sediment trap observation and bed material sedimentation test. Correlations among sediment particle size、 density and fall velocity were examined. Sedimentation rates have little correlation with neither particle size nor density in sediment trap test. But in sedimentation tests、 falling process of particle can be classified into three different motions. (1)Particles larger than 0.075mm (defined as sand) reach bed rapidly so they can be assumed to follow Stokes’ law of soil. (2)Particles smaller than 0.010mm falls very slowly so they can be assumed to keep suspended and can be treated as base turbidity. (3)Other particles obey to Stokes’ law、 but density was smaller than soil and it depends on particle size. The resuspension and sedimentation model was developed based on these tests. Next water quality simulation was based on 2-D vertical model was conducted. Mixed-size sediment particle was considered in turbidity calculation. Simulation can reproduce long-term turbidity generally. However、 accuracy of simulation is still insufficient in short-term turbidity change in some cases. Further discussion on particle movement modeling will be needed. |
