| We have conducted study on developing 3D water quality simulation model for an oxbow lake close to urban district. Our research field is the Barato River which is an oxbow of the Ishikari River. Pollution load have been flown into the Barato River from Sapporo metro area with a population of approximately two millions. Sediment containing much nutrient and organic materials has been accumulated on the oxbow bed. It is known that resuspension of sediment in this oxbow is a major resource of phosphorus、 which may causes water quality problems such as algal bloom. Resuspension rate is important parameter for improving water quality simulation accuracy. We have been studied simulation model for oxbow lake considering sediment movement in an oxbow.[*]As a first step、 modeling of particle movement in the Barato Rivre was investigated. We investigated resuspension rate and sedimentation rate. Resuspension rate was examined on some observation sites by collecting sinking particles into sediment traps. Correlation between resuspension rate and wind velocity was investigated. Correlation between resuspension rate and wind velocity were modeled. However、 more important parameter in our research is sedimentation rate. Because resuspension rate depends on fall velocity of particles. Generally、 fall velocity of sediment follows Stokes low of soil. However、 particles in the Barato River contain much organic materials and density of sediment is smaller than soil. Actual data of particles fall velocity was needed for modeling process. Sedimentation rate was investigated in two different methods: sediment trap observation and sedimentation tests. Sedimentation tests were conducted to many kinds of samples; sediment material、 high-turbidity water near oxbow bed and river water. 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 by considering particle size. The resuspension and sedimentation model was developed based on these series of observation and tests.[*]As a second step、 water quality simulation model for the Barato River was investigated. The Barato River has not only complicated sediment particle movement but also complicated hydraulic and geometric conditions. This oxbow is in tidal compartment and has two bottlenecks. It is separated into three areas which has different hydraulic characteristics. Additionally、 because the oxbow is once a meandering river channel、 wind affect to oxbow stream may change on longitudinal or cross-sectional directions. Water quality simulation based on 3-D model was needed for analyzing hydraulic and water quality conditions. 3-D simulation model with mixed-size sediment particle for turbidity calculation were developed. 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 and simulation will be needed. |
