Study of Two-Dimensional Shear Effects on Flow and Pollutant Transport in Meandering Channels

다국어 초록 (Multilingual Abstract)

In this study, the depth-averaged two-dimensional model was used to simulate the shear effects on meandering river flows and pollutant transport with reasonable accuracy and efficiency. To validate the numerical model, large-scale experiments were conducted in the River Experiment Channel (REC) large-scale meandering channels, in which the data of flow by acoustic Doppler current profiler (ADCP) measurement and pollutant transport by tracer tests in meandering channels were collected. Using velocity measurements, the effect of the secondary current on the primary flow distribution was analyzed. First, the relations of the secondary flow strength to the depth to radius-of-curvature of the channel and channel roughness were found. Then, the vertical profile equation for the secondary flow was developed reflecting the nonlinear term effects on the secondary flow which were omitted in the previous studies. The proposed equation generated a vertical profile that showed a decrease in the maximum secondary flow strength at the top and bottom of the profile, which is different from the existing equations.
The proposed velocity profile equation was inserted into the momentum equations with the dispersion stress method for the two-dimensional flow solver HDM-2D, in order to induce the shear effect of secondary flow, which is normally neglected in the depth averaging process. The simulation results using the proposed equation to the dispersion stress method showed that the simulation of the Rozovskii channel showed improvement over the dispersion stress model using deVriend and Kikkawa’s velocity equation, which was based on the linear behavior between the secondary flow and primary flow, as well as over the model with no-dispersion stress in the distributions of primary flow velocity. The validation results with REC meandering channels revealed that the 2D hydrodynamic model with dispersion stress term gave a better fit of primary flow distribution to the experimental data than the simulation without the dispersion stress term.
To find the characteristics of pollutant transport in meandering channels, the two-dimensional dispersion coefficients were calculated using the vertical velocity profiles which represents shear dispersion from the ADCP measurements, and the results were compared with the values calculated by applying 2D stream-tube routing procedure to the concentration curves obtained from 2D transient tracer experiments. The velocity driven two-dimensional dispersion coefficient in rivers which was calculated using the shear flow dispersion coefficient equation developed by Fischer et al. (1979) is easy to obtain since it did not require the tracer test experiments. The results showed that non-dimensional longitudinal dispersion coefficient by velocity profile ranges from 4 to 6 which is close to Elder’s result, while non-dimensional transverse dispersion coefficient ranges from 0.05 to 0.4. However, the dispersion coefficients calculated using the 2D stream-tube routing procedure were quite large: 4-5 times larger than velocity driven values for longitudinal dispersion coefficients, 1-3 times larger for the transverse dispersion coefficients. These differences could be explained by the fact that the concentration-driven dispersion coefficient included the mixing effects due to the irregularities of the channel, storage zones and numerical dispersion while velocity-driven coefficients only accounted for shear flow effects. Then, CTM-2D advection-dispersion model was applied to simulate mixing in meandering channels and the dispersion stress term improved accuracy by 1%. The calibrated dispersion coefficients by the CTM-2D model were between the velocity-driven results and concentration driven results. The simulation results proved the applicability of the CTM-2D model in reproducing concentration curves in meandering channels.

목차 (Table of Contents)

• 1. Introduction 1
• 1.1 Research Background and Necessity 1
• 1.2 Objectives and Scope 8
• 2. Theoretical Research 11
• 2.1 Meandering Channel Classification 11
• 2.2 Theoretical Concept of Secondary Flow 13
• 2.3 Vertical Profile Equation of the Secondary Flow 17
• 2.4 Methods of Secondary Flow Application 24
• 2.4.1 Secondary Flow Modeling 24
• 2.4.2 Moment of Momentum Method 31
• 2.4.3 Dispersion Stress Method 35
• 2.5 Dispersion Coefficients 43
• 2.5.1 Theoretical and experimental approaches 43
• 2.5.2 Velocity driven dispersion coefficient 56
• 2.5.3 STRP method 58
• 3. Model Development 64
• 3.1 2D Shallow Water Model 64
• 3.1.1 Governing Equations 64
• 3.1.2 Dispersion stress method 66
• 3.2 2D Advection Dispersion Model 70
• 4. Field Experiments 71
• 4.1 Field Site 71
• 4.2 Experimental Set-up 74
• 4.3 Data Collection 76
• 4.4 Analysis for Velocity Data 81
• 4.5 Analysis for Concentration Data 95
• 5. Derivation of the Transverse Velocity Profile 103
• 5.1 Development of the Velocity Profile Equation 103
• 5.2 Validation of Profile 110
• 6. Applications of 2D Flow Model 115
• 6.1 Development of DS Model 115
• 6.2 Laboratory Channels 118
• 6.2.1 SNU M2 Channel 118
• 6.2.2. Shumate Channel 122
• 6.2.3 Rozovskii Channel 129
• 6.3 REC Channels 134
• 6.4. Application to Virtual Meandering Channels 143
• 7. Derivation of Dispersion Coefficients 148
• 7.1 Dispersion Coefficient Derivation using Velocity Profile 148
• 7.2 Dispersion Coefficient Derivation using Concentration Curves 163
• 7.3 Empirical Equation for Dispersion Coefficients 169
• 8. Application of 2D Mixing Model 175
• 8.1 Calibration of Dispersion Coefficients 175
• 8.2 Concentration Distributions 188
• 9. Conclusions and Future Study 194
• References 198
• Appendix 211
• A.1 Velocity distributions for Andong Channel Experiment (R315-2) 211
• A.2 Velocity distributions for Andong Channel Experiment (R317-2) 214
• A.3 Concentration data for Andong Channel Experiment (R315-2) 217
• A.4 Concentration data for Andong Channel Experiment (R317-2) 239
• 국문초록 273