Influences of Momentum Ratio on Transverse Dispersion for Intermediate-Field Mixing Downstream of Channel Confluence

Abstract

This study aims to analyze the influences of momentum ratio (Mr) and confluence angle (α) on the transverse dispersion in an urban scale confluence channel from the numerical simulation results using the Environmental Fluid Dynamics Code model. By changing the momentum flux and confluence angle from the simulation results, the analysis focused on the relations between the vertical variations of transverse velocity and transverse dispersion. The high momentum tributary aligned the mixing interface toward the outer bank and created a strong helical motion, which transported the contaminated water along the channel bed and inflows into the recirculation zone. The high momentum ratio induced the large vertical shear in transverse velocity with a strong helical motion and increased the transverse dispersion. However, the helical motion persistence rapidly decreased as the flow reached downstream and led to a decrease in the transverse dispersion for the large confluence angle. Thus, the transverse dispersion coefficient increased with a high momentum ratio and low confluence angle, and the dimensionless transverse dispersion coefficient was in the range of 0.39–0.67, which is observed in meandering channels, for Mr > 1 and α = 45°.

An Investigation of Contaminant Transport and Retention from Storage Zone in Meandering Channels

“…After reaching the peak, each curve descends at different rates, with the higher discharge rates generally declining more quickly, which may suggest that the substance stays shorter in the recirculation zone at higher discharge rates. This may possibly be due to the greater momentum generated by higher velocity overcoming the recirculating motion [36], which may retain the contaminant shorter before it is transported out of the zone. Additional analysis was carried out, with the results shown in Figure 14.…”