Effect of floodplain submergence and riparian vegetation on flow dynamics and mass transport near a spur dike

Spur dikes, designed for river flow diversion, alter flow dynamics by creating low-velocity dead zone storages that influence the longitudinal dispersion of contaminants. Seasonal variations in flow depth also modify flow patterns and mass transport around the spur dike. This study investigates three-dimensional (3D) flow and mass transport near a spur dike in a straight channel using numerical simulations. Three configurations were examined: an unsubmerged spur dike, a submerged spur dike with overbank flow, and a submerged spur dike with floodplain vegetation. The results show that in unsubmerged cases, streamwise velocities accelerate in the main channel region due to blockage by the spur dike, with a strong recirculation behind it. The shear layer is stronger but positioned far from the spur dike, leaving it uninvolved with a long, low-velocity zone. On the other hand, in submerged cases, the recirculation length is reduced, and vertical velocity gradients are strengthened. Due to horizontal and vertical separation, a horizontal shearing vortex forms over the spur dike, alongside the typical horseshoe and tube vortices. The shear layer extends into the recirculation zone behind the spur dike, promoting rapid mass mixing. Finally, floodplain vegetation was found to cause flow instabilities that grow with stem density, affecting flow and transport. Scalar transport analysis revealed stronger anomalous transport in unsubmerged flows, with submergence and vegetation reducing this behavior and accelerating scalar decay. This analysis of spur dike-induced interactions between the vegetated floodplain and main channel offers insights into restoring flow dynamics in low-velocity zones.