Seepage Intensities Increase Roughness in Steady Non-Uniform Flow Within Rigid Cylindrical Vegetation

Theories for flow conveyance within aquatic vegetation have predominantly been developed for impermeable beds thereby overlooking key interactions between hydraulic and hydrologic drivers. To begin addressing this overlook, the hydrodynamic properties of emergent rigid cylindrical vegetation covering a pervious gravel bed are experimentally explored under varying planar vegetation densities ((Formula presented.), 0.150, and 0.096 (Formula presented.) (Formula presented.)) and controlled seepage intensities (Formula presented.), where seepage flow (Formula presented.) constitutes approximately 0%–95% of the inflow (Formula presented.). The focus is on longitudinal variations of the roughness coefficient as impacted by variations in (Formula presented.) and (Formula presented.). The vegetation-gravel system is represented by cylindrical rods in a laboratory flume where the water surface profiles were imaged for different configurations of (Formula presented.) and (Formula presented.) but at a constant inflow rate. The longitudinal variations of the effective vegetation drag coefficient and the equivalent Manning roughness (Formula presented.) were inferred from a modified Saint-Venant equation where (Formula presented.) was included in the continuity as a mass sink and in the horizontal momentum balance as analogous to a mean vertical advection. The (Formula presented.) exhibited three distinct longitudinal trends: gradual decrease, initial increase followed by a decrease, and initial decrease followed by an increase. These trends were then explained by the pressure term, longitudinal acceleration term, and the newly proposed seepage term. Under low-to-medium seepage conditions, the pressure term-dominated the drag coefficient as expected. However, under high seepage conditions, the seepage term intensified, leading to effective drag values overshooting their pressure-term counterparts. These findings indicate structural modifications in the resistance mechanisms occur owing to seepage-induced vertical momentum exchanges.

Duke Scholars

Author Gabriel G. Katul Civil and Environmental Engineering