Determining aeration properties over vegetated embankments

This study examines the air–water flow characteristics of overflowing vegetated embankments through laboratory experiments with submerged natural vegetation of varying heights (3–12 cm) on an 18° slope. Double-tip conductivity probe measurements at 50 kHz and high-speed imaging were used to determine aeration properties, the inception point of air entrainment, and vegetation-induced roughness and velocities. The results show that flow-induced vegetation deflection governs roughness, with taller vegetation reaching maximum deflection even at lower flow rates, while shorter vegetation bends progressively with increasing discharge. The resulting friction factor and roughness parameter decreased with decreasing bending ratios (hd/hi). Increasing vegetation height shifted the inception point upstream under lower flow rates and enhanced air entrainment deeper into the flow. Differences in air entrainment between vegetation heights, despite similar friction factors, were linked to observed swaying motions of the deflected vegetation blades, which intensified turbulence, bubble count frequency, and interfacial area. Bubble size distributions indicated larger bubbles near the inception point and progressive breakup downstream. Findings highlight the interplay between vegetation dynamics, roughness, and aeration, and suggest further investigation into turbulence and velocity profiles within the vegetated layer to better understand swaying and bending effects on air entrainment.