Canopy Randomness, Scale, and Stem Size Effects on the Interfacial Transfer Process in Vegetated Flows

Aquatic vegetation plays an important role in natural water environments by interacting with the flow and generating turbulence that affects the air-water and sediment-water interfacial transfer. Regular and staggered arrays are often set as simplified layouts for vegetation canopy to study both mean flow and turbulence statistics in vegetated flows, which creates uniform spacing between vegetation elements, resulting in preferential flow paths within the array. Such preferential paths can produce local high velocity and strong turbulence, which do not necessarily happen in natural environments where vegetation is randomly distributed. How the randomness of the canopy affects interfacial processes by altering spatial turbulence distribution, which can potentially lead to different turbulence feedback on the interfacial transfer process, remains an open question. This study conducted a series of laboratory experiments in a race-track flume using rigid cylinders as plant surrogates. Mean and turbulent flow statistics were characterized by horizontal- and vertical-sliced PIV. Based on the measured flow characteristics under different stem diameters and array configurations, we propose a method to quantify the randomness of the vegetation array and update a sediment-water-air interfacial gas transfer model with the randomness parameter to improve its accuracy. The updated model agrees well with the dissolved oxygen experimental data from our study and data from existing literature at various scales. The study provides critical insight into water quality management in vegetated channels with improved dissolved oxygen predictions considering vegetation layout as part of the interfacial transfer model. Aquatic vegetation plays an important role in natural water environments by interacting with the flow and generating turbulence, which affects the interfacial gas transfer across the air-water and sediment-water interfaces. Researchers often used regular and staggered arrays as simplified vegetation layouts in laboratory experiments to study the flow hydrodynamics of vegetated channels. However, such regular vegetation pattern creates uniform spacing between stem elements, resulting in preferential flow paths within the canopy, which is unrealistic in nature where plants are randomly distributed. To understand the discrepancies between the idealized and the actual field cases, laboratory experiments were conducted in a race-track flume using arrays of rigid cylinders as plant surrogates. We investigate the effects of randomly distributed vegetation on hydrodynamics and how spatial heterogeneity can alter turbulence feedback on interfacial transfer processes. A randomness index was proposed based on the measured flow characteristics under different stem diameters and array configurations to update previous interfacial transfer models. The study provides helpful insight into water quality management in vegetated channels, with improved dissolved oxygen predictions via a more accurate and universal interfacial transfer model with different vegetation distribution patterns. An index based on lateral flow variations is proposed to quantify the randomness in the distribution of vegetation elements in a canopy Layout of emergent vegetation does not affect surface gas transfer rates, as the average of the horizontal-shear turbulence remains the same Contributions from coherent structures by flow-stem-bed interaction need to be corrected by the randomness index for sediment-water transfer