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Abstract Dissolved Oxygen (DO) fluxes across the air‐water and sediment‐water interface (AWI and SWI) are two major processes that govern the amount of oxygen available to living organisms in aquatic ecosystems. Aquatic vegetation generates different scales of turbulence that change the flow structure and affect gas transfer mechanisms at AWI and SWI. A series of laboratory experiments with rigid cylinder arrays to mimic vegetation was conducted in a recirculating race‐track flume with a lightweight sediment bed. 2D Planar Particle Image Velocimetry was used to characterize the flow field under different submergence ratios and array densities to access the effect of vegetation‐generated turbulence on gas transfer. Gas transfer rate across AWI was determined by DO re‐aeration curves. The effective diffusion coefficient for gas transfer flux across SWI was estimated by the difference between near‐bed and near‐surface DO concentrations. When sediment begins to mobilize, near‐bed suspended sediment provides a negative buoyancy term that increases the critical Reynolds number for the surface gas transfer process according to a modified Surface Renewal model for vegetated flows. A new Reynolds number dependence model using near‐bed turbulent kinetic energy as an indicator is proposed to provide a universal prediction for the interfacial flux across SWI in flows with aquatic vegetation. This study provides critical information and useful models for future studies on water quality management and ecosystem restoration in natural water environments such as lakes, rivers, and wetlands.
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Canopy Randomness, Scale, and Stem Size Effects on the Interfacial Transfer Process in Vegetated Flows
Abstract 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.
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- Award ID(s):
- 1753200
- PAR ID:
- 10494946
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Water Resources Research
- Volume:
- 60
- Issue:
- 3
- ISSN:
- 0043-1397
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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