From Substrate to Surface: A Turbulence-based Model to Predict Interfacial Gas Transfer across Sediment-water-air Interfaces in Vegetation Streams with Sediments

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The experimental result shows that turbulence generated from the bed does not affect the surface transfer process directly.However, the near-bed suspended sediment provides a negative buoyancy term that reduces the transfer efficiency according to the predictions by a modified Surface Renewal model for vegetated flows.The measured interfacial transfer fluxes across the SWI show a clear dependence on the within-canopy flow velocity, indicating that bed shear turbulence and within-canopy turbulence are critical indicators of transfer efficiency at SWI in 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 the SWI in flows with aquatic vegetation.Our study provides critical insight for future studies on water quality management and ecosystem restoration in natural water environments such as lakes, rivers, and wetlands.

Abstract
• Turbulence generated by aquatic vegetation can alter flow structures throughout the whole water column, affecting gas transfer mechanisms across the air-water and sediment-water interfaces (Fig. 1).
• The experiment result shows that turbulence generated from the bed does not affect the surface transfer process directly, but the near-bed suspended sediment reduces the gas transfer efficiency.
• A new Reynolds number dependence model using nearbed turbulent kinetic energy as an indicator is proposed to provide a universal prediction for the interfacial flux across the SWI in vegetated flows.

Methodology
• Experiments were conducted in a recirculating race-track flume with staggered arrays of rigid cylinders (  = 0.64 ) to mimic aquatic vegetation (Fig. 2).

Department of Civil and Environmental Engineering, University of Illinois at Urbana-Champaign
From Substrate to Surface: A Turbulence-based Model to Predict Interfacial Gas Transfer across Sediment-water-air Interfaces in Vegetation Streams with Sediment • Vegetation-generated turbulence drives both air-water and sediment-water interfacial gas transfer.
• The near-bed suspended sediment concentration due to higher turbulent kinetic energy reduces surface gas transfer rates.
• A new Reynolds number dependence model based on turbulent kinetic energy provides consistent predictions for sediment-water interfacial gas transfer fluxes.

Conclusions
Contact information: cytseng2@illinois.eduØ Stem-scale turbulence dominates the mixing and exchange processes within the canopy.
Ø Canopy-scale turbulence dominates the mixing and exchange processes above the canopy.
CT acknowledges funding support from Taiwan-UIUC Fellowship.This study was supported by NSF through CAREER EAR 1753200.Any Opinions, findings and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect those of the National Science Foundation.• By using Sodium Sulfite (Na 2 SO 3 ) as an oxygen depletion agent, surface gas transfer rates can be fitted by DO reaeration curves in water.

Figure 1 .Figure 4 .
Figure 1.Sketch of interfacial gas transfer in vegetated flows and the associated dissolved oxygen concentration profile.

Figure 2 .
Figure 2. (a) Top-view sketch of the recirculating flume.(b) Side-view sketch of the test section (not to scale).(c) The vegetation array configuration, where the green dashed line shows the location of the PIV laser slice focusing on the center of the array.

Figure 3 .
Figure 3. (a) The fittings of re-aeration process for the surface gas transfer rate,  " , estimation.(b) The relation between surface gas transfer rate,  " , and timeaveraged mean flow velocity, .

Figure 6 .-ØØFigure 7 .
Figure 6.The linear fitting results of the emergent (left) and submerged (right) canopy data by the modified SR model.The critical stem-scale Reynolds number, Re dc , for the emergent case shifts from 200 on a fixed smooth bed(Tseng & Tinoco, 2000)  to 300 on a sediment bed, while the critical mean flow Reynolds number, Re Hc , shifts from 7000 on a fixed smooth bed(Tseng & Tinoco, 2000)  to 9000 on a sediment bed.

Figure 8 .
Figure 8.Comparison of (a)  ' -dependence model (Voermans et al., 2018) (b) the proposed  (') -dependence model for the sediment-water interfacial gas transfer diffusion coefficient,  #$$ with previous observation data (bare-bed open-channel flows) and the current experimental data (vegetated flows).The three regimes: molecular, dispersion, and turbulence are separated by the dotted vertical lines.The current experimental data in vegetated flows are represented by the color markers (red circles).