Wind‐driven water motions in wetlands with emergent vegetation

Abstract

Abstract.Wetland biogeochemical transformations are affected by flow and mixing in wetland surface water. We investigate the influence of wind on wetland water flow by simultaneously measuring wind and surface water velocities in an enclosed freshwater wetland during one day of strong-wind conditions. Water velocities are measured using a Volumetric Particle Imager while wind velocities are measured via sonic-anemometer. Our measurements indicate that the wind interacting with the vegetation canopy generates coherent billows and that these billows are the dominant source of momentum into the wetland water column. Spectral analysis of velocity timeseries shows that the spectral peak in water velocity is aligned with the spectral peak of in-canopy wind velocity, and that this peak corresponds with the Kelvin-Helmholtz billow frequency predicted by mixing layer theory. We also observe a strong correlation in the temporal pattern of velocity variance in the air and water, with high variance events having similar timing and duration both above and below the air-water interface. Water-side variance appears coupled with air-side variance at least down to 5 cm, while the theoretical Stokes' solution predicts momentum transfer down to only 2 mm assuming transfer via molecular viscosity alone. This suggests that the winddriven flow contributed to significant mixing in the wetland water column.

Air‐water gas exchange by waving vegetation stems

“…This effect of waving on the gas transfer velocity is much smaller than the effect of wind would be in the absence of vegetation. The canopy greatly reduces wind speed [ Tse et al ., ], and the small amount of wind shear acting on the surface drives a typical gas transfer velocity around 0.1 cm/h [ Poindexter and Variano , ]. This value is negligible compared to the gas transfer velocities observed in open waters, which are certainly above 1 cm/h and typically on the order of 10 cm/h [ Crusius and Wanninkhof , ].…”

“…This coupling changes the influence of wind from direct shear, as expected on a lake, to damped bursts of momentum and to wind‐driven vegetation movement. Previous work has explored the relative importance of these momentum bursts for water column mixing and has shown they are nonnegligible for wetland environments [ Tse et al ., ]. Here we focus on wind‐driven vegetation movement and its effect on gas transport.…”

Turbulent Kinetic Energy in Submerged Model Canopies Under Oscillatory Flow

“…The interaction between the meadow and waves can also generate a mean current in the direction of wave propagation (e.g., Abdolahpour et al, 2017;Luhar et al, 2010). Previous studies have demonstrated that flow-vegetation interaction can be a significant source of turbulence within a canopy (e.g., Banerjee et al, 2015;Nepf, 1999;Tanino & Nepf, 2008;Tse et al, 2016). Stem-generated turbulence has been shown to play a role in sediment mobilization under both unidirectional current (Yang et al, 2016) and oscillatory flow (Tinoco & Coco, 2014).…”

On Factors Influencing Air‐Water Gas Exchange in Emergent Wetlands

“…Because very few systematic laboratory and field experiments have been conducted specifically to examine gas exchange in wetlands (e.g., Happell et al, ; Poindexter & Variano, ; Variano et al, ), wind speed/gas exchange parameterizations developed for lakes have been used to determine gas transfer velocity in these environments (e.g., Hagerthey et al, ). However, the relationship between wind speed and momentum flux has been found to be significantly different between open water environments, such as lakes and the ocean, and wetlands with emergent vegetation (Tse et al, ). Because of the existence of vegetation (floating, emergent, and submerged), limited fetch, and shallow depths, the relationship between wind speed and gas exchange in wetlands could be significantly different than in open waters.…”