Quantifying the Effect of Aquatic Vegetation on Interfacial Gas Transfer
in Streams
Abstract
Turbulence generated by aquatic vegetation in rivers, lakes, and
estuaries, can significantly alter the flow structure throughout the
whole water column, affecting gas transfer mechanisms at the air-water
interface, driving changes in indicators of water quality. We conducted
a series of laboratory experiments with rigid cylinder arrays to mimic
vegetation using a staggered configuration in a recirculating
Odell-Kovasznay type race-track flume. 2D planar Particle Image
Velocimetry (PIV) was used to characterize the mean flow field and
turbulent flow statistics, to characterize the effect of emergent and
submerged vegetation in terms of turbulent kinetic energy, Reynolds
stresses, and turbulent shear production. The surface gas transfer rate
was determined by measuring the dissolved oxygen (DO) concentration
during the re-aeration process in water based on the methodology
proposed by the American Society of Civil Engineers (ASCE). Our data
provide new insight on how stem- and canopy- scale turbulence affect the
surface gas transfer rate at different submergence ratios and array
densities. The relation between mean flow velocity and turbulent shear
production in these scenarios is used to develop a modified surface
renewal (SR) model using turbulent shear production as an indicator of
gas transfer efficiency, which allows us to more accurately predict
surface gas transfer rates in vegetated flows.