A model to predict surface gas transfer rate in streams based on turbulence production by aquatic vegetation

Abstract

Turbulence generated by aquatic vegetation in lakes, estuaries, and rivers can significantly alter the flow structure throughout the entire water column, affecting gas transfer mechanisms at the air-water interface, thus modifying indicators of water quality. A series of laboratory experiments with rigid, acrylic cylinder arrays to mimic vegetation was conducted in a recirculating Odell-Kovasznay type race-track flume. Particle Image Velocimetry was used to characterize mean and turbulent flow statistics, to investigate the effect of emergent and submerged vegetation on gas transfer rate in terms of turbulent kinetic energy (TKE), Reynolds stresses, and TKE production. Surface gas transfer rates were determined by measuring dissolved oxygen concentration during re-aeration using an optical sensor. The results provided new insights 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 TKE production in each scenario is discussed, and a modified surface renewal model using TKE production as an indicator of gas transfer efficiency is developed to more accurately predict surface gas transfer rates in vegetated streams.