Effects of Aquatic Vegetation on Gas Exchange Process Across Air-Water
and Sediment-Water Interface
Abstract
Aquatic vegetation alters the hydrodynamics of natural waters, such as
rivers, lakes, and estuaries. Plants can generate turbulence that
propagates throughout the entire water column, which affects gas
transfer mechanisms at both air-water and water-sediment interfaces,
driving changes of dissolved oxygen (DO), an important indicator of
water quality. We conducted a series of laboratory experiments with
rigid cylinder arrays to mimic vegetation using a staggered
configuration in a recirculating race-track flume. Walnut shells were
chosen as the sediment substrate, which interacts with DO in water. 2D
planar Particle Image Velocimetry was used to characterize the flow
field under various submergence ratios, highlighting the effect of
vegetation on turbulence quantities. Gas transfer rates were determined
by measuring the DO concentration during the re-aeration process based
on the methodology proposed by the American Society of Civil Engineers.
Our data provide new insight on Air-Water-Vegetation-Sediment
interactions in streams as a function of submergence ratio, array
density, and flow turbulence. A modified surface renewal model using
turbulence production as an indicator of gas transfer efficiency is used
to predict surface gas transfer rates. A delayed time of re-aeration
between the bulk and the near-bed region was observed and varies with
flow velocities and submergence ratios, which controls the oxygen flux
from water to sediment. Future studies are required to investigate the
cause of the delayed time to incorporate sediment oxygen demand in a
substrate-to-surface transfer model.