Longitudinal dispersion in open channel flow with suspended canopies

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Courtesy of IWA Publishing

Suspended canopies can cause flow disturbances such as reducing velocities within the canopy, and increasing flow beneath the canopy. Flow modifications by canopies dramatically affect the fate and transport of sediment, nutrients, contaminants, dissolved oxygen, and fauna in aquatic systems. A three-zone model is presented here to predict the longitudinal dispersion coefficient by simplifying Chikwendu's N-zone model. To validate the model, both flow field and tracer experiments were conducted using a straight rectangular Plexiglas flume, with rigid circular rods as the modeled suspended canopies. The result shows that velocities increased above the flume bed and maximized at a point between the canopies and flume bed. Above that point, streamwise velocities decreased into and within the canopies. Reynolds shear stresses were largest at the canopy interface and smallest (zero) at the velocity maximum point. Good agreement between the modeled results and experimental data shows that the model can effectively predict the longitudinal dispersion coefficient in open channels with suspended canopies.

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