Flow structure in depth-limited, vegetated flow

Aquatic vegetation controls the mean and turbulent flow structure in channe ls and coastal regions and thus impacts the fate and transport of sediment and contaminants. Experiments in an open-channel flume with model vegetatio n were used to better understand how vegetation impacts flow, In particular , this study describes the transition between submerged and emergent regime s based on three aspects of canopy flow: mean momentum, turbulence, and exc hange dynamics. The observations suggest that flow within an aquatic canopy may be divided into two regions. In the upper canopy, called the "vertical exchange zone", vertical turbulent exchange with the overlying water is dy namically significant to the momentum balance and turbulence; and turbulenc e produced by mean shear at the top of the canopy is important, The lower c anopy is called the "longitudinal exchange zone" because it communicates wi th surrounding water predominantly through longitudinal advection. In this region turbulence is generated locally by the canopy elements, and the mome ntum budget is a simple balance of vegetative drag and pressure gradient. I n emergent canopies, only a longitudinal exchange zone is present. When the canopy becomes submerged, a vertical exchange zone appears at the top of t he canopy and deepens into the canopy as the depth of submergence increases .