TURBULENCE STRUCTURE IN DEPTH-LIMITED, VEGETATED FLOW: TRANSITION BETWEEN EMERGENT AND SUBMERGED REGIMES

摘要

Laboratory experiments were used to explore the flow characteristics and turbulencernstructure within and above submerged vegetation. The study focused on the role ofrnwater depth in limiting shear-layer development above the vegetation, and inrnparticular on the as yet unexplored transition between submerged and emergentrnregimes. The experiments were carried out in an open channel flume with a modelrnvegetative meadow. Velocity was measured using both acoustic (threecomponents)rnand laser (two-components) Doppler velocimetry. The momentum andrnturbulence fields within the canopy were characterized by the penetration ofrnturbulent stress from the overlying flow; the balance of sweep and ejection events;rnand the relative contribution of terms to the turbulent kinetic energy budget.rnTurbulence production within the canopy was associated with two components:rngeneration within the strong shear-layer at the top of the canopy, and generationrnwithin the stem wakes. The relative importance of these sources varied withrncharacteristic depth, H/h, where H was the total flow depth and h the canopy height.rnAcross the range of H/h considered [1 (emergent) to 2.75], the transition fromrnemergent to submerged conditions was captured by a shift in penetration depth, hp,rna measure of the region within the canopy for which vertical turbulent exchange withrnthe overlying water was dynamically significant to the momentum and turbulencernstructure within the canopy.