A MODEL FOR DIFFUSION WITHIN EMERGENT VEGETATION

Detailed velocity measurements made with laser Doppler velocimetry hav e shown that, except very close to the bed, the production of turbulen ce within a stand of emergent vegetation is dominated by the stem wake s rather than by the bottom-boundary shear, as in open-channel flows. This observation formed the basis for a modified random-walk model tha t describes the contribution of stem wakes to the turbulent diffusivit y within marsh passes. The model was validated by comparison to observ ed diffusivity over a range of population and flow conditions within a simple plantlike array of circular cylinders. The diffusion model was also evaluated for a more complex morphology that included a flexible canopy. Laser-induced fluorescence and image-processing techniques we re used to measure the diffusivity as well as to examine turbulence st ructure within the experimental system. The latter analysis documented changes in turbulence scale that arise as larger eddies are broken ap art by the stems and smaller eddies (comparable to the stem diameter) are produced within the wakes.