Effects of vegetation on turbulence, sediment transport, and stream morphology

Vegetation as multiple stems in various configurations or an isolated stem profoundly alters turbulent flows. Past research has shown that these alterations influence sediment transport and stream morphology, but entail complex interactions and feedbacks between flow, vegetation, and sediment processes that involve many parameters. These interactions are examined here for a variety of macrophyte patterns and scales in riverine environments. Flow Reynolds number, canopy density, and submergence ratio are just a few of the key parameters that influence the spatial variability of the flow, momentum transfer, vortex shedding and dissipation, and instantaneous stresses that are known to affect sediment and morphological processes in rivers. Knowledge gaps, though, still remain. A taxonomy that classifies vegetated flows as dense, sparse, or isolated on the basis of threshold parameters like the ratio of stem diameter to stem spacing would be useful for comparing studies among researchers and predicting likely morphological pathways. More research is needed to quantify thresholds and empirical relationships for flow-vegetation-sediment interactions so that aquatic macrophyte plantings can be used more effectively in water resource management. Field measurements of plant, canopy, and plant patch characteristics for these macrophytes would also be desirable.