Flow hydrodynamics in tidal marsh canopies

The transport of particulate and dissolved matter on the surface of coastal marshes is controlled by the hydrodynamic characteristics of over-marsh flows. High-frequency (5 Hz) in situ measurements of flow speed were collected in Spartina alterniflora, Juncus roemerianus, and Distichlis spicata canopies using hot-film anemometry sensor arrays. These data indicate that mean flow speed, turbulence intensity, and the shape of the vertical speed profile are influenced by variations in plant morphology and stem density. Mean flow speed and turbulence intensity are inversely related to stem density and to distance from the creek edge. Flow energies decrease by about one order of magnitude when flows encounter the vegetated marsh surface and continue to decrease as vegetation density increases. Turbulent flow energy also decays exponentially with increasing distance from the creek edge. Reductions in flow speed coupled with energy decay provide a hydrologic mechanism for sediment deposition patterns commonly observed in marsh systems. Suspended matter transport is also affected by plant-flow interactions. Vertical flow structure is strongly influenced by canopy morphology (plant type and plant shape). Plant-flow interactions result in vertical speed profiles whose shapes deviate from the logarithmic profile typical in free-stream conditions and in the development of transitional flow regimes (i.e. neither laminar nor fully turbulent).