Prediction of velocity profiles and longitudinal dispersion in salt marsh vegetation

Abstract

To predict the behavior of solutes and suspended particles in wetlands, it is necessary to estimate advection and longitudinal dispersion. To better understand these processes, measurements were taken of stem frontal area, velocity, vertical diffusion, and longitudinal dispersion in a Spartina alterniflora salt marsh in the Plum Island Estuary in Rowley, Massachusetts. Vegetation volumetric frontal area peaked at 0.067 Ϯ 0.007 cm Ϫ1 near 10 cm from the bed. If the velocity profile in a dense emergent marsh canopy depends on the local balance between pressure forcing and vegetation drag, the velocity will vary inversely with canopy drag (i.e., velocity is minimum where the frontal area is maximum). In fact, the minimum velocity was observed at 10 cm from the bed. The momentum balance therefore provides a way to predict the velocity profile structure from canopy morphology. The vertical diffusion coefficient also depends on canopy characteristics, such that the vertical diffusion coefficient normalized by the velocity and stem diameter had a constant value of 0.17 Ϯ 0.08 at this study site. The canopy morphology also controls the longitudinal dispersion, observed in this study to be 4 to 27 cm 2 s Ϫ1. However, theoretical considerations show that dispersion coefficients of at least 540 cm 2 s Ϫ1 can occur under typical marsh conditions. Comparisons to other canopies indicate that the prediction of the velocity profile and shear dispersion from canopy morphology can be extended to other emergent canopies and that shear dispersion may vary widely between stands with different physical characteristics.

Effects of warming and altered precipitation on plant and nutrient dynamics of a New England salt marsh

“…A more productive marsh, as we observed with drought and warming, could trap more inorganic sediments during tidal flushing (Leonard and Croft 2006) and directly contribute more organic inputs to the sediment. Although we did not observe differences in stem density among our treatments, such differences have been shown to reduce wave energy and contribute to sedimentation (Lightbody and Nepf 2006). We speculate that increased allocation to leaves or increased stem widths may have occurred, both of which could also contribute to sedimentation to a lesser degree.…”

Flow and Transport in Regions with Aquatic Vegetation

“…A self-similar velocity structure 14 was confirmed by measurements in a coastal marsh (Lightbody & Nepf 2006) and in the freshwater 15 wetlands of the Everglades (Huang et al 2008). The normalization in (9) provides an important tool 16 for extrapolating a full velocity profile from records at a single vertical position.…”

Hydrologic and Vegetative Removal of Cryptosporidium parvum, Giardia lamblia, and Toxoplasma gondii Surrogate Microspheres in Coastal Wetlands

“…However, different aquatic plants may vary in their ability to remove parasites due to distinct surface properties, unique biofilms, and differential effects on water flow and drag (36,37). Therefore, plant selection is an important consideration for wetlands managers in understanding how to best restore degraded wetlands or conserve existing ones (4,5,38).…”