Prediction of velocity profiles and longitudinal dispersion in salt marsh vegetation

TLDR

Predicting solute and particle transport in wetlands requires estimating advection and longitudinal dispersion, and theory indicates dispersion coefficients can reach at least 540 cm⁻² s⁻¹ under typical marsh conditions. The study measured stem frontal area, velocity, vertical diffusion, and longitudinal dispersion in a Spartina alterniflora marsh and modeled velocity as inversely related to canopy drag, linking pressure forcing and vegetation drag. The measurements revealed a peak frontal area of 0.067 cm⁻¹ at 10 cm above the bed, a corresponding minimum velocity, a constant vertical diffusion coefficient (0.17 cm s⁻¹ cm⁻¹) when normalized by velocity and stem diameter, and longitudinal dispersion ranging from 4 to 27 cm⁻² s⁻¹, confirming that canopy morphology predicts velocity profiles and shear dispersion, which can vary widely among different marsh stands.

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 ± 0.007 cm−1 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 ± 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−1. However, theoretical considerations show that dispersion coefficients of at least 540 cm−2 s−1 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.