Transverse and longitudinal mixing in real emergent vegetation at low velocities

F Sonnenwald,I Guymer,P West,V R Stovin,J R Hart

doi:10.1002/2016wr019937

F Sonnenwald, I Guymer + Show 3 more

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https://doi.org/10.1002/2016wr019937

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Abstract

AbstractUnderstanding solute mixing within real vegetation is critical to predicting and evaluating the performance of engineered natural systems such as storm water ponds. For the first time, mixing has been quantified through simultaneous laboratory measurements of transverse and longitudinal dispersion within artificial and real emergent vegetation. Dispersion coefficients derived from a routing solution to the 2‐D Advection Dispersion Equation (ADE) are presented that compare the effects of vegetation type (artificial, Typha latifolia or Carex acutiformis) and growth season (winter or summer). The new experimental dispersion coefficients are plotted with the experimental values from other studies and used to review existing mixing models for emergent vegetation. The existing mixing models fail to predict the observed mixing within natural vegetation, particularly for transverse dispersion, reflecting the complexity of processes associated with the heterogeneous nature of real vegetation. Observed stem diameter distributions are utilized to highlight the sensitivity of existing models to this key length‐scale descriptor, leading to a recommendation that future models intended for application to real vegetation should be based on probabilistic descriptions of both stem diameters and stem spacings.

Highlights

Natural and engineered environmental systems such as wetlands or storm water ponds typically contain vegetation and are often designed to treat pollutants [Shilton, 2000]
Validation of the 2-D Routing Equation Figure 3 shows an example upstream, downstream, and routed concentration distribution for one artificial and one real emergent vegetation test case. Both the upstream and downstream distributions are close to Gaussian and there is relatively little spread
From the 2-D Routing Analysis Figure 5 shows the mean optimized transverse and longitudinal dispersion coefficients obtained from 2-D optimization of the pulse injection data

Summary

Introduction

Natural and engineered environmental systems such as wetlands or storm water ponds typically contain vegetation and are often designed to treat pollutants [Shilton, 2000]. To protect natural systems and design effective engineered systems, it is necessary to quantify the impact of aquatic vegetation on solute mixing. Understanding mixing in these flows is relevant to a number of other environmental applications, e.g., seed dispersal [Merritt and Wohl, 2002] or nutrient transport [Nishihara and Terada, 2010]. Vegetation is characterized by small leaves and slightly larger stems, located in bodies of water much larger than a single plant, resulting in a multiscale problem that cannot be directly modeled. An approach that spatially averages the effects of the vegetation, i.e., a bulk mixing characterization, is generally preferred [e.g., Nepf, 1999]

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