Predicting Current-Induced Drag in Emergent and Submerged Aquatic Vegetation Canopies

Arnold Van Rooijen,Liming Tan,Ryan Lowe,Mario Conde-Frias,Marco Ghisalberti

doi:10.3389/fmars.2018.00449

Abstract

Canopies formed by aquatic vegetation, such as mangroves, seagrass and kelp, play a crucial role in altering the local hydrodynamics in rivers, estuaries and coastal regions, and thereby influence a range of morphodynamic and biophysical processes. Prediction of the influence of canopies on these hydrodynamic processes requires a fundamental understanding of canopy drag, which varies significantly with both flow conditions and canopy properties (such as density and submergence). Although our knowledge on canopy drag has increased significantly in recent decades, a conclusive, physics-based description for canopy drag that can be applied to both emergent and submerged canopies is currently lacking. Here, we extend a new theoretical canopy drag model (that employs the velocity between canopy elements as the reference velocity) to submerged aquatic canopies. The model is validated for the first time with direct measurements of drag forces exerted by canopies across broad ranges of flow conditions and canopy density and submergence. The skill and broader applicability of the model are further assessed using a comprehensive set of existing experimental data, covering a broad range of natural conditions (including flexible vegetation). The resulting model provides a simple tool to estimate canopy drag forces, which govern hydraulic resistance, sediment transport and biophysical processes within aquatic ecosystems.

Highlights

It is widely recognized that aquatic vegetation, such as seagrass, reeds, kelp, and mangroves, greatly influences hydrodynamic processes within rivers, estuaries, and coastal regions (e.g., Nepf, 2012)
The isolated cylinder drag coefficients were compared to theory (Equation 2) to gain confidence in the experimental methodology
Emergent Canopies As discussed in the Introduction, for emergent canopies, both the bulk velocity and pore velocity are often used as the reference velocity in Equation (3) to relate a given flow condition to the canopy drag force through a drag coefficient (i.e., Cd,b and Cd,p, respectively)

Summary

Introduction

It is widely recognized that aquatic vegetation, such as seagrass, reeds, kelp, and mangroves, greatly influences hydrodynamic processes within rivers, estuaries, and coastal regions (e.g., Nepf, 2012). The drag exerted by emergent and submerged vegetation impacts the local hydrodynamics, morphodynamics, and ecology over a range of spatial scales (Koch et al, 2007). The canopies formed by vegetation can affect the local flow environment at the smallest scale (i.e., the plant scale, mm to cm) to the larger-scale (>1 km) flows that occur across benthic ecosystems. The flow reduction induced by canopy drag can, in turn, influence a number of morphodynamic and biophysical processes (Koch et al, 2007). Aquatic ecosystems are under increasing pressure from anthropogenic and climate change impacts (Duarte, 2002), and it is crucial we increase our understanding of canopy drag as it directly influences many important biophysical processes in aquatic environments

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Conclusion