Transport in aquatic canopies

Abstract

As a bridge to Chapter 4 in this book, it is useful to begin with a comparison of terrestrial and aquatic systems. The range of flow condition observed in aquatic systems is depicted in Figure 6.1. Conditions span from the unconfined canopy, in which the flow depth is much greater than the canopy height, to the emergent canopy, in which the canopy fills the entire flow depth. Consider Figure 6.1a, which depicts an unconfined canopy that closely resembles terrestrial conditions (see also Figure 4.1a). The discontinuity in drag at the top of the canopy creates a region of strong shear. This region of the velocity profile resembles a free-shear-layer and includes an inflection point just within the canopy. A free-shear-layer is characterized by large coherent vortices that form via Kelvin-Helmholtz (K-H) instability and which dominate the transport across the layer [86, 649]. In terrestrial canopies, these coherent structures have been shown to play an important role in transport between the canopy and overlying atmosphere [187, 194, 530]. Far above the canopy the flow returns to a boundary layer structure. The in-canopy flow is driven by the turbulent stress at the top of the canopy, and it decays with distance from the top of the canopy due to the significant momentum