Flow structure in depth‐limited, vegetated flow

TLDR

Aquatic vegetation controls mean and turbulent flow structure in channels and coastal regions, thereby influencing sediment and contaminant transport. This study aims to characterize the transition between submerged and emergent vegetation regimes by examining mean momentum, turbulence, and exchange dynamics. The authors conducted open‑channel flume experiments with model vegetation to investigate how vegetation affects flow. Observations reveal that canopy flow separates into a vertical exchange zone at the canopy top, where vertical turbulent exchange and shear‑generated turbulence dominate, and a longitudinal exchange zone below, where local turbulence balances vegetative drag and pressure gradient, with emergent canopies lacking the vertical zone and submerged canopies developing a deeper vertical zone as submergence increases.

Abstract

Aquatic vegetation controls the mean and turbulent flow structure in channels and coastal regions and thus impacts the fate and transport of sediment and contaminants. Experiments in an open‐channel flume with model vegetation were used to better understand how vegetation impacts flow. In particular, this study describes the transition between submerged and emergent regimes based on three aspects of canopy flow: mean momentum, turbulence, and exchange dynamics. The observations suggest that flow within an aquatic canopy may be divided into two regions. In the upper canopy, called the “vertical exchange zone”, vertical turbulent exchange with the overlying water is dynamically significant to the momentum balance and turbulence; and turbulence produced by mean shear at the top of the canopy is important. The lower canopy is called the “longitudinal exchange zone” because it communicates with surrounding water predominantly through longitudinal advection. In this region turbulence is generated locally by the canopy elements, and the momentum budget is a simple balance of vegetative drag and pressure gradient. In emergent canopies, only a longitudinal exchange zone is present. When the canopy becomes submerged, a vertical exchange zone appears at the top of the canopy and deepens into the canopy as the depth of submergence increases.