Comparison of Flexible and Rigid Vegetation Induced Shear Layers in Partly Vegetated Channels

Gerardo Caroppi,Kaisa Västilä,Paweł M Rowiński,Juha Järvelä,Maurizio Giugni,Paola Gualtieri

doi:10.1029/2020wr028243

Abstract

AbstractNatural riparian vegetation generally presents a complex hydrodynamic behavior governed by plant morphology and flexibility. By contrast, hydrodynamic processes in partly vegetated channels are conventionally simulated by using simplified model vegetation, such as arrays of rigid cylinders. The aim of this study is to investigate the impacts of embedding natural plant features in the experimental simulation of flow in partly vegetated channels. Unique comparative experiments were carried out with both reconfiguring vegetation made of natural‐like shrubs and grasses, and with rigid cylinders. While the lateral distributions of flow properties presented a high similarity governed by the shear layer differential velocity ratio, the bulk vegetative drag, and the presence of large‐scale vortices, the flexibility‐induced mechanisms of natural‐like vegetation markedly affected the flow at the interface. Differences in plant morphology and spacing, and the dynamic motion of flexible foliated plants induced deeper vortex penetration into the vegetation. The normalized shear penetration was 6–10 times greater than observed for rigid cylinders, resulting in wider zones significantly exchanging momentum with the adjacent open water. The efficiency of lateral momentum transport for flexible foliated vegetation was up to 40% greater than the corresponding rigid cylinder case. Overall, the results indicated that improving the representativeness of model vegetation is a critical step toward the accurate simulation of hydrodynamic and transport processes in natural settings.

Highlights

In a variety of environmental settings, including vegetated channel banks and rivers, and their associated floodplains and riparian areas, vegetation occurs along river margins, partially obstructing the cross-section
For slightly and moderately reconfigured vegetation (F1 and F2), more of the shear layer developed within the vegetation than for the corresponding rigid cylinder cases (R1 and R2), for which the shear mostly developed into the main channel
A novel experimental setting closely representing conditions found along river margins was investigated (Figure 6)

Summary

Introduction

In a variety of environmental settings, including vegetated channel banks and rivers, and their associated floodplains and riparian areas, vegetation occurs along river margins, partially obstructing the cross-section In these systems, referred to as partly vegetated channels, the long regions of finite width of emergent vegetation laterally interact with the flow, deeply altering the mean and turbulent flow structure with implications on the conveyance capacity of the channel, the water levels, and the mass and momentum exchange processes (Aberle & Järvelä, 2013; Naot et al, 1996; Nepf, 2012; Rowiński et al, 2018). These two processes have been found to deeply influence the flow-vegetation interaction, affecting the exchange processes occurring at the interface between vegetation and the adjacent open water (Abdolahpour et al, 2018; Caroppi, Västilä, Järvelä, et al, 2019; Ghisalberti & Nepf, 2006) and are expected to markedly impact the flow structure in partly vegetated channels in presence of non-submerged foliated vegetation

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