Does the canopy mixing layer model apply to highly flexible aquatic vegetation? Insights from numerical modelling

Abstract

Vegetation is a characteristic feature of shallow aquatic flows such as rivers, lakes and coastal waters. Flow through and above aquatic vegetation canopies is commonly described using a canopy mixing layer analogy which provides a canonical framework for assessing key hydraulic characteristics such as velocity profiles, large-scale coherent turbulent structures and mixing and transport processes for solutes and sediments. This theory is well developed for the case of semi-rigid terrestrial vegetation and has more recently been applied to the case of aquatic vegetation. However, aquatic vegetation often displays key differences in morphology and biomechanics to terrestrial vegetation due to the different environment it inhabits. Here we investigate the effect of plant morphology and biomechanical properties on flow-vegetation interactions through the application of a coupled LES-biomechanical model. We present results from two simulations of aquatic vegetated flows: one assuming a semi-rigid canopy and the other a highly flexible canopy and provide a comparison of the associated flow regimes. Our results show that while both cases display canopy mixing layers, there are also clear differences in the shear layer characteristics and turbulent processes between the two, suggesting that the semi-rigid approximation may not provide a complete representation of flow-vegetation interactions.

Simplification bias: lessons from laboratory and field experiments on flow through aquatic vegetation

“…Moreover, while some progress has also been made incorporating more complex parameters such as vegetation flexibility (Dijkstra and Uittenbogaard, 2010), these effects are not yet widely included in freely available or commercial models. Efforts by Boothroyd et al (2017) and Marjoribanks et al (2017) have successfully included flexibility effects to predict turbulence and mean velocities within and around patches of flexible vegetation, allowing identification of deposition and erosion zones. However, even if advances in computational resources allow for more accurate solutions, the scale separation still prohibits DNS at field scales.…”

Wood‐induced backwater effects in lowland streams

“…Wood in streams can be modelled using physics‐based, empirical or conceptual models. Physics‐based models attempt to incorporate all involved physical processes, aiming to resolve the detailed flow patterns (Baptist et al, ; Huthoff, Augustijn, & Hulscher, ; Marjoribanks, Hardy, Lane, & Parsons, ; Nepf & Vivoni, ; Verschoren et al, ). Therefore, these models require detailed input data for the wood elements, the stream geometry and the flow velocity, which are not always available at large spatial and temporal scales (Vargas‐Luna, Crosato, & Uijttewaal, ).…”

Flexural Rigidity and Shoot Reconfiguration Determine Wake Length Behind Saltmarsh Vegetation Patches

“…As such, while the model was able to capture static reconfiguration of the vegetation to the time-averaged flow conditions, it could not capture the impacts of time-varying plant motion. In reality, shoots undergo dynamic reconfiguration (Siniscalchi & Nikora, 2013) in response to the turbulent flow which may act to either further dampen turbulence within the wake (Ghisalberti & Nepf, 2006) or introduce higher levels of turbulent kinetic energy and turbulent events which may decrease deposition (Ghisalberti & Nepf, 2009;Marjoribanks, Hardy, Lane, & Parsons et al, 2017).…”