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

Abstract

We present a critical analysis of experimental findings on vegetation-flow-sediment interactions obtained through both laboratory and field experiments on tidal and coastal environments. It is well established that aquatic vegetation provides a wide range of ecosystem services (e.g. protecting coastal communities from extreme events, reducing riverbank and coastal erosion, housing diverse ecosystems), and the effort to better understand such services has led to multiple approaches to reproduce the relevant physical processes through detailed laboratory experiments. State-of-the-art measurement techniques allow researchers to measure velocity fields and sediment transport with high spatial and temporal resolution under well-controlled flow conditions, yielding predictions for hydrodynamic and sediment transport scenarios that depend on simplified or bulk vegetation parameters. However, recent field studies have shown that some simplifications on the experimental setup (e.g. the use of rigid elements, a single diameter, a single element height, regular or staggered layout) can bias the outcome of the study, by either hiding or amplifying some of the relevant physical processes found in natural conditions. We discuss some observed cases of bias, including general practices that can lead to compromises associated with simplified assumptions. The analysis presented will identify potential pathways to move forward with laboratory and field measurements, which could better inform predictors to produce more robust, universal and accurate predictions on flow-vegetation-sediment interactions.

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

“…For R tests, the flow within the vegetated region patterned the cylinder lateral distribution, and local regions of high shear stress, drag and turbulence intensity ( Figures S6-S8) developed within the array. These flow features, usually associated with staggered and aligned cylinders arrangement (Tinoco et al, 2020 and reference therein), locally alter the mass and energy transport processes within the vegetated region.…”

“…Recent studies have shown that using rigid, cylindrical elements for simulating natural vegetation can bias the study outcomes, by either hiding or amplifying some of the relevant physical processes found in natural conditions (Tinoco et al., 2020). For example, for vertical shear layers induced by submerged vegetation, significant differences in turbulent flow structure and momentum exchange have been observed for natural‐like vegetation (dynamically scaled vegetation prototypes of aquatic seagrasses, blade‐shaped tensile vegetation) relative to rigid cylinders, both for current‐ (Ghisalberti & Nepf, 2006; Termini, 2015) and wave‐dominated flows (Abdolahpour et al., 2018).…”

Relating millimeter‐scale turbulence to meter‐scale subtidal erosion and accretion across the fringe of a coastal mangrove forest

“…Many laboratory studies have used regular, idealized vegetation analogues, and future examination of irregular and patchy vegetation may be justified (e.g. Horstman et al, 2015; Tinoco et al, 2020). (ii) Consistent with recent laboratory studies (Tinoco & Coco, 2016, 2018; Yang & Nepf, 2018), our field observations support further examination of TKE, rather than averaged currents, as a promising predictor of sediment mobilization and deposition across both vegetated and unvegetated environments.…”

Identifying turbulence features hindering swimming capabilities of grass carp larvae (Ctenopharyngodon idella) through submerged vegetation

“…In the case of vegetated flows, it is known that the type of vegetation, stem density, and submergence conditions will determine the hydrodynamic conditions (i.e., velocity gradients, turbulence features, coherent flow structures, and relevant timeand length-scales) within and around the vegetated patches (e.g., reviews by Nepf 2012;Tinoco et al 2020). The complexity of the vegetation arrays will drive a range of turbulent eddies of multiple sizes, from individual stem-scale eddies to canopy-scale eddies at the top of the array in submerged vegetation, if the array is dense enough (i.e., for ah > 0.1, where a is the volumetric frontal area and h is the array height, Nepf 2012).…”