Hydrodynamic performance of vegetation surrogates in hydraulic studies: a comparative analysis of seaweed blades and their physical models

Vegetation surrogates have been extensively used in laboratory experiments for studying flow-vegetation interactions. However, it remains unclear how accurately the surrogates replicate the prototype vegetation in terms of hydrodynamic performance, even when similarity conditions are followed. To address this matter, we compare the hydrodynamic performance of seaweed blades of the species Saccharina latissima with performance of their surrogates, which were designed based on similarity considerations. To assess the hydrodynamic performance of samples, we measured flow velocities upstream and downstream of the samples, their vertical movements, and the drag forces exerted on them. The obtained data reveal that the mechanisms governing flow-blade interactions are essentially the same for live blades and their surrogates. Even though the surrogates successfully replicate many aspects of live blade dynamics, they experience weaker drag force and reconfiguration, likely because of their simplified morphologies that differed from the live blades at small scales. To enhance similarity in hydrodynamic performances, we suggest employing comprehensive similarity conditions at all relevant scales

The effects of abiotic factors on plant health and biomechanics: a mesocosm study on potamogeton crispus

Interactions between flow and vegetation are widely investigated because vegetation is a primary factor controlling channel ecohydraulics, nearshore hydraulics and flood risk. Laboratory experiments are a critical tool in this research area and, to adequately represent the complexity of natural ecosystems, live plants, rather than artificial surrogates, are often used. In the present work, we expose a freshwater macrophyte (Potamogeton crispus) to a range of environmental conditions commonly found in ecohydraulic laboratories to investigate how these affect the level of plant health and associated variations in plant biomechanical properties. This is motivated by a need to understand how deterioration in live plants that are used in flume facilities affects their hydraulic performance and therefore the verisimilitude of the data they provide on flow interactions. Results show that short-medium term exposition to tap water or low irradiance levels is stressful for plants and can induce modifications in their biomechanics, with a potential effect on their hydrodynamic performance

Flow-seaweed interactions of Saccharina latissima at a blade scale: turbulence, drag force, and blade dynamics

Physical interactions between seaweed blades of Saccharina latissima and unidirectional turbulent fow were examined in an open-channel fume, focussing on fow velocities, drag force acting on a blade, and blade reconfguration. The data reveal that seaweed blades adjust to high-energy fow conditions relatively quickly, efciently reducing fow-induced drag via compaction, a mechanism of blade reconfguration. The drag coefcient of blades of S. latissima varied between 0.02 and 0.07 over a range of mean fow velocities from 0.1 to 0.55 m/s. Both fow action and blade biomechanical characteristics infuenced the blade dynamics, with the fow role being predominant in highly energetic conditions. The interaction mechanisms and their strength were found to be scale-dependent, with the combined efect of reduced mean fow velocity and enhanced turbulence in blade wakes. The thickness of the difusive boundary layer, an important factor in nutrient uptake from the surrounding water, was estimated to be in the range from 0.010 to 0.067 mm. Mechanisms of blade adjustment to the fow and scale-dependent dynamic interactions between blades and turbulent eddies have direct implications for seaweed growth, acclimation, and survival. The estimates of the drag coefcient and the thickness of the difusive boundary layer will be useful for the development of bio-physical models, environmental assessments, and design of seaweed farms

Assessing the hydrodynamic performance of vegetation physical models: an experimental study of seaweed blades

Seaweed blades and seaweed blade physical models designed following a similarity theory were tested in a flume facility. The measured drag coefficient of physical models is found to be significantly biased low compared to that of natural seaweed blades

Innovative approaches for measuring organism stress and behavioural integrity in flume facilities: Deliverable D8-IV

HYDRALAB+ aims to improve the usefulness and value of hydraulic laboratory facilities and is developing experimental guidelines that will allow researchers to successfully investigate complex scenarios representative of natural environments in a context of climate change. Within this framework it is often important to incorporate relevant biological elements in physical experiments, including the use of live vegetation. Notwithstanding efforts to maintain their health by careful husbandry, plants typically degenerate when introduced to flume settings. Physiological responses to degenerating health can affect their interactions with the flow so that experimental conditions are not representative of healthy specimens in situ. There is therefore a need to measure and evaluate the health of plants being used in hydraulic facilities, especially since behavioural integrity might be reduced before there are obvious signs of degeneration. Such measurements are not routinely made so there is a need to identify measurement techniques and methodological protocols for assessing vegetation health status in hydraulic laboratories. This deliverable identifies a technique established in plant physiology and horticulture for monitoring vegetation health status and shows how it can be applied in hydraulic laboratories with minimal impact on organisms. A simple and suitable test among those established in the relevant literature is validated by conducting experiments on freshwater macrophytes. From the relevant literature and the results of experiments reported herein, this deliverable provides an overview of the technique identified and establishes practical guidance on how to properly apply it in hydraulic experiments. The methodological protocol developed can potentially be integrated into established protocols used in ecohydraulics studies as a simple proxy of vegetation health status

Supplementary information files for 'Flow-seaweed interactions of Saccharina latissima at a blade scale: turbulence, drag force, and blade dynamics'

Supplementary information files for 'Flow-seaweed interactions of Saccharina latissima at a blade scale: turbulence, drag force, and blade dynamics'Abstract:Physical interactions between seaweed blades of Saccharina latissima and unidirectional turbulent fow were examined in an open-channel fume, focussing on fow velocities, drag force acting on a blade, and blade reconfguration. The data reveal that seaweed blades adjust to high-energy fow conditions relatively quickly, efciently reducing fow-induced drag via compaction, a mechanism of blade reconfguration. The drag coefcient of blades of S. latissima varied between 0.02 and 0.07 over a range of mean fow velocities from 0.1 to 0.55 m/s. Both fow action and blade biomechanical characteristics infuenced the blade dynamics, with the fow role being predominant in highly energetic conditions. The interaction mechanisms and their strength were found to be scale-dependent, with the combined efect of reduced mean fow velocity and enhanced turbulence in blade wakes. The thickness of the difusive boundary layer, an important factor in nutrient uptake from the surrounding water, was estimated to be in the range from 0.010 to 0.067 mm. Mechanisms of blade adjustment to the fow and scale-dependent dynamic interactions between blades and turbulent eddies have direct implications for seaweed growth, acclimation, and survival. The estimates of the drag coefcient and the thickness of the difusive boundary layer will be useful for the development of bio-physical models, environmental assessments, and design of seaweed farms.</div

Temporal variability and within‐plant heterogeneity in blade biomechanics regulate flow‐seagrass interactions of Zostera marina

Seagrasses are marine flowering plants that have important roles in the ecological and physical processes of many coastal areas. Seagrass modelling to date has mostly assumed that seagrasses have uniform biomechanical traits in space and time. In this study we compare the biomechanical traits of Zostera marina blades collected in late summer and spring from a lagoon in southern Denmark. Then, we describe how biomechanics vary depending on (i) seasonality, (ii) storage in laboratory conditions with high nutrient levels, (iii) blade rank and (iv) position along blades. The data collected with these direct measurements are fed into a numerical structural model that simulates seagrass response to an idealized flow and accounts for plant non uniformity. The model is used to assess the effects of temporal variability and within-plant heterogeneity in blade biomechanics on flow-seagrass interactions. Results show that seagrass biomechanics are affected considerably by seasonality and laboratory storage. This biomechanical variability has a key role in defining flow-seagrass interactions, enhancing light availability in summer and reducing potential drag force in spring. Significant within-plant heterogeneity associated with both blade rank and along-blade position is reported. Compared to temporal variability, within-plant heterogeneity has a secondary role in determining flow seagrass interactions; however, blade rank is associated with a consistent reduction in the drag force. The results presented improve the understanding of flow-seagrass interactions by clarifying the importance of variations in seagrass blade biomechanical traits and their origin

Implications of environmental conditions for health status and biomechanics of freshwater macrophytes in hydraulic laboratories

Submerged freshwater macrophytes are frequently used in hydraulic laboratories to study flow–plant interactions and the role of plants in aquatic ecosystems, but environmental conditions in flume facilities are often suboptimal for plants and can cause plant stress. Physiological responses of plants under stress can trigger modifications in plant biomechanics, which may affect plant–flow interactions and compromise experimental results. In the extreme, dead plants cannot be expected to reveal how live plants interact with flowing water, but stressed plants that are not visibly unhealthy may also affect experimental results. The present work aims to assess if and how environmental conditions typical of flume facilities can impact plant health status and induce variations in plant biomechanics. Using chlorophyll fluorescence analysis, a standard method for assessing plant health, we found that freshwater macrophytes can be significantly stressed under conditions typically found in hydraulic laboratories. Even though the abiotic factors investigated affected different species in different ways, exposure to tap water and low irradiance were the most stressful conditions for freshwater macrophytes. Biomechanical properties with a primary role in flow–plant physical interactions (e.g. flexural rigidity) changed significantly as a result of exposure to stressful conditions. In general, plant stress was associated with a reduction in flexural rigidity at the top of plant stems, suggesting a potential effect on plant hydrodynamics when leaves and petioles are considered. The maximum quantum yield of photosystem II, used as proxy of plant health status, was positively correlated with flexural rigidity of plant stems

Assessing the hydrodynamic performance of vegetation physical models: an experimental study of seaweed blades

Seaweed blades and seaweed blade physical models designed following a similarity theory were tested in a flume facility. The measured drag coefficient of physical models is found to be significantly biased low compared to that of natural seaweed blades

A link between plant stress and hydrodynamics? Indications from a freshwater macrophyte

Live plants are increasingly used in hydraulic laboratories to investigate flow-vegetation interactions. In such experiments, they are often exposed to stressful handling and storage that can cause strong physiological responses and modifications in plant biomechanics. Little is known about the potential effect of these impacts on the performance of plants during hydraulic experiments. In this multidisciplinary study with a freshwater macrophyte (Potamogeton natans) we assess whether the duration and the conditions in which plants are stored in a laboratory prior to testing can impact plant stress, biomechanics and hydrodynamics, and quantify this impact. Plant stress was evaluated using chlorophyll fluorescence analysis (and the maximum quantum yield of photosystem II as specific indicator). Plant hydrodynamics were assessed using the drag coefficient calculated from drag force measurements at two flow scenarios. The results show that different plant handling/storage procedures can have a significant impact on plant hydrodynamics even within a short time frame, with a variation of the mean drag coefficient of approximately 30% across groups, which is comparable to the variation found across different species of freshwater macrophytes in previous studies. Plants with the highest level of stress were also characterized by the lowest drag coefficient across the groups considered, suggesting a potential link between plant stress and hydrodynamics