Sediment Transport of Fine Sand to Fine Gravel on Transverse Bed Slopes in Rotating Annular Flume Experiments

Large‐scale morphology, in particular meander bend depth, bar dimensions, and bifurcation dynamics, are greatly affected by the deflection of sediment transport on transverse bed slopes due to gravity and by secondary flows. Overestimating the transverse bed slope effect in morphodynamic models leads to flattening of the morphology, while underestimating leads to unrealistically steep bars and banks and a higher braiding index downstream. However, existing transverse bed slope predictors are based on a small set of experiments with a minor range of flow conditions and sediment sizes, and in practice models are calibrated on measured morphology. The objective of this research is to experimentally quantify the transverse bed slope effect for a large range of near‐bed flow conditions with varying secondary flow intensity, sediment sizes (0.17–4 mm), sediment transport mode, and bed state to test existing predictors. We conducted over 200 experiments in a rotating annular flume with counterrotating floor, which allows control of the secondary flow intensity separate from the streamwise flow velocity. Flow velocity vectors were determined with a calibrated analytical model accounting for rough bed conditions. We isolated separate effects of all important parameters on the transverse slope. Resulting equilibrium transverse slopes show a clear trend with varying sediment mobilities and secondary flow intensities that deviate from known predictors depending on Shields number, and strongly depend on bed state and sediment transport mode. Fitted functions are provided for application in morphodynamic modelin

Making realistic wave climates in low-cost wave mesocosms: A new tool for experimental ecology and biogeomorphology

Wave flume facilities that are primarily designed for engineering studies are often complex and expensive to operate, and hence not ideal for long-term replicated experiments as commonly used in biology. This study describes a low-cost small wave flume that can be used for biological purposes using fresh- or seawater with or without sediment. The wave flume can be used as a mesocosm to study interactions between wave hydrodynamics and benthic organisms in aquatic ecosystems. The low-costs wave maker (< 2000 USD) allows for experimental setups which can be easily replicated and used for longer term studies; hence the term wave mesocosm. Waves were generated with a pneumatic piston and wave heights ranged between 3 and 6 cm. Maximum orbital flow velocities ranged between 10 and 50 cm s−1 representing shallow coastal areas with a short fetch. The system can generate both regular waves (i.e., the wave period and orbital velocity remains constant), using a wave absorber, and irregular waves (i.e., varying wave period and orbital velocity) using a fast push and slow pull motion of the wave paddle. This wave mesocosm system is particularly useful in biogeomorphology to quantify interactions between organisms, sediment, and hydrodynamics and for aquatic ecologist aiming to simulate realistic bed shear stress where short- and long-term experiments (weeks–months) can be replicated

Making realistic wave climates in low-cost wave mesocosms: A new tool for experimental ecology and biogeomorphology

Wave flume facilities that are primarily designed for engineering studies are often complex and expensive to operate, and hence not ideal for long-term replicated experiments as commonly used in biology. This study describes a low-cost small wave flume that can be used for biological purposes using fresh- or seawater with or without sediment. The wave flume can be used as a mesocosm to study interactions between wave hydrodynamics and benthic organisms in aquatic ecosystems. The low-costs wave maker (< 2000 USD) allows for experimental setups which can be easily replicated and used for longer term studies; hence the term wave mesocosm. Waves were generated with a pneumatic piston and wave heights ranged between 3 and 6 cm. Maximum orbital flow velocities ranged between 10 and 50 cm s−1 representing shallow coastal areas with a short fetch. The system can generate both regular waves (i.e., the wave period and orbital velocity remains constant), using a wave absorber, and irregular waves (i.e., varying wave period and orbital velocity) using a fast push and slow pull motion of the wave paddle. This wave mesocosm system is particularly useful in biogeomorphology to quantify interactions between organisms, sediment, and hydrodynamics and for aquatic ecologist aiming to simulate realistic bed shear stress where short- and long-term experiments (weeks–months) can be replicated

Wind exposure and sediment type determine the resilience and response of seagrass meadows to climate change

10.1002/lno.11865Limnology and Oceanograph

Key Bioturbator Species Within Benthic Communities Determine Sediment Resuspension Thresholds

Abundant research has shown that macrobenthic species are able to increase sediment erodibility through bioturbation. So far, however, this has been at the level of individual species. Consequently, we lack understanding on how such species effects act on the level of bioturbator communities. We assessed the isolated and combined effects of three behaviorally contrasting macrobenthic species, i.e., Corophium volutator, Hediste diversicolor, and Limecola balthica, at varying densities on the critical bed shear stress for sediment resuspension (τcr). Overall, the effect of a single species on sediment erodibility could be described by a power function, indicating a relatively large effect of small bioturbator densities which diminishes toward higher individual density. In contrast to previous studies, our results could not be generalized between species using total metabolic rate, indicating that metabolic rate may be only suitable to integrate bioturbation effects within and between closely related species; highly contrasting species require consideration of species-specific bioturbation strategies. Experiments at the benthic community level revealed that the ability of a benthic community to reduce τcr is mainly determined by the species that has the largest individual effect in reducing τcr, as opposed to the species that is dominant in terms of metabolic rate. Hence, to predict and accurately model the net effect of bioturbator communities on the evolution of tidal flats and estuaries, identification of the key bioturbating species with largest effects on τcr and their spatial distribution is imperative. Metabolic laws may be used to describe their actual activity

Key Bioturbator Species Within Benthic Communities Determine Sediment Resuspension Thresholds

Abundant research has shown that macrobenthic species are able to increase sediment erodibility through bioturbation. So far, however, this has been at the level of individual species. Consequently, we lack understanding on how such species effects act on the level of bioturbator communities. We assessed the isolated and combined effects of three behaviorally contrasting macrobenthic species, i.e., Corophium volutator, Hediste diversicolor, and Limecola balthica, at varying densities on the critical bed shear stress for sediment resuspension (τcr). Overall, the effect of a single species on sediment erodibility could be described by a power function, indicating a relatively large effect of small bioturbator densities which diminishes toward higher individual density. In contrast to previous studies, our results could not be generalized between species using total metabolic rate, indicating that metabolic rate may be only suitable to integrate bioturbation effects within and between closely related species; highly contrasting species require consideration of species-specific bioturbation strategies. Experiments at the benthic community level revealed that the ability of a benthic community to reduce τcr is mainly determined by the species that has the largest individual effect in reducing τcr, as opposed to the species that is dominant in terms of metabolic rate. Hence, to predict and accurately model the net effect of bioturbator communities on the evolution of tidal flats and estuaries, identification of the key bioturbating species with largest effects on τcr and their spatial distribution is imperative. Metabolic laws may be used to describe their actual activity

Habitat-forming species trap microplastics into coastal sediment sinks

Nearshore biogenic habitats are known to trap sediments, and may therefore also accumulate biofouled, non-buoyant microplastics. Using a current-generating field flume (TiDyFLOW), we experimentally assessed the mechanisms of microplastic trapping of two size classes, 0.5 mm and 2.5 mm particle size, by three contrasting types of biogenic habitats: 1) seagrasses, 2) macroalgae, and 3) scleractinian corals. Results showed that benthic organisms with a complex architecture and rough surface – such as hard corals – trap the highest number of microplastics in their aboveground structure. Sediment was however the major microplastic sink, accumulating 1 to 2 orders of magnitude more microplastics than the benthic structure. Microplastic accumulation in the sediment could be explained by near-bed turbulent kinetic energy (TKE), indicating that this is governed by the same hydrodynamic processes leading to sediment trapping. Thus, the most valuable biogenic habitats in terms of nursery and coastal protection services also have the highest capacity of accumulating microplastics in their sediments. A significantly larger fraction of 0.5 mm particles was trapped in the sediment compared to 2.5 mm particles, because especially the smaller microplastics are entrained into the sediment. Present observations contribute to explaining why especially microplastics smaller than 1 mm are missing in surface waters

Habitat-forming species trap microplastics into coastal sediment sinks

Nearshore biogenic habitats are known to trap sediments, and may therefore also accumulate biofouled, non-buoyant microplastics. Using a current-generating field flume (TiDyFLOW), we experimentally assessed the mechanisms of microplastic trapping of two size classes, 0.5 mm and 2.5 mm particle size, by three contrasting types of biogenic habitats: 1) seagrasses, 2) macroalgae, and 3) scleractinian corals. Results showed that benthic organisms with a complex architecture and rough surface – such as hard corals – trap the highest number of microplastics in their aboveground structure. Sediment was however the major microplastic sink, accumulating 1 to 2 orders of magnitude more microplastics than the benthic structure. Microplastic accumulation in the sediment could be explained by near-bed turbulent kinetic energy (TKE), indicating that this is governed by the same hydrodynamic processes leading to sediment trapping. Thus, the most valuable biogenic habitats in terms of nursery and coastal protection services also have the highest capacity of accumulating microplastics in their sediments. A significantly larger fraction of 0.5 mm particles was trapped in the sediment compared to 2.5 mm particles, because especially the smaller microplastics are entrained into the sediment. Present observations contribute to explaining why especially microplastics smaller than 1 mm are missing in surface waters

Sediment shell-content diminishes current-driven sand ripple development and migration

Shells and shell fragments are biogenic structures that are widespread throughout natural sandy shelf seas and whose presence can affect the bed roughness and erodibility of the seabed. An important and direct consequence is the effect on the formation and movement of small bedforms such as sand ripples. We experimentally measured ripple formation and the migration of a mixture of natural sand with increasing volumes of shell material in a racetrack flume. Our experiments reveal the impacts of shells on ripple development in sandy sediment, providing information that was previously lacking. Shells expedite the onset of sediment transport while simultaneously reducing ripple dimensions and slowing down their migration rates. Moreover, increasing shell content enhances near-bed flow velocity due to the reduction of bed friction that is partly caused by a decrease in average ripple size and occurrence. This, in essence, limits the rate and magnitude of bed load transport. Given the large influence of shell content on sediment dynamics as well as the high shell concentrations found naturally in the sediments of shallow seas, a significant control from shells on the morphodynamics of sandy marine habitats is expected