This paper is closed access until 28th June 2018.Interactions between water flow and aquatic vegetation strongly depend on morphological and biomechanical characteristics of vegetation. Although any physical or numerical model that aims to replicate flow-vegetation interactions requires these characteristics, information on morphology and mechanics of vegetation living in coastal waters remains insufficient. The present study investigates the mechanical properties of blades of Saccharina latissima, a seaweed species spread along the shores of the UK and North East Atlantic. More than 50 seaweed samples with lengths spanning from 150 mm to 650 mm were collected from Loch Fyne (Scotland) and tested. Seaweed blades had a natural ‘stretched droplet’ shape with bullations in the central fascia and ruffled edges in the area close to the stipe. Their morphological features showed high variability for samples longer than 400 mm. The blades were almost neutrally buoyant, their material was found to be very flexible and ductile, being stiffer in longer blades. The laboratory tests showed that estimates of tensile Young's modulus appeared to be similar to bending Young's modulus suggesting a reasonable degree of isotropy in studied seaweed tissues

This paper reports the morphology of a natural patch of Ranunculus penicillatus and presents high-resolution measurements of flow velocities in its wake using a stereoscopic PIV field measurement system. The patch was 3.80 m long, 1.24 m wide and caused substantial changes to downstream mean velocities and turbulence. Vertical profiles of streamwise mean velocity were not logarithmic and flow was redirected under the positively buoyant canopy, enhancing vertical turbulent mixing in the wake and generating a large region where the velocity covariance $\overline{u^{\mathrm{\prime }}{w}^{\mathrm{\prime }}}$ was positive. Turbulent kinetic energy was enhanced downstream from the patch lateral shear layer, but not at the centre of the wake. Spectra downstream from the patch showed that turbulence was neither dominated by fine-scale nor large-scale structures, likely due to the low energy of the flow conditions and lack of a developed vortex street within the measurement domain. Sedimentation was observed at the upstream end of the patch, but not underneath the floating canopy. The methods and results of this work will be useful for planning other in situ studies. Also, the reported data on macrophyte geometry and biometrics will assist with the design of more realistic replicas for use in laboratory studies.

This work investigates how turbulence in open-channel flows is altered by the passage of surface waves by using experimental data collected with laboratory tests in a large-scale flume facility, wherein waves followed a current. Flow velocity data were measured with a laser Doppler anemometer and used to compute profiles of mean velocity and Reynolds stresses, and pre-multiplied spectra. The velocity signal containing contributions from the mean flow, wave motion and turbulence was decomposed using the empirical mode decomposition (EMD), which is considered a promising tool for the analysis of velocity time series measured in complex flows. A novel outer length scale ) appears. These longitudinal structures present in the wave-dominated flow region seem to share many features with Langumir-type cells.

Flow–vegetation interactions is an interdisciplinary research area with applications in the management of coastal waters, lakes, and watercourses. Due to an emerging interest in the cultivation of seaweeds, this study seeks to develop a sound understanding of the physical interactions between flow and seaweeds. This is achieved via experiments in a laboratory flume using plastic-made models of blades of the seaweed species Saccharina latissima. In the experiments, strain gages, a digital camera, and acoustic Doppler velocimeters were used for measuring drag forces, blade movements (reconfiguration), and flow velocities. The study involved experiments with single blades and with pairs of tandem blades at different spacing between the blades. The revealed mechanisms controlling the dynamics of seaweed blade models varied depending on the ratio of blade length to eddy length scale. The drag coefficient of seaweed blade models appeared to be dependent on the Reynolds number, the Cauchy number, and the ratio of blade length to integral turbulence length scale. Turbulence had a primary role in controlling blade model dynamics and its drag coefficient. Seaweed blade models affected the flow in their wakes by increasing the turbulence intensity and reducing the mean longitudinal velocity. These effects on the flow are the reason for which, in a pair of tandem blades, the drag force experienced by the downstream blade is lower than that experienced by the upstream blade.

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.