This study proposes a free energy centred approach to surface runoff and morphological development of hillslopes and rivers. The starting point is the strong analogy between the functioning of engines on the one hand and watersheds on the other hand. Like an engine converts energy input into motion/kinetic energy, watersheds and hillslopes convert potential energy inputs by rainfall into potential energy and kinetic energy of surface runoff. The latter determines the maximum work water can perform on the sediments. Similar to the energy efficiency of an engine, relating the free energy/work output to the energy input, we define energy efficiencies for each process in the aforementioned cascade of rainfall-runoff formation and sediment transport. The efficiency in generating potential energy of surface runoff depends on precipitation, the runoff coefficient, topography and landforms. While the vast amount of the runoffs potential energy is dissipated, a minute amount sustains the kinetic energy of surface runoff and stream flow, driving erosive changes in watersheds. Here the efficiency depends on the controls of driving and frictional forces: the geo-potential gradient, material roughness and hydraulic radius of the river. We applied this framework to surface runoff at the hillslope scale and the Amazon basin. At the hillslope scale we found that typical morphological stages of hillslope forms and related transitions of dominant erosion processes (from soil creep, rain splash, to soil wash) evolve towards a declining energy efficiency in surface runoff. This implies a reduction in power to trigger future landform changes. However, rill and river networks do essentially the opposite. By reducing the specific dissipation, they increase the efficiency in the conversion of potential into kinetic energy of overland and streamflow. In several cases, rill networks were found to even maximize total power of surface runoff in the sheet and rill domains. For the Amazon and its tributaries, we found distinct self-similar patterns of stream flow potential energy along each river course. Starting from the source, potential energy in stream flow was growing with downstream distance, up to a maximum value, and exhibited from there an almost linear decline to the river mouth. This implies that the maximum work the river can perform is growing from its source to this maximum, as the rapid growth in the stream flow mass over-weights the steep downstream decline in geopotential. We found the same behavior at the hillslope scale. In a third step, we found that for the largest terrestrial river networks on the world that Horton’s laws of stream area and length were close the Feigenbaum constants characterising bifurcations of logistic growth at related deterministic chaos. This suggest parallels between the interplay of growth and mortality of populations, and the interplay of stream power generation and its turbulent dissipation.

The formation and evolution of sediment ribons over a uniform sediment bed in an open-channel flow was investigated via a stereo-photogrammetric system to measure the bed evolution in combination with a stereo-PIV system to measure the three-component velocity field in a cross-sectional plane above the bed. The formation of ribbons is observed to be triggered by the initially meandering low and high-speed streaks sharing the same spanwise wavelength as the fully-developed ribbons. When the ribbons are fully developed, the streaks are locked in place with low-speed streaks over the ridges and high-speed streaks over the troughs with strong secondary flows. The lateral stabilization appears to be facilitated by the stable streaks near the wall.

The impingement of a compound open-channel flow upon vertical and streamwise-oriented plates of different solidities located at the edge between main channel and floodplain are investigated experimentally. The plates model finite-length thin hedges along the river bank. The incident flow is characterised by a turbulent shear layer dominated by Kelvin-Helmholtz structures of length Ls. The solidity of the plates is varied from 17% (i.e. 83% porosity) to 100% (solid plate) in order to represent different vegetation densities, while the plate length is fixed to 0.56Ls. The velocity field at the free-surface is measured by means of Large-Scale Particle Image Velocimetry (LS-PIV) with a field of view of length 3Ls and covering the channel width. While the solid plate is shown to induce a significant decrease in the incoming turbulent shear stress, the porous plates instead lead to an overshoot in the turbulent shear stress. All plates induce a reduction of the lateral turbulence intensity, while the longitudinal turbulence intensity is never reduced and even strongly overshoots the incident value for the porous plates. The individual Kelvin-Helmholtz structures are altered when passing the plates but are not destroyed. The alteration of the structures increases with the plates' solidity, but the vortex cores as well as their spatial periodicity are always maintained, and the structures reform to their original state within a relatively short distance downstream of the plates of about 1.5Ls. While for the solid plate the vorticity within the vortex cores decreases by the passage of the plate, in the case of the porous plates the vorticity increases. Vorticity shed from the holes of the porous plates is surmised to be the cause of this increase. The ensemble-averaged Kelvin-Helmholtz structure downstream of each plate reveals that for the least solid plates, the upstream ejection associated with the vortex core is strongly strengthened, which accounts for the observed overshoots in the turbulent stresses. When the plate's solidity is higher than 67% however, the Kelvin-Helmholtz vortex downstream of the plate has shrunk and weakened, but is accompanied by secondary vortices, generated by the interaction with the plates.

This letter investigates converged statistics in three-dimensional deep-canopy-dominated flows under two low relative submergence conditions: h/k=1.5 and h/k=1.2. Using a multi-plane telecentric PIV setup, time-averaged velocity fields were obtained across nine planes. For h/k=1.5, the flow structure exhibited a classical three-region behavior: a uniform emergent zone, a mixing zone and a logarithmic free-flow zone. In contrast, the extreme low submergence case (h/k=1.2) revealed significant flow modifications, including seiching - a periodic free-surface oscillation typically associated with emergent flows. Local double-averaged velocity profiles showed a unique undershoot near z/k=0.8, reflecting enhanced momentum transfer from cavity flows to alleys of the canopy. These findings highlight the transition mechanisms from submerged to emergent flow regimes, providing insights into canopy hydrodynamics and the influence of submergence on momentum transport.

Refractive-index matching (RIM) techniques are often employed in order to perform optical flow measurements around solid bodies: the RI of a transparent solid object is matched with that of the working fluid. The solid becomes "invisible" and full optical access inside or behind the object is granted free of distortions. In this paper, a low-cost and low-viscosity RIM pair is proposed. It combines the polymer THV (tetrafluoroethylene, hexafluoropropylene and vinylidene fluoride), which is commercially available and manufactured in various shapes, with NaCl (table salt) added to water for the working fluid. The low cost of table salt combined with the viscosity of the solution close to water allows this RIM pair to be extended for large scale, high Reynolds number experiments, such as open-channel flows. Here, a set of validation experiments is presented which was designed to analyse the errors and matching sensitivity of PIV measurements in an actual flow rather than using a still target pattern -- the common testing method. The flow -- a dipole impacting a patch of vertical tubes -- is measured simultaneously by two PIV systems, where one camera takes images through a patch of THV tubes while the other camera has a free optical path to the same measurement laser plane. Comparing the results between the two cameras gives an estimate of the PIV measurement error. To further analyse the PIV errors and their dependence on the tubes' geometry and their sensitivity to slight differences in the refractive indices between the solid and the fluid, a mathematical model of light refraction is employed. The higher thickness of the tube is found to increase the PIV error less than expected and sometimes even decrease it for a given resolution. It is found that the PIV error is inversely proportional to the PIV correlation box size relative to the tube size. This conclusion is also valid for other geometries, where edges or curved surfaces are present. That is, close-up measurements (higher resolution) around objects require higher precision of the RI matching.

Measuring the flow field not just above but also inside the canopy of rough beds in open-channel flows is challenging when optical access from below or the side is not possible. To this end, a new stereo-PIV arrangement was implemented and tested. To optically access the area in between the roughness elements, a top-viewing stereo-PIV system was installed at a steep viewing angle of 70 ^∘ . Furthermore, a glass plate was installed at the water surface to avoid random image distortions from surface waves. Finally, in-house-produced low-cost fluorescent particles were used to filter out laser reflections on the canopy’s surface. To validate the stereo-PIV measurements with respect to the steep viewing angle of the stereo cameras, the instantaneous flow field above a bed of spheres was measured simultaneously with the stereo-PIV system and with a third side-looking camera for standard 2D-PIV processing. Two different water depths and two different Reynolds numbers were investigated under uniform flow conditions. Finally, to assess the influence of the glass plate on the flow, 2D-PIV measurements were performed with and without the glass plate. It is shown that the steep viewing stereo PIV can properly reconstruct the instantaneous, mean and turbulent statistics except for the vertical normal turbulent stresses, which are overestimated due to peak-locking errors. Also, the boundary layer developing below the glass plate induces a slight acceleration in the bulk flow, which can be considered negligible for higher water depths. In all instances, the flow acceleration does not affect the near-bed region.

Experiments are conducted in an open-channel flow where half of the section is smooth and the other half consists of an array of cubes, which are either submerged or emergent. A shear layer featuring large-scale Kelvin–Helmholtz structures develops between the two subsections. The flows are first analysed in the framework of the double-averaging method (averaging of the flow both in time and space). Double averaging could be performed thanks to an experimental set-up (three-dimensional, two-component telecentric scanning particle image velocimetry) that allows to measure the velocity field in a large volume, including the interstices between the cubes. A momentum balance performed on the smooth subsection indicates that the loss of momentum towards the rough subsection has the same order of magnitude than the momentum loss through bed friction. This lateral momentum flux occurs nearly exclusively through turbulent shear stress, whereas secondary currents plays a minor role and dispersive shear stress is negligible. A pattern recognition technique is then applied to investigate statistically the large-scale Kelvin–Helmholtz structures that develop in the shear layer. The structures appear to be coherent over the water depth and to be strongly inclined in the vertical, the top part being ahead. The educed coherent structure is responsible by itself for the shape of the velocity profile across the shear layer and for a large part of the turbulence (up to 60 % for the turbulent shear stress). Finally, a coupling is identified between the passage of the Kelvin–Helmholtz structures and the instantaneous wake flow around the cubes at the interface.

Flow experiments are conducted in a two-stage compound open-channel, with varying intensity of the velocity difference between the main channel (deep part) and the floodplain (shallower part), using a large-scale free surface PIV technique (LS-PIV). For all investigated flows, a shear layer develops at the interface between main channel and floodplain, characterised by a peak of turbulent shear stress. Yet, two different kinds of shear layer could be identified. The first kind is characterised by the presence of large-scale quasi-periodic structures of Kelvin-Helmholtz type which are growing in downstream direction, whereas the second kind is characterised by smaller-scale vortical structures without quasi-periodicity and which do not grow in downstream direction. The shear parameter λ =(U_2-U_1)/(U_2+U_1) , where U_1 and U_2 are defined as the velocities outside the shear layer, is identified as a key parameter to distinguish between these two types of shear layers, supporting a result from Proust et al. (Water Resour Res 53: 3387–3406, 2017). A physical interpretation of the λ -criterion is proposed, based on the inhibiting effect of ambient turbulence (the turbulence level outside the shear layer) on the emergence of Kelvin-Helmholtz structures. Accordingly, the threshold value of λ , above which large-scale structures can develop, is dependent on the level of the ambient turbulence. Despite their very different behaviours, the two types of shear layer have the same efficiency to generate turbulent shear stress for a given velocity difference across the shear layer, except for λ -values close to the threshold value.

<p>A laboratory study is reported on the flow perturbations induced by a cylinder across an open-channel flow at a subcritical Reynolds number of <em>Re<sub>D</sub></em> = 10<sup>4</sup> (based on the cylinder diameter). Inspired by field measurements on the Plizska River, Poland (Blanckaert et al., 2014) the investigated configuration is representative of large wood trunks that traverse a river.</p><p>The flow perturbation induced by cylinders has been abundantly investigated in infinite unbounded configurations. Cylinders placed in a bottom boundary layer, such as found in rivers, have hardly been investigated. Previous investigations have demonstrated the importance of different parameters such as the cylinder-based Reynolds number <em>Re<sub>D </sub></em>, the gap ratio <em>G<sub>b</sub></em>, defined as the ratio between the distance from the flume bed to the lower edge of the cylinder and the cylinder diameter <em>D</em>, the blockage ratio <em>Br</em>, defined as the ratio between <em>D</em> and the boundary layer thickness, and the boundary layer turbulence. However, these studies mainly focused on the frequency of the vortex shedding, characterized by the Strouhal number <em>St </em>and not on the characteristics of the wake.</p><p>Thus, a first aim of the present study is to analyze the flow characteristics in the wake of the cylinder, such as the half-width of the wake <em>L0 </em>, the velocity deficit <em>Us </em>, the decay of the perturbations in the streamwise velocity <em>ū</em>, turbulent kinetic energy <em>k</em> and Reynolds stresses.</p><p>A second aim is to analyze the effect of the bed boundary layer on the perturbations induced by the cylinder, and the effect of the latter on the bed boundary layer. This is important as these mutual interactions can have implications on the bed morphology, fluxes of matter including wake retention and hyporheic exchange and stream habitats in general.</p><p>The study reveals that important differences exist between the flow perturbations in unbounded and bounded configurations. In a bounded environment, the wake half-width <em>L0</em> increases at a slower rate along the flow direction and is limited by the boundaries (the free-surface and the bed). Moreover, a faster recovery of the streamwise velocity <em>ū</em> and a faster decay of the velocity deficit <em>Us</em> were observed. These differences with the unbounded case are due to the confinement of the wake and the blockage created by the cylinder. Furthermore, the turbulence generated by the interaction of the wake and the boundary layer, as well as the ambient turbulence also have an impact on these differences.</p><p> </p><p>Blanckaert, K., Han, R., Pilotto, F., and Pusch, M. (2014). Effects of Large Wood on Morphology, Flow and Turbulence in a Lowland River. In International Conference on Fluvial Hydraulics, River Flow 2014, pages 2493–2501</p>

The paper examines the flow through a highly porous canopy patch made of streamwise-oriented thin plates arranged in a staggered configuration and placed in a rough-bed open channel. This patch geometry contrasts with the patches made of bluff bodies, which are nearly exclusively used in the literature. Particle Image Velocimetry was used to measure the flow upstream, within and downstream of the patch. The canopy patch has the effect of drastically reducing the turbulence level of the incoming flow, especially the turbulence shear stress, which is reduced by 85%. Spectral analysis of the velocity shows that the reduction in turbulent kinetic energy occurs at all length scales. Yet, at the entrance of the patch, the energy from the smallest scales up to the scale of the water surface increases. This suggests a spectral shortcut mechanism by which the large-scale structures of the incoming flow are disintegrated by the group of plates instead of decaying through the energy cascade. The increased small-scale turbulent energy then dissipates through the patch.

The concept of canopy-scale resistances was developed to investigate and evaluate the transfer of momentum, heat and mass from the leaf surface to the canopy air space and to the atmosphere. Therefore, reliable estimates of resistances are of fundamental importance for studying the ecosystem scale fluxes and land-atmosphere interaction. The canopy-scale resistance has two components: the leaf boundary layer resistance and canopy-air-to-atmosphere resistance. In big-leaf conceptualizations, canopy-scale resistances are represented in a single term called aerodynamic resistance, which refers to the resistance between an idealized ‘big-leaf’ and the atmosphere for the transfer of momentum, heat and mass. A decent amount of literature exists on the estimation of aerodynamic resistances for various ecosystems based on the roughness length parametrizations and atmospheric stability correction. Most of these parametrizations do not include the leaf boundary layer explicitly and therefore rely on a conceptual 'aerodynamic temperature' at some distance above the leaf surface. This gap hampers reliable modelling of canopy gas exchange (transpiration and CO2 assimilation) as these processes happen directly at the leaf surface and strongly rely on accurately capturing the leaf surface temperature. To bridge this gap, an additional resistance based on a ‘kB-1' parametrization is commonly added to the classical aerodynamic resistance. The objective of the present study is to estimate the total resistance to heat transfer from the heat exchanging surfaces to the measurement height and to find the most appropriate mathematical formulation for this resistance. We used radiometric and eddy covariance (EC) measurements from a wide range of land cover types and estimated the total resistance to heat transport using measured fluxes and radiometric surface temperatures by inverting the flux-profile equation. We also performed a comprehensive comparison of total resistance estimates with commonly used stability and roughness-based resistance formulations, including ‘KB-1' parametrizations and the momentum flux resistance inverted from EC measurements. We found that total resistances were consistently greater than the roughness length-based resistance parametrizations at most of the study sites. We further found that the difference between the total and aerodynamic resistance can be largely explained by dominant leaf sizes at the individual sites. Based on these results, we propose a consistent canopy resistance formulation by explicitly considering leaf sizes and leaf boundary layer resistances in combination with an adequate representation of aerodynamic canopy-atmosphere resistance. This approach will enable a consistent coupling of the aerodynamic process with physiological leaf-scale processes such as photosynthesis and stomatal control, which depend on and interact with leaf temperature, and aerodynamic stability.

The research concerns the hydrodynamic processes around obstacles of cylindrical shape installed across an open channel flow at a subcritical Reynolds number of ReD = 1 x 104 (based on the cylinder diameter), and the forces exerted by the turbulent flow on these obstacles. Based on field measurements performed on the Plizska River, Poland, this study is mainly on cylinders representing large wood trunks that traverse a river.The first aim of the study is to reproduce the flow pattern around an inclined single tree trunk of quasi constant diameter and without branches measured in the field and to enable a more detailed analysis of the underlying turbulent flow processes. These field measurements have shown that horizontal near bank recirculation zones, scour below the trunk and plunge scour overtopping it occurred.The second aim is to compare the mean flow and vortex shedding around inclined and horizontal cylinders across the flow. The effects of inclined and horizontal cylinders on the flow field are very different: the former create a higher variability in flow processes. These configurations differ in gap width below the cylinder and in approach velocity, as the inclined cylinder is located at different elevations in the bottom boundary layer. Both parameters affect the vortex shedding frequency and the wake structure.Results show that a transversally inclined cylinder generates more complex flow patterns and creates a high heterogeneity in the flow as well as the depth. The analysis of the dimensionless shedding frequency also suggests the suppression of vortex shedding near both banks when the gap ratio is small. However, vortex shedding characteristics in the central part of the cross-section are similar for the horizontal and inclined cylinders, i.e. the changing gap ratio below the inclined cylinder does not affect significantly the vortex shedding. In the central part of the cross-section, the wake flow is governed by the interaction of the nearly symmetrical shear layers generated above and below the cylinder. Near the banks, the shear layer near the bed or water surface is suppressed, which could explain the suppression of the vortex shedding.

Recent research explored an alternative energy-centred perspective on hydrological processes, extending beyond the classical analysis of the catchment's water balance. Particularly, streamflow and the structure of river networks have been analysed in an energy-centred framework, which allows for the incorporation of two additional physical laws: (1) energy is conserved and (2) entropy of an isolated system cannot decrease (first and second law of thermodynamics). This is helpful for understanding the self-organized geometry of river networks and open-catchment systems in general. Here we expand this perspective, by exploring how hillslope topography and the presence of rill networks control the free-energy balance of surface runoff at the hillslope scale. Special emphasis is on the transitions between laminar-, mixed- and turbulent-flow conditions of surface runoff, as they are associated with kinetic energy dissipation as well as with energy transfer to eroded sediments. Starting with a general thermodynamic framework, in a first step we analyse how typical topographic shapes of hillslopes, representing different morphological stages, control the spatial patterns of potential and kinetic energy of surface runoff and energy dissipation along the flow path during steady states. Interestingly, we find that a distinct maximum in potential energy of surface runoff emerges along the flow path, which separates upslope areas of downslope potential energy growth from downslope areas where potential energy declines. A comparison with associated erosion processes indicates that the location of this maximum depends on the relative influence of diffusive and advective flow and erosion processes. In a next step, we use this framework to analyse the energy balance of surface runoff observed during hillslope-scale rainfall simulation experiments, which provide separate measurements of flow velocities for rill and for sheet flow. To this end, we calibrate the physically based hydrological model Cat-flow, which distributes total surface runoff between a rill and a sheet flow domain, to these experiments and analyse the spatial patterns of potential energy, kinetic energy and dissipation. This reveals again the existence of a maximum of potential energy in surface runoff as well as a connection to the relative contribution of advective and diffusive processes. In the case of a strong rill flow component, the potential energy maximum is located close to the transition zone, where turbulence or at least mixed flow may emerge. Furthermore, the simulations indicate an almost equal partitioning of kinetic energy into the sheet and the rill flow component. When drawing the analogy to an electric circuit, this distribution of power and erosive forces to erode and transport sediment corresponds to a maximum power configuration.

In rivers, fine sediments are often transported over immobile coarse grains. With low sediment supply, they tend to aggregate in longitudinal ribbons. Yet, the long‐term evolution of such ribbons and the influence of immobile grains on the erosion of fine sediments are still not well understood. Flume experiments without sediment supply were therefore performed to investigate the erosion of an initially uniform fine‐sediment bed covering an immobile bed of staggered spheres through topographic and flow measurements. The topographic measurements yielded the spheres' protrusion above the fine sediment (P) and revealed long‐lived ribbons with ridges and troughs. The ridges are the main long‐term sediment source as the troughs are quickly eroded to a stable bed level resulting from the spheres' sheltering. The ridges stabilize with a spacing of 1.3 effective water depths, their number resulting from the integer number of wavelengths fitting into the effective channel width which excludes side‐wall accumulations. The ridges' erosion is damped by the local upflow of secondary current cells, which displaces the strongest sweep events above the bed. The upflow intensity is controlled by the ridges' height for low P, while for high P by the lateral roughness heterogeneity. The trends in erosion rates over ridges and troughs are similar and characterized by the following sequence of four regimes with increasing P: a drag sheltering, a turbulence‐enhancement, a wake‐interference sheltering, and a skimming‐flow sheltering regime. The critical P levels at the transitions are independent of the flow above the canopy, depending only on the geometrical configuration of the immobile bed.

Because of the absence of the forcing effective rainfall intensity term, Peff, an additional, and simpler, analysis by the authors will likely lead to linearization of the recession hydrograph by an inverse fractional-power transformation of the discharge, 1/Q . This was derived earlier for a nonlinear storage-discharge function by this writer, e.g., Ding (1974, Eq. 4; second equation after Eq. 11).

Abstract. Recent developments in hydrology have led to a new perspective on runoff processes, extending beyond the classical mass dynamics of water in a catchment. For instance, stream flow has been analyzed in a thermodynamic framework, which allows the incorporation of two additional physical laws and enhances our understanding of catchments as open environmental systems. Related investigations suggested that energetic extremal principles might constrain hydrological processes, because the latter are associated with conversions and dissipation of free energy. Here we expand this thermodynamic perspective by exploring how macro and micro hillslope structures control the free energy balance of Hortonian overland flow. This may ultimately help understanding why these structures have evolved to their present shape. To this end, we develop a general theory of surface runoff and of the related conversion of geopotential energy gradients into other forms of energy, particularly kinetic energy as driver of erosion and sediment transport. We then use this framework to analyze how combinations of typical hillslopes profiles and width distributions control the spatial patterns of steady state stream power and energy dissipation along the flow path. Additionally, we provide a first order estimate whether and when rills reduce the overall energy dissipation compared to sheet flow. Finally, we relate accumulated stream power of linear hillslopes to slope angles, closing the loop to Horton's original formulation of erosion force. The analytical analysis of stream power reveals that the common formulation, a function of the depth-discharge product is a reduced version of the more general equations if we neglect changes in velocity and discharge in space. The full equations of stream power result in maximum energy fluxes in space for sinusoidal and exponential hillslope profiles, while linear and negative exponential forms unlimitedly increase these fluxes in the downstream direction. Depending on geometry, rill flow increases or decreases kinetic energy fluxes downslope, effectively counteracting or increasing the dissipation of potential energy. For accumulated power in space for steady state runoff, we find that on linear hillslopes a slope angle of 45° maximizes the conversion of potential energy into dissipation and an angle of 35° maximizes the conversion of potential energy into kinetic energy.

In sediment supply-limited river systems, fine sediments are often transported over a layer of immobile gravel. The level of protrusion of the coarse immobile gravel strongly influences the transport rate of fine sediments which can be sheltered or scoured. This laboratory study examines the effect on the local hydrodynamics of the increasing protrusion of immobile gravel as the fine sediment erodes. The fine sediment and coarse gravel are modelled by fine plastic granulate and two layers of spheres arranged in a staggered pattern, respectively. Simultaneous stereoscopic PIV and stereo-photogrammetric measurements show that the bed is characterized by high erosion rates for mean levels of protrusion P = k/R < 0.7, where is the height of the protruding sphere above the sediment bed and R is its spheres' radius. This is consistent with the observation, that the spheres locally enhance the shear stress for low levels of protrusion. For P = 0.7, though, local sheltering regions appear in the lee of the spheres, which reduce the shear stress acting on the fine sediments. Consistently, the evolution of the bed is seen to slow down and effectively stops for P = 1, when the whole region in between the spheres is sheltered.

Multi-plane telecentric 2D particle image velocimetry (PIV) measurements were performed over and within the interstices of rough beds in an open-channel flow with low relative submergence. Two rough beds constituted of cubes and elongated cuboid obstacles in a square array were investigated with the same modified relative submergence. The telecentric method allowed the complete fluid space within the interstices and above to be measured without disturbing the flow. Nine planes were measured yielding a total of 17 planes by symmetry consideration. This allowed double-average computations to be performed with a 17 $$\times$$ 17 horizontal spatial resolution yielding allowing to resolve adequately the shear stress terms contributing to the total shear stress. It is shown that subsampling with a coarse grid of 1 $$\times$$ 5 yields acceptable spatially converged results, but only for the double-averaged longitudinal velocities. Together, the spatially averaged turbulent shear stresses and the dispersive stresses require a sampling of at least $$9\times 9$$ within the canopy and $$5\times 5$$ above for the errors relative to the total shear stress to remain below 5%. Above the canopy, the dispersive shear stress depends largely on the aspect ratio of the roughness obstacles, being much higher for the cubes than for elongated cuboids. Finally, the converged total shear stress is shown to yield the vertical integrated drag stress as well as the repartition of the flat-bed shear stress to the total shear stress.

The feedback mechanisms within the “triangle” flow-biofilm-sediment are crucial for a variety of important ecosystem functions within the aquatic habitat. With great progress made in measurement techniques in fluid mechanics as well as microbiology, but also in biomechanical understanding, it is now possible and timely to address “old questions” with new approaches within this triangle research. Nevertheless, setting new high standards in spatial resolution (e.g., measuring small scales relevant to biofilm) or addressing natural conditions (e.g., generating controlled, but fully turbulent flow conditions) brings new challenges. To address these issues, a joint workshop was held in Stuttgart, Germany between June 2018 and February 2019, bringing experts from Germany in these key experimental areas together. The main goals of the workshop were i) to consolidate knowledge and identify knowledge gaps in understanding flow-biofilm-sediment interactions and ii) to perform a pilot experiment to demonstrate how the identified knowledge gaps can be addressed by co-application of cutting-edge methods in the fields of hydraulics, sedimentation engineering, microbial ecology and biochemistry. In the pilot experiment, we applied two different Particle Image Velocimetry (PIV) systems to measure flow field in the vicinity of the biofilm at contrasting flow conditions. Surface roughness of the biofilm was quantified through measurements by laser triangulation system and digital microscopy. This combination allowed showing that flow speed affects time of settlement, growth direction and subsequent topography of biofilm. In turn, the biofilm was found to impact flow pattern leading to significant increase in Reynolds stress. With varying flow regimes, we also observed significant shifts in species composition and diversity at both prokaryotic and eukaryotic level by using modern molecular techniques. The molecular microbiological analyses coupled with biochemical characterization of biofilm suggest that quantities of polymeric substances play a minor role in explaining the mechanisms of attachment and binding. Furthermore, microalgae appeared to play a dominant role in biostabilizing the sediment, with more or less impact according to the particular groups or species involved. Thus, the possibility to monitor by hyperspectral imaging the density, composition and distribution of phototrophic biofilms creates a strong predictor for sediment stability and metabolic activity. Microsensors provided measurements of metabolic activity (photosynthesis, respiration) and allowed for novel observations of potential prey-predator relation. Microsensor measurements could be used together with PIV, hyperspectral imaging and optical coherence tomography (OCT) measurements in analysing the external and internal mass transfer by combining substrate distribution, flow dynamics and biofilm structure. Bringing these tools together for improved understanding of flow-biofilm-sediment interactions and ecosystem functions would be a great research concept for hypothesis building. In reference to the workshop results above, we present a timely update on the current knowledge, research approaches and methodological progress that have been witnessed over the last years, especially relevant to biofilm, hydraulics and fine sediment dynamics. The presentation will be concluded by highlighting the important forefront research lines to be answered within the forthcoming years in the flow-biofilm-sediment triangle.