Tidal flow past offshore wind farm (OWF) infrastructure generates a turbulent vortex wake. The wake is hypothesised to enhance seabed stress and water column turbulence mixing, thereby affecting seabed mobility, water column stratification and the transport of nutrients and oxygen and resulting in ecological impacts. We collect novel hydrodynamic data 40 m from an OWF monopile over a spring–neap cycle and use high-frequency velocity measurements to quantify turbulence. Outside of the wake, we observe a classical depth-limited boundary layer, with strong turbulence production and dissipation forced by tidal shear at the seabed. Inside the wake, turbulence production, dissipation and stress are enhanced throughout the full water column and are maximised in the upper half of the water column, where they correspond to a strong mean velocity deficit. Our results show that the seabed drag coefficient is doubled from Cd=3.5×10-3 to 7.8×10-3, suggesting greater seabed mobility, and the eddy viscosity is increased by 1 order of magnitude, indicating enhanced water column mixing. This research provides some valuable insight as OWFs expand into deeper seasonally stratified waters using both bottom-fixed and floating structures, where the addition of enhanced wake turbulence may have broad impacts as the additional mixing energy is added to regions with low rates of background mixing.

There is an urgent need to address coastal dynamics as a fundamental interaction between physical and biological processes, particularly when trying to predict future biological-physical linkages under anticipated changes in environmental forcing. More integrated modelling, support for observational networks and the use of management interventions as controlled experimental exercises should now be vigorously pursued.

The world's shallow continental shelves are currently experiencing a rapid pace of development from the growth of offshore renewable energy. The emplacement of infrastructure on the seabed can change the morphology of the bed, the nature of the flow above it the transport of sediment, and so complicate the assessment of seabed stability for planning and designing offshore renewable infrastructure. To ascertain how much of an impact these natural processes have on the stability of cables, we present the first field observations made directly over a section of subsea cable, from two deployments in the Eastern Irish Sea at a location of current and planned offshore windfarms. Profiles of flow, turbulence and suspended sediment concentration were measured over a section of typical high voltage electricity cable. Upon deployment our observations showed that sediment was deposited around the cable and self-burial occurred. The rate of deposition varied between surveys dependent on forcing and local bed conditions. Turbulence generated from the cable itself reduced as the embedment depth increased, but the relationship between bed shear stress and suspended sediment concentration was not consistent between surveys. We discuss several processes potentially responsible for the prevalence of deposition around the cable, and the difference in seabed mobility between the surveys.

With lack of convergence on any single wave or tidal technology, test centres have a unique role in the marine renewable energy industry. Test centres facilitate real testing at sea for devices and components at various TRLs (Technology Readiness Level), reducing the time, cost, and risks faced by marine energy developers. META (Marine Energy Test Area) is a £2.7M project managed by Marine Energy Wales (MEW), consisting of eight test areas in the Milford Haven Waterway and surrounding waters (Pembrokeshire, Wales). Although various datasets have been collected from the META test areas over the last decade, and some aspects of these data have been published in various reports, the data has not been gathered together, systematically analysed and critically assessed – the aim of this study. Here, we describe and interpret the various META datasets, including multibeam, ADCP (acoustic Doppler current profiler), and wave buoy data. We report the key parameters of relevance to testing at META, including bathymetry, the nature and magnitude of the tidal currents, turbulence, and wave climates. We make recommendations on future priorities for data collection at META, and discuss the future of the test areas, including expansion into floating wind and other evolving marine energy technologies.

Insights from a geoscience communication activity, verified using preliminary investigations with an artificial neural network, illustrate that observation of humans' abilities can help design an effective artificial intelligence or "AI". Even given only one set of "training" examples, survey participants could visually recognize which flow conditions created bedforms (e.g. sand dunes and riverbed ripples) from their shapes, but an interpreter's geoscience expertise does not help. Together, these observations were interpreted as indicating that a machine learning algorithm might be trained successfully from limited data, particularly if it is "helped" by pre-processing bedforms into a simple shape familiar from childhood play.

Predicting sediment transport near the threshold of mobility is a particular challenge in coastal environments, due in part to turbulence in the wake of bedforms and infrastructure but also due to variable grain size distributions and biological processes affecting mobility. Understanding the relevant processes and having the ability to accurately predict sediment transport in shallow shelf seas are currently of pivotal importance due to the prevalence of offshore wind infrastructure being built on mobile seabeds with mixtures of sediment grain sizes. Bridging the gap between the small-scale detail of sediment transport to large-scale modelling is a key challenge for the community. Using a set of novel observations of suspended sediment concentration (via a multifrequency acoustic backscatter system) and turbulence (via Nortek’s Aquadopp High Resolution Doppler Profiler) from a coastal site (~15 m depth) with sandy bed sediments, we revisit the threshold of motion from the perspective of Grass’ 1970’s work by investigating the overlaps of bed shear stress and initiation of motions for the bed sediments. A section of electricity cable was attached to the seabed instrument frame so that on ebb tides turbulent wakes and sediment suspensions from interactions with the cable and frame were measured, and on flood tides a clear boundary layer flow was measured. We create a distribution of initiation of motions from bed sediment data, and from the ADCP data we calculate distributions of bed shear stresses using a temporal filter based on the large eddy turnover time. We investigate the overlap between the two distributions to assess the temporal mobility of the sediments, and discuss how estimating these distributions (and their overlap) can be an important way of improving our predictive capability of sediment transport beyond the usual median grain size and bed shear stress methods – especially important when there are turbulent wakes from bedforms and sea bed infrastructure.

During this pivotal time of energy transition, it is of crucial importance to unlock the potential of the seabed for offshore energy conversion and electrical power transport. With the construction of ever larger offshore windfarms plus other coastal infrastructure, a better understanding of the interactions between the infrastructure and the flow, the flow and the seabed, and all the above with marine life has never been more pressing, as they define feasibility and sustainability of the offshore projects. To better understand the dynamics of the flow in the wake of a large object, the School of Ocean Sciences at Bangor University deployed a bed frame with an Acoustic Doppler Current profiler in the wake of a 10 m-high and steep-crested sediment wave on a seabed 60 meters deep. Vessel-mounted ADCP data was collected simultaneously in orthogonal transects. Velocity profiles near the seabed diverge from the standard law of the wall. On the flood tides, when the flow interacted with the large bedform, increased turbulence in the water column vertically mixed the suspended sediments (measured via the ADCP) into a vertically uniform suspension. On the ebb tides, without any interactions with the bedform, the backscatter shows a boundary layer bursting structure. The enhanced turbulence can affect the sediment composition and bed mobility in these large wakes whether they are natural or anthropogenic, and to numerically model these effects is complex. We discuss the wider impacts of this work, as changes to sediment, seabed and water column properties can affect aggregations of prey that crucially depend on it. These changes can then extend through the food chain and contribute to the ecological impacts of windfarms, both as risks and as opportunities.

Abstract. 42 survey participants demonstrate that it is visually possible to recognise the type of flow that created bedforms (e.g. sand dunes, riverbed ripples) from short distance-depth profiles, but this is much harder for individual forms. An interpreter's geoscience expertise does not help, indicating a machine learning or 'AI' algorithm might be trained well from the data alone, especially if multiple bedforms are used.

Current understanding of the role that dunes play in controlling bar and channel‐scale processes and river morphodynamics is incomplete. We present results from a combined numerical modeling and field monitoring study that isolates the impact of dunes on depth‐averaged and near‐bed flow structure, with implications for morphodynamic modeling. Numerical simulations were conducted using the three‐dimensional computational fluid dynamics code OpenFOAM to quantify the time‐averaged flow structure within a 400 m × 100 m channel using digital elevation models (DEMs) for which (i) dunes and bars were present within the model and (ii) only bar‐scale topographic features were resolved (dunes were removed). Comparison of these two simulations shows that dunes enhance lateral flows and reduce velocities over bar tops by as much as 30%. Dunes influence the direction of modeled sediment transport at spatial scales larger than individual bedforms due to their effect on topographic steering of the near‐bed flow structure. We show that dunes can amplify, dampen, or even reverse the deflection of sediment down lateral bar slopes, and this is closely associated with 3‐D and obliquely orientated dunes. Sediment transport patterns calculated using theory implemented in depth‐averaged morphodynamic models suggest that gravitational deflection of sediment is still controlled by bar‐scale topography, even in the presence of dunes. However, improved parameterizations of flow and sediment transport in depth‐averaged morphodynamic models are needed that account for the effects of both dune‐ and bar‐scale morphology on near‐bed flow and sediment transport.

Images from specially-commissioned aeroplane sorties (manned aerial vehicle, MAV), repeat unmanned aerial vehicle (UAV) surveys, and Planet CubeSat satellites are used to quantify dune and bar dynamics in the sandy braided South Saskatchewan River, Canada. Structure-from-Motion (SfM) techniques and application of a depth-brightness model are used to produce a series of Digital Surface Models (DSMs) at low and near-bankfull flows. A number of technical and image processing challenges are described that arise from the application of SfM in dry and submerged environments. A model for best practice is presented and analysis suggests a depth-brightness model approach can represent the different scales of bedforms present in sandy braided rivers with low-turbidity and shallow (< 2 m deep) water. The aerial imagery is used to quantify the spatial distribution of unit bar and dune migration rate in an 18 km reach and three 1 km long reaches respectively. Dune and unit bar migration rates are highly variable in response to local variations in planform morphology. Sediment transport rates for dunes and unit bars, obtained by integrating migration rates (from UAV) with the volume of sediment moved (from DSMs using MAV imagery) show near-equivalence in sediment flux. Hence, reach-based sediment transport rate estimates can be derived from unit bar data alone. Moreover, it is shown that reasonable estimates of sediment transport rate can be made using just unit bar migration rates as measured from 2D imagery, including from satellite images, so long as informed assumptions are made regarding average bar shape and height. With recent availability of frequent, repeat satellite imagery, and the ease of undertaking repeat MAV and UAV surveys, for the first time, it may be possible to provide global estimates of bedload sediment flux for large or inaccessible low-turbidity rivers that currently have sparse information on bedload sediment transport rates. (c) 2018 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.

In this short paper, we demonstrate a new method of deciphering the state of nonequilibrium bedforms using Semi-Variograms. Semi-variograms allow us to take a measure of uniformity in repeating features; therefore, the technique implicitly assumes that equilibrium is defined by uniformity. Comparison of the semi variogram produced from 5 selected profiles of dunes at different states of equilibrium and boundary conditions from the Mekong River (Cambodia) is compared against the semi variogram produced from an idealised and identical train of bedforms. We find that the empirical semi-variogram of a bedform profile is more than adequate at estimate the mean length of the bedforms, can decipher the dominant scales of bedforms. Moreover, this technique opens up the possibility of quantifying the degree of dis-equilibrium in the bed state that departs from basic geometric measurement of height, length and aspect ratio, and also does not require a series of repeated measurements: which is uncommon in large MBES surveys. Marine and River Dune Dynamics – MARID VI – 1-3 April 2019 Bremen, Germany 226 1981; Kadota & Nezu, 1999; Unsworth et al., 2018). 2) changing the rate of sediment supply though altering the ratio of suspended to bedload fraction (Naqshband et al., 2014; Reesink et al., 2018; Schindler & Robert, 2005; Wren et al., 2007), largely a response to bedform interactions in or out of equilibrium (Blois et al., 2012; Ewing & Kocurek, 2010; Reesink et al., 2018). The semi-variance  of a random function  is described by half the variance of the increment: 2       (1) Where G is the random variable of interest, h is the lag or distance with respect to x. The empirical semi variance is estimated by: 2     ∑        (2) Where N is the number of observations. The semi-variogram of three types of dune shapes is described in Figure 1. In this figure the maximum lag distance  has been set to the same length scale as the length of the bedform profiles so any variation in the shape of the dunes would be picked up. The shape of the variogram appears to not correspond to the shape or asymmetry of the bedform – it is always a sine wave when given identical repeating bedform. This is a useful feature as we wish to test this method of quantifying equilibrium on a range of bedform shapes. To provide a range of realistic bed states to test these ideas on, Multibeam echosounder (MBES) bathymetry from the Mekong River in Cambodia is used as it provides a range of flow discharges, bedform states and is large enough that superimposed bedforms are well resolved in the bathymetry. Figure 1. Three sets of idealised repeating dune shapes and their semi-variogram. The triangular dunes in a) are those form (Unsworth et al., 2018); b) half-sine wave shape is from (Nelson et al., 1993). The repeating dune shapes produce a sine-wave in the semi-variogram that is correlated to the dune troughs – where the profiles begin. The location of each trough in the semi-variograms directly corresponds to the wavelength of the bedform. The sine wave bedforms in c) provide a symmetrical bedform profile, the semi-variogram of which produces a sine wave 45 degrees out of phase.

Most past experimental investigations of flow over river dunes have focused on conditions that match semiempirical flow-depth scaling laws, yet such equilibrium conditions are of limited value because they rarely occur in natural channels. This paper quantifies the structure of mean and turbulent flow over fixed 2-D laboratory dunes across a range of nonequilibrium conditions within the dune flow regime. The flow field was quantified using 2-D particle imaging velocimetry for 12 conditions, including flows that are too deep, too shallow, too fast, or too slow for the size of the fixed dunes. The results demonstrate major departures in the patterns of the mean flow and structure of turbulence when compared to dunes formed under equilibrium flow conditions. The length of flow reattachment scales linearly with the ratio of mean depth-averaged streamwise velocity to shear velocity at the dune crest ((U) over bar (c) / u*c), which provides a new predictive measure for flow reattachment length. Depth-averaged vertical velocities at the dune crest ((V) over bar (c)) show a parabolic relationship with (U) over bar (c), peaking at (U) over bar (c) similar to 0.60 m/s, which matches the relationship of dune aspect ratio with transport stage present in mobile bed conditions. The spatial location of the turbulent wake was found to vary with flow depth and velocity, with lower Uc and greater flow depths causing the wake to rise toward the free surface. Deeper flows are likely to show less flow convergence over the crests of dunes due to reduced interaction of turbulence with the free surface, resulting in a reduction of transport stage. Plain Language Summary This piece of research expands our description of how rivers flow over dunes on a river bed. Most of the scientific communities' research to date has used unnaturally steady conditions to measure how water moves over dunes. Yet these flow conditions are not strictly true to the variety of conditions nature produces, most importantly during floods. This research is the first detailed description of a wide range of flow states over dunes and changes our present understanding of the structure of flow over dunes in rivers. Consequently, the scientific community will be able to use this new information to better model and simulate how rivers work, how they flood, and how they transport sediment toward the world's deltas. Plain Language Summary This piece of research expands our description of how rivers flow over dunes on a river bed. Most of the scientific communities' research to date has used unnaturally steady conditions to measure how water moves over dunes. Yet these flow conditions are not strictly true to the variety of conditions nature produces, most importantly during floods. This research is the first detailed description of a wide range of flow states over dunes and changes our present understanding of the structure of flow over dunes in rivers. Consequently, the scientific community will be able to use this new information to better model and simulate how rivers work, how they flood, and how they transport sediment toward the world's deltas.

The dunes that cover the beds of most alluvial channels change in size and shape over time and in space, which in turn affects the flow and sediment-transport dynamics of the river. However, both the precise mechanisms of such adaptation of dunes, and the hydraulic variables that control these processes, remain inadequately understood. This paper provides an overview of the processes involved in the maintenance and adaptation of dunes, provides new tools for the analysis of dune dynamics, and applies these to a series of bespoke experiments. Dunes that grow compete for space, and dunes that decay need to shed excess sediment. Therefore, dune adaptation necessarily involves the redistribution of sediment over and among dunes. The details of sediment redistribution are not captured by mean geometric parameters such as dune height and wavelength. Therefore, new analyses of dune kinematics, bed-elevation distributions, and dune deformation are presented herein that aid the identification and analysis of dune dynamics. Dune adaptation is often described as a morphological response to changes in water depth at a rate that depends on sediment mobility, which itself is a product of flow depth and velocity. However, depth and velocity are out-of-phase during the passage of flood waves, and they vary spatially across rivers from the thalweg to bar tops, and downstream along the river profile. In order to improve our understanding of the hydraulic controls on dune morphology and kinematics, a series of experiments was performed to investigate the response of dunes in fully-mobile sand (D-50 = 240 pm) to changes in flow depth and velocity. The experimental results illustrate that water depth and flow velocity have separate effects on the processes that control dune adaptation, and that the crests and troughs of dunes do not respond simultaneously to changes in flow. Trough scour increases with flow velocity, but superelevation of the dune crests appear to show only a weak relation with flow depth. Flattening-out of dune crests is related to decreasing depth and increasing flow velocity. Bedform superimposition, a key feature of bedform kinematics, was associated with increased flow depth, but was also systematically associated with local increases in the crest-to-crest distance following the dissipation of an upstream dune. Thus, local flow-form interactions have a significant effect on the manner in which sediment is redistributed over and among dunes. The splitting of dunes decreased in the downstream direction along the length of the flume, illustrating that the dunes continue to interact even after dune height has stabilised. Other processes, such as differential migration and dune merging, are ubiquitous during all flow conditions. These varied responses support the notion that the processes of dune adaptation vary over time and in space. Analysis of dune deformation through examination of the residuals of cross-correlations between successive dune profiles illustrates that local sources and sinks of sediment exist within mobile dune fields. These findings highlight that dune adaptation to changes in flow is a dynamic response involving multiple interconnected dunes. The redistribution of sediment that is required for dunes to change shape and adapt to new conditions is expected to be an important cause of variability in sediment transport. These detailed analyses and findings provide a foundation for further study of dune dynamics in different environments on Earth as well as other planetary bodies.

The morphodynamics of sandy braided rivers are complicated by the presence of bedforms, which alter the spatial distribution of momentum, shear and sediment transport. These effects are not understood well and are typically simplified in morphodynamic models. This paper presents acoustic Doppler Current Profiler (aDcp) measurements of flow, and Unmanned Aerial Vehicle (UAV) photo-grammetric surveys of a 600 m reach of the sandy braided South Saskatchewan River, Canada. The response of alluvial bedforms to varying flow direction, bed-slope and sediment availability is examined, and the use of bedform crestline tracking to estimate sediment transport direction is discussed.

Flow processes measured in the laboratory over fixed, 2D or 3D bedforms have mostly been conducted at one flow depth and with bedform dimensions set by scaling laws based upon “equilibrium” flow conditions. These results thus have limited applicability to many natural situations where bedforms and flow fields are co-evolving at different rates in response to transient conditions, such as changes in flow depth and flow discharge associated with a flood. The research presented herein investigates flow processes over 2D fixed bedforms under a range of non-equilibrium, transient, states in order to quantify the spatio-temporal changes in turbulence associated with steady conditions that are set at non­ equilibrium depths and velocities. Flow field information was obtained at steady states for a range of flow depths and mean flow velocities, mimicking conditions during the transient evolution of flow and bedforms during a flood wave. This allowed quantification of flow fields over bedforms under transient boundary conditions, including shear stress profiles and the spatial variation in the dynamics of the separation zone. These findings provide data for a preliminary assessment of the link between sediment transport lag and transient flow dynamics, and facilitate an analysis of the implications of variable dune height: flow depth for flood wave propagation and bedform response.