The offshore wind industry is rapidly expanding to meet clean and secure energy needs. New developments are now increasingly constrained to deeper waters, where the water column is seasonally stratified. Here flows past offshore wind infrastructure will increase water column mixing, although such processes and their extent are poorly understood. Studies have so far been limited to field-scale simulations, which make sweeping assumptions regarding flow-structure interactions and fine-scale stratified turbulence, and field observations, which are limited by the sparsity of measurement campaigns and data captured. To isolate and quantify the key processes governing water column mixing by infrastructure, we present fully structure-resolved direct numerical simulations of two-layer stratified flow past a vertical cylinder. We identify two wake regimes by systematically varying the flow Reynolds and Richardson numbers: (1) a weakly stratified regime, characterized by a narrow but highly energetic wake dominated by horizontal shear and (2) a strongly stratified wake, characterized by the emergence of a thermocline-spanning recirculation cell attached to the cylinder. Here strong vertical motions develop from the recirculation cell which are responsible for the formation of large-scale stationary internal waves. These waves account for up to 10% of the total energy budget and provide a mechanism for far-field energy propagation. The weakly stratified wake regime is characteristic of existing offshore wind sites where temperature gradients are relatively weak; the identified strongly stratified regime describes the dynamics to be expected in future deep water offshore wind sites. This difference between the two regimes offers an explanation for the previously enigmatic discrepancies in field observations regarding wake persistence and detectability. Future work must focus on narrowing the gap between idealized simulations and field-scale flows, for which the datasets herein will provide a critical benchmark for validation.

As global energy demands and climate concerns continue to grow, the need for renewable energy is becoming increasingly clear and wave energy converter (WEC) systems are receiving growing interest. WECs often utilize optimal control techniques for power take-off operation and leverage a prediction of the upcoming wave force to ensure power production optimization. Prior work has clearly demonstrated that high power production can be achieved when an exact system model is used and the upcoming wave conditions are known, but uncertainty in the underlying model or the wave prediction can degrade performance. The uncertainty in these predictions and the model could degrade the WEC’s power output. This work examines the impact of uncertainty on the control of a WEC system that leverages machine learning to predict wave forces over the upcoming time horizon. This paper quantifies wave prediction uncertainty and its seasonal variation and illustrates that this uncertainty may only minimally degrade power output on complex multi-axis WECs due to the strong influence of constraints in the system.

Rivers are primary vectors of plastic debris to oceans, but sources, transport mechanisms, and fate of fluvial microplastics (<5 mm) remain poorly understood, impeding accurate predictions of microplastic flux, ecological risk and socio-economic impacts. We report on microplastic concentrations, characteristics and dynamics in the Mekong River, one of the world's largest and polluting rivers, in Cambodia and Vietnam. Sampling throughout the water column at multiple localities detected an average of 24 microplastics m(-3) (0.073 mg l(-1)). Concentrations increased downstream from rural Kampi, Cambodia (344 km from river mouth; 2 microplastics m(-3,) 0.006 mg l(-1)), to Can Tho, Vietnam (83 km from river mouth; 64 microplastics m(-3), 0.182 mg l(-1)) with most microplastics being fibres (53 %), followed by fragments (44 %) and the most common polymer being polyethylene terephthalate (PET) or polyester. Pathways of microplastic pollution are expected to be from urban wastewater highlighting the need for improved wastewater treatment in this region. On average, 86 % of microplastics are transported within the water column and consequently we identified an optimum sampling depth capturing a representative flux value, highlighting that sampling only the water surface substantially biases microplastic concentration predictions. Additionally, microplastic abundance does not linearly follow discharge changes during annual monsoonal floods or mirror siliciclastic sediment transport, as microplastic concentrations decrease rapidly during higher monsoon flows. The findings reveal complex microplastic transport in large rivers and call for improved sampling methods and predictive models to better assess environmental risk and guide policy.

The threshold hydraulic condition of wave ripple initiation has been essential in a wide range of disciplines, including coastal engineering and geology, but remains unclear for the case of sand-mud mixtures as substrates. Scale experiments were conducted using an oscillating-bed to study wave ripple initiation in sand-mud mixtures, considering bed material, wave period and maximum orbital velocity. Four kinds of sand-mud mixtures were employed as bed material with different muds (cohesive kaolin and non-cohesive silt) and mixing ratios. Ripple initiation in the cases with sand-mud mixtures tended to require larger orbital velocity than pure sand, indicating that mud mixed in bed material hinders ripple formation and spreading of the ripple field. The results of the threshold for pure sand and 10% kaolin mixture show that cohesive mixed sand-mud hindered ripple formation and spreading. In contrast, in the case of the non-cohesive sand-silt mixture beds, the results differed depending on the proportion of sand and silt, which implied the effect of the strength of network structure. The inhibition of ripple initiation due to mud mixing observed in the present experiments is considered to have a significant effect, particularly in environments with limited wave action duration, such as intertidal zone.

The interplay between seafloor sediment laden density-driven flows, turbidity currents, and topography helps to shape continental margins. However, these interactions are poorly understood, especially those within enclosed depressions termed minibasins. In this study, novel experiments quantify the three-dimensional (3-D) dynamics of turbidity currents interacting with a range of minibasin geometries that, for the first time, scale within the parameter space of natural systems. Controls on flow dynamics are quantified by measuring the evolving velocity and sediment transport fields, in addition to maps of bathymetry. This study focuses on three aspects of turbidity current interactions with minibasins. First, the results suggest that sediment transport and deposition in minibasins is likely dominated by evolving flow conditions. Contrary to earlier studies in two-dimensional (2-D) flumes, this study supports a time-to-flow equilibrium in mini basins that scales with the time to replace ambient fluid with turbid influx, and this replacement time likely takes days to achieve in many field-scale minibasins. Second, in all experiments, horizontal flow circulation is observed, which is critical for distributing sediment throughout minibasins. However, the strength of the horizontal circulation reduces as the ratio of minibasin length to width increases, which leads to stagnant or even upstream-directed flow near the bed, elevated height of the velocity maximum in flows, the lowering of near-bed shear stresses, and more homogeneous deposits through a reduction in bed reworking. Finally, the results indicate that fluid detrainment from minibasins significantly reduces sediment fall velocities, severely lowering the sediment trapping efficiency for small or light particles. This reduction in effective fall velocities of sediments suggests a mechanism that fractionates fine particulates (e.g., clays), nutrients (e.g., organic carbon), and pollutants (e.g., microplastics) along transport paths down topographically complex margins.

This work is focussed on understanding the fundamental fluid dynamics of tidal wakes generated by offshore wind infrastructure in stratified waters, using direct numerical simulations. The tidal flows past the structures are approximated by a uniform quiescent background flow with a two-layer density profile, interacting with a vertically oriented cylinder. Through these simulations we identify the processes through which turbulence generated in the wake of the structures leads to vertical mixing across the thermocline.We identify two fundamentally different flow regimes, dependent on both the stratification strength and the flow Reynolds number. The 'weakly stratified' wake is characterised by a highly energetic wake and a dominance of horizontal shear. As a result, vertical mixing occurs much further downstream than the region of maximum turbulent kinetic energy production. In contrast, the `strongly stratified' wake regime is characterised by a large-scale recirculation region that develops across the thermocline which generates significant vertical shearing. This subsequently leads to time-independent standing waves which account for up to 10

Floating offshore wind turbine (FOWT) platforms are subject to a wide range of hydrodynamic loading and dynamic movement, making hydrodynamic force evaluation difficult. Amongst various floating platforms, submersible platforms are structurally complex, with multiple members held together by cross-braces. The influence of these members on hydrodynamic loading is poorly understood. An investigation of the effect of these members on loads is essential to optimise the design of FOWT platforms, mooring systems, and protective coatings, leading to a reduction in construction and maintenance costs. This paper numerically investigates the effect of structural members on the forces acting on a static semi-submersible platform in a unidirectional current flow of Reynolds number (Re) ranging from 2000 to 200,000, based on structural diameter and tidal velocity. The OC4 semi-submersible is chosen as the baseline platform. For each Re, this study is divided into three stages, such that in each stage, the number of members increased. These stages are as follows: (1) a finite cylinder (FC), (2) a finite cylinder with a heave plate (FCHP), (3) three cylinders with heave plates (TCHP) in an equilateral triangle arrangement, and (4) the OC4 semi-sub. The drag coefficient (C¯d) increases with increasing structural members and weakly varies with increasing Re. However, the viscous drag coefficient (C¯f) decreases with increasing Re, and a reverse trend is seen in the case of the pressure drag coefficient (C¯p), with pressure drag dominating over friction drag. Further, the contribution of individual members is observed to vary with Re. The contribution of cylinders towards C¯d is higher than heave plates, showing that contributions directly depend on the aspect ratio of members. In the case of TCHP and OC4, the contribution of the rear members is higher than that of the leading members due to the strong wake effect of the former. Also, the braces and pontoons of OC4 have contributed substantially towards total C¯d, unlike the central cylinder, which has experienced low drag due to the wake effect of the front cylinder and heave plate. Also, flow visualisation has shown vortex cores, and recirculating flows in the near wake of the cylinders and under the heave plates. Recirculation zones under the heave plates lead to vertical pressure on the structures. This vertical pressure increases with the number of structural members and the vertical pressure coefficient (C¯v), varying with Re due to three-dimensionality in the wake. Further, this pressure varies across the bottom surfaces of structures. Analyses of the streamwise pressure coefficient have shown it is highest on the front surfaces of cylinders. The highest friction is on the top and sides of the heave plates, and there is considerable friction on the sides of the cylinder.

Renewable energy sources, including offshore wind energy, are fundamental to reducing fossil fuelconsumption and greenhouse gas emissions. Many countries are planning for a rapid and massive ex-pansion of the offshore wind sector to meet the NetZero goals. So far, the installation of offshore windturbines (OWT) has been restricted to near-shore shallow water (≤ 60m). However, future expansionof the sector will be in deep waters, away from the shore, where the wind speed is stronger and moreconsistent. Monopiles, the most commonly used foundations for OWT, become uneconomical or tech-nologically unfeasible in deep waters. Therefore, OWT supported by floating platforms is the way to goforward. The initial platform designs and construction were based on the experience obtained from theoil and gas industry (O&G). However, the load acting and the movement of the floating offshore windturbine (FOWT) platforms are vastly different from the O&G platforms. In addition to the aerodynamicloading, these platforms are subjected to hydrodynamic loading, making platform design a complex task.Evaluating the forces acting on these platforms, even under idealistic conditions, is challenging. Althoughsignificant progress has been made, platform, anchor, mooring, and turbine design improvement dependson accurate load calculation. Further, understanding hydrodynamic loading is essential to evaluate theenergy losses due to the FOWT system and, therefore, the mixing of the water column behind the struc-ture. In this research, the effect of increased geometric complexity on load acting on a semi-submersibleplatform is numerically investigated. Three unidirectional flow regimes of Reynolds number (Re) = 2900,43000, and 200000 are investigated, using the OC4 semi-submersible platform as the reference. The OC4semi-submersible platform was developed by the OC4-DeepCWind consortium to obtain experimentaldata and validate numerical models for FOWT. The results show that the drag force acting on the plat-form increases as the Re and number of members in the platform increases. These findings are importantin understanding the hydrodynamic loading on FOWT platforms under static conditions and designingthe platform, mooring and anchoring systems. Further, this is essential for the sustainable developmentof the offshore wind energy sector.

Rivers polluted by plastics have become sites where mixtures of microplastics and sediment particles are transported by the river current and deposited in the riverbed. A hydromorphodynamic numerical model was developed using Delft3D (software specialized in simulating natural water systems), to simulate the sedimentation, erosion, resuspension and transport of microplastics together with sediment particles, introducing an innovative model with an active riverbed. The model was used to understand the distribution patterns, morphological changes and load balances of plastic debris in a river. The study case is an artificial braided river with a non-buoyant suspended microplastic load. The results simulate a sediment bed that acts as a source of microplastic storage near the point of release. The high deposition of microplastics increases the capacity of the river flow to erode the banks and channels, resulting in deeper channels and larger river bars. The highest amounts of microplastics were deposited in the inner channel banks, and the highly suspended microplastic load is transported in the main channel thalweg. The model can be used as a more accurate method to predict the dynamics of microplastic fluxes in rivers, providing better tools to understand how much plastic enters the ocean from the river environment.This article is part of the Theo Murphy meeting issue 'Sedimentology of plastics: state of the art and future directions'.

Natural depressions on continental margins termed minibasins trap turbidity currents, a class of sediment-laden seafloor density driven flow. These currents are the primary downslope vectors for clastic sediment, particulate organic carbon, and microplastics. Here, we establish a method that facilitates long-distance self-suspension of dilute sediment-laden flows, enabling study of turbidity currents with appropriately scaled natural topography. We show that flow dynamics in three-dimensional minibasins are dominated by circulation cell structures. While fluid rotation is mainly along a horizontal plane, inwards spiraling flow results in strong upwelling jets that reduce the ability of minibasins to trap particulate organic carbon, microplastics, and fine-grained clastic sediment. Circulation cells are the prime mechanism for distributing particulates in minibasins and set the geometry of deposits, which are often intricate and below the resolution of geophysical surveys. Fluid and sediment are delivered to circulation cells by turbidity currents that runup the distal wall of minibasins. The magnitude of runup increases with the discharge rate of currents entering minibasins, which influences the amount of sediment that is either trapped in minibasins or spills to downslope environs and determines the height that deposits onlap against minibasin walls.

Rivers are primary vectors of plastic debris to oceans, but sources, transport mechanisms, and fate of fluvial microplastics (<5mm) remain poorly understood, impeding accurate predictions of microplastic flux, ecological risk and socio-economic impacts. We report, for the first time, on microplastic concentrations, characteristics and dynamics in the Mekong River, one of the world’s largest and polluting rivers, in Cambodia and Vietnam. Sampling throughout the water column at multiple localities detected an average of 24 microplastics m-3 (0.073mg l-1). Concentrations increased downstream from Kampi, Cambodia (344km from river mouth; 2 microplastics m-3, 0.006mg l-1), to Can Tho, Vietnam (83km from river mouth; 64 microplastics m-3, 0.05mg l-1) and most microplastics were fibres (53%). On average, 86% of microplastics are transported within the water column and consequently we identified an optimum sampling depth capturing a representative flux value, highlighting that sampling only the water surface substantially biases microplastic concentration predictions. Additionally, microplastic abundance does not linearly follow discharge changes during annual monsoonal floods or mirror siliclastic sediment transport, as concentrations decrease rapidly during higher monsoon flows. The findings reveal complex microplastic transport in large rivers and call for improved sampling methods and predictive models to better assess environmental risk and guide policy.

The controls on the development of submarine channel sinuosity are contested: slope gradient and Coriolis forcing have both been recognized as key governing factors: gradient via an inverse relationship (low sinuosity at high slope and vice versa), and Coriolis forcing through its effect on sedimentation patterns (reducing lateral bend migration, and hence sinuosity development, at high latitudes and/or in large channels). Using theoretical models to calculate the bulk properties of channelized turbidity currents, this study investigates the joint role of the Coriolis force and parameters including channel size, downchannel slope and turbidity current properties in the development of submarine channel sinuosity. Model validation is undertaken through the comparison of the calculated turbidity current tilting against the measured tilting of channel levees in the Northwest Atlantic Mid-Ocean Channel; this approach is then used to evaluate the controls on channel sinuosity in nine other modern seafloor channels. The results indicate that the Coriolis force only becomes significant when the size of the channel, the slope gradient and flow conditions are within appropriate ranges instead of solely being dependent on latitude. Thus, thick and dense (>= 1% bulk sediment concentration) flows traveling within steep-gradient, small-scale channels were shown to be relatively less susceptible to flow modification by Coriolis forcing even at high latitudes. On the other hand, thin and dilute (<< 1% bulk sediment concentration) flows in shallow-gradient, large-scale channels showed susceptibility to Coriolis forcing at all latitudes. These results offer new insights into submarine channel evolution and intra-channel sedimentation patterns. Sediments are widely distributed in the oceans by underwater currents akin to powder snow avalanches. These "turbidity currents" may sculpt the sea floor to build submarine channels which, like rivers, may range in sinuosity from being virtually straight to highly sinuous. Several competing controls have been suggested to explain this variation. Some argue that slope is most important, with low sinuosity channels forming on high angle slopes and vice versa. Others claim that the Coriolis force, which affects flows moving across a rotating surface (such as the Earth), is the main control - either via latitude alone (with high sinuosity channels restricted to lower latitudes) or only affecting channels that are large enough. To test these ideas we developed a new numerical modeling approach that looks at the combined effects of channel axis gradient, channel size and flow conditions. By modeling the tilt of turbidity currents flowing around bends we show that single factors cannot be used to explain channel sinuosity. The model is tested with real world data. Although sinuosity is generally greater at low latitudes there are exceptions; variations across a range of controlling factors can produce channels of any sinuosity at any latitude. We present a novel methodology to assess the contested controlling factors on the development of sinuous channels Deep and dense flows that pass through small channels in steep slopes, regardless of the latitude, promote sinuosity Dilute and shallow flows that pass through large channels in low gradient systems hinder sinuosity development at any latitude

Offshore wind farms are becoming an increasingly common feature in the marine environment as a renewable energy source. There is a growing body of evidence on the effects of wind farms on the seabed and its organisms. However, an important and understudied aspect of site development is the interaction of turbine foundations on the surrounding marine environment. Structures exert significant disturbance on tides, waves and currents; these are visible as optically-distinct, elongate wakes at the sea surface with elevated suspended particulate matter. Despite this, there is uncertainty on the mechanisms that lead to the visible manifestation of wakes at turbine foundations, primarily due to a lack of direct measurements. Here, in situ measurements along with a 15-year time series of satellite images of the Thanet offshore wind farm, located within the Thames Estuary, were used to investigate the formation of visible monopile wakes, and the effects these have on the surrounding water column. We show the optically distinct wakes are near-constant at Thanet; visible in >90% of all satellite images, yet no regional change in sea surface turbidity could be attributed to wind farm construction or operation. Monopile wake in situ water samples and acoustic Doppler current profiler (ADCP) backscatter measurements demonstrated colour change related to elevated sea surface sediment concentration. However, averaged water column measurements of suspended sediment within wakes, and upstream of monopiles, remained consistent. These measurements demonstrate that sediment was redistributed towards surface waters, rather than additional sediment becoming suspended in the wake. ADCP velocity measurements supported a mechanism of sediment lofting towards the surface, with enhanced vertically upwards flow recorded in wakes.

Abstract Submarine channels are key features for the transport of flow and nutrients into deep water. Previous studies of their morphology and channel evolution have treated these systems as abiotic, and therefore assume that physical processes are solely responsible for morphological development. Here, a unique dataset is utilised that includes spatial measurements around a channel bend that hosts active sediment gravity flows. The data include flow velocity and density, alongside bed grain size and channel‐floor benthic macrofauna. Analysis of these parameters demonstrate that while physical processes control the broadest scale variations in sedimentation around and across the channel, benthic biology plays a critical role in stabilising sediment and trapping fines. This leads to much broader mixed grain sizes than would be expected from purely abiotic sedimentation, and the maintenance of sediment beds in positions where all the sediment should be actively migrating. Given that previous work has also shown that submarine channels can be biological hotspots, then the present study suggests that benthic biology probably plays a key role in channel morphology and evolution, and that these need to be considered both in the modern and when considering examples preserved in the rock record.

We investigate a turbulent stratified plane Poiseuille flow using linear models and nonlinear simulations. We propose the first complete explanation for the prolific and coherent backward (BWs)- and forward-propagating waves (FWs), which have been observed in these flows. We demonstrate a significant presence of oblique waves in the channel core, particularly for the FWs. Critically, we show that neglect of spanwise structure leads to a distorted dispersion relation due to its strong dependence on the angle of obliquity. Interestingly, solutions to the Taylor-Goldstein equations show that wave dynamics is strongly dependent on shear, with only a weak dependence on buoyancy for the BWs at low and order-one wavenumbers, when the wavenumber is scaled by the channel half-height. As the wavenumber increases, waves transition from a shear-dominated regime to a buoyancy-dominated regime, with their dispersion relation tending towards that of idealised internal waves subject to a shear-free and constant-buoyancy-gradient flow, with a characteristic velocity and buoyancy frequency corresponding to respective centreline values in the channel. Finally, we show that the dominance of the BWs arises due to the external forcing of the system, whereby turbulent fluid ejected into the core has a lower momentum when compared with the local flow, therefore preferentially generating BWs in the channel. Qualitatively, channel-core dynamics can be reproduced with low-momentum forcing to a velocity profile with a velocity maximum and a corresponding negative second derivative intersecting a region of strong buoyancy gradient. This structure is inherent to a wide variety of jet-like environmental, atmospheric and industrial flows, suggesting that BWs are a critical control on dynamics of such flows.

ABSTRACTSedimentary bedforms such as ripples and dunes are generated both by river flows and sediment‐laden gravity currents. Gravity current deposits are usually parameterized using existing bedform phase diagrams which are based on data from laboratory experiments and field observations of open‐channel flows. Yet, it is not evident that open‐channel flow bedform phase diagrams are applicable to gravity current deposits. Gravity current hydrodynamics are dependent on vertical density variation, that is density stratification, and therefore are fundamentally different from open‐channel flows. New experiments to produce gravity current deposits are conducted and compared to existing open‐channel flow data. It is shown that a parameter phase‐space based on the lower layer of stratified gravity currents (i.e. that part below the velocity maximum) significantly improves the prediction of bedform type compared to bedform phase diagrams derived from layer‐averaged parameters. These results confirm that bedforms produced by gravity currents can only be predicted accurately using the characteristics of the lower layer of stratified flow.

We investigate the flow over smooth (non-ribletted) shark skin denticles in an open-channel flow using direct numerical simulation (DNS) and two Reynolds averaged Navier–Stokes (RANS) closures. Large peaks in pressure and viscous drag are observed at the denticle crown edges, where they are exposed to high-speed fluid which penetrates between individual denticles, increasing shear and turbulence. Strong lift forces lead to a positive spanwise torque acting on individual denticles, potentially encouraging bristling if the denticles were not fixed. However, DNS predicts that denticles ultimately increase drag by 58% compared to a flat plate. Good predictions of drag distributions are obtained by RANS models, although an underestimation of turbulent kinetic energy production leads to an underprediction of drag. Nevertheless, RANS methods correctly predict trends in the drag data and the regions contributing most to viscous and pressure drag. Subsequently, RANS models are used to investigate the dependence of drag on the flow blockage ratio (boundary layer to roughness height ratio), finding that the drag increase due to denticles is halved when the blockage ratio δ / h is increased from 14 to 45. Our results provide an integrated understanding of the drag over non-ribletted denticles, enabling existing diverse drag data to be explained.

Flow in the body of gravity currents is typically assumed to be statistically two-dimensional, and cross-stream flow is often neglected (Simpson 1997 ; Meiburg et al. 2015 ). Here, we assess the validity of such assumptions using Shake-the-Box particle tracking velocimetry measurements of experimental gravity current flows. The resulting instantaneous, volumetric, whole-field velocity measurements indicate that cross-stream and vertical velocities (and velocity fluctuations) are equivalent in magnitude and thus are key to energy distribution and dissipation within the flow. Further, the presented data highlight the limitations of basing conclusions regarding body structure on a single cross-stream plane (particularly if that plane is central). Spectral analysis and dynamic mode decomposition of the fully three-dimensional, volumetric velocity data suggests internal waves within the current body that are associated with coherent three-dimensional motions in higher Reynolds number flows. Additionally, a potential critical layer at the height of the downstream velocity maximum is identified.

Cohesive sediment particles are ubiquitous in environmental flows. The cohesive properties of clay promote the formation of clay flocs and gels and relatively small suspended clay concentrations can enhance or suppress turbulence in a flow. Furthermore, flows are naturally non‐uniform, varying in space and time, yet the dynamics of non‐uniform open‐channel clay suspension flows is poorly understood. For the first time, the adaptation time and length scales of non‐uniform clay suspension flows were quantified using novel experiments with spatially varying but temporally uniform flow. Different levels of turbulence enhancement and attenuation were identified as the flow decelerates or accelerates. Results highlight that decelerating clay suspension flows crucially have a longer adaptation time than accelerating clay suspension flows. This is explained by the longer timescale required for the formation of bonds between cohesive particles in turbulence attenuated flows after deceleration than the rapid breakdown of bonds in turbulent flows after acceleration of clay suspension flows. This hysteresis is more pronounced for higher concentration decelerating flows that pass through a larger variety of clay flow types of turbulence enhancement and attenuation. These different adaptation time scales and associated clay flow type transitions are likely to affect clay flow dynamics in a variety of fluvial and submarine settings.

Minibasins on continental margins trap turbidity currents transporting material downslope, but little is known about the inherently three-dimensional (3-D) mechanics of these confined flows. Utilizing new methodology, experimental results quantify flow dynamics in minibasins for the first time. It is shown that dynamics are dominated by 3-D circulation cell structures, across the fill-to-strip-to-spill transition that are controlled by flow discharge. Measurements of velocity throughout circulation cells indicate vorticity dominates strain rate with fluid rotating into the center of cells where it upwells: this influences minibasin sediment trapping potential and deposit heterogeneity. Flow properties link to depositional patterns on minibasin slopes. Specifically, higher input discharges are correlated with higher fluxes into the center of minibasins and reduced deposit tapering on minibasin slopes. This geometry is linked to the amount of sediment rich flow runup on the distal minibasin wall, where flow and sediment is delivered to circulation cells.