Stuart J. McLelland

Three‐dimensional measurement of river channel flow processes using acoustic doppler velocimetry

This paper describes and assesses: (i) the use of a new instrument for the determination of three-dimensional flow velocities in natural rivers, the acoustic Doppler velocimeter (ADV); and (ii) a method for positioning and orienting such measurements relative to a single local coordinate system to relate flow velocity vectors with the bed and water surface. The ADV uses the Doppler shift principle to measure the velocity of small particles, assuming to be moving at velocities similar to the fluid. Velocity is resolved into three orthogonal components, and measured in a volume 5 cm below the sensor head, minimizing interference of the flow field, and allowing measurements to be made close to the bed. A simple method for positioning and orienting the instrument using digital tacheometry is described, and is used to obtain velocity measurements concurrently with measurements of both bed and water surface topography. The paper includes a preliminary field assessment of the ADV by comparing velocity profiles with those generated from Marsh McBirney electromagnetic current meters, and a full field assessment of the position and orientation methodology. These results suggest that the recommended methods in combination with an ADV are able to provide reliable mean three-dimensional velocity field information and accurate bed and surface topography. Copyright

A new method for evaluating errors in high‐frequency ADV measurements

The acoustic Doppler velocimeter (ADV) measures three-dimensional velocities in a small, remote sampling volume at high frequencies, however, these measurements incorporate errors that are intrinsic to the measurement technique. This paper demonstrates a new method for calculating the total measurement errors, including sampling errors, Doppler noise and errors due to velocity shear in the sampling volume associated with single-point ADV measurements. This procedure incorporates both the effects of instrument configuration and the distribution of errors between velocity components for any probe orientation. It is shown that the ADV can characterize turbulent velocity fluctuations at frequencies up to the maximum sampling rate and that Reynolds shear stress errors are very small. Copyright

Computational fluid dynamics modelling of three-dimensional processes on natural river floodplains

Abstract Results are presented from numerical simulations of overbank flows on a natural river floodplain. The model used here solves the three-dimensional Navier-Stokes equations with an RNG k-ϵ turbulence closure. Channel roughness is treated using a wall function approach while a drag-law is used to represent the effects of floodplain vegetation. The model is applied in situations with complex boundary conditions that are specified using a two-dimensional depth-averaged hydraulic model. Simulation results are compared with field measurements of three-dimensional flow velocity and turbulence obtained using an array of Acoustic Doppler Velocimeters (ADV). The model is shown to reproduce the overall spatial patterns in the flow data and the shape of vertical profiles of velocity and turbulent kinetic energy. Differences between model results and ADV measurements reflect small-scale local variability in field conditions and systematic variations in surface roughness. Results illustrate that natural floodpl...

Physical modelling of water, fauna and flora: knowledge gaps, avenues for future research and infrastructural needs

Physical modelling is a key tool for generating understanding of the complex interactions between aquatic organisms and hydraulics, which is important for management of aquatic environments under environmental change and our ability to exploit ecosystem services. Many aspects of this field remain poorly understood and the use of physical models within eco-hydraulics requires advancement in methodological application and substantive understanding. This paper presents a review of the emergent themes from a workshop tasked with identifying the future infrastructure requirements of the next generation of eco-hydraulics researchers. The identified themes are: abiotic factors, adaptation, complexity and feedback, variation, and scale and scaling. The paper examines these themes and identifies how progress on each of them is key to existing and future efforts to progress our knowledge of eco-hydraulic interactions. Examples are drawn from studies on biofilms, plants, and sessile and mobile fauna in shallow water fluvial and marine environments. Examples of research gaps and directions for educational, infrastructural and technological advance are also presented.

Time development of scour around a cylinder in simulated tidal currents

A laboratory flume experiment was performed to investigate the time development of scour around a vertical cylinder acting as a scaled model of an offshore wind turbine monopile in tidal currents. The tidal current was simulated by resolving each half-cycle into three time steps, between which flow velocity and depth were varied. Flow direction was reversed between half-cycles, which were otherwise identical. Between them, the three time steps exhibited clear water, transitional, and live-bed conditions. The experiment was run over two full-simulated tidal cycles. The scour hole formed tended to a symmetrical shape after two half-cycles and was both shallower and slower developing than the scour hole in a unidirectional current test carried out in the same flume. This was due mainly to the variable rates of scour caused by the variable flow conditions within each half-cycle, and to a lesser extent to the infilling of the scour hole when the current direction reversed. The lower scour depth recorded in tidal conditions implies that the amount of scour protection required may be less than previous studies suggest.

Pressurized groundwater outflow experiments and numerical modeling for outflow channels on Mars

The landscape of Mars shows incised channels that often appear abruptly in the landscape, suggesting a groundwater source. However, groundwater outflow processes are unable to explain the reconstructed peak discharges of the largest outflow channels based on their morphology. Therefore, there is a disconnect between groundwater outflow processes and the resulting morphology. Using a combined approach with experiments and numerical modeling, we examine outflow processes that result from pressurized groundwater. We use a large sandbox flume, where we apply a range of groundwater pressures at the base of a layer of sediment. Our experiments show that different pressures result in distinct outflow processes and resulting morphologies. Low groundwater pressure results in seepage, forming a shallow surface lake and a channel when the lake overflows. At intermediate groundwater pressures, fissures form and groundwater flows out more rapidly. At even higher pressures, the groundwater initially collects in a subsurface reservoir that grows due to flexural deformation of the surface. When this reservoir collapses, a large volume of water is released to the surface. We numerically model the ability of these processes to produce floods on Mars and compare the results to discharge estimates based on previous morphological studies. We show that groundwater seepage and fissure outflow are insufficient to explain the formation of large outflow channels from a single event. Instead, formation of a flexure-induced subsurface reservoir and subsequent collapse generates large floods that can explain the observed morphologies of the largest outflow channels on Mars and their source areas.

Hydrodynamics of a floodplain recirculation zone investigated by field monitoring and numerical simulation

Abstract Results are presented from a combined field monitoring and numerical modelling study of flow dynamics within a floodplain recirculation zone. An array of four acoustic Doppler velocimeters (ADVs) was employed to monitor 3D flow velocities within a backwater site on the River Culm, Devon, UK. Patterns of measured mean velocity and turbulent kinetic energy are compared with the results of a numerical simulation of 3D flow carried out using the computational fluid dynamics software package fluent. Simulation results illustrate the presence of flow recirculation within the backwater zone, which is consistent with lateral flow convergence monitored in the field along the upstream portion of the channel-backwater interface. Both the simulation results and ADV data highlight the existence of a free-shear layer at the mixing interface between these flows, which is characterized by high levels of turbulent kinetic energy. However, simulated turbulent kinetic energy levels are significantly lower than those monitored in the field. In addition, ADV data illustrate deviations from isotropic turbulence that may not be simulated using a two-equation k-ɛ turbulence model. These results provide quantitative evidence of the hydraulic mechanisms responsible for promoting high rates of suspended sediment deposition in such backwater recirculation zones, and highlight several modelling issues that require further attention.

The impact of macroalgae on mean and turbulent flow fields

In this paper, we attempt to quantify the mean and turbulent flow fields around live macroalgae within a tidal inlet in Norway. Two Laminaria digitata specimens ~ 0.50m? apart were selected for detailed study and a profiling ADV was used to collect 45 velocity profiles, each composed of up to seven 0.035 m-high profiles collected for 240 s at 100 Hz, at a streamwise spacing of 0.25 m and cross-stream spacing of 0.20 m. To quantify the impact of the macroalgae, measurements were repeated over a sparser grid after the region had been completely cleared of algae and major roughness elements.

Large-scale laboratory study of breaking wave hydrodynamics over a fixed bar

A large-scale wave flume experiment has been carried out involving a T = 4 s regular wave with H = 0.85 m wave height plunging over a fixed barred beach profile. Velocity profiles were measured at 12 locations along the breaker bar using LDA and ADV. A strong undertow is generated reaching magnitudes of 0.8 m/s on the shoreward side of the breaker bar. A circulation pattern occurs between the breaking area and the inner surf zone. Time-averaged turbulent kinetic energy (TKE) is largest in the breaking area on the shoreward side of the bar where the plunging jet penetrates the water column. At this location, and on the bar crest, TKE generated at the water surface in the breaking process reaches the bottom boundary layer. In the breaking area, TKE does not reduce to zero within a wave cycle which leads to a high level of “residual” turbulence and therefore lower temporal variation in TKE compared to previous studies of breaking waves on plane beach slopes. It is argued that this residual turbulence results from the breaker bar-trough geometry, which enables larger length scales and time scales of breaking-generated vortices and which enhances turbulence production within the water column compared to plane beaches. Transport of TKE is dominated by the undertow-related flux, whereas the wave-related and turbulent fluxes are approximately an order of magnitude smaller. Turbulence production and dissipation are largest in the breaker zone and of similar magnitude, but in the shoaling zone and inner surf zone production is negligible and dissipation dominates.

Near-Bed Turbulent Kinetic Energy Budget Under a Large-Scale Plunging Breaking Wave Over a Fixed Bar: TKE BUDGET UNDER BREAKING WAVES

Hydrodynamics under regular plunging breaking waves over a fixed breaker bar were studied in a large-scale wave flume. A previous paper reported on the outer flow hydrodynamics; the present paper focuses on the turbulence dynamics near the bed (up to 0.10 m from the bed). Velocities were measured with high spatial and temporal resolution using a two component laser Doppler anemometer. The results show that even at close distance from the bed (1 mm), the turbulent kinetic energy (TKE) increases by a factor five between the shoaling, and breaking regions because of invasion of wave breaking turbulence. The sign and phase behavior of the time-dependent Reynolds shear stresses at elevations up to approximately 0.02 m from the bed (roughly twice the elevation of the boundary layer overshoot) are mainly controlled by local bed-shear-generated turbulence, but at higher elevations Reynolds stresses are controlled by wave breaking turbulence. The measurements are subsequently analyzed to investigate the TKE budget at wave-averaged and intrawave time scales. Horizontal and vertical turbulence advection, production, and dissipation are the major terms. A two-dimensional wave-averaged circulation drives advection of wave breaking turbulence through the near-bed layer, resulting in a net downward influx in the bar trough region, followed by seaward advection along the bars shoreward slope, and an upward outflux above the bar crest. The strongly nonuniform flow across the bar combined with the presence of anisotropic turbulence enhances turbulent production rates near the bed.