Monitoring of the water particle velocity field near the seabed under different wave and tidal scenarios: a real case

Monitoring of water particle velocity on the sea bed is crucial to study morphological shore changes in a coast at intermediate and shallow water depth under progressive surface waves and tidal flow current. Therefore, 3-D particle velocity was monitored continuously at the bottom of Santa Maria del Mar (SMM) beach (SW Spain) by means of a single point current meter during 3 weeks in 2007 since August 28. The current meter was placed at 0.45m above the seabed in order to acquire instantaneous velocity. Wave properties (height and period) were taken from the nearby wave buoy and tidal data were taken from a tidal gauge station. Wave-induced bottom particle velocities were obtained during spring and neap tides at a d/L (depth over wave length) parameter ranging from 0.06 to 0.3. Bottom water particle velocity near the seabed ranges from 0 to ± 0.5 m/sec of which about 82% does not exceed 0.2 m/sec during monitoring. Therefore, only 18% of the surveyed water particle velocities exceed the critical Shield parameter of the beach sand (d50 = 0.23mm) which is about 0.05-0.2 depending on Reynolds number. Results show that maximum horizontal speed is obviously lower during the slack tide (high or low tide) in comparison with flood tide and ebb tide. Moreover, speed is higher during ebb tide in comparison to adjacent flood tide, with steady wave climate. Finally and among other conclusions, the maximum real values of the bottom current surveyed in SMM, as well as the Shield parameter, substantially coincide with the theoretical estimates calculated for a given wave and tidal climate

First-principles scattering matrices for spin-transport

Details are presented of an efficient formalism for calculating transmission and reflection matrices from first principles in layered materials. Within the framework of spin density functional theory and using tight-binding muffin-tin orbitals, scattering matrices are determined by matching the wave-functions at the boundaries between leads which support well-defined scattering states and the scattering region. The calculation scales linearly with the number of principal layers N in the scattering region and as the cube of the number of atoms H in the lateral supercell. For metallic systems for which the required Brillouin zone sampling decreases as H increases, the final scaling goes as H^2*N. In practice, the efficient basis set allows scattering regions for which H^{2}*N ~ 10^6 to be handled. The method is illustrated for Co/Cu multilayers and single interfaces using large lateral supercells (up to 20x20) to model interface disorder. Because the scattering states are explicitly found, ``channel decomposition'' of the interface scattering for clean and disordered interfaces can be performed.Comment: 22 pages, 13 figure

Near-Bed Turbulent Kinetic Energy Budget Under a Large-Scale Plunging Breaking Wave Over a Fixed Bar

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 bar's 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

Analysis of wave and current data in a tidal energy test site

Characterisation of a tidal stream site before device deployment is important for the marine tidal industry, in order to optimise the device design and accurately predict its performance during operation. Understanding the short-term uctuations in tidal stream velocity, resulting from turbulence and wave-current interactions, is essential for proper evaluation of the transient performance of a tidal stream turbine. Several aerodynamic models have been proposed for the design of tidal stream turbines and energy production calculation, most of which have proved to predict accurately mean quantities within the ow regime. Unfortunately, these models cannot accurately predict the instantaneous ow variation and resulting forces within the ow regime acting on tidal stream systems which is of central importance to the tidal energy industry. The reasons for this may not be unconnected to the signi cant differences between wind and tidal turbines. Given the uncertainties which exist in the assumptions of resource modelling and the need for an accurate energy capture assessment at a speci c site, accurate on-site measurements that can predict the ow velocity (and its directional component) at a given location, are needed. This thesis is devoted to characterisation of ow in a typical tidal stream site (the Fall of Warness, of the European Marine Energy Center (EMEC), Orkney) through measured data. The high-frequency Acoustic Doppler Current Pro ler (ADCP) surveys were conducted at different locations within this site by EMEC. This dataset allows for the rst time a statistical and thorough analysis of the vertical turbulence structure at the EMEC's tidal stream test site. Detailed analyses of the directional components of the wave elds using a non-phase-locked method and in uences of seabed and tidal stream on wave elds, were performed. The bulk turbulence parameters within the tidal streams were estimated using the variance method. The results suggest a signi - cant effect of directionality and short-term uctuations in stream velocity resulting from turbulence and wave-current interactions, on the hydrodynamics in a typical tidal energy test site. The results from this study can be used to validate and improve/develop proposed hydrodynamic models and can play a vital role in tidal energy resource planning

QuantumATK: An integrated platform of electronic and atomic-scale modelling tools

QuantumATK is an integrated set of atomic-scale modelling tools developed since 2003 by professional software engineers in collaboration with academic researchers. While different aspects and individual modules of the platform have been previously presented, the purpose of this paper is to give a general overview of the platform. The QuantumATK simulation engines enable electronic-structure calculations using density functional theory or tight-binding model Hamiltonians, and also offers bonded or reactive empirical force fields in many different parametrizations. Density functional theory is implemented using either a plane-wave basis or expansion of electronic states in a linear combination of atomic orbitals. The platform includes a long list of advanced modules, including Green's-function methods for electron transport simulations and surface calculations, first-principles electron-phonon and electron-photon couplings, simulation of atomic-scale heat transport, ion dynamics, spintronics, optical properties of materials, static polarization, and more. Seamless integration of the different simulation engines into a common platform allows for easy combination of different simulation methods into complex workflows. Besides giving a general overview and presenting a number of implementation details not previously published, we also present four different application examples. These are calculations of the phonon-limited mobility of Cu, Ag and Au, electron transport in a gated 2D device, multi-model simulation of lithium ion drift through a battery cathode in an external electric field, and electronic-structure calculations of the composition-dependent band gap of SiGe alloys.Comment: Submitted to Journal of Physics: Condensed Matte

Sediment Resuspension in a Microtidal Estuary: Causative Forces and Links with Algal Blooms

After years of efforts to restore the Chesapeake Bay, bacterial levels are down and species diversity has increased, however, algal blooms (primarily dinoflagellates) persist, occurring nearly every summer. Dinoflagellates produce resting cysts that accumulate in the bottom sediments and are thought to provide seed populations for future algal blooms when they are resuspended. When estuarine sediments are advected from a bed, other materials, such as pollutants, nutrients, and organic matter are also released into the water column. Thus, resuspended sediments can contribute to the degradation of water quality, habitat, and aquatic life, and impart negative impacts on local ecosystems and economies. To investigate the causes of sediment resuspension in a shallow, tidal system and the potential role of sediment resuspension on algal production, time-series measurements of current velocity, wave height, and suspended sediment concentrations were recorded using acoustic, optical, and pressure sensors, in conjunction with a temporal and spatial survey of conductivity, temperature, suspended sediment concentration profiles, benthic sediment samples, and water samples. Regional meteorological data including hourly wind speed and direction and precipitation totals were also compiled for comparison with sediment resuspension, chlorophyll a (Chl a) concentrations and dissolved nutrient concentrations. Sediment resuspension in estuaries typically results from wind-driven waves, tidal currents, or wind-driven currents. Results from this study found maximum wave orbital velocities (Ubm) to be an order of magnitude less than current velocities (Uc), however, periods of elevated Ubm, were associated with the majority of observed resuspension events. Despite surface gravity waves primarily causing resuspension, currents (tidal and wind-driven), as well as water depth, appeared to mitigate or even negate wave induced resuspension. Overall, resuspension most often resulted when Ubm \u3e~2 cm/s, coinciding with southwesterly winds ≥ 5 m/s, during periods of relatively weak current speeds and water depth. Observation of increased nutrient concentrations and/or Chl a concentrations followed numerous resuspension events, suggesting that resuspension likely aided in the growth of algal blooms observed in the Lafayette River. The link between sediment resuspension, elevated nutrient and Chl a concentrations, was supported via observations of elevated near-bed concentrations of ammonium and nitrate/nitrite concentrations. Nutrient concentrations in bottom waters then declined as Chl a concentrations increased. The timing of this sequence of events (2-7 days) was on the order observed previously. This study suggests that sediment resuspension may be an import factor for stimulating algal production in shallow, eutrophic, microtidal estuaries

Practical sand transport formula for non-breaking waves and currents

Open Access funded by Engineering and Physical Sciences Research Council Under a Creative Commons license Acknowledgements This work is part of the SANTOSS project (‘SANd Transport in OScillatory flows in the Sheet-flow regime’) funded by the UK's EPSRC (GR/T28089/01) and STW in The Netherlands (TCB.6586). JW acknowledges Deltares strategic research funding under project number 1202359.09. Richard Soulsby is gratefully acknowledged for valuable discussions and feedback on the formula during the SANTOSS project.Peer reviewedPostprin

Exploitation of SAR data for measurement of ocean currents and wave velocities

Methods of extracting information on ocean currents and wave orbital velocities from SAR data by an analysis of the Doppler frequency content of the data are discussed. The theory and data analysis methods are discussed, and results are presented for both aircraft and satellite (SEASAT) data sets. A method of measuring the phase velocity of a gravity wave field is also described. This method uses the shift in position of the wave crests on two images generated from the same data set using two separate Doppler bands. Results of the current measurements are pesented for 11 aircraft data sets and 4 SEASAT data sets