ADCP Data Research Articles

Importance of station-to-station line orientation in performing coastal acoustic tomography to obtain sea current speed

To perform the coastal acoustic tomography measurement, the locations of stations should be first selected at some appropriate sites on or near the coast. When selecting the locations, it is important to consider vehicle accessibility, the presence of structures such as barges, and the absence of obstructions to sound transmission and reception. Based on these conditions, the five stations were selected for the coastal acoustic tomography observation in Yeosu Bay of South Korea in 2023. In this observation, total six sound propagation lines were used for the inversion of the sea current speed. The sea current speed estimated for each sound propagation line was compared to that from ADCP installed in Yeosu Bay. As a result, when the angle between a line along which the Yeosu Bay's dominant sea current flows and the sound propagation line was small, the estimated current speed was similar to that from ADCP data. This can be considered to be because the smaller the time difference obtained from tomography measurement, the greater the impact of arrival time measurement error on the current speed. This result shows that the orientation of the lines between stations should also be considered important for selecting the locations of stations.

FAIR ADCP data with OSADCP: a workflow to process ocean current data from vessel-mounted ADCPs

This paper presents the open-source Python software OSADCP developed for the processing of vessel-mounted Acoustic Doppler Current Profiler (VMADCP) data. At this stage, the toolbox is designed for processing VMADCP measurements from open-ocean applications of Teledyne RDI Ocean Surveyor ADCPs and the data acquisition software VMDAS. Based on the VMDAS ENX binary output format, the software contains implementations for cleaning and vector-averaging of single-ping velocity data, verification of the position data, and applying misalignment and amplitude corrections. The procedures of OSADCP are described in detail to encourage the scientific community to use it for their own purposes. The toolbox is an integral part of a workflow implemented on the German marine research vessels in the framework of the Underway Research Data project of the German Marine Research Alliance (DAM). It aims to ensure standardized data acquisition measures, reliable data transfer from the ADCP to shore both near-real-time and in delayed-mode, processing and quality control, and dissemination of the curated data product in the data repository PANGAEA. From PANGAEA, data sets are forwarded to the European marine data hubs Copernicus Marine Service and EMODnet. The workflow that forms the framework for OSADCP is described here as an example of scientific data management that follows the FAIR data guidelines.

Characterization of the wake generated downstream of a MW-scale tidal turbine in Naru Strait, Japan, based on vessel-mounted ADCP data

With tidal energy demonstration projects with one or a small number of turbines having provided very positive results, the technology is moving to the commissioning and operation of tidal energy farms. For this next step, the understanding of the wakes generated downstream of the turbines is crucial to optimize the array performance. To date, the analysis of wakes of MW-scale tidal turbines has been made by numerical methods or experimentally with downscaled rotors. However, no consensus on a methodology to characterize wakes generated by full-scale turbines based on data measured on-site has yet been reached. The present paper introduces a new method to compare current velocity data measured before and during turbine operation that minimizes the impact of the spatial and temporal variability of tidal currents, thus enabling the estimation of the velocity deficit caused downstream of the turbine. Through this method, a characterization of the near wake was possible, with velocity deficits of 0.537, 0.463, 0.469 and 0.431 at 2D, 3D, 4D and 5D from the turbine. Results from this paper present a very valuable tool for the validation of numerical models aiming to estimate the wake losses in tidal energy farms.

Preliminary Performance Assessment of the Wave Parameter Retrieval Algorithm from the Average Reflected Pulse

To obtain new information about surface waves, it is proposed to use an underwater acoustic wave gauge, and an assessment of its effectiveness can be performed using a numerical simulation and field experiment. A new device, an underwater acoustic wave gauge named “Kalmar”, was developed by the Institute of Applied Physics of the Russian Academy of Sciences for long-term, all-weather monitoring of wind waves. The instrument uses ultrasound to probe the water surface from underwater and can be used to verify remote sensing data. In this work, the capabilities of the device are tested and compared with ADCP data. Two independent methods for processing underwater acoustic wave gauge data are discussed and compared. One of them is completely new for acoustic measurements and is based on the analysis of the shape of the reflected acoustic pulse averaged over space and time. The other allows processing individual reflected pulses and calculating the time implementation of the distance to the water surface. It is shown that two independent methods of significant wave height retrieval from the acoustic wave gauge measurements are highly correlated. The “Kalmar” acoustic wave gauge and the RDI WH-600 acoustic Doppler current profiler operated simultaneously at the test site in Gelendzhik from 1 February to 10 February 2020. The significant wave heights measured by the two instruments are in good agreement.

The impact of topography and rotation in shaping the basin-scale circulation in Lough Corrib, Ireland, under homogenous conditions

Abstract: The impact of the Earth's rotation on circulation patterns in homogenous basins has been well studied using numerical models. This contribution addresses the need for supporting observational studies. Bottom mounted acoustic doppler current (ADCP) velocity profiles are analysed alongside drifter tracks and numerical solutions in order to demonstrate how rotation modifies circulation patterns and horizontal velocity profiles in Lough Corrib, Ireland. The wind driven circulation in Lough Corrib forms a system of topographic gyres, shaped by the complex bathymetry of the basin. Surface and benthic frictional boundary layers are demonstrated to be closely described by classical Ekman theory, despite the system being constrained by shallow water depths (mean depth = 8.4m) and small horizontal dimensions (∼10km). Observed velocity profiles can be derived theoretically by summing the two driving forces of the flow (the barotropic pressure gradient force and the surface wind stress force) and their opposing forces (internal friction and Coriolis acceleration). Rotation introduces a significant cross-wind component to the flow, which is thus described as being quasi-geostrophic. The barotropic pressure gradient force is empirically quantified as a function of wind stress by relating surface set-up to the surface water shear velocity. The vertical eddy viscosity profile, derived from ADCP data, increases approximately linearly with increasing depth and is largest at the upper interface of the benthic boundary layer.

The Impact of Topography and Rotation in Shaping the Basin-Scale Circulation in Lough Corrib, Ireland, Under Homogenous Conditions

The impact of the Earth’s rotation on circulation patterns in homogenous basins has been well studied using numerical models. This contribution addresses the need for supporting observational studies. Bottom mounted acoustic doppler current (ADCP) velocity profiles are analysed alongside drifter tracks and numerical solutions in order to demonstrate how rotation modifies circulation patterns and horizontal velocity profiles in Lough Corrib, Ireland. The wind driven circulation in Lough Corrib forms a system of topographic gyres, shaped by the complex bathymetry of the basin. Surface and benthic frictional boundary layers are demonstrated to be closely described by classical Ekman theory, despite the system being constrained by shallow water depths (mean depth = 8.4m) and small horizontal dimensions (~10km). Observed velocity profiles can be derived theoretically by summing the two driving forces of the flow (the barotropic pressure gradient force and the surface wind stress force) and their opposing forces (internal friction and Coriolis acceleration). Rotation introduces a significant cross-wind component to the flow, which is thus described as being quasi-geostrophic. The barotropic pressure gradient force is empirically quantified as a function of wind stress by relating surface set-up to the surface water shear velocity. The vertical eddy viscosity profile, derived from ADCP data, increases approximately linearly with increasing depth and is largest at the upper interface of the benthic boundary layer.

Multiple island effects shape oceanographic processes and zooplankton size spectra off an oceanic archipelago in the Tropical Atlantic

Pelagic ecosystems around tropical oceanic islands are considered oases of high plankton biomass in the middle of oligotrophic “blue deserts”. To understand the dynamics of such a pelagic ecosystem, we used CTD and ADCP data and zooplankton samples taken from the waters off the remote Fernando de Noronha Archipelago (FNA), Tropical Atlantic. We analyzed how the flow and island topography interactions influence the spatial variability of chlorophyll-a fluorescence and zooplankton abundance, biovolume, and normalized biovolume size spectra (NBSS). We used a 500-μm mesh bongo net to obtain plankton samples in July and August 2010 in areas upstream and downstream of FNA. Zooplankton samples were analyzed with a ZooScan device. Chlorophyll-a peaks and a rise in thermocline indicated a topographic uplift and turbulence downstream of the island, i.e., the “Island Mass Effect”. The NBSS presented a mean slope of −1.19 ± 0.28, and a mean intercept of 3.98 ± 0.87. There were no significant differences in NBSS slopes and intercepts between upstream and downstream areas. Nevertheless, zooplankton and decapod community structures were significantly different between areas: meroplanktonic communities showed higher abundances and biovolumes downstream (p < 0.05). Zoeae of stenopodid cleaner shrimps, anomuran and brachyuran crabs, and fish eggs were significantly more abundant downstream, indicating a “Larval Island Effect”. Distinct peaks in size spectra due to teleost eggs and decapod larvae, downstream of FNA, also indicated a Larval Island Effect. Upstream of the island, there was a higher abundance of gelatinous organisms, holoplanktonic decapods, and advanced stages of brachyuran crabs (“Upstream Island Effect”). Also, copepods, gelatinous organisms, large-sized “other crustaceans” (e.g., euphausiids, amphipods, stomatopod larvae, mysids, etc.), teleost eggs, and stenopodids caused abundance peaks in the size spectra upstream. Our study highlights the need for careful management and conservation of parental spawning stocks of fishes and macroinvertebrates surrounding tropical oceanic islands.

Estimation and characterisation of the wave-induced turbulent kinetic energy and turbulent dissipation from ADCP data

Turbulence in the water flow causes small-scale variations in the mechanical stress acting on submerged tidal turbines. As such it increases their fatigue loading and impacts greatly their lifetime. It is therefore essential for engineers to have an accurate knowledge and characterisation of turbulence at a given site as they design the structures to be deployed. The strength of the tidal currents is the main parameter influencing the intensity of turbulence through their friction with the sea bed. However, most potential tidal energy sites are located in a coastal environment with shallow enough water depths so that the direct impact of waves on turbulence can not be overlooked. Steepness-induced wave breaking is indeed observed to increase the turbulent mixing for such applications. In this context, we propose to estimate the contribution of surface processes to the total turbulence in Alderney Race, France, the most energetic tidal site in western Europe. The turbulent kinetic energy (TKE) specifically induced by waves and wind is characterised using measurements from a 5-beams ADCP deployed between 27/02/2018 and 06/07/2018.Analytical profiles are fitted to the data, the only fitting parameter of the model is an evaluation of the turbulence penetration depth, it determines how deep surface processes impact the water column. Its dependence towards mean wave and current parameters is studied. The results do not allow to conclude on the nature of turbulence observed in the mid water column.

Tidal turbine wake characterization by vessel-mounted ADCP data analysis

Wakes generated downstream turbines are one of the key points to be considered for tidal stream energy farm layout design. Distance between consecutive turbines must be optimized considering available resource spatial distribution and the current velocity reduction caused by upstream turbines. Tidal turbine wake characteristics have been analyzed experimentally and numerically, however, there is still a lack of studies presenting data measured in the wake of large-size turbines in tidal sites.&#x0D; In this paper, the wake generated downstream of a SIMEC Atlantis Energy 0.5 MW turbine in Naru Strait (Nagasaki Prefecture, Japan) is analyzed using data measured with a vessel-mounted ADCP. This analysis is based on the comparison between data measured before turbine installation (September 2020) and during turbine operation (March 2021, May 2021, and September 2021). &#x0D; Due to the difficulties that measuring at the same position and the same tide moment with a vessel-mounted ADCP would mean, a direct comparison between these four datasets is unviable. To solve this issue, data were pretreated to calculate a ratio (RD) resulting from the division of current velocity values at every vertical layer by the current velocity measured at the shallowest layer. After that, this ratio is interpolated to a regular 3D mesh. Assuming that the current velocity profile shape for one point does not change with time, resulting RD values interpolated from data measured in different fieldworks can be compared.&#x0D; The impact of tidal turbine operation on the current velocities is quantified with a variation ratio RV calculated by dividing RD during turbine operation (RD*) by RD before turbine installation (RD). These results show a clear velocity deficit even at 18D from the turbine, which clearly contrasts with results from other similar studies available in the literature, which may be related to the low turbulence intensity measured at the turbine installation point.

Morphology, flow structure, and sediment transport in the Paraná and Piquiri Rivers confluence, Brazil

Confluences are places of complex interaction of flow and sediments coming from different channels. Studies at large confluences have advanced the understanding of these environments on a larger scale. This study is carried out at the confluence formed by the Paraná and Piquiri rivers in the state of Paraná - Brazil. It characterizes the morphology, flow structure and sediment dispersion for these two channels. The methodology consists of: interpoling bathymetric data; analyze the ADCP data – Acoustic Doppler Current Profiler and quantify the suspended and bottom sediments. ADCP data was processed in VMT software and generated the following information: flow direction, sediments load (bed and suspended) and also used to calculate the moment ratio (Mr), discharge ratio (Qr), flow power (Ω), and specific flow power (ꞷ) in both channels. The results reveal the variation of channel depth, and specific morphological areas for the confluence with a sediment accumulation zone, submerged lateal bar and excavation area. The flow dynamics shows an increase in velocity in the Paraná River downstream of the confluence, and a decrease in the flow velocity of the Piquiri River as it approaches the junction. The confluence has flow stagnation zone, acceleration and shear layer. The concentration of suspended sediment is higher in the Piquiri River, on the other hand the Paraná River presents thicker fractions of bed material.

Circulation, stratification and salt dispersion in a former estuary after reintroducing seawater inflow

Around the world, estuaries have been partially or completely closed-off from the sea and their number may increase with rising sea levels. Concurrently, there is a trend to reintroduce seawater inflow into enclosed former estuaries for ecosystem improvement. This is also the case in the Haringvliet, a former estuary in the Rhine-Meuse Delta, closed-off in 1970 with floodgates blocking seawater inflow and regulating outflow. As the reintroduced salt water inflow can threaten fresh water intake, inflow, flushing and dispersion need to be well understood and carefully managed.Here we investigate stratification, flow circulation and salt transport in the Haringvliet by analyzing ADCP data collected in two former tidal channels, together with salinity time series and profiles. The profiles show that the incoming water reaches the deeper parts and that the system tends to be strongly stratified. Over time, the interface levels deepen in steps, mainly coinciding with floodgate discharge events, which are strongly correlated with the primary current velocities in the channels. However, even floodgate discharges for above average Rhine discharge conditions were insufficient to quickly flush or mix the salt out of the channels. This is consistent with calculated gradient Richardson numbers, which barely get in the range of critical values.For closed floodgates with no outward discharge, we found considerable depth-averaged upwind currents in the channels for axial winds. This reveals a dominant horizontal circulation, with downwind currents over the shallow parts and upwind currents over the deep parts of the system, explained by a local imbalance between the wind stress and pressure gradient force at both shoals and channels. This horizontal circulation is an important driver for inland salt transport, as increased salinity values were found at landward locations for seaward wind. This implies this is a condition with increased risks for fresh water availability. Analytical calculations confirmed the upwind currents in the channels can become sufficiently strong to transport salt mixed up at one side of the system to the other within the duration of a wind event. However, the current-related shear is likely not strong enough to induce interfacial mixing directly above the deep parts, and we hypothesize mixing mostly occurs when salt water reaches less deep areas after tilting of the pycnocline.The insights from this study are relevant for other formerly enclosed estuaries for which reintroduction of seawater inflow is considered, as well as presently open systems for which (partial) closure is discussed.

Investigation of stage‐discharge model performance for streamflow estimating: A case study of the Gono River, Japan

AbstractIn water resource studies, long‐term measurements of river streamflow are essential. They allow us to observe trends and natural cycles and are prerequisites for hydraulic and hydrology models. This paper presents a new application of the stage‐discharge rating curve model introduced by Maghrebi et al. (2016) to estimate continuous streamflow along the Gono River, Japan. The proposed method, named single stage‐discharge (SSD) method, needs only one observed data to estimate the continuous streamflow. However, other similar methods require more than one observational data to fit the curve. The results of the discharge estimation by the SSD are compared with the improved fluvial acoustic tomography system (FATS), conventional rating curve (RC), and flow‐area rating curve (FARC). Some statistical indicators, such as the coefficient of determination (R2), root mean square error (RMSE), percent bias (PBAIS), mean absolute error (MAE), and Kling‐Gupta efficiency (KGE), are used to assess the performance of the proposed model. ADCP data are used as a benchmark for comparing four studied models. As a result of the comparison, the SSD method outperformed of FATS method. Also, the three studied RC methods were highly accurate at estimating streamflow if all observed data were used in calibration. However, if the observed data in calibration was reduced, the SSD method by R2 = 0.99, RMSE = 2.83 (m3/s), PBIAS = 0.715(%), MAE = 2.30 (m3/s), and KGE = 0.972 showed the best performance compared to other methods. It can be summarized that the SSD method is the feasible method in the data‐scarce region and delivers a strong potential for streamflow estimation.

Application of Delft3D computational model to estimate the influence of El Bala run-of-the-river dam on the morphological activity of Beni River, Bolivia

Beni river is an important tributary in the Amazon basin. There, the Bolivian government decided the construction of a two-dam cascade hydropower system, including Chepete dam and El Bala run-of-the-river dam. The main concern is the high sediment transport in Beni river. Then, the aim of the research is the application of Delft3D model to estimate the influence of El Bala run-of-the-river dam on the sediment transport and morphological activity of Beni river. The model was calibrated for natural conditions based on ADCP data and sediment transport measurements collected from the river. The implemented model was able to reproduce hydrodynamics and sediment transport in natural conditions. Hence, several scenarios were tested. A hydrodynamic and morphological model was set to test the influence of the dam on the river for hydrodynamics, sedimentation and erosion processes in Beni river. Results show that Delft3D performed very well in modelling sedimentation and erosion processes in Beni river, the model was able to predict the expected erosion process due to the presence of Chepete dam, upstream of El Bala, which retains the sediment load of Beni river affecting the morpho-dynamics of the river. Additionally, the model was able to predict a sedimentation in the reservoir.

Bubble-Turbulence Dynamics and Dissipation Beneath Laboratory Breaking Waves

Abstract Bubbles directly link sea surface structure to the dissipation rate of turbulence in the ocean surface layer through wave breaking, and they are an important vehicle for air–sea transfer of heat and gases and important for understanding both hurricanes and global climate. Adequate parameterization of bubble dynamics, especially in high winds, requires simultaneous measurements of surface waves and breaking-induced turbulence; collection of such data would be hazardous in the field, and they are largely absent from laboratory studies to date. We therefore present data from a series of laboratory wind-wave tank experiments designed to observe bubble size distributions in natural seawater beneath hurricane conditions and connect them to surface wave statistics and subsurface turbulence. A shadowgraph imager was used to observe bubbles in three different water temperature conditions. We used these controlled conditions to examine the role of stability, surface tension, and water temperature on bubble distributions. Turbulent kinetic energy dissipation rates were determined from subsurface ADCP data using a robust inertial-subrange identification algorithm and related to wind input via wave-dependent scaling. Bubble distributions shift from narrow to broadbanded and toward smaller radius with increased wind input and wave steepness. TKE dissipation rate and shear were shown to increase with wave steepness; this behavior is associated with a larger number of small bubbles in the distributions, suggesting shear is dominant in forcing bubbles in hurricane wind-wave conditions. These results have important implications for bubble-facilitated air–sea exchanges, near-surface ocean mixing, and the distribution of turbulence beneath the air–sea interface in hurricanes. Significance Statement Bubbles are a vehicle for the flux of heat, momentum, and gases between the atmosphere and ocean. These fluxes contribute to the energy budgets of hurricanes, climate, and upper-ocean biology. Few to no simultaneous measurements of surface waves, bubbles, and turbulence have been made in hurricane conditions. To improve numerical model representation of bubbles, we performed laboratory experiments to parameterize bubble size distributions using physical variables including wind and waves. Bubble distributions were found to become broadbanded and shift toward smaller radius with increased wind stress and wave steepness. Turbulence dissipation rate and shear were shown to increase with wave steepness. Our results give the first physically based bubble distribution parameterization from naturally breaking waves in hurricane-force conditions.

Evaluation of discrete target detection with an acoustic Doppler current profiler

AbstractAcoustic Doppler current profilers (ADCPs) use the signal scattered back from small suspended drifting particles, such as suspended sediments and planktonic species, to measure water current velocity. Additionally, ADCPs detect signals scattered from fish and other pelagic organisms, but these signals are generally treated as noise and rejected during data processing. However, these rejected signals can contain information on fish movement, which presents an opportunity to extend the application of ADCP technology as a tool for monitoring fish activity. We compare discrete target counts made using an ADCP with those of a split‐beam echosounder in a region of high tidal currents in the Bay of Fundy. The comparison was achieved using a self‐contained bottom‐mounted frame equipped with both a 600‐kHz Teledyne RD Instruments Workhorse ADCP and a 120‐kHz BioSonics DTX Submersible split‐beam echosounder system. The discrete targets were identified using a combination of signal correlation and volume backscatter (SV) thresholds in the ADCP data and using a fish tracking algorithm in the split‐beam echosounder data. The resulting ADCP discrete target counts agreed with fish tracking counts made from the co‐located split‐beam echosounder. Notably, discrete targets recorded by the ADCP could be differentiated from entrained air using a high signal correlation threshold. This method can expand the application of ADCP data for biophysical assessments for fisheries management and could be of particular use in rivers and tidal channels.

3D turbulent currents simulation in the coastal zone using the LES approach based on filtered ADCP data

The interest in turbulence in shallow waters, such as the Azov Sea, is caused by the fact that in the places of its existence there is an intensive transfer of the amount of motion and heat, the spread of passive impurities, the transfer of suspended particles. These processes significantly affect the formation and spatial structure of physical, chemical and biological fields of reservoirs and their spatial and temporal changes. Depending on the smoothing scale of the series of initial readings of the flow meters, the pulsation characteristics of the flow velocity were determined, which were processed in order to obtain data on turbulence and its scales. The research results provided empirical data on the conditions of generation and existence of small-scale turbulence. The collected empirical material is currently being processed to study the internal structure of the recorded disturbances of small-scale turbulence. In this paper, small-scale motion is excluded from the Navier-Stokes equations by applying the filtration operation and is modeled using subgrid models. To do this, in two-dimensional and three-dimensional cases, various types of filters are used: a box filter, a Gaussian filter and a Fourier filter, with a gradual decrease in the filter width, which allows you to reproduce a wider frequency range of fluctuations of the solution. The obtained data are planned to be used for numerical simulation of three-dimensional turbulent flows using the LES approach and comparison with the results of averaging by RANS. The article considers the possibilities of using various types of approximations for parametrization of vertical turbulent exchange. Algebraic models for calculating the coefficient of vertical turbulent exchange and semi-empirical turbulence models are compared.

CFD-FEM Modeling of a Floating Foundation under Extreme Hydrodynamic Forces Generated by Low Sea States

The high development of the offshore industry for supporting new marine and renewable energy projects requires a constant improvement of methods for structure designing. Because recent studies warned that maximum environmental loads occur during low sea states and not during extreme sea states as recommend by the offshore standards (e.g., RP 2AWSD-2014), this study used measured wave and current data for analyzing that warning. The Colombian Caribbean coast was selected as the study area, and in situ ADCP data combined with Reanalysis and numerical data was used for identifying proper sea states for the analysis. Then, two low and one extreme sea states were selected and their associated current profiles were extracted, for providing input data for Computational Fluid Dynamics (CFD) and Finite Element Method (FEM) simulations to evaluate the effect of the hydrodynamic forces over a floating structure. The results showed that low sea states generated maximum loads and rotations in the floating structure, and the extreme sea states caused high-frequency vibrations that could provoke structural dynamics problems such as failures due to fatigue or sudden collapse by resonance and amplification.

Modeling of Plastic Debris Particle Trajectory During Pre and Post Reclamation in Jakarta Bay

The primary source of environmental problems in Jakarta Bay’s waters is the change in hydrodynamics caused by reclamation and land-derived waste from the 13 watersheds that discharge into the bay. In June and December 2006 and 2015, hydrodynamic and trajectory simulations were conducted in Jakarta Bay to determine the distribution of plastic debris in pre- and post-reclamation conditions. The hydrodynamic conditions and particle trajectory in Jakarta Bay were described numerically using the Hamburg Shelf Ocean Model (HAMSOM). Secondary tidal data and primary ADCP data are used to validate the simulation results. The distribution of plastic debris released during the simulation depicts a distribution that follows the monsoon pattern, moving west during the east monsoon and moving east in the west monsoon and spreading along the coastline in both conditions. The percentage of plastic debris increased by 21.42% in June and 4.07% in December. The increase in the percentage of plastic debris that remains in the waters of Jakarta Bay is due to a 0.03 - 0.05 m/s decrease in current velocity following the formation of the reclamation island. Due to the decrease in current velocity within the bay, trajectory simulations indicate that plastic debris will take an additional 69.52 – 304.25 hours to exit. The accumulation of plastic debris around the reclamation islands demonstrates that the islands act as waste traps, necessitating proper waste cleanup, particularly in the canal area and around the reclamation island.

Mobile bedform dynamics approaching a bedload parting site: Pentland Firth, northeast UK

The Pentland Firth between mainland Scotland and Orkney is an area of the UK continental shelf that experiences extreme tidal flows (>3 m/s). This study presents a time series of bathymetric data coupled with a tidal flow model to examine hydrodynamics and bedform response at the eastern approaches to the Pentland Firth. These observations were additionally augmented by sediment grain type and ADCP data to validate the numerical flow model. Tidal flows of the Pentland Firth result from a tidal phase difference between the east and west approaches to the channel. A resulting barotropic pressure gradient leads to flow accelerations that locally exceed 5 m/s. The extreme tidal setting of the Pentland Firth's eastern approach was found to promote distinct bedforms that are spatially varied in geomorphology with distance from the Pentland Firth. Sediment analysis also showed a decreasing grain size trend also with distance from the Pentland Firth. The modelled residual tidal current shows strong agreement with the sediment transport pathways, supported by the bedform migration direction. The energetic tidal flows of the Pentland Firth's eastern approach interacts with the highly irregular coastline, generating residual tidal counter currents leeward of flow obstructions. These counter currents (i.e. residual tidal current vortices) were reflected in bedform migrations and thence the sediment transport pathways. As the Pentland Firth is considered as a bedload parting site, the residual tidal current vortices are expected to influence the rate of erosion at the bedload parting site, by recirculating sediment back upstream as a counter current inshore of the main flow. The modification of sediment transport pathways by residual tidal current vortices may affect the development of nearshore and offshore engineering, and should be considered in any initial site assessment.

High-amplitude internal waves southeast of Spitsbergen

We investigate tidal and higher frequency internal waves southeast of Hopen Island of the Svalbard archipelago using shipborne CTD and ADCP data, model simulations, and satellite observations. The experiment was carried out in the marginal ice zone region. Previous numerical simulations predicted high amplitudes of internal tides here. A series of repeated CTD-casts provide estimates of the vertical displacements of water particles caused by internal waves generated over the continental slope. Peak to peak displacements caused by forced non-propagating internal tides were as high as 50 m. Generation of strong internal tides is also shown based on numerical model revealing high amplitudes (50 m) of vertical displacement over the submarine slope. The development of forced internal tides in the vicinity of the M2 critical latitude is accompanied by generation of short-wave trains of internal solitary waves depicted in satellite observations.