Depth-averaged Current Velocities Research Articles

Comparison of Summer Coastal Currents off the East Coast of Korea in 2021 and 2022

This study investigates the variability of coastal currents off the east coast of Korea, specifically in the Wangdolcho area of the East/Japan Sea, from June 2021 to October 2022. The observations revealed significant fluctuations in depth-averaged current velocity, with a peak of 0.81 m/s in August 2021 and a minimum of −0.05 m/s in August 2022. These year-to-year variations highlight the complex dynamics of coastal currents influenced by regional wind patterns and mesoscale eddies. In the summer of 2021, the development of offshore eddies, coupled with variable alongshore wind stress, led to increased current velocities and alternating upwelling and downwelling conditions, resulting in abrupt changes in current intensity. Conversely, in the summer of 2022, the negative vorticity in the Korea Strait and negative wind stress curl along the coast likely caused the East Korean Warm Current to shift further offshore, resulting in a weakened southward flow along the coast. This study emphasizes the need for long-term monitoring to better understand the coastal current dynamics and their environmental impacts. The anticipated completion of the Wangdolcho Ocean Research Station is expected to greatly improve monitoring capabilities, providing continuous and comprehensive data that will enhance our understanding of coastal currents and their broader impacts on the marine environment.

Different effects between cold front and tropical cyclone on short-term morphodynamics in the Changjiang Delta

Global climate change is altering the frequency and intensity of extreme weather events such as typhoons and cold fronts, and this is inducing physical changes and adaptions in estuaries and coasts. We conducted a field campaign on the subaqueous Changjiang Delta front in September 2019 to improve understanding of storm impact on short-term hydro-morphodynamics. Over an 11-day period, during which both typhoon and cold front events occurred, in-situ data including flow velocities, suspended sediment concentrations (SSC), and bed-level changes were acquired using bottom tripod and buoyage systems, Significant wave height reached 5.0 m during the typhoon, and the depth-averaged current velocity increased to 1.7 m/s. The net near-bottom sediment flux was in the same direction as the wind and was 5.5 times of that under calm weather. During the cold-front, significant wave height reached 2.5 m, and the near-bottom SSC increased to 7.0 kg/m3. Bed-level changes were small (<2 cm) under pre-storm weather, while net deposition and erosion reached 15.8 cm and 16.8 cm, respectively, during the typhoon. Short-term changes in the sediment source-to-sink regime were detected in the subaqueous Changjiang Delta. The cold front enhances seaward sediment flushing from the delta towards the sea, while the typhoon drives sediment transport from the subaqueous delta towards Hangzhou Bay. We also observed rapid bed-level recovery following these extreme events. These findings improve our understanding of sediment transport under stormy conditions.

A hybrid forecasting model for depth-averaged current velocities of underwater gliders

A hybrid forecasting model for depth-averaged current velocities of underwater gliders

A combined Gaussian process regression and one-dimensional least squares harmonic method for tidal current prediction

Predictions of tidal currents are required for various activities in the maritime industry, including marine power exploitation, operational forecasting, and coastal engineering. In addition to the conventional least squares harmonic method (LSHM) and numerical simulation, a machine learning algorithm has recently been developed and applied for predicting tidal currents in time series. Gaussian Process Regression (GPR) is one of the most important machine learning algorithms due to its ability to process time-series data. In this paper, we propose a combined one-dimensional LSHM and GPR method for predicting depth-averaged current velocity vectors according to time-series changes of water surface elevation due to tide. LSHM firstly decomposes water surface elevation time series into a number of possible constituents (i.e., amplitudes, phase-lags) according to known tidal frequencies. For each of the contributing constituents, GPR is then applied to produce the corresponding current generator using known tidal current as training data. The training data are generated by numerical simulations representing different coastal settings. The predicted tidal current is calculated according to the previously trained GPR model. As a comparison, we also generated tidal current predictions using a conventional two-dimensional LSHM. It is found that the tidal currents predicted using the sequence of computations proposed in this paper show comparable results with slightly better accuracy than conventional two-dimensional LSHM. The results from this study could provide an alternative method of tidal current prediction, minimising the need for the laborious re-runs of numerical simulations and lengthy current observations, as well as removing the requirement to carry out a two-dimensional analysis of the tidal current using LSHM.

Novel Method for the Estimation of Vertical Temperature Profiles Using a Coastal Acoustic Tomography System

Systems based on remote sensing technology, which use reciprocal acoustic signals to continuously monitor changes in the coastal oceanic environment, are referred to as coastal acoustic tomography (CAT) systems. These systems have been applied in regions in which heavy ship traffics, fishing and marine aquaculture activities make it difficult to establish in situ oceanic sensor moorings. Conventionally, CAT measurements were used to successfully produce horizontal maps of the depth-averaged current velocity and temperature in these coastal regions without attempting to produce a vertical temperature profile. This prompted us to propose a new method for vertical temperature profile estimation (VTPE) from CAT data using the available sea surface temperature (SST), near-bottom temperature (NBT), and water depth. The VTPE method was validated using data-assimilated and tide-included high-resolution ocean model outputs, including tide data, by comparing the estimated and simulated temperatures. Measurements were performed in the southern coastal region of Korea, where two CAT stations were moored to establish a continuous coastal ocean monitoring system. The validation results revealed that the algorithm performed well across all seasons. Sensitivity tests of the VTPE method with reasonable realistic random errors in the SST, NBT, and acoustic travel time measurements demonstrate that the method is applicable to CAT observation data because the monthly mean root-mean-squared difference (RMSD) for the vertical profiles for February, May, August, and November were 0.23, 0.30, 0.50, and 0.24°C, respectively. The VTPE method was applied to the CAT observation datasets acquired in February and August. The transceivers at the CAT stations were at depths 11 and 6 m on average. The RMSD between the estimated and observed temperatures in the middle layer (∼3 m depth) between two stations in February and August were 0.08 and 0.60°C, respectively, the accuracy of which is sufficient in largely time-varying coastal environments. We provide a novel method for continuous coastal subsurface environmental monitoring without interrupting maritime traffic, fishing, and marine aquaculture activities.

Method for predicting depth-averaged current velocities of underwater gliders based on data feature analysis

In this paper, the data feature of depth-averaged current velocities (DACVs) derived from underwater gliders is analyzed for the first time. Two features of DACVs have been proposed: one is the complex ingredients and small samples, and the other is the stationarity that occurs as the length of a DACV sequence increases. With these features in mind, a set of methods combining statistical analysis and machine learning are proposed to realize the prediction of DACVs. Four groups of DACV data of different gliders from sea trials in the South China Sea are used to verify the prediction method. Based on three general error criteria, the prediction performance of the proposed model is demonstrated. The persistence method is used as a comparison model. The results show that the prediction methods proposed in this paper are effective.

An integrated marine data collection for the German Bight – Part 2: Tides, salinity, and waves (1996–2015)

Abstract. Marine spatial planning requires reliable data for, e.g., the design of coastal structures, research, or sea level rise adaptation. This task is particularly ambiguous in the German Bight (North Sea, Europe) because a compromise must be found between economic interests and biodiversity since the environmental status is monitored closely by the European Union. For this reason, we have set up an open-access, integrated marine data collection for the period from 1996 to 2015. It provides bathymetry, surface sediments, tidal dynamics, salinity, and waves for the German Bight and is of interest to stakeholders in science, government, and the economy. This part of a two-part publication presents data from numerical hindcast simulations for sea surface elevation, depth-averaged current velocity, bottom shear stress, depth-averaged salinity, wave parameters, and wave spectra. As an improvement to existing data collections, our data represent the variability in the bathymetry by using annually updated model topographies. Moreover, we provide data at a high temporal and spatial resolution (Hagen et al., 2020b); i.e., numerical model results are gridded to 1000 m at 20 min intervals (https://doi.org/10.48437/02.2020.K2.7000.0004). Tidal characteristic values (Hagen et al., 2020a), such as tidal range or ebb current velocity, are computed based on numerical modeling results (https://doi.org/10.48437/02.2020.K2.7000.0003). Therefore, this integrated marine data collection supports the work of coastal stakeholders and scientists, which ranges from developing detailed coastal models to handling complex natural-habitat problems or designing coastal structures.

Numerical simulation of tides for the assessment of tidal in-stream energy in selected sites in the Philippines

The Philippines has an untapped potential of ocean renewable energy, and one of the appropriate tidal energy converters for the country is in-stream tidal energy. The archipelagic nature of the country along with its position between the Pacific Ocean and West Philippine Sea results in high tidal currents due to the narrow transitions between the two large bodies of water. This study aims to assess the potential in-stream energy in the Visayas-Mindanao region using numerical simulation. Using the Advanced Circulation Model (ADCIRC) in the modelling of tides, four (4) locations in the study area with the highest simulated depth-averaged current velocities are identified, namely, Liloan Port, Hilutungan Channel, Surigao Strait and Banug Strait. Among the selected stations, Banug Strait between the Islands of Bayagnan and Awasan produces the highest energy density of 253.2 kW-h/m2. Using an 18-m diameter turbine, it is estimated to have a monthly energy output of 64.4 MW-h, which is estimated to support 260 households on average. The annual energy output of the said 18-m diameter turbine is 773.3 MW-h.

From Ripples to Large-Scale Sand Transport: The Effects of Bedform-Related Roughness on Hydrodynamics and Sediment Transport Patterns in Delft3D

Bedform-related roughness affects both water movement and sediment transport, so it is important that it is represented correctly in numerical morphodynamic models. The main objective of the present study is to quantify for the first time the importance of ripple- and megaripple-related roughness for modelled hydrodynamics and sediment transport on the wave- and tide-dominated Ameland ebb-tidal delta in the north of the Netherlands. To do so, a sensitivity analysis was performed, in which several types of bedform-related roughness predictors were evaluated using a Delft3D model. Also, modelled ripple roughness was compared to data of ripple heights observed in a six-week field campaign on the Ameland ebb-tidal delta. The present study improves our understanding of how choices in model set-up influence model results. By comparing the results of the model scenarios, it was found that the ripple and megaripple-related roughness affect the depth-averaged current velocity, mainly over the shallow areas of the delta. The small-scale ripples are also important for the suspended load sediment transport, both indirectly through the affected flow and directly. While the current magnitude changes by 10–20% through changes in bedform roughness, the sediment transport magnitude changes by more than 100%.

Model-Aided Localization and Navigation for Underwater Gliders Using Single-Beacon Travel-Time Differences.

An accurate motion model and reliable measurements are required for autonomous underwater vehicle localization and navigation in underwater environments. However, without a propeller, underwater gliders have limited maneuverability and carrying capacity, which brings difficulties for modeling and measuring. In this paper, an extended Kalman filter (EKF)-based method, combining a modified kinematic model of underwater gliders with the travel-time differences between signals received from a single beacon, is proposed for estimating the glider positions in a predict-update cycle. First, to accurately establish a motion model for underwater gliders moving in the ocean, we introduce two modification parameters, the attack and drift angles, into a kinematic model of underwater gliders, along with depth-averaged current velocities. The attack and drift angles are calculated based on the coefficients of hydrodynamic forces and the sensor-measured angle variation over time. Then, instead of satisfying synchronization requirements, the travel-time differences between signals received from a single beacon, multiplied by the sound speed, are taken as the measurements. To further reduce the EKF estimation error, the Rauch-Tung-Striebel (RTS) smoothing method is merged into the EKF system. The proposed method is tested in a virtual spatiotemporal environment from an ocean model. The experimental results show that the performance of the RTS-EKF estimate is improved when compared with the motion model estimate, especially by 46% at the inflection point, at least in the particular study developed in this article.

Hydrodynamic Characteristics of a Pneumatic Breakwater with Combined Wave–Current Actions: A Numerical Investigation

Yin, Z.; Wang, Y., and Jia, Q., 2020. Hydrodynamic characteristics of a pneumatic breakwater with combined wave–current actions: A numerical investigation. Journal of Coastal Research, 36(1), 196–203. Coconut Creek (Florida), ISSN 0749-0208.A pneumatic breakwater is a new type of coastal protection structure with mobility and environment-friendly advantages. A series of physical experiments with two-dimensional (2-D) numerical modeling was conducted to investigate the hydrodynamic characteristics of a pneumatic breakwater under wave–current action. The numerical model consists of the Reynolds-averaged Navier-Stokes (RANS) equations, the renormalization group (RNG) k-e turbulence model, and the volume of fluid (VOF) method. An additional mass source term was added to the continuity equation to generate the desired air bubbles, and it was well validated with the related experimental data. The wave transmission coefficient (Kt) of a pneumatic breakwater was explored in relation to the incident wave height (Hi), wave period (T), air flow rate per unit width (q), and depth-averaged current velocity to wave phase velocity ratio (U/C) in both wave-alone and wave–current fields. The results indicated that Kt of the pneumatic breakwater in a wave-alone field increased with increasing Hi and T and decreased as q increased. Additionally, Kt of waves following a current increased with increasing U/C, while that of waves opposing a current increased up to a certain value before decreasing with U/C. Finally, the wave energy flux parameter was used to investigate the energy damping effect of the pneumatic breakwater in a wave–current field.

Time Series Prediction Methods for Depth-Averaged Current Velocities of Underwater Gliders

In this paper, we propose time series prediction methods for depth-averaged current velocities (DACVs) of underwater gliders. Based on historical DACV data, these methods can predict the DACVs of future profiles with good performance. Regarding DACVs as time series, we use backpropagation neural network and least squares support vector machine (LSSVM) methods to predict the DACVs. To obtain better prediction performance, the features of DACVs are considered, and we use empirical mode decomposition (EMD) to decompose the time series into several sub-series. Then, the two methods are reused to predict each sub-series, and the results of all the sub-series with each method are added. Based on the real-time DACVs obtained from the simulation environment and the DACVs obtained from sea trials, we test and verify the four methods. The results demonstrate that all the methods exhibit a good prediction performance for conditions in which ocean currents are relatively regular; whereas in other cases, EMD-LSSVM shows inherent robustness and superiority compared with the other three methods.

An experimental study on near-orthogonal wave–current interaction over smooth and uniform fixed roughness beds

An experimental study of periodic waves interacting with near-orthogonal turbulent currents is presented in this paper. Mean velocity profiles for current-alone, wave-alone and combined wave–current flows are measured with Acoustic Doppler Velocimeters to resolve the changes in mean flow kinematics due to wave–current interaction. In this study, the near-bottom wave orbital velocity is approximately 1.5 times greater than the depth-averaged current velocity, and the log-profile method is used to determine the bottom roughness from the measured velocity profiles. The primary focus is on 90° wave–current interaction, while selected findings for 60° and 120° wave–current orientations are also presented. In the smooth bed experiment, the interaction is shown to be linear due to the relatively low Reynolds-number flows generated in the facility — a relatively common scenario in small-scale laboratory setups that has not received much attention in previous studies. The smooth turbulent flow equation, with modification of the input shear velocity, is found to accurately predict the mean flow roughness for linear interaction. The bottom roughness is subsequently increased with the introduction of a layer of uniform 12.5mm marbles to achieve a more realistic rough turbulent flow regime. The results agree qualitatively with previous experimental findings that showed a reduction in the near-bottom mean velocity due to a wave-enhanced (apparent) roughness. However, the Grant–Madsen (GM) model is found to over-estimate the apparent roughness when the angle of wave–current interaction is large, implying a lack of directional sensitivity of this model under “strong-wave, weak-current” conditions. A tentative explanation of this shortcoming is given in the paper. However, it is not possible to extend the GM model analysis to the present near-orthogonal wave–current flows, as results suggest that both 60° and 120° wave–current cases are sufficiently contaminated by the wave-induced mass transport component in the current direction to invalidate the use of the log-profile method to resolve the bottom roughness. In addition to the modification of current profiles by waves, the present study also shows a drastic transformation of wave-induced mass transport profiles by the external turbulent currents. The veering of mean flow over depth is consistent with the superposition of a wave-induced return flow on the external current, while additional veering in the near-bottom region may be attributed to turbulence asymmetry induced by the current component in the direction of the near-bottom wave orbital velocity which is shown to vary locally by ±10° due to parasitic waves emanating from the current inlet and outlet.

Going with the flow: Tidal influence on the occurrence of the harbour porpoise (Phocoena phocoena) in the Marsdiep area, The Netherlands

One of the most important factors explaining the distribution and behaviour of coastal marine mammals are tides. Tidal forces drive a large number of primary and secondary processes, such as changes in water depth, salinity, temperature, current velocity and direction. Unravelling which tidal process is the most influential for a certain species is often challenging, due to a lack of observations of all tide related covariates, strong correlation between them, and the elusive nature of most marine organisms which often hampers their detection.In the Marsdiep area, a tidal inlet between the North Sea and the Dutch Wadden Sea, the presence of harbour porpoises (Phocoena phocoena) was studied as a function of tide related covariates. Observations were carried out in early spring from a ferry crossing the inlet on a half hourly basis. Environmental and sightings data were collected by one observer, while an on-board Acoustic Doppler Current Profiler (ADCP) and temperature sensor continuously recorded current velocity profiles and temperature, respectively. Sea surface temperature and salinity were measured at a nearby jetty. Sightings (n=134) were linked to tidal elevation, geographical position, local depth-averaged current velocity, water temperature (with and without trend correction) and salinity.Variation in sighting rate was best described by salinity, with highest sighting rate at high levels of salinity (>30gkg−1), indicating that porpoises enter the area in bodies of (more saline) North Sea water. Second best variable was time of day, with the highest sighting rate early morning, and decreasing during the day. However, surveys in the morning happened to coincide more often with high water and hence, the apparent time of day effect could be due to collinearity. Most porpoises were present in the northern part of the Marsdiep, particularly during high tide.Tide dependent sighting rates confirmed that porpoises reside in the North Sea, and enter the western Wadden Sea during the flood and leave during ebb. This tidal influx is most likely related to prey availability, which corresponds to other recent studies in this area showing higher fish abundance during high tide. Documenting information on tide related patterns could be used in practice, when e.g. planning anthropogenic activities or assessing critical habitats for this species.

Hydrodynamic modeling of Changjiang Estuary: Model skill assessment and large-scale structure impacts

An unstructured grid, Finite Volume Coastal Ocean Model (FVCOM) is used to study hydrodynamics and large-scale structure impacts in Changjiang Estuary. Field measurements conducted after the construction of the large-scale channel–jetty system are used to assess numerical results. The agreements between simulated and measured water levels, depth-averaged current velocities and tidally averaged longitudinal salinity distributions are excellent, as indicated by predictive skills higher than 0.94. The predictive skills for time series of salinity show a large range, from 0.97 to 0.62, in different measurement locations. The impacts of two 50 km long jetties and tens of spurs on the estuarine circulations as well as salinity distribution are investigated using comparisons of numerical results with and without jetty structures. Results reveal that the jetty structures intensify currents in the navigation channel and generate strong shear and vortices in jetty–spur blocks, which enhance turbulent mixing and degrade salinity stratification in the channel. The large-scale structures not only affect the flow field in the northern passage, but also play an important role in redistributing freshwater runoff in the multi-channel system, resulting in estuary-scale adjustments of circulations and salinity distribution.

A real-time, event-triggered storm surge forecasting system for the state of North Carolina

A new real-time, event-triggered storm surge prediction system has been developed for the State of North Carolina to assist emergency managers, policy-makers and other government officials with evacuation planning, decision-making and resource deployment during tropical storm landfall and flood inundation events. The North Carolina Forecast System (NCFS) was designed and built to provide a rapid response assessment of hurricane threat, accomplished by driving a high-resolution, two-dimensional, depth-integrated version of the ADCIRC (Advanced Circulation) coastal ocean model with winds from a synthetic asymmetric gradient wind vortex. These parametric winds, calculated at exact finite-element mesh node locations and directly coupled to the ocean model at every time step, are generated from National Hurricane Center (NHC) forecast advisories the moment they are inserted into the real-time weather data stream, maximizing the number of hours of forecast utility. Tidal harmonic constituents are prescribed at the open water boundaries and applied as tidal potentials in the interior of the ocean model domain. A directional surface roughness parameterization that modulates the wind speed at a given location based on the types of land cover encountered upwind, a forest canopy sheltering effect, and a spatially varying distribution of Manning’s–n friction coefficient used for computing the bottom/channel bed friction are also included in the storm surge model. Comparisons of the simulated wind speeds and phases against their real meteorological counterparts, of model elevations against actual sea surface elevations measured by NOAA tide gauges along the NC coast, and of simulated depth-averaged current velocities against Acoustic Doppler Current Profiler (ADCP) data, indicate that this new system produces remarkably realistic predictions of winds and storm surge.

Estimating suspended sediment concentration using backscatterance from an acoustic Doppler profiling current meter at a site with strong tidal currents

Since 1998, ferry observations have been carried out in the Marsdiep tidal inlet (Dutch Wadden Sea), using amongst other instruments a vessel-mounted acoustic Doppler current profiler (ADCP). Besides 32 cross-sections a day of current velocity data, the instrument also records the echo intensity, which has been interpreted in terms of suspended sediment concentration (SSC) before (Thorne and Hanes, Cont Shelf Res 22:603–632, 2002). However, we show herein that the random phase scattering model as outlined by Thorne and Hanes (Cont Shelf Res 22:603–632, 2002), predicts unrealistically high values of SSC if the depth-averaged current velocity exceeds approximately 0.7 m/s. Therefore, we extended the random phase scattering model by including scattering by turbulence-induced variability in SSC. The important mechanism is that when SSC fluctuations are present at length, scales of the order of the acoustic wavelength, the phase of the returned signal is no longer random and causes stronger backscatter. Such SSC fluctuations occur as a result of turbulent eddies in combination with an SSC gradient. The extended model was compared with data of two field surveys carried out in the Marsdiep inlet. The extended model, when compared with the classical random phase model, showed a large improvement of accuracy of the estimated SSC, which allows us to apply the model to the ferry data set to analyse suspended sediment transports through the Marsdiep tidal inlet.

Estimating surface oil spill transport due to wind in the Arabian Gulf

Oil spill models typically compute advection of an oil spill on the sea surface as a simple vectorial weighted sum of the wind velocity and the depth-averaged current velocity due to forces other than wind. Appropriate values for the weights needed to estimate wind-induced surface oil spill speed and the associated deflection angle suitable for the Arabian Gulf were computed. The estimates were based on the results of a series of drifting buoy experiments carried out in the Arabian Gulf during 1983–1984. Two approaches for estimating the deflection angle were proposed: the first assumed the angle to be constant, whereas the second assumed the angle to vary with wind.

SUSPENDED SEDIMENT CONCENTRATION AND SEDIMENT TRANSPORT RATE IN WAVE AND CURRENT CO-EXISTING SYSTEM

Numerical analysis was carried out to obtain the near-bottom flow velocity and the suspended sediment concentration in the wave and current co-existing system. The near-bottom flow velocity was computed by the turbulent boundary layer theory. The shear stress was evaluated by using the time-independent eddy viscosity proposed by Grant-Madsen.Suspended sediment concentration was computed by the turbulent diffusion theory. The diffusion coefficient was evaluated by using the horizontal and vertical components of flow velocity. The results for near-bottom flow velocity were compared with the laboratory data obtained by Asano et al. For opposing current, as the depth-averaged current velocity exceeds to 1/10-1/5 of the maximum bottom velocity in wave field, the direction of sediment transport is reversed.