Simulated Current Velocities Research Articles

Characteristics of ocean current meandering and potential efficacy of maximizing power capacity by tracking short-term meanders with hydro sail enabled active mooring

This research studies the characteristics of ocean current meanders from the viewpoint of power generation engineering and the potential of maximizing ocean current power generation capacity by making generator turbines to track the meanders applying a recently proposed Cross-stream Active Mooring concept. The current meanders will hurt capacity factor of marine turbines moored at fixed locations, somewhat like the issue of wind availability for wind farms. The new mooring concept features a hydro sail system that can deploy generator turbines transversely across streams in a marine current. The hydro sail system can further adjust the horizontal position of the turbines, by changing sail angle of attack, to actively track fast streams in an ocean current. This paper first reviews and discusses the characteristics of short-term track variations of the Florida Current and the Kuroshio near Taiwan and then analyzes potential improvement in power generation using measured and simulated current velocity data in the two ocean currents, suggesting a potential increase of 10–40 % power generation achievable by tracking the meanders. Operations of the hydro sail were demonstrated by tests using remote controllable and passive scale models. Control methods are discussed. Approaches to observe, predict and track the meanders are conceptualized.

Long-term mean circulation in the Japan Sea as reproduced by multiple eddy-resolving ocean circulation models

The capacity of four eddy-resolving ocean circulation models—HYCOM (HYbrid Coordinate Ocean Model), MRI.COM (Meteorological Research Institute Community Ocean Model), OFES (Ocean General Circulation Model for the Earth Simulator), and NEMO (Nucleus for European Modeling of the Ocean)—to simulate the long-term mean hydrographic conditions and circulation patterns in the Japan Sea is investigated in this study. The assessment of this study includes the evaluation of mean vertical profiles and time series of temperature and salinity at the representative monitoring stations. Different model products from 1993 to 2015 are compared with in situ measurements provided by historical cruises and monitoring stations. After that, we compared the observed and simulated surface current velocities over the basin and volume transports through the key straits in the Japan Sea. Simulated current velocities are validated against 15 years of Acoustic Doppler Current Profiler (ADCP) measurements near the longshore and offshore branches of the East Korea Warm Current (EKWC). Furthermore, the atmospheric forcing data of the four ocean circulation models are validated against the satellite wind product. We found that the vertical profiles and long-term variations of temperature and salinity reproduced by MRI.COM and HYCOM are closer to in situ measurements. All models simulate temperature well in upper ocean, but salinity simulations are of lower quality from OFES and NEMO at several stations. Simulated current velocities predominantly lie within the standard deviation of ADCP measurements at two locations. However, the sea surface currents are underestimated by four models compared with Drifter data. Although simulated hydrographic profiles agree well with in situ observations, the mean circulation patterns greatly differ between the models, which highlight the need for additional evaluation and corrections based on the long-term current measurements. Because of the lack of ocean current measurements, only the baroclinic velocities simulated by each model are reliable. The substantial part of the differences in barotropic velocities among the simulate result of four models is explained by the differing wind velocities from the corresponding atmospheric forcing datasets.

Simulation of Marine Debris Path Using Mathematical Model in the Gulf of Thailand

Marine debris is an important environmental problem that affects aquatic animals, ecosystems, economy, society, and humans. This research aims to simulate the path of marine debris in the Gulf of Thailand using a mathematical model that includes two models: the Oceanic Model (OCM), which is based on the Shallow Water Equations (SWE), and the Lagrangian Particle Tracking (LPT) model. The OCM is the partial derivative equation system solved by the finite difference method to satisfy the Arakawa C-grid and the splitting method. The LPT model includes the current velocity, wind velocity at 10 m above sea level, random walk term, and the buoyancy ratio of marine debris with six cases, which are 100:1, 10:1, 1:1, 0:1, 1:10, and 1:100. The current velocity from OCM is applied to the LPT model. This research uses a garbage boat that capsized near Koh Samui on 1 August 2020 as a case study. The simulated current velocity of OCM is compared with Ocean Surface Current Analyses Real-time (OSCAR) data. The Root Mean Square Error (RMSE) of u-velocity is 0.070 m/s, and that of v-velocity is 0.058 m/s. The simulation of the marine debris’s path from the LPT model demonstrates the movement to Koh Samui, Koh Taen, Koh Wang Nai, Koh Wang Nok, Koh Rap, the east coast of Nakorn Si Thammarat province, Phu Quoc Island of Vietnam and the middle of the Gulf of Thailand with the different buoyancy ratios and time durations.

Investigating shallow water bottom feature using SAR data along Gulf of Khambhat, India

The coastal bathymetry often forms complex and transient morphology, especially along the tide-dominated regions like the Gulf of Khambhat (GoK), west coast of India. The high tidal currents within the gulf relentlessly modify the morphology of the bathymetric features that prompts for a frequently updated bathymetric information. The SAR estimated bathymetry represent updated depth values based on the transient bathymetric features observed from the SAR images. The present study has used ALOS PALSAR-1/2 data to update the depth of shallow water bottom features of GoK. The method involves modeling the gradient in the backscatter coefficient (σ°) modulated by the slope of the bathymetric features with the satellite images. The slope corresponding to the bathymetric feature from hydrographic charts (initial guess bathymetry) and the simulated current velocity forms the input to the model. The bathymetry is updated by varying the slopes of the bathymetric features through iteration until the σ° converges. The method is applied to update the depth of various bathymetric features observed from the SAR images of 2008, 2010, 2014 and 2018. We validated the updated depth from 2018 satellite data using available in situ measurements. The comparison of in situ observation with the hydrographic chart has resulted in RMSE of 3.2 m, while the RMSE have improved to 1.5 m when compared with the SAR updated bathymetry. The temporal dynamics of the bathymetric features are studied using the pair of 2008–10 and 2010-14 SAR images. Analysis has comprehended the morphodynamics of bathymetric features like lateral migration of tidal bars, erosion along the tidal channel periphery, and deposition at the tidal spits. We updated the depth of rapidly changing bathymetric features using the 2018 SAR images. The study shows its potential in operational usage to update the complex nearshore bathymetry from the SAR image.

Long-Term Mean Circulation of the Baltic Sea as Represented by Various Ocean Circulation Models

The skill of the state-of-the-art ocean circulation models GETM (General Estuarine Transport Model), RCO (Rossby Centre Ocean model), and MOM (Modular Ocean Model) to represent hydrographic conditions and the mean circulation of the Baltic Sea is investigated. The study contains an assessment of vertical temperature and salinity profiles as well as various statistical time series analyses of temperature and salinity for different depths at specific representative monitoring stations. Simulation results for 1970-1999 are compared to observations from the Baltic Environmental Database (BED). Further, we analyze current velocities and volume transports both in the horizontal plane and through three transects in the Baltic Sea. Simulated current velocities are validated against ten years of Acoustic Doppler Current Profiler (ADCP) measurements in the Arkona Basin and five years of mooring observations in the Gotland Basin. Furthermore, the atmospheric forcing datasets, which drive the models, are evaluated using wind measurements from 28 automatic stations along the Swedish coast. We found that the seasonal cycle, variability, and vertical profiles of temperature and salinity are simulated close to observations by RCO with an assimilation setup. All models reproduce temperature well near the sea surface. Salinity simulations are of lower quality from GETM in the northern Baltic Sea and from MOM at various stations. Simulated current velocities lie mainly within the standard deviation of the measurements at the two monitoring stations. However, sea surface currents and transports in the ocean interior are significantly larger in GETM than in the other models. Although simulated hydrographic profiles agree predominantly well with observations, the mean circulation differs considerably between the models highlighting the need for additional long-term current measurements to assess the mean circulation in ocean models. With the help of reanalysis data ocean state estimates of regions and time periods without observations are improved. However, due to the lack of current measurements only the baroclinic velocities of the reanalyses are reliable. A substantial part of the differences in barotropic velocities between the three ocean models and reanalysis data is explained by differences in wind velocities of the atmospheric forcing datasets.

Empirical Orthogonal Function Analysis of Suspended Sediment Concentration in Gulf of Kachchh, India, and Its Tidal Influence

Complexity of the temporal sediment variability within the Gulf of Kachchh is resolved into major modes of variations using empirical orthogonal function (EOF) analysis on sequential OCM derived suspended sediment concentration (SSC) images during spring intermonsoon period of 2011. Variance accounted collectively by the first four modes is around 80% and the principal component (PC) of each mode is correlated with different hydro-meteorological forces influencing the hydrodynamics of the gulf. Hydro-meteorological forces are classified in the order of precedence of influencing the sediment dynamics of the region. PC1 contributing 50% of the total variance is correlated with average of the simulated current velocities between consecutive images, indicating the variation of tides from neap-spring ranges (r2 = 0.86). The ocean state parameters like tidal height, wind speed, and current speed at the time of OCM pass contribute to around 20%, 6%, and 4%, respectively, to the total variability of the temporal dynamics of SSC. The interchange of tides from neap and spring is the major contributor toward the temporal variability of sediment concentration within the gulf, where the sediments are subjected to deposition and resuspension. The gulf being highly tide dominated, the suspended sediment variability also reverberates with the variability of tidal characteristics and subtly with the wind conditions.

Numerical modeling of cohesive sediment transport in a tidal bay with current velocity assimilation

Tidal currents play an important role in sediment dynamics in coastal and estuarine regions. The goal of this study is to investigate the effects of current velocity assimilation (CVA) on sediment transport modeling in tide-dominated waters. A hydrodynamic and sediment transport model for Deep Bay, Hong Kong, was established based on a three-dimensional primitive equation Finite Volume Coastal Ocean Model. An additional numerical simulation was conducted with in situ current velocity measurements sequentially assimilated into the model using a three-dimensional optimal interpolation scheme. The performance of CVA shows improvements in the root-mean-square errors and average cosine correlations of simulated current velocity by at least 9.1 % and 10.3 %, respectively. Moreover, the root-mean-square error of the simulated sediment concentration from the model with CVA was decreased by at least 7 %. A reasonable enhancement in the vertical and spatial distributions of sediment concentrations was demonstrated from the simulation results from the model with CVA. It was found that the bottom shear stress changed significantly when the simulated velocities were corrected with CVA. The results suggest that CVA has the potential to improve sediment transport prediction because tidal currents dominate sediment dynamics in the studied areas.

Boulder and fine sediment transport and deposition by the 2004 tsunami in Lhok Nga (western Banda Aceh, Sumatra, Indonesia): A coupled offshore–onshore model

Estimating the magnitude of a past tsunami from its deposits is one of the major topics to be developed in future studies on tsunami hazard assessment. Main limitations are (1) the great variability of tsunami sandy sheets deposited on land (the sediment source and the topography controlling many aspects of the deposition), (2) the preservation of these soft sediments, and (3) the controversial interpretations of coastal boulder accumulations. In this paper, we investigate sediment transport and deposition during the December 26, 2004 tsunami inflow and outflow in the Lhok Nga Bay, located 10 km west of the city of Banda Aceh (northwest Sumatra, Indonesia). Side-scan sonar data of the near shore area are used to study the morphometry and distribution of boulders offshore. Entrainment of finer sediments offshore is inferred by estimating the movable grain sizes based upon the simulated current velocities of the tsunami waves. Results demonstrate that the tsunami waves raised the bed shear velocities to levels above critical values for the entrainment of coarse sands on the continental shelf. Most of the sediments deposited on land came from offshore, from fine sands to coral boulders. With very high values of u ⁎ (> 30 cm/s), the outflow (backwash) reworked and re-deposited large volume of sediments offshore. All rocky outcrops offshore were affected by the tsunami (down to 25 m deep). The fraction of boulders transported from offshore and deposited inland represents only 7% of the total number of boulders moved during the tsunami. Characteristics of the boulders can help to estimate flow velocities required for detaching them, and their imbrication (if any) to infer flow directions. However, calculations of flow depth and transport distance do not provide convincing results. Future studies coupling offshore–onshore mapping of boulder accumulations with reconstitutions of the morphological history (sea-level variations, coastal sediment discharge and landform evolution) may allow distinguishing storms and tsunami deposits.

Spatially explicit model predicting the spawning habitat and early stage mortality of Northern pike (Esox lucius) in a large system: the St. Lawrence River between 1960 and 2000

Discharge fluctuations in the St. Lawrence River (Canada) affect reproduction habitat for Northern pike (Esox lucius Linnaeus). We developed a spatially explicit model for that large-scale river system to predict spawning habitat surfaces available for egg deposition and the potential mortality by dewatering occurring during the embryonic-larval stages. The spatial model used simulated current velocity, wetland type and water temperature at a high spatial resolution over the entire fluvial St. Lawrence River, Canada. Those three variables were integrated into a habitat suitability index (HSI) and weighted according to the literature. A new thermal preferendum curve, based on a field experiment, was included in the HSI, reflecting that the probability of observing pike spawners in a given area increased with temperature. The reproduction chronology was predicted every year with an original model based on air temperature in order to reconstitute the historic habitat surfaces for the period 1960–2000. The results revealed that discharge had a substantial effect on both suitable habitat for egg deposition and potential mortality following dewatering. The best and the largest spawning habitats were identified, as well as the most limiting regions in the river. The present findings have already been used to prepare a new discharge regulation plan for the Lake Ontario–St. Lawrence River system.

Bottom Shear Stress Under Wave-Current Interaction

The present work adopts the COHERENS-SWAN model developed by the first author through coupling three-dimensional hydrodynamic model (COHERENS) and third-generation wave model (SWAN). Inside the COHERENS-SWAN, the SWAN is regarded as a subroutine and the time- and space-varying current velocity and surface elevation are obtained from the COHERENS. Wave-enhanced bottom shear stress, wave induced surface mixing length and wave dependent surface drag coefficient have been introduced into the COHERENS. Secondly, as wave-enhanced bottom shear stress (“bottom shear stress” described as BSS sometimes in this article) is concerned, a modified bottom shear stress Grant and Madsen model which introduces random wave field is given and introduced to COHERENS-SWAN. COHERENS-SWAN is also adopted to simulate three-dimensional flow in the Yellow River Delta with wave-current co-existing. Four numerical experiments were given to study the effects of wave-current interaction on enhancing bottom shear stress. The simulated current velocities, wave height and wave period match well with field measurement data. The simulated significant wave height and wave period for the case with considering the effects of current can give better agreement with measurement data than the case without involving the effects of current. The introduction of random wave generates lower the bottom shear stress than the case without introducing it. There are obvious differences between bottom shear stress of two way interaction and one way interaction. Velocity field obtained by the COHERENS-SWAN is reasonable according to previous studies and measurements.