Tidal Propagation Research Articles

Wake field study of tidal turbines under realistic flow conditions

Numerical modelling of the flow interactions between tidal turbines in arrays is a prerequisite to assess the energy production potential and to optimise the layout of tidal stream energy farms. It requires (i) a refined representation of the tide propagation and (ii) a reliable estimate of the flow characteristics around the turbines. The Actuator Disk (AD) theory is recognised as a reliable parameterisation to approach the far wake of fixed horizontal-axis turbines and was therefore implemented to provide recommendations as regards to the design of arrays, especially the optimal spacing between devices. However, analysis of the arrangement of tidal stream turbines are typically restricted to schematic test cases with idealised bathymetry and inflow conditions. The present study aims to investigate the flow interactions between turbines in a stream energy site with spatially- and time-varying tidal conditions. The site of application is the Alderney Race (English Channel), an area with strong potential for the exploitation of tidal energy in European shelf seas. The three-dimensional numerical model Telemac3D is used to capture the flow at both the regional and the array scales. Predictions are first assessed against ADCP measurements acquired around the island of Alderney. A series of simulations, representing turbines with the AD theory, are then performed to investigate the influence of the wake interactions on the energy production of individual devices. In those simulations, the turbines are represented with the AD theory. The zone occupied by the turbines is 20D-long and 15D-wide (D is the turbine diameter). Different layouts (isolated, aligned and staggered turbines) and different longitudinal and lateral spacings are considered to investigate the effect of the turbines’ arrangement and density on the array energy production. The results show that, for a given turbines density within the array, the staggered layout produces more than the aligned layout (+16%). When the turbines are staggered, a minimal lateral spacing of 5D is required to avoid wake overlapping. However, when the turbines are aligned, the lateral spacing has a small influence on the turbines production. It is therefore possible to pack the turbines close to each other in the lateral direction. The simulations also outline that the turbulence develops much faster within the array when the turbines are aligned.

Tidal and non-tidal components of water level and currents in the Sundarbans ecosystem

The present study is an effort to understand the complex tidal characteristics and flow conditions at Kaikhali station located in the Matla River of the Indian Sundarbans ecosystem. The study region is located south of the low-lying Ganges–Brahmaputra deltaic environment that is highly vulnerable to the impact of sea level rise. The maximum recorded tidal range at Kaikhali location is about 6.35 m, and the current speed varied between 0.03 to 1.18 m s−1 with an average speed of 0.48 m s−1 during the observation period. The currents closely followed the flood and ebb phases of the tidal cycle with a maxima recorded during the spring high tide conditions. The predominant current direction was northeast (NE) and southwest (SW) during the flood and ebb phases of the tidal cycle, respectively. The tidal characteristics are semidiurnal in nature, and among the five major tidal constituents M2, S2, K1, O1 and N2, the relative influence of principal lunar (M2) and principal solar (S2) components contributes 58% and 25%, respectively. The circulation characteristic in this ecosystem is a result of the combined effects of freshwater discharge from riverine tributaries, creeks and tidal propagation from the open sea. A detailed investigation performed in the present study using the measured data signify that the tidal currents dominate as compared to the non-tidal components in this region. The study has practical applications in understanding the estuarine hydrodynamics over this region.

Numerical study of the tidal refraction effect on the open coast in southeastern Zhejiang

In this study, the tidal refraction effect on the open coast in southeastern Zhejiang, China, is numerically investigated using an MIKE21-based model. The accuracy of this numerical model was verified by comparing with the measured results. The model was then employed to simulate the tidal propagation and the impacts of intense reclamation planned in the study area. The co-tidal chart and iso-amplitude curves of M2 (principal lunar semi-diurnal), S2 (principal solar semi-diurnal), O1 (lunar diurnal), K1 (lunar diurnal), and the tidal current ellipses of the M2 constituent illustrate the different tidal refraction phenomena from underwater shoals to tidal flats on the open coast. The results showed that the tide spreads along the coast in underwater shoals, while it spreads toward the coast in tidal flats. The spreading direction of the tide was congruent with the main spindle of the M2 constituent tidal current ellipses. Furthermore, the tidal range decreased; tidal refraction effects were weakened after reclamation, and the tidal direction changed.

Study of high tide level changes in estuary area in response to coastline deformation in Wenzhou Bay

Wenzhou Bay, which is located in the southern coast of China with rich tidal flat resources, is frequently influenced by human activities, leading coastline deformation and high tide level changes in estuary area. A two-dimensional tidal numerical model of high resolution including Wenzhou Bay and East China Sea was established based on the MIKE21 FM model. The model was calibrated with field data and then was employed to predict high tide level changes under the development plan of coastline deformation. According to the simulating results, high tide level and tidal range in the Ou River and the Feiyun River estuary area will decrease about 0.01~0.13 m and 0.01~0.18 m, whether in dry season or flood season. Further analysis from tide wave point illustrates that weaken of the tidal system outside estuary area will occur after the coastline deformation. The direction of tidal propagation will change from vertical to parallel to coastline, causing weaken reflection, leading to the decreases of tidal range and high tidal level.

Seasonal variations of river and tidal flow interactions in a tropical estuarine system

The tidal hydrodynamics of a tropical estuarine system with river outflow seasonal variations was studied. Water level observations from eleven sites within the system were analyzed over one year in a restricted and shallow tropical estuary. The analysis was performed using wavelets, power spectral density analysis and a linear analytical model. This methodology gives the principal tidal components, their amplitude, phase and the interactions between the tide and the seasonal variations of the river outflow. The analytical model included the river discharge as an extra force on the frictional parameter to help understand the seasonal variations in tidal propagation within an estuary. The system was divided into three zones based on its bathymetry, geometry and river influence. Each zone presented different seasonal responses. River influence showed a 30% decrease in the amplitude of tidal signal from the mouth to the head, and the hydrodynamics were driven by the balance between the pressure gradient and frictional forces. This high tide attenuation in the river influence zone showed seasonal variations, therefore this research proposes adding into the frictional forces the river discharge to the frictional parameter in the analytical solution. The results show that although river discharge has a marked sub-tidal periodicity, river discharge has a strong influence on tidal hydrodynamics and inhibits the propagation of the tide toward the head of highly frictional estuarine systems.

Modal structure and propagation of internal tides in the northeastern South China Sea

The evolution of energy, energy flux and modal structure of the internal tides (ITs) in the northeastern South China Sea is examined using the measurements at two moorings along a cross-slope section from the deep continental slope to the shallow continental shelf. The energy of both diurnal and semidiurnal ITs clearly shows a ~14-day spring-neap cycle, but their phases lag that of barotropic tides, indicating that ITs are not generated on the continental slope. Observations of internal tidal energy flux suggest that they may be generated at the Luzon Strait and propagate west-northwest to the continental slope in the northwestern SCS. Because the continental slope is critical-supercritical with respect to diurnal ITs, about 4.6 kJ/m2 of the incident energy and 8.7 kW/m of energy flux of diurnal ITs are reduced from the continental slope to the continental shelf. In contrast, the semidiurnal internal tides enter the shelf because of the sub-critical topography with respect to semidiurnal ITs. From the continental slope to the shelf, the vertical structure of diurnal ITs shows significant variation, with dominant Mode 1 on the deep slope and dominant higher modes on the shelf. On the contrary, the vertical structure of the semidiurnal ITs is stable, with dominant Mode 1.

Validation of the 3D-MOHID Hydrodynamic Model for the Tagus Coastal Area

The hydrodynamics of the TagusROFI (Regions of Freshwater Influence) is affected by the coastal upwelling, the estuarine tidal flow, the thermohaline circulation that is modulated by the Tagus freshwater discharge, and by its complex bathymetry. The use of numerical models is the best way to explain the processes that characterize this region. These models are also crucial to answer important scientific and management questions. Nevertheless, the robustness of the products derived from models depend on their accuracy and therefore models must be validated to determine the uncertainty associated. Time and space variability of the driving forces and of bathymetry enhance flow complexity increasing validation difficulties, requiring continuous high-resolution data to describe flow and thermohaline horizontal and vertical variabilities. In the present work, to increase the precision and accuracy of the coastal processes simulations, the sub-systems coastal area and the Tagus estuary were integrated into a single domain, which considers higher resolution grids in both horizontal and vertical directions. The three-dimensiosal (3D)-MOHID Water model was validated for the TagusROFI by comparing statistically modelling results with in situ and satellite L4 data. Validation with a conductivity, temperature, and depth probe (CTD), an acoustic doppler current profiler (ADCP) and satellite data was performed for the first time. Validation against tidal gauges showed that the model is able to simulate tidal propagation inside the estuary with accuracy. A very good agreement between CTD data and surface sea water temperature (SST) and salinity simulations was observed. The validation of current direction and velocity from ADCP data also indicated a high model accuracy for these variables. Comparisons between model and satellite for SST also showed that the model produces realistic SSTs and upwelling events. Overall results showed that MOHID setup and parametrisations are well implemented for the TagusROFI domain. These results are even more important when a 3D model is used in simulations due to its complexity once it considers both horizontal and vertical discretization allowing a better representation of the heat and salinity fluxes in the water column. Moreover, the results achieved indicates that 3D-MOHID is robust enough to run in operational mode, including its forecast ability, fundamental to be used as a management tool.

The impacts of tidal energy development and sea-level rise in the Gulf of Maine

In this study, we employed a 3-D and two-way nested Regional Ocean Modeling System (ROMS) to address several important outstanding issues regarding tidal energy development in the Gulf of Maine. We investigated the impact of projected sea-level rise (SLR) on the energy resources of the region, and examined how tidal dynamics will be influenced by energy extraction and/or SLR. Further, we assessed whether the effect of SLR on the generation of tides in the ocean (hence at the boundary of the region) is significant in these assessments. We find that the impact of SLR exceeds the impact due to energy extraction in the Gulf of Maine - even when considering very large energy extraction, of order 3.0 GW, in the Minas Passage. Although results showed that energy extraction does not significantly increase the amplitude of the tides in the far-field, a drastic change in the Bay of Fundy (e.g. full blockage) can lead to considerably higher amplitudes of tides (around 35 cm, or 12%) in the western Gulf of Maine. As a result of 1 m SLR, the theoretical tidal energy resources in some areas, including the Bay of Fundy can increase noticeably while any significant change in extracted energy highly depends on the turbine technology.

Mangrove density impacts on tidal dynamic in Segara Anakan Lagoon, Indonesia

The present study aims to understand the tidal dynamics in a tropical coastal lagoon, Segara Anakan, Java, Indonesia. It is a shallow lagoon, with complex tidal channels and connected to the Indian Ocean by two channel outlets. It covers an overall area of about 12.000 ha. Three–quarters of the lagoons area have mangroves and the remaining quarter is covered by water. The Delft3D model has been implemented and validated using observation data. Scenario with uniform and spatially bottom drag coefficient were created in order to investigate the influence of the mangrove density on tidal dynamics. Based on tidal harmonic analysis, the M2 amplitude attenuates from both the western and the eastern lagoon inlets to the interior of the lagoon as the tidal wave is constricted by the narrow lagoon inlet and shallowness of the lagoon. For the case of uniform mangrove density, tidal harmonic analysis reveals M2 amplitude decreasing of 0.5 to 0.25 m from lagoon inlets toward central lagoon due to bottom friction effect. The tidal propagation into lagoon show increasing M2 tidal phase with maximum delay of 2.5 hours at the central lagoon. For the case of spatially mangrove density, the dampening of the tidal wave is stronger of 18% and the phase delay is longer 0.8 hours compare to the case of uniform mangrove density.

Internal Tide Generation and Dissipation by Small Periodic Topography in Deep Ocean

Internal tides generated by a rough sea floor are an important source of mixing in the abyssal ocean. Two linear models are employed to evaluate the conversion rate from barotropic tides to internal tides and the energy distribution in each mode. Considering the periodicity of internal tides, the topography is represented by periodically distributed knife edges and sinusoidal ridges within one wavelength of mode-1 internal tides. The knife edges generate greater internal tides than the sinusoidal ridges due to their sharp shape, which approximates an extremely supercritical condition. Energy flux concentrates in modes whose numbers are multiples of the knife edge or ridge number. Then, a fully nonlinear model that integrates viscosity and diffusion is implemented, and its results are compared with those of the linear model. Internal wave rays generated in the nonlinear model show a distribution similar to the linear models’ prediction. High dissipation rates coincide with the rays, suggesting that nonlinear wave-wave interaction is a dominant mechanism for internal tide dissipation in the abyssal ocean.

Coastal Semidiurnal Internal Tidal Incoherence in the Santa Maria Basin, California: Observations and Model Simulations

AbstractA series of five realistic, nested, hydrostatic numerical ocean model simulations are used to study semidiurnal internal tide generation and propagation from the continental slope, through the shelf break and to the midshelf adjacent to Point Sal, CA. The statistics of modeled temperature and horizontal velocity fluctuations are compared to midshelf observations (30‐ to 50‐m water depth). Time‐ and frequency‐domain methods are used to decompose internal tides into components that are coherent and incoherent with the barotropic tide, and the incoherence fraction is 0.5–0.7 at the midshelf locations in both the realistic model and observations. In contrast, the incoherence fraction is at the most 0.45 for a simulation with idealized stratification, and neither atmospheric forcing nor mesoscale currents. Negligible conversion from barotropic to baroclinic energy occurs at the local shelf break. Instead, the dominant internal tide energy sources are regions of small‐scale near‐critical to supercritical bathymetry on the Santa Lucia escarpment (1,000–3,000 m), 70–80 km from the continental shelf. Near the generation region, semidiurnal baroclinic energy is primarily coherent and rapidly decays adjacent to the shelf break. In the realistically forced model, incoherent energy is less than 10% in the generation region, with a steady increase in incoherence fraction from the continental slope to the midshelf. Backward ray tracing from the midshelf to the Santa Lucia escarpment identifies multiple energy pathways potentially leading to spatial interference. As internal tides shoal on the predominantly subcritical slope/shelf system, temporally variable stratification and Doppler shifting from mesoscale and submesoscale features appear equally important in leading to the loss of coherence.

The Closure of a Shallow Tidal Inlet Promoted by Infragravity Waves

AbstractTidal inlets connect the ocean to inner water bodies and are present worldwide. Shallow inlets display fast morphological changes, due to complex interactions between tides, waves, and shallow depths. Their closure is commonly observed under storm waves, but the underlying processes remain only partly understood. Here, we present new field evidence that infragravity waves contribute to the closure of shallow inlets. The analysis of new field data collected at a shallow inlet under storm waves reveals that infragravity waves up to 0.4 m high can propagate inside the lagoon during flood but are blocked by opposing currents during ebb. At the passage of an infragravity wave crest, currents peak over 2.5 m/s and increase instantaneous sand fluxes by 2 orders of magnitude. Large accumulations of sand at the lagoon entrance damp tidal propagation until full inlet closure. This mechanism provides a new explanation for the closure of shallow inlets observed worldwide.

Propagation of tides along a river with a sloping bed

Conceptually, tidal rivers are seen as narrow channels along which the cross-section geometry remains constant and the bed is horizontal. As tidal waves propagate along such a channel, they decrease exponentially in height. The more rapid the decrease, the stronger the river flow. Near the coast, the tidally averaged width and depth change little throughout the year, even if the river discharge varies strongly between the seasons. However, further upstream, the water depth varies considerably with the river discharge. Recent observations from the Kapuas River, Indonesia, show that the water surface forms a backwater profile when the river flow is low. In this case, the depth converges, i.e. it gradually decreases between the river mouth and the point where the bed reaches sea level. This effect distinctly influences how tidal waves propagate up river so that their wave height does not decrease exponentially any more. We present a theoretical analysis of this phenomenon, which reveals several so far overlooked aspects of river tides. These aspects are particularly relevant to low river flow. Along the downstream part of the tidal river, depth convergence counteracts frictional damping so that the tidal range is higher than expected. Along the upstream parts of the tidal river, the low depth increases the damping so that the tide more rapidly attenuates. The point where the bed reaches sea level effectively limits the tidal intrusion, which carries over to the overtide and the subtidal water level set-up.

Experimental study on inner slope failure mechanism of seawall by coupling effect of storm surge and wave

In the context of global climate change, the impact of group-occurring ocean dynamic disasters on China’s offshore areas is becoming more and more intense. The study of the effect of existing ocean dynamic disasters on offshore hazard-bearing bodies mostly focuses on the effect of single disaster-causing factors, and it is still insufficient to study storm surge and dynamic wave coupling & reinforcement effects as well as the process of the dynamic response of such hazard-bearing bodies as seawalls. This study firstly realized the synchronous process of water level and wave through continuous tide generation and wave generation by the wave maker and tide generating device, so as to realize the dynamic coupling simulation of storm surge and wave in the laboratory. Then the physical model test of the typical seawall section was carried out under the dynamic coupling of storm surge and wave as well as at a conventional fixed water level respectively. In the process of test wave overtopping discharge and the damage process of the levee crown and backwall of seawalls were observed and compared, and their damage mechanism was also studied.

The Effects of Remote Internal Tides on Continental Slope Internal Tide Generation

AbstractInternal tide generation at sloping topography is nominally determined by the local slope geometry, density stratification, and tidal forcing. Recent global ocean models have revealed that remotely generated internal tides (RITs) can also influence locally generated internal tides (LITs). Field measurements with through-the-water column moorings on the southern portion of the Australian North West Shelf (NWS) suggested that RITs led to local regions with either positive or negative barotropic to baroclinic energy conversion. Three-dimensional numerical simulations were used to examine the role of RITs on local internal tide climatology on the inner slope and shelf portion of the NWS. The model demonstrated the principle remote generation site was the western portion of the offshore Exmouth Plateau. Extending the model domain to include this offshore plateau region increased the local net energy conversion on the inner shelf by 13.5% and on the slope by 8%. Simulations using an idealized 2D model configuration aligned along the principal direction of RIT propagation demonstrated that the sign and magnitude of the local energy conversion was dependent on the distance between the remote and local generation sites, the phase difference between the local barotropic tide and the RIT, and the amplitude of both the local barotropic tide and the RIT. For RITs with a low-wave Froude number (Fr < 0.05), where Fr is the ratio of the internal wave baroclinic velocity to the linear wave speed, the conversion rates were consistent with kinematic predictions based on the phase difference only. For stronger flows with Fr > 0.05, the conversion rates showed a nonlinear dependence on Fr.

Spatial and Temporal Variability of Semidiurnal Internal Tide Energetics in the Western Bay of Bengal

The three-dimensional Massachusetts Institute of Technology general circulation model (MITgcm) with time-dependent forcing is implemented to investigate the spatial and temporal variability of internal tides over the eastern coast of India. Numerical simulations are conducted for a number of different months of the year for which in situ observations are available at high temporal resolution. During the months of November–December and March–April, peak spectral estimates of density variability are evident in the semidiurnal frequency band in both observations and simulations, while during August, variability is dominantly in the near-inertial frequency band. Empirical orthogonal functions (EOF) analysis is applied to decompose the baroclinic tidal currents into vertical modes. The results show that about 70–80% of the total variance is in the first mode, while the first three modes represent 90–95% of the total variance in all seasons. Internal tide characteristics such as phase speed, group speed, and wavelength are largest in the postmonsoon season. The magnitude of the computed energy flux is greatest during November, while the direction of propagation of internal tides is almost unchanged through the year. The available potential energy peaks in November (20 kJ m−2) and is smallest in August (14 kJ m−2). Calculation of the energy budget shows that the energy conversion rate from barotropic forcing to internal tides is about 85% in November but only about 46% in August.

Deep-ocean mixing driven by small-scale internal tides

Turbulent mixing in the ocean is key to regulate the transport of heat, freshwater and biogeochemical tracers, with strong implications for Earth’s climate. In the deep ocean, tides supply much of the mechanical energy required to sustain mixing via the generation of internal waves, known as internal tides, whose fate—the relative importance of their local versus remote breaking into turbulence—remains uncertain. Here, we combine a semi-analytical model of internal tide generation with satellite and in situ measurements to show that from an energetic viewpoint, small-scale internal tides, hitherto overlooked, account for the bulk (>50%) of global internal tide generation, breaking and mixing. Furthermore, we unveil the pronounced geographical variations of their energy proportion, ignored by current parameterisations of mixing in climate-scale models. Based on these results, we propose a physically consistent, observationally supported approach to accurately represent the dissipation of small-scale internal tides and their induced mixing in climate-scale models.

Optimization of salinity intrusion control through freshwater and tidal inlet modifications for the Indus River Estuary

Increasing salinity levels in the tidal rivers are becoming a significant challenge to inland water quality. Salinization of the adjoining water resources and agricultural lands have prompted the water managers to optimize the solutions for mitigation of adverse effects. Keeping this in view, a hydrodynamic model coupled with a water quality model was used to analyze the propagation of tides from the Arabian Sea into the Indus River and their subsequent effect over longitudinal salinity distribution. Field observations of hydraulic parameters and salinity were used for validation of the model predictions in dry (no river flow) and wet (specific river flow) hydrological conditions. Results suggest that the tidal amplitude dissipates landward; salinity intrudes about 58 km (near Dandari) into the Indus River Estuary (IRE) at spring tide. Salinity in the proximity of Dandari point is sensitive to variations in fresh water supplies. A complete neap-spring tidal cycle was chosen for scenario-based evaluation of the impact of fresh water over the longitudinal salinity distribution. A daily river discharge 250 m3/s was found optimum for limiting the salinity interface (≤1) up to 31 km from the estuary mouth. In addition, tidal inlet modifications based on the theoretical framework of stable channel design combined with the optimum river discharge were tested. The cross-sectional modifications based on the regime/Lacey theory proven better and salinity interface was further pushed back up to 27 km. Thus, it is maintained that synergic impact of the optimized freshwater quantity and the estuary mouth modifications are considered as one of the pragmatic solutions for control of salinity intrusion and achieving the specific environmental conditions along the alluvial estuary.

Experimental Study on the Propagation and Stability of Tidal Vortices

AbstractLaboratory studies of tidal vortex formation and propagation through an idealized inlet configuration are presented for various water depths over flat bathymetry. Using surface particle ima...

River-tide-storm surge interaction characteristics for the Hooghly estuary, East coast of India

ABSTRACT The head Bay of Bengal region that borders the North Indian Ocean is crisscrossed with numerous tidal creeks, riverine systems and channels that play an important role in storm surge studies. In particular, the storm surge resulting from the landfall of tropical cyclone over this region can interact with the river systems and thereby modify the characteristics of storm surge. Several studies were attempted to understand the behaviour of storm surges and associated inundation for the Indian coast; however, the role of hydrologic component in modifying storm surge received less attention. The present study considers a stretch of Hooghly River channel in West Bengal that provides the navigable route to two important national ports, and important in context to socio-economic implications. The state-of-art advanced circulation model (ADCIRC) hydrodynamic model used in this study incorporates the riverine stretch and investigates the effect of river discharge on storm surge propagation along the upstream reaches of the Hooghly River. In addition, the river-tide interaction was studied with differential river discharge volumes of 1000 and 3000 m3 s−1 that corresponds to the respective minimum and maximum discharges. Model simulations well represented the upstream tidal propagation as well the reversal of tidal currents in the river. Also the effect of variable river discharge volumes in modifying the limit of flood flow was clear from simulation. The results signify that limit of flood flow was seen nearly 70 km in the river upstream for discharge volume corresponding to 1000 m3 s−1 and reduced to 62 km on increasing the discharge to 3000 m3 s−1. The effect of wind stress on storm surge penetration into river was also investigated using the hypothetical Aila cyclone wind field. Moreover, the combined effect due to varying river discharge volumes and wind stress on storm surge characteristics was also studied. The study is an effort to understand the role of river-tide-storm surge interaction very useful in a real-time storm surge forecasting system.