Assessment of the Tidal Current Energy Resources and the Hydrodynamic Impacts of Energy Extraction at the PuHu Channel in Zhoushan Archipelago,China

An unstructured model FVCOM(The Unstructured Grid Finite Volume Community Ocean Model) with sink momentum term was applied to simulate the tidal current field in Zhoushan Archipelago, China, with focus on the region named PuHu Channel between Putuo Island and Hulu Island. The model was calibrated with several measurements in the channel, and the model performance was validated. An examination of the spatial and temporal distributions of tidal energy resources based on the numerical simulation revealed that the greatest power density of tidal energy during spring tide is 3.6 kW m-2 at the northern area of the channel. Two parameters were introduced to characterize the generation duration of the tidal array that causes the temporal variation of tidal current energy. The annual average available energy in the channel was found to be approximately 2.6 MW. The annual generating hours at rated power was found to be 1800 h when the installed capacity of tidal array is approximately 12 MW. A site for the tidal array with 25 turbines was selected, and the layout of the array was configured based on the EMEC specifications. Hydrodynamic influence due to the deployment of the tidal array was simulated by the modified FVCOM model. The simulation showed that the tidal level did not significantly change because of the operation of the tidal array. The velocity reduction covered a 2 km~2 area of the downstream the tidal array, with a maximum velocity reduction of 8 cm s-1 at mid-flood tide, whereas the streamwise velocity on both sides of the farm increased slightly.     

Estimates of reynolds stress and TKE production in the seasonally stratified East China Sea

During the two cruises in March and July of 2011, the tidal cycling of turbulent properties and the T/S profiles at the same location in seasonally stratified East China Sea (ECS) were measured synchronously by a bottom-mounted fast sampling ADCP (acoustic Doppler current profiler) and a RBR CTD (RBR-620) profiler. While focusing on the tide-induced and stratification’s impact on mixing, the Reynolds stress and the turbulent kinetic energy (TKE) production rate were calculated using the ‘variance method’. In spring, the features of mixing mainly induced by tides were clear when the water column was well-mixed. Velocity shear and turbulent parameters intensified towards the seabed due to the bottom friction. The components of the velocity shear and the Reynolds stress displayed a dominant semi-diurnal variation related to velocity changes caused by the flood and ebb of M₂ tide. Stratification occurred in summer, and the water column showed a strongly stratified pycnocline with a characteristic squared buoyancy frequency of N² ～ (1–6) × 10^?3 s^?2. The components of the velocity shear and the Reynolds stress penetrated upwards very fast from the bottom boundary layer to the whole water column in spring, while in summer they only penetrated to the bottom of the pycnocline with a relatively slow propagation speed. In summer, the TKE production within the pycnocline was comparable with and sometimes larger than that in the well-mixed bottom layer under the pycnocline. Considering the associated high velocity shear, it is speculated that the mixing in the pycnocline is a result of the local velocity shear.     

A two-dimensional numerical model of the tidal motions in the Bohai sea

The motions of diurnal, semidiurnal, and shallow-water tides and tidal current in the Bohai Sea are computed using a finite-difference method based on two-dimensional tidal wave equations. Good agreement of the computed results with the observed data is achieved for diurnal and semidiurnal tides. The general pattern of the computed quarterdiurnal tide conforms to the observed pattern, but the computed amplitudes are on the high side. This is attributed to the ineligibility of the friction terms in the two-dimensional governing equations to dissipate the energy of high frequency tidal waves. It is found that the existing semidiurnal cotidal charts have considerable differences in Laizhou Bay. The difference is likely caused by the movement of the coastline of the Yellow River Delta. The present result coincides with the recent empirical cotidal chart. The computation shows a new current-amphidromic point for both semidiurnal and durnal tidal currents. The diurnal current has two current-amphidromic points in the Bohai Sea with co-phase lines progressing clockwise round these points. The semi-diurnal current has also two current-amphidromic points with co-phase lines progressing counterclockwise. The distributions of tide-induced residual elevation and currents are illustrated, and the tidal energy fluxes are computed. Institute of Marine Scientific and Technological Information, State Oceanic Administration Contribution No. 1125, Institute of Oceanology, Academia Sinica, Qingdao, China.     

南海西北部正压潮的数值模拟

利用1/30°分辨率三维POM(Princeton Ocean Model)模式,以M2、S2、K1、O14大分潮作为潮汐边界条件,模拟南海西北部(105.5-115°E,16-23°N)海域正压潮,分析琼州海峡及其附近区域正压潮能通量分布特征。结果表明,研究海域内M2分潮和全日潮都是顺时针传入北部湾,然后自西向东通过琼州海峡,直至琼州海峡东口;计算所得穿过琼州海峡中部(110°E断面)能通量为M2,0.2GW或m1,0.47GW;穿过北部湾湾口(18.5°N断面)能通量为M2,1.0GW或m1,2.5GW;海南岛西部和琼州海峡处潮能耗散最强。     

Estimates of global M_2 internal tide energy fluxes using TOPEX/POSEIDON altimeter data

TOPEX/POSEIDON altimeter data from October 1992 to June 2002 are used to calculate the global barotropic M2 tidal currents using long-term tidal harmonic analysis. The tides calculated agree well with ADCP data obtained from the South China Sea (SCS). The maximum tide velocities along the semi-major axis and semi-minor axis can be computed from the tidal ellipse. The global distribution of M2 internal tide vertical energy flux from the sea bottom is calculated based on a linear internal wave generation model. The global vertical energy flux of M2 internal tide is 0.96 TW, with 0.36 TW in the Pacific, 0.31 TW in the Atlantic and 0.29 TW in the Indian Ocean, obtained in this study. The total horizontal energy flux of M2 internal tide radiating into the open ocean from the lateral boundaries is 0.13 TW, with 0.06 TW in the Pacific, 0.04TW in the Atlantic, and 0.03 TW in the Indian Ocean. The result shows that the principal lunar semi-diurnal tide M2 provides enough energy to maintain the large-scale thermohaline circulation of the ocean.     

Evaluation of tidal stream energy and its impacts on surrounding dynamics in the Eastern Region of Pingtan Island,China

Using an improved FVCOM numerical model, combined with the momentum-sinking scheme based on the structural characteristics of specific turbines, this study analyzed the temporal and spatial distributions of tidal energy resources before and after the deployment of tidal turbines near Pingtan Island, China. Considering factors such as the distribution of tidal stream energy, bathymetry, topography, and the design parameters of the turbines, an appropriate location for a demonstration tidal turbine was selected and the corresponding energy resource was evaluated. Several sites with strong tidal streams were considered: south of the northern cape, east of the southern cape, and the southern end of Haitan Bay. The former was thought most suitable for the deployment of a tidal energy turbine, with projected power generation for approximately 470 h per month. The average power of this demonstration was about 2.4 k W, and the annual electricity output was approximately 17.47 MWh. The intervention of the turbine device had little influence on the near-field tidal stream or water level. The tidal stream was reduced slightly in the area south of the northern cape, although the ef fect weakened further from the turbine. Conversely, the velocity increased slightly on both sides of the demonstration site. The dif ference in current speed with and without the turbine was greater at slack tide than still tide. The influence of turbine operation on water level was minor. The method adopted in this study can be considered a reference for the selection of sites for the demonstration of tidal stream energy. However, the method is unable describe the dynamic characteristics of the turbulent flow surrounding the deployed turbines, which has an important role regarding the optimal designs of the turbine blade and pile foundations. Therefore, we will continue to work to improve this model in future research.     

Estimation of annual energy output from BCM tidal barrage and the corresponding marine environmental impact

Based on the finite-volume coastal ocean model(FVCOM),a three-dimensional numerical model FVCOM was built to simulate the ocean dynamics in pre-dam and post-dam conditions in Bachimen(BCM).The domain decomposition method,which is effective in describing the conservation of volume and non-conservation of mechanical energy in the utilization of tidal energy,was employed to estimate the theoretical tidal energy resources and developable energy resources,and to analyze the hydrodynamic effect of the tidal power station.This innovative approach has the advantage of linking physical oceanography with engineering problems.The results indicate that the theoretical annual tidal energy resources is about 2×10~8 k Wh under the influence of tidal power station;Optimized power installation is confirmed according to power generation curve from numerical analysis;the developable resources is about 38.2% of theoretical tidal energy resources with the employment of one-way electricity generation.The electricity generation time and power are 3479 hours and 2.55×10~4 KW,respectively.The power station has no effect on the tide pattern which is semi-diurnal tide in both two conditions,but the amplitudes of main constituents apparently decrease in the area near the dam,with the M_2 decreasing the most,about 62.92 cm.The tidal prism shrinks to 2.28×10~7 m~3,but can still meet the flow requirement for tidal power generation.The existence of station increases the flow rate along the waterway and enhances the residual current.There are two opposite vortexes formed on the east side beside the dam of the station,which leads to pollutants gathering.     

Seasonal Variability of Internal Tides Northeast of Taiwan

The spatial-temporal characteristics of the barotropic tides and internal tides(ITs) northeast of Taiwan Island are examined, based on a 1-year mooring current observations from May 23, 2017 to May 19, 2018. The results of harmonic tidal analysis show that the barotropic tides are dominated by semidiurnal tides, which is mainly controlled by M_2 tidal components. Moreover, the vertical structures of diurnal and semidiurnal ITs show that the semidiurnal IT shows notable seasonal variation, whereas seasonal variations of the diurnal IT energy is not significant. The semidiurnal IT energy in winter half year is twice that in summer half year. The seasonal variation of semidiurnal IT is mainly modulated by the direction change of the current rather than by the topographic features and stratification. In summer(winter) half year cyclonic(anti-cyclonic) eddies meanly control at this point, so the flow direction is mainly in the southwest(northeast) direction, causing the background flow to flow along(perpendicular to) the isobath. When crossing the isobath, the ITs are generated by the interaction of the barotropic tide and the topography, resulting in the increase of the tidal energy in the winter half year.     

Estimates of global M 2 internal tide energy fluxes using TOPEX/POSEIDON altimeter data

TOPEX/POSEIDON altimeter data from October 1992 to June 2002 are used to calculate the global barotropic M ₂ tidal currents using long-term tidal harmonic analysis. The tides calculated agree well with ADCP data obtained from the South China Sea (SCS). The maximum tide velocities along the semi-major axis and semi-minor axis can be computed from the tidal ellipse. The global distribution of M ₂ internal tide vertical energy flux from the sea bottom is calculated based on a linear internal wave generation model. The global vertical energy flux of M ₂ internal tide is 0.96 TW, with 0.36 TW in the Pacific, 0.31 TW in the Atlantic and 0.29 TW in the Indian Ocean, obtained in this study. The total horizontal energy flux of M ₂ internal tide radiating into the open ocean from the lateral boundaries is 0.13 TW, with 0.06 TW in the Pacific, 0.04TW in the Atlantic, and 0.03 TW in the Indian Ocean. The result shows that the principal lunar semi-diurnal tide M ₂ provides enough energy to maintain the large-scale thermohaline circulation of the ocean. Supported by the National Basic Research Program of China (973 Program, No. 2005CB422303), the International Cooperation Program (No. 2004DFB02700), and the National Natural Science Foundation of China (No. 40552002). The TOPEX/POSEIDON data are provided by Physical Oceanography Distributed Active Archive Center (PO DACC)     

一种新的固体潮观测数据特征量提取方法

将经验模态分解方法应用于固体潮观测数据的处理,通过对模态分量从频域上以及从固体潮调和分析结果上与原始观测数据进行对比证明：经验模态分解可以将固体潮观测数据完整地分离成5种特征量：潮汐观测高频信号、半日波信号、周日波信号、潮汐观测低频信号和观测数据的长趋势变化。     

Effects of Sediment-induced Stratification on Floc Breakup in an Idealized Tidal Estuary： A Numerical Modelling Study

Floc breakup dynamics are studied by a sediment transport numerical model in an idealized tidal estuary that has a constant water depth and rapid flocculation of cohesive sediments. The focus is placed on the effects of boundary layer stratification induced by a bottom nepheloid layer on floc breakup and size distribution in the water column. In a neutrally stratified estuary, the floc size distribution follows a parabolic function with maximum values at the surface and bottom. The sediment-induced stratification in the bottom boundary layer increases the median floc sizes. Furthermore, sediment-voided convection caused by the settling lutocline generates significant turbulent kinetic energy dissipation and reduces floc size at the depth where the convective mixing happens. Below that depth, a weak local maxima in the floc size is predicted due to presence of the lutocline. The effect of sediment-stratified bottom boundary layer on the floc breakup can be consistently approximated by a linear regression between the maximal floc size and flux Richardson number.     

Effects of sediment-induced stratification on floe breakup in an idealized tidal estuary: A numerical modelling study

Floe breakup dynamics are studied by a sediment transport numerical model in an idealized tidal estuary that has a constant water depth and rapid flocculation of cohesive sediments. The focus is placed on the effects of boundary layer stratification induced by a bottom nepheloid layer on floe breakup and size distribution in the water column. In a neutrally stratified estuary, the floe size distribution follows a parabolic function with maximum values at the surface and bottom. The sediment-induced stratification in the bottom boundary layer increases the median floe sizes. Furthermore, sediment-voided convection caused by the settling lutocline generates significant turbulent kinetic energy dissipation and reduces floe size at the depth where the convective mixing happens. Below that depth, a weak local maxima in the floe size is predicted due to presence of the lutocline. The effect of sediment-stratified bottom boundary layer on the floe breakup can be consistently approximated by a linear regression between the maximal floe size and flux Richardson number.     

Effects of internal tidal dissipation and self-attraction and loading on semidiurnal tides in the Bohai Sea,Yellow Sea and East China Sea: a numerical study

A parameterized internal tide dissipation term and self-attraction and loading(SAL) tide term are introduced in a barotropic numerical model to investigate the dynamics of semidiurnal tidal constituents M_2 and S_2 in the Bohai Sea, Yellow Sea and East China Sea(BYECS). The optimal parameters for bottom friction and internal dissipation are obtained through a series of numerical computations. Numerical simulation shows that the tide-generating force contributes 1.2% of M_2 power for the entire BYECS and up to 2.8% for the East China Sea deep basin. SAL tide contributes 4.4% of M_2 power for the BYECS and up to 9.3% for the East China Sea deep basin. Bottom friction plays a major role in dissipating tidal energy in the shelf regions, and the internal tide eff ect is important in the deep water regions. Numerical experiments show that artifi cial removal of tide-generating force in the BYECS can cause a signifi cant dif ference(as much as 30 cm) in model output. Artifi cial removal of SAL tide in the BYECS can cause even greater diff erence, up to 40 cm. This indicates that SAL tide should be taken into account in numerical simulations, especially if the tide-generating force is considered.     

Numerical hydrodynamics study around turbine array of tidal stream farm in Zhoushan,China

In recent decades, great efforts have been made to efficiently explore tidal stream energy due to its unique advantages of easy prediction and great potential. China recently launched a national tidal stream farm demonstration project in the waterway between Putuoshan and Hulu Islands in the Zhoushan area. Before deployment of the turbine array, it is necessary to understand the hydrodynamic changes associated with the construction of a turbine array. In this study, we developed a depth-averaged hydrodynamics model that solves the shallow water governing equations to simulate the tidal hydrodynamics around the Zhoushan Archipelago. The simulation results agree with field data in terms of the water elevation and stream velocity. We considered two types of turbine arrays in this study and investigated their impacts on the local hydrodynamics. In general, the stream velocity in the northern and southern areas is reduced due to the power take-off of the turbine array, whereas stream velocity in the western and eastern areas is slightly increased due to the blockage impact of the turbine array.     

Test and evaluation of a moored microstructure recorder

A new moored microstructure recorder （MMR） is designed, developed, tested, and evaluated. The MMR directly measures the high-frequency shear of velocity fluctuations, with which we can estimate the dissipation rate of turbulent kinetic energy. We summarize and discuss methods for estimating the turbulent kinetic energy dissipation rate. Instrument body vibrations contaminate the shear signal in an ocean field experiment, and a compensating correction successfully removes this contamination. In both tank test and ocean field experiment, the dissipation rate measured with the MMR agreed well with that measured using other instruments.     

Internal Waves Generated by Tidal Flows over a Triangular Ridge with Critical Slopes

Direct numerical simulations are performed to investigate the generation of internal waves by tide-topography interaction in a lab-scale model. The bottom topography is a triangular ridge with two critical slopes. With increasing tidal forcing, subharmonic instabilities are identified, which cause internal wave beams to become unstable and turbulent. Kinetic energy densities in the upward going beams from the ridge top are stronger than those from the ridge bottom, whereas the reverse is true for the energy flux. This disparity between energy and energy flux is due to the existence of strong pressure disturbances near the ridge bottom. On each side of the critical ridge, there exists an amphidromic point, from which internal wave beams are emitted in opposite directions. The calculated energy conversion rate scales linearly with the square of the forcing amplitude and agrees within 13% of theoretical prediction, even when turbulence occurs. The fraction of radiated baroclinic energy becomes saturated in the range of low excursion parameter considered, which agrees with the behavior in large-scale systems wherein mixing parameterizations must be used. The present work enriches the studies on the generation of internal waves over a critical triangular ridge.     

Tidal analysis in the venturi-shaped area between Zhenhai and Shenjiamen in the East China Sea

In order to better understand the general tidal features in the venturi-shaped area between Zhenhai and Shenjiamen in the northern coastal region of Zhejiang Province in the East China Sea, the tidal data were obtained from both the three permanent tide stations of Zhenhai, Dinghai and Shenjiamen, and four temporary tide stations of Mamu, Chuanshan, Guoju and Liuheng, along with the current speed being observed at Luotou Waterway. Results from harmonic analysis show that: (1) The area was dominated by shallow water tides with irregular semi-diurnal features, and the smallest tidal range occurred in the area near a crossing line between Zhenhai and Dinghai stations, indicating that a tidal node existed in the southern Hangzhou Bay; (2) Formulae, HS2/HM2 >0.4 and gM2-(gK1+gO1)=270° (where H and g are harmonic constants), could be used as judging criteria for high and low tidal level diurnal inequalities; (3) The duration difference between ebb and flood tides could be roughly assessed by the ratio of HM4 vs. HM2; and the larger the ratio is, the bigger the duration difference is. At the same time, the duration period could be assessed by 2gM2-gM4, the epoch difference between M2 and M4 tidal constituents. If 2gM2-gM4 <180°, then the ebb duration is longer than the flood duration; if 180°< 2gM2-gM4 <360°, the result is reversed; (4) Taking Dinghai station as a center point, the highest tidal levels and the average high tidal levels, as well as the average tidal ranges at all stations became higher and larger both southeastwards and northwestwards, while the lowest tidal levels and the average low tidal levels appeared to be lower both southeastwards and northwestwards; and (5) The tidal patterns were not all in line with the tidal current patterns. As a conclusion, the smallest tidal range occurred in the narrow part of the venturi-shaped area. Along the both sides of the area, the highest tidal level and tidal range became higher and larger, while the lowest tidal level became lower with the increase of the distance from the narrow throat area. This is somehow different from the theory that the tidal level increases gradually when it moves towards the top narrow area of a V-shaped bay or estuary.     

The upper mixed layer during coastal upwelling events on the northern portugal shelf

The upper mixed layer (UML) depth obtained from temperature is very close to that from density:the maximum is about 15m. This indicates that temperature is a good indicator of mixed layer during measurements. When the surface heat flux is balanced by a cross-shore heat flux, the surface mixed layer depth obtained from the WM model (Weatherly and Martin, 1978),hPRT, is roughly the same as observed. The mixed layer depth calculated from the PWP model (Price, Weller and Pinkel, 1986) is close to the depth obtained from thermistor chain temperature data. The results show that both the WM model and PWP model can provide a good estimate of stratification in the study area during the cruise. The value of log( h/u3) is about 9.5 in the study area, which shows that the study area is strongly stratified in summer. Observations on the northern Portugal shelf reveal high variability in stability, giving rise to semi-diurnal, semi-monthly and diurnal oscillations, and long term variations. The fortnightly oscillatio     

Bottom Currents Observed in and Around a Submarine Valley on the Continental Slope of the Northern South China Sea

Bottom currents at about 1000 m depth in and around a submarine valley on the continental slope of the northern South China Sea were studied by a 14-month long experiment from July 2013 to September 2014. The observations reveal that bottom currents are strongly influenced by the topography, being along valley axis or isobaths. Power density spectrum analysis shows that all the currents have significant peaks at diurnal and semi-diurnal frequencies. Diurnal energy is dominant at the open slope site, which is consistent with many previous studies. However, at the site inside the valley the semi-diurnal energy dominates, although the distance between the two sites of observation is quite small(11 km) compared to a typical horizontal first-mode internal tide wavelength(200 km). We found this phenomenon is caused by the focusing of internal waves of certain frequencies in the valley. The inertial peak is found only at the open slope site in the first deployment but missing at the inside valley site and the rest of the deployments. Monthly averaged residual currents reveal that the near-bottom currents on the slope flow southwestward throughout the year except in August and September, 2013, from which we speculate that this is a result of the interaction between a mesoscale eddy and the canyon/sag topography. Currents inside the valley within about 10 mab basically flow along slope and in the layers above the 10 mab the currents are northwestward, that is, from the deep ocean to the shelf. The monthly mean current vectors manifest an Ekman layer-like vertical structure at both sites, which rotate counter-clockwise looking from above.     

The observed currents in summer in the Bohai Sea

A harmonic method was used to analyze the tidal currents observed in summer at 11 stations made from 1996 to 2001 in the Bohai Sea, China. Data was compared among different instruments and intervals. Elliptic elements were calculated based on harmonic constants, of which vertical distributions of the maximum speed and rotation direction were discussed for understanding the characteristics of diurnal and semi-diurnal tidal current components. The results indicate that the maximum speed of M2 tidal current component is much larger than that of K1; the rotation direction of M2 tidal current constituent is clockwise in the central part of the Bohai Sea and in the Laizhou Bay, but anticlockwise in the Liaodong Bay and Bohai Bay. For K1 tidal current constituent, it is clockwise in the central Bohai Sea but anti-clockwise in the Laizhou Bay and Liaodong Bay. The tidal currents in most stations in the Bohai Sea were regular semidiurnal except for those in the central Bohai Sea, being irregular semidiurnal.