Tidal Phase Difference Research Articles

On the Dependency of Bottom Drag and the Eddy Viscosity upon Flow Structure in the Coastal Boundary Layer

The physical processes governing coastal exchange between the surf zone, the inner shelf, and the open ocean is critical for estimating mass exchange and its impact on ecological processes. The present study combined field measurements and theoretical approaches to explore the hydrodynamics in the coastal boundary layer (CBL) in which both bottom drag and shore friction affect the transport and mixing processes. Observed drifter-cluster trajectories in a nearly alongshore-uniform coastal area showed that the occurrence of current reversal varies with cross-shore distance, which confirmed the tidal phase difference between different cross-shore distances predicted by the proposed CBL model. According to the CBL model, tidal phase difference is affected by the bottom drag coefficient and horizontal eddy viscosity coefficient. With the results of three experiments under different wave conditions, this study also discusses the effects of waves on the CBL. Data analysis based on observations indicates that the bottom drag term is closely related to the bottom shear stress induced by the interactions of waves and currents. The bottom drag coefficient under the more energetic wave condition was much greater than that under milder wave conditions during the experiment. The study also suggests that in addition to pressure gradient and bottom drag, flow structure is subject to lateral stress, which reflects the impact of shoreline roughness in the nearshore region and that the estimated eddy viscosity coefficient decreases linearly with distance from the shoreline.

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

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

The Potential for Tidal Range Energy Systems to Provide Continuous Power: A UK Case Study

The extraction of tidal energy from head differences represents a predictable and flexible option for generating electricity. Here, we investigate the generation potential of prospective tidal power plants in the UK. Originally conceived as separate projects, operating these schemes as a cooperative system could prove beneficial. Combined with the inherent operational flexibility of tidal range-based schemes, a notable tidal phase difference in selected sites allows for the system to spread power generation over a larger proportion of the day. Using depth-averaged modelling and gradient-based optimisation techniques, we explore how a flexible cumulative operation schedule could be applied to provide a degree of continuous supply if desirable. While fully continuous operation is not achieved, a number of different optimisation schedules deliver cumulative continuous supply for over half of the year. The average minimum cumulative power output on these days is consistently over 500 MW out of a total installed capacity of 6195.3 MW. Furthermore, by introducing financial incentives associated with reliable, baseload supply, we provide an economic assessment of the tidal power plant system. The daily minimum cumulative power output determines income in the modelled idealised baseload market, while excess supply is traded in an hourly variable wholesale energy market. Results indicate that subsidies would be required in order to make a pursuit of continuous generation financially advantageous over energy maximisation strategies.

Bathymetrically controlled velocity‐shear front at a tidal river confluence

AbstractNonbuoyant front formation at the confluence of Nanjemoy Creek and the main Potomac River (MD) channel is examined. Terra satellite ASTER imagery reveals a sediment color front emerging from Nanjemoy Creek when the Potomac is near maximum ebb. Nearly contemporaneous ASTER and Landsat ETM+ imagery are used to extract surface velocities, which suggest a velocity shear front is collocated with the color front. In situ velocities (measured by RiverRay traverses near the Nanjemoy Creek mouth) confirm the shear front's presence. A finite‐element simulation (using ADCIRC) replicates the observed velocity‐shear front and is applied to decipher its physics. Three results emerge: (1) the velocity‐shear front forms, confined to a shoal downstream of the creek‐river confluence for most of the tidal cycle, (2) a simulation with a flat bottom in Nanjemoy Creek and Potomac River (i.e., no bathymetry variation) indicates the velocity‐shear front never forms, hence the front cannot exist without the bathymetry, and (3) an additional simulation with a blocked‐off Creek entrance demonstrates that while the magnitude of the velocity shear is largely unchanged without the creek, shear front formation is delayed in time. Without the Creek, there is no advection of the M6 tidal constituent (generated by nonlinear interaction of the flow with bottom friction) onto the shoals, only a locally generated contribution. A tidal phase difference between Nanjemoy and Potomac causes the ebbing Nanjemoy Creek waters to intrude into the Potomac as far south as its deep channel, and draw from a similar location in the Potomac during Nanjemoy flood.

On the Flushing of Tampa Bay

A high-resolution, three-dimensional coastal ocean circulation model is used to estimate the bulk residence time distributions for the entirety of Tampa Bay and for five of its sub-regions using an Eulerian passive tracer technique. With a resolution as high as 20 m, the model domain includes all of the important mass conveyances by the channels and inlets, plus the impediments to flow by bridge causeways and natural constrictions. The non-tidal circulation is found to be the primary agent for the flushing of Tampa Bay. Tides alone have a minor effect. Exceptions pertain to within a tidal excursion from the mouth of the bay and sub-regions with multiple inlets, where differential flows related to tidal phase differences aid in flushing. By resolving the main shipping channels, we also address the effect of channel deepening and widening on the flushing of Tampa Bay. Bulk residence time is found to decrease due to an increase in the non-tidal circulation.

염하수로 인근에서 조석 변형과 장주기 조류성분의 변동 특성

경기만과 같이 지형이 복잡하고 조차가 큰 해역에서는 활발한 물질순환이 나타난다. 이러한 물질순환에 영향을 주는 장주기 흐름은 다양한 외력에 의하여 형성되며 시공간적으로 독특한 특성이 나타난다. 본 연구에서는 경기만에 위치하는 인천항부터 잠실수중보 사이에서 동기간에 관측된 조위와 유속자료를 분석하여 장주기 조류성분의 주기적인 변화 특성과 발생 기작을 파악하였다. 염하수로와 한강하구에서는 배조나 복합조와 같은 비선형분조의 증가로 인하여 조석파의 변형이 발생하고 이에 따른 평균해면 상승이 나타난다. 경기만으로부터 한강으로 전파하는 조석파는 수로폭과 수심이 크게 감소하는 염하수로로 진입하면서 급격히 변형되어 저조위와 평균해면이 상승하며, 상류로 갈수록 조석변형이 커진다. 이러한 조석변형은 조차가 큰 대조기에 증가하고 조차가 작은 소조기에 감소하며 주기적인 변동을 나타내는데, 이는 장주기 조석성분인 <TEX>$M_{sf}$</TEX> 분조로 해석될 수 있다. 조석변형의 공간적 차이에 의해 발생하는 해면 경사는 대조-소조의 주기와 동일한 주기를 보이며 대조기에 증가한다. 해면 경사의 장주기 변동에 반응하여 조류의 장주기 변화가 나타난다. 이러한 장주기 조류성분은 상류에서 크게 증가하여 한강하류인 전류리 인근에서는 주요 분조인 <TEX>$S_2$</TEX> 분조보다 크게 나타난다. The mass transport is very complicated at the area which has the macro tide and complex geometry such as Gyeonggi bay. Especially, the long period current has a strong influence on the estuarine ecosystem and the long-term distribution of substances. The long period current is caused by several external forcing, whose unique characteristic varies spatially and temporally. The variation characteristics of long period current is analysed and its generation mechanism is studied. The tidal nonlinear constituents such as overtide and compound tide are generated due to nonlinear interaction and it causes mean sea level setup. The tidal wave propagating up into estuary is transformed rapidly by decrease of cross-sectional area and depth. Therefore the mean sea level is getting rise toward upriver. The high and low tide level is similar between down-river(Incheon) and up-river(Ganghwa) during neap tide when the tidal deformation is decreased. The tidal phase difference between two tidal stations causes a periodic fluctuation of sea level difference. The low water level of Ganghwa station during spring tide does not descend under EL(-)2.5 m, but the low water level of Incheon fall down under EL(-)4.0 m. The variation of tidal range and its sea level are increased during spring tide. It is found that the long period current <TEX>$M_{sf}$</TEX> is quite similar to that of sea level difference between the two tidal stations. It means that the sea surface inclination caused by the spatial difference of tidal deformation is important forcing for the generation of long period current.

Phase relationships between tidal horizontal wind components observed in the mid-latitude winter upper mesosphere

Wind measurements obtained from the Wuhan/China MF radar during winter 2001 are used to study the phase relationships between tidal horizontal winds in the upper mesosphere. The tidal initial phases in any time window are uniformly defined in radians. The meridional and zonal initial phases of each tide exhibit similar time variation trends at each height, which is associated with near tidal horizontal phase differences at three continuous heights. However, different tides have different phase variation trends and phase differences. Tidal meridional components generally lead the zonal. The domains of tidal phase ‘quasi-quadrature’ and ‘quasi-in-phase’ are strictly specified. In most of ∼14 days, the diurnal ( Δ ϕ 24 ) and semidiurnal ( Δ ϕ 12 ) phase differences are ‘quasi-quadrature’ while the terdiurnal phase difference ( Δ ϕ 8 ) is surprisingly ‘quasi-in-phase’. The appropriate tidal meridional and zonal quadratic phase coupling (QPC) equations are estimated, respectively. The calculated terdiurnal phases and those observed agree well. The tidal phases calculated by discrete Fourier transform (DFT) and least-squares fitting (LSF) are equivalent as are the tidal QPC revealed, respectively, by bicoherence spectrum and LSF analyses. By subtracting the estimated tidal zonal QPC equation from the meridional, a tidal phase difference equation is obtained. The calculated terdiurnal phase differences and those observed again agree with each other. Within the 14th time window, all three tides are generally elliptically rather than circularly or linearly polarized. However, both Δ ϕ 24 and Δ ϕ 12 in this window are ‘quasi-quadrature’ at all heights while Δ ϕ 8 are ‘quasi-in-phase’ at 92.0 and 94.0 km. So the estimated tidal QPC equations, the deduced tidal phase difference equation, the observed terdiurnal ‘quasi-in-phase’ phase differences and the typical tidal polarization plots are all indications of real QPC between the observed 24-, 12- and 8-h tides.

MF Radar Observations of Phase Relationships Between Tidal Horizontal Wind Components in the Mid-latitude Winter Lower Thermosphere

采用武汉(30°N,114°E)MF雷达在2001年冬季的风场观测数据研究中纬度低热层大气潮汐水平风分量之间的相位关系.统一用弧度定义的各潮汐经纬向分量的拟合初相位在三个连续的高度上分别显示出相同的时间变化倾向和相近的相位差,但是在绝大多数观测时间△ψ24和△ψ12准正交,而△ψ8出乎意料地准同相.周日、半日和8 h潮汐经纬向分量的二次相位耦合(QPC)方程被分别估计出来,利用它们相减还得到一个潮汐相位差相关方程.推测的8 h潮汐相位和相位差与相应的观测值很好地符合.在第14个时间窗内,三个潮汐一般表现为椭圆偏振而不是圆偏振或线偏振,但是△ψ24和△ψ12在三个连续的高度上准正交,而△ψ8在92.0和94.0 km上准同相.因此估计的潮汐QPC方程、推导的潮汐相位差相关方程、观测的8 h潮汐准同相相位差以及典型的潮汐偏振图都是观测的周日、半日和8 h潮汐之间真实QPC的反映.

Tidal and sedimentary response to the Late Quaternary closure and opening of Cook Strait, New Zealand: Results from numerical modeling

Evidence from numerical modeling of the M2 tide, supplemented by geological data, reveals marked changes in the tidal and sedimentary regimes of Cook Strait over the last postglacial rise in sea level. Approximately 18,000 years B.P., when sea level was approximately −113 m, the strait was reduced to a narrow bay by the emergence of a land bridge connecting the North and South islands of New Zealand. High sediment supply and low, tidally induced bottom stress encouraged widespread deposition of fine sediment except at the bay mouth (the Narrows) where localized high stress resulted in the transport of sediment to the deep ocean via Cook Strait canyons. At 17,000 years B.P. (approximately −88 m), the bay reached maximum size and was near resonance with the oceanic tide resulting in high tidal amplitudes, bottom stress, and sediment movement at the Narrows and bay head. The land bridge was breached and Cook Strait was formed about 15,000 years B.P. (approximately −75 m). Both the Narrows and breach were subject to high bottom stress, whereas the intervening area was a zone of convergent low stress and fine sediment deposition. By 11,000 years B.P. (approximately −40 m) Cook Strait was well established and had a tidal response similar to the modern strait. The rising sea levels inundated remnants of the land bridge to form the broad continental shelf approaches to Cook Strait. Today, overall tidal stress is reduced, and mud deposition prevails except on the inner shelf where periodic forcing by wind‐induced currents maintains a sandy substrate, and in the Narrows where a tidal phase difference of approximately 120° maintains a zone of high sediment movement.

Suspended-sediment exchanges between the St. Lawrence estuary and a coastal embayment

Rapid local changes in the bathymetry of the St. Lawrence estuary define several physiographic units which act as distinct reservoirs of suspended sediments. Tidal exchanges between three reservoirs (the South Channel, Baie de Ste. Anne and Rivière Ouelle) lead to a marginal increase of the turbidity and to the reinforcement of the main plume in the South Channel. Baie de Ste. Anne is a large (175 km 2) subtidal platform less than 8 m below MLLW along the South Channel. A two-step intensification of turbidity occurs inside the bay. Estuarine processes in Rivière Ouelle — a small coastal plain estuary — build up a turbidity maximum which provides a local source of suspended sediments to intertidal flats in the bay. Tidal exchanges between these flats and the bay, in turn contribute to maintaining high turbidity levels over the Ste. Anne platform. A total mass of suspended sediments equivalent to more than half the mean solid discharge per tide of the St. Lawrence River may be exchanged during the ebb and flood. Waters in the South Channel are themselves enriched during each tidal cycle by lateral inputs from the platform. On a long-term basis, this multicomponent system appears to have reached a steady state, with no sediments accumulating over the flats. It provides additional suspended sediments to the downstream advection of turbidity in the estuary. Slight tidal phase differences between the three reservoirs are important in controlling the exchanges.

On the Advection of Turbidity in the Saint Lawrence Middle Estuary

The suspension transport away from the extensive turbidity zone of the St. Lawrence estuary is largely determined by the channel topography. The suspended sediments are advected downstream by a 40 km long turbid plume which takes its source in a turbidity maximum at the head of the estuary and flows downstream partly confined by the South Channel. During the ebbing phase of tidal cycles, the turbid waters of the plume are forced downstream through narrow converging sections of the channel, and slowed down through more opened diverging regions, particularly down-stream of the St. Roch Traverse. These, large fluctuations in stratification modulate the vertical transport of suspended material from the bottom to the surface layer. Midway down the estuary, dispersion of the plume occurs along a frontal zone which seasonally migrates 30 km or more in response to changes in fresh water discharge. The plume is reinforced and the turbidity gradient is intensified by local injections of inshore waters from Ste. Anne Bay, a subtidal platform highly enriched in suspended material by intertidal exchanges with large mudllats. Lateral erosion of the plume and cross-channel transport of suspended matter from the South into the North Channel is made possible by large horizontal shears developing in the central part of the middle estuary during the early flood. These are created by a one-hour tidal phase difference between the North and the South Channel.