Tidal Dynamics Research Articles

Identifying economically viable tidal sites within the Alderney Race through optimization of levelized cost of energy.

Costs of tidal stream energy generation are anticipated to fall considerably with array expansion and time. This is due to both economies of volume, where arrays comprising of large numbers of turbines can split fixed costs over a greater number of devices, and learning rates, where the industry matures and so arrays of the same size become cheaper due to lessons learned from previous installations. This paper investigates how tidal energy arrays can be designed to minimize the levelized cost of energy (LCOE), by optimizing not only the location but also the number of devices, to find a suitable balance between decreased costs due to economies of volume and diminishing returns due to global blockage effects. It focuses on the Alderney Race as a case study site due to the high velocities found there, making it a highly suitable site for large-scale arrays. It is demonstrated that between 1 and 2 GW could be feasibly extracted as costs in the tidal industry fall, with the LCOE depending greatly on the assumed costs. A Monte-Carlo analysis is undertaken to account for variability in capital and operational cost data used as inputs to the array optimization. Once optimized, the estimated P50 LCOE of an 80 MW array is £110/MWh. This estimate aligns closely with the level of subsidy considered for tidal stream projects in the Alderney Race in the past. This article is part of the theme issue 'New insights on tidal dynamics and tidal energy harvesting in the Alderney Race'.

Surface currents in the Alderney Race from high-frequency radar measurements and three-dimensional modelling.

Two weeks of high-frequency radar measurements collected at the Alderney Race are compared with the results of a three-dimensional fully coupled wave-current model. Spatial current measurements are rare in this site, otherwise well investigated through modelling. Thus, the radar measurements offer a unique opportunity to examine the spatial reliability of numerical results, and can help to improve our understanding of the complex currents in the area. Comparison of observed and modelled surface current velocities showed a good agreement between the methods, represented by root mean squared errors ranging from 14 to 40 cm s-1 and from 18 to 60 cm s-1 during neap and spring tides, respectively. Maximum errors were found in shallow regions with consistently high current velocities, represented by mean neap and spring magnitudes of 1.25 m s-1 and 2.7 m s-1, respectively. Part of the differences between modelled and observed surface currents in these areas are thought to derive from limitations in the k-epsilon turbulence model used to simulate vertical mixing, when the horizontal turbulent transport is high. In addition, radar radial currents showed increased variance over the same regions, and might also be contributing to the discrepancies found. Correlation analyses yielded magnitudes above 0.95 over the entire study area, with better agreement during spring than during neap tides, probably because of an increase in the phase lag between radar and model velocities during the latter. This article is part of the theme issue 'New insights on tidal dynamics and tidal energy harvesting in the Alderney Race'.

Characterization of the vertical evolution of the three-dimensional turbulence for fatigue design of tidal turbines.

A system of two coupled four-beam acoustic Doppler current profilers was used to collect turbulence measurements over a 36-h period at a highly energetic tidal energy site in Alderney Race. This system enables the evaluation of the six components of the Reynolds stress tensor throughout a large proportion of the water column. The present study provides mean vertical profiles of the velocity, the turbulence intensity and the integral lengthscale along the streamwise, spanwise and vertical direction of the tidal current. Based on our results and considering a tidal-stream energy convertor (TEC) aligned with the current main direction, the main elements of turbulence prone to affect the structure (material fatigue) and to alter power generation would likely be: (i) the streamwise turbulence intensity (Ix), (ii) the shear stress, [Formula: see text], (iii) the normal stress, [Formula: see text] and (iv) the vertical integral lengthscale (Lz). The streamwise turbulence intensity, (Ix), was found to be higher than that estimated at other tidal energy sites across the world for similar height above bottom. Along the vertical direction, the length (Lz) of the large-scale turbulence eddies was found to be equivalent to the rotor diameter of the TEC Sabella D10. It is considered that the turbulence metrics presented in this paper will be valuable for TECs designers, helping them optimize their designs as well as improve loading prediction through the lifetime of the machines. This article is part of the theme issue 'New insights on tidal dynamics and tidal energy harvesting in the Alderney Race'.

Turbulence characterization at a tidal energy site using large-eddy simulations: case of the Alderney Race.

Sites suitable for the deployment of tidal turbines generally show a combination of complex seabed morphologies and extreme current magnitudes. Such configurations favour the formation of vortices, which can be very powerful. Anticipating the vortex effect on the turbine performance and/or lifespan requires refined description of the turbulence. Thanks to increased calculation resources, large-eddy simulation (LES) can now be applied to natural flow. An LES approach developed within the TELEMAC-3D open-source software is presented here. After validating the model with in-situ measurements, the model is applied to characterize the flow statistics of the Alderney Race. This article is part of the theme issue 'New insights on tidal dynamics and tidal energy harvesting in the Alderney Race'.

The Alderney Race: general hydrodynamic and particular features.

This study presents an overview of the main hydrodynamic features of the Alderney Race strait based on in situ measurements and two-dimensional hydrodynamic model simulations. The strait encompasses a large amplitude of tidal properties (tidal range and tidal wave propagation) and particularly strong currents exceeding 5 m s−1 with associated counter currents and gyres. Variations in depth, sea bottom roughness, coastal topography and current orientation around the La Hague Cape provide access to a large variety of original hydrodynamic regimes. Some are revealed as locations with a 0.4 m drop in the mean sea level associated with strong average currents. A resonance effect associated with the offshore currents can also be observed close to the coasts. The ‘St Martin whistle’ occurs in a bay whose gyre centre oscillates with a reversal of the measured current every 5–7 min. The Alderney Race represents a particular area of interest for coastal hydrodynamic studies. The available in situ measurement datasets are rich with recordings of: sea levels; acoustic Doppler current profiler current profiles; surface radar currents; waves; dye experiments; surface and in-depth dissolved tracer surveys. Combined with hydrodynamic models, the complexity of this area can be further understood and knowledge of the hydrodynamic process and forcing parameters can be refined, which can be applied to other coastal areas.This article is part of the theme issue ‘New insights on tidal dynamics and tidal energy harvesting in the Alderney Race’.

Tidal current power effects on nearby sandbanks: a case study in the Race of Alderney.

A validated numerical model of tidal flows and sediment transport around the Alderney South Banks was used to investigate the potential effects of large (300 MW) tidal turbine arrays at different locations in Alderney territorial waters. Two methods were used, firstly looking at hydrodynamic changes only and secondly modelling sediment transport over a non-erodible bed. The baseline hydrodynamic model was validated relative to ADCP velocity data collected in the immediate vicinity of the sandbank. Real-world sand transport rates were inferred from sand-wave migrations and agree favourably with sediment transport residuals calculated from model outputs. Outputs from the sediment model reproduced realistic morphological behaviours over the bank. Seventeen different locations were considered; most did not result in significant hydrodynamic changes over the South Banks; however, three array locations were singled out as requiring extra caution if development were to occur. The results provide a case for optimizing the array locations for twin objectives of maximizing array power and minimizing impacts on the sandbanks. This article is part of the theme issue 'New insights on tidal dynamics and tidal energy harvesting in the Alderney Race'.

Underway velocity measurements in the Alderney Race: towards a three-dimensional representation of tidal motions.

The Alderney Race, located northwest of the Cotentin Peninsula (France), is a site with high tidal-stream energy potential. Circulation through the Alderney Race is complex, with current speed exceeding 3 m s-1 at neap tide. Towed acoustic Doppler current profiler (ADCP) measurements and static point velocity measurements were performed in July 2018 focusing on assessment of circulation and vertical structure of tidal currents. Transect surveys revealed peculiar features of local dynamics such as change in location of the tidal jet on ebb and flood flow. The spatial expanse of the tidal jet was quantified and regions with largely sheared or nearly homogeneous velocity distributions were identified on the cross-sections. Velocity profiles acquired along the cross-sections were accurately characterized using a power law. The spatial variability of the power-law exponent α was found to be large and correlated with the tidal conditions. The largest variation in profile shape was observed in the northern sector and assumed to be generated by the current interaction with a bathymetric constriction. The velocity profiles were found to vary from highly sheared on flood flow to nearly homogeneous on ebb flow, with corresponding range of power-law exponent α variation from 6 to 14. In the southern sector, over a relatively smooth bathymetry, the velocity profile shape was accurately approximated using the 1/7 power law with a range of variation of α from 6.5 to 8, with respect to the tidal conditions. To our knowledge, this is the largest field survey done using towed ADCP and the results could represent a significant advance in tidal site characterization and provide advanced information to turbine developers. This article is part of the theme issue 'New insights on tidal dynamics and tidal energy harvesting in the Alderney Race'.

Experimental analysis of the shear flow effect on tidal turbine blade root force from three-dimensional mean flow reconstruction.

In the main tidal energy sites like Alderney Race, turbulence intensity is high and velocity fluctuations may have a significant impact on marine turbines. To understand such phenomena better, a three-bladed turbine model is positioned in the wake of a generic wall-mounted obstacle, representative of in situ bathymetric variation. From two-dimensional Particle Image Velocimetry planes, the time-averaged velocity in the wake of the obstacle is reconstructed in the three-dimensional space. The reconstruction method is based on Proper Orthogonal Decomposition and enables access to a representation of the mean flow field and the associated shear. Then, the effect of the velocity gradient is observed on the turbine blade root force, for four turbine locations in the wake of the obstacle. The blade root force average decreases whereas its standard deviation increases when the distance to the obstacle increases. The angular distribution of this phase-averaged force is shown to be non-homogeneous, with variation of about 20% of its time-average during a turbine rotation cycle. Such force variations due to velocity shear will have significant consequences in terms of blade fatigue. This article is part of the theme issue 'New insights on tidal dynamics and tidal energy harvesting in the Alderney Race'.

Numerical modelling of hydrodynamics and tidal energy extraction in the Alderney Race: a review.

The tides are a predictable, renewable, source of energy that, if harnessed, can provide significant levels of electricity generation. The Alderney Race (AR), with current speeds that exceed 5 m s-1 during spring tides, is one of the most concentrated regions of tidal energy in the world, with the upper-bound resource estimated at 5.1 GW. Owing to its significance, the AR is frequently used for model case studies of tidal energy conversion, and here we review these model applications and outcomes. We examine a range of temporal and spatial modelling scales, from regional models applied to resource assessment and characterization, to more detailed models that include energy extraction and array optimization. We also examine a range of physical processes that influence the tidal energy resource, including the role of waves and turbulence in tidal energy resource assessment and loadings on turbines. The review discusses model validation, and covers a range of numerical modelling approaches, from two-dimensional to three-dimensional tidal models, two-way coupled wave-tide models, Large Eddy Simulation (LES) models, and the application of optimization techniques. The review contains guidance on model approaches and sources of data that can be used for future studies of the AR, or translated to other tidal energy regions. This article is part of the theme issue 'New insights on tidal dynamics and tidal energy harvesting in the Alderney Race'.

Assessing the turbulent kinetic energy budget in an energetic tidal flow from measurements of coupled ADCPs.

Two coupled four-beam acoustic Doppler current profilers were used to provide simultaneous and independent measurements of the turbulent kinetic energy (TKE) dissipation rate ε and the TKE production rate [Formula: see text] over a 36 h long period at a highly energetic tidal energy site in the Alderney Race. The eight-beam arrangement enabled the evaluation of the six components of the Reynolds stress tensor which allows for an improved estimation of the TKE production rate. Depth-time series of ε, [Formula: see text] and the Reynolds stresses are provided. The comparison between ε and [Formula: see text] was performed by calculating individual ratios of ε corresponding to [Formula: see text]. The depth-averaged ratio [Formula: see text] averaged over whole flood and ebb tide were found to be 2.2 and 2.8 respectively, indicating that TKE dissipation exceeds TKE production. It is shown that the term of diffusive transport of TKE is significant. As a result, non-local transport is important to the TKE budget and the common assumption of a local balance, i.e. a balance between production and dissipation, is not valid at the measurement site. This article is part of the theme issue 'New insights on tidal dynamics and tidal energy harvesting in the Alderney Race'.

The energy yield potential of a large tidal stream turbine array in the Alderney Race

This research provides an updated energy yield assessment for a large tidal stream turbine array in the Alderney Race. The original array energy yield estimate was presented in 2004. Enhancements to this original work are made through the use of a validated two-dimensional hydrodynamic model, enabling the resolution of flow modelling to be improved and the impacts of array blockage to be quantified. Results show that a range of turbine designs (i.e. rotor diameter and power capacity) are needed for large-scale development, given the spatial variation in bathymetry and flow across the Alderney Race. Array blockage causes a reduction in flow speeds in the array of up to 2.5 m s-1, increased flow speeds around the array of up to 1 m s-1 and a reduction in the mean volume flux through the Alderney Race of 8%. The annual energy yield estimate of the array is 3.18 TWh, equivalent to the electricity demand of around 1 million homes. The capacity factor of the array is 18%, implying sub-optimal array design. This result demonstrates the need for turbine rated speed to be selected based on the altered flow regime, not the ambient flow. Further enhancement to array performance is explored through increases to rotor diameter and changes to device micro-siting, demonstrating the significant potential for array performance improvement. This article is part of the theme issue 'New insights on tidal dynamics and tidal energy harvesting in the Alderney Race'.

Dynamics of the Estuarine Turbidity Maximum Zone from Landsat-8 Data: The Case of the Maroni River Estuary, French Guiana

The estuarine turbidity maximum (ETM) zone occurs in river estuaries due to the effects of tidal dynamics, density-driven residual circulation and deposition/erosion of fine sediments. Even though tropical river estuaries contribute proportionally more to the sediment supply of coastal areas, the ETM in them has been hardly studied. In this study, surface suspended particulate matter (SPM) determined from OLI (Operational Land Imager)-Landsat 8images was used to gain a better understanding of the spatio-temporal dynamics of the ETM of the tropical Maroni estuary (located on the Guianas coast, South America). A method to estimate the remotely-sensed ETM location and its spatiotemporal evolution between 2013 and 2019 was developed. Each ETM was defined from an envelope of normalized SPM values > 0.6 calculated from images of the estuary. The results show the influence of the well-marked seasonal river discharge and of tides, especially during the dry season. The ETM is located in the middle estuary during low river-flow conditions, whereas it shifts towards the mouth during high river flow. Neap–spring tidal cycles result in a push of the ETM closer to the mouth under spring-tide conditions or even outside the mouth during the rainy season. An increase in SPM, especially since 2017, coincident with an extension of the ETM, is shown to reflect the periodic influence of mud banks originating from the mouth of the Amazon and migrating along the coast towards the Orinoco (Venezuela). These results demonstrate the advantages of ocean color data in an exploratory study of the spatio-temporal dynamics of the ETM of a tropical estuary, such as that of the Maroni.

Moderate deviations for stochastic tidal dynamics equations with multiplicative Gaussian noise

In this article, we consider the stochastic tidal dynamics equations perturbed by multiplicative Gaussian noise and discuss some asymptotic behaviors. A central limit theorem and a moderate deviation principle are derived for such systems. The results are proved using a variational method (based on weak convergence approach) developed by Budhiraja and Dupuis.

Study on the Characteristics of Tides and Storm Surge in Dongji Islands

Storm surge has posed a great threat to the safe development of marine ranch in Dongji Islands. Research on the tidal dynamics and storm surge of Dongji Islands is more and more urgent. For this reason, based on measured data of tidal level in 2012, this paper conducts research on the variations of tidal level and storm surge in Dongji Islands. Main conclusions are: Affected by the topography and tidal waves in the East China Sea, temporal and spatial distribution of tidal level and storm surge is asymmetric. Tidal intensity is strong in the eastern sea and weak in the western. The amplitudes of main semi-diurnal tides (M2, S2, N2) in the western sea have a significant attenuation, with the average tidal range decreased by about 16%. Cycle of the practical storm surge is consistent with the tidal constituents, for the cycle at Daishan station was close to 24 h, while Miaozihu station was about 12 h.

Seasonal Estuarine Turbidity Maximum under Strong Tidal Dynamics: Three-Year Observations in the Changjiang River Estuary

The estuarine turbidity maximum (ETM) under strong tidal dynamics (during spring tides) was investigated along the Deepwater Navigation Channel (DNC) in the North Passage (NP) of the Changjiang River Estuary (CRE) in wet and dry seasons of 2016, 2017 and 2018. The observed water current, salinity, stratification and suspended sediment concentration (SSC) were illustrated and analyzed. Results show that the SSC was lower in wet seasons than dry seasons in 2016 and 2017 because of the weak influence of typhoons before observations in wet seasons. On the contrary, the SSC was higher in the wet season than the dry season in 2018 because of the strong influence of typhoons in the wet season. Our observations challenged the common perspective that SSC in the NP is higher in wet seasons than dry seasons, because the magnitudes of SSC were found to be easily influenced by strong winds before observations. The along-channel distribution of high SSC was determined by the location of salt wedge, and consequently, the ETM was further upstream in dry seasons than wet seasons. The observed SSC was more concentrated in lower water layers in wet seasons (“exponential” profile) than dry seasons (“linear” profile). This seasonal difference of vertical SSC was related to the flocculation setting velocity influenced by temperature rather than the weak stratification during spring tides. Moreover, on the basis of the net water/sediment transport and flux splitting, large river discharge and a low-SSC condition could reduce siltation in the middle DNC. The former vanished the convergence of water transport, and the latter reduced landward tidal pumping sediment transport. Sediment trapping and siltation in the dry seasons occurred in the seaward segment of the upper reach because of the decrease in the river discharge.

The challenges of constraining shelf sea tidal models using seabed sediment grain size as a proxy for tidal currents

Past major changes in sea level have had a significant influence on global- and shelf sea tidal dynamics. Some of these changes are preserved in sedimentary records from the shelf seas, and so appropriate proxy data have the potential to constrain tidal model outputs over the recent geological past. Tidal models which simulate the evolution of tide-dependent parameters over geological timescales are fundamental to understanding the response of the tides to sea-level rise and climate change. This study explores a potential new sedimentary proxy for validating past shelf sea tidal dynamics, interrogating the relationship between tidally-modulated bed shear stress and seabed sediment grain size at discrete sediment core locations over the northwest European shelf seas. Radiocarbon-dated sediment grain size profiles were generated for four British Geological Survey UK shelf sediment vibrocores, spanning a range of physical environments. Changes in observed sediment grain size through time were compared with simulated changes in tidal-induced bed shear through time, using temporal and spatial outputs from the most recently developed palaeotidal model of the Northwest European shelf seas. Although a positive correlation between observed grain size and simulated bed shear stress was observed at three of the four sediment cores sites, no robust relationship could be quantified. The palaeotidal model output failed to resolve the details of the actual sediment dynamics, since only tidal-induced bed shear stresses were considered. Wave processes were neglected, and the model was not sensitive enough to constrain simulated past tidal conditions at point locations; rather it is suitable for examining general trends. There remains a need to develop new proxies for past shelf sea hydrodynamic conditions which can be used to constrain numerical model output of tidal currents at regional scales.

Tidal response to sea level rise and bathymetric changes in the German Wadden Sea

Tidally dominated coasts are directly affected not only by projected rise in mean sea level, but also by changes in tidal dynamics due to sea level rise and bathymetric changes. By use of a hydrodynamic model, which covers the entire German Bight (South-Eastern North Sea), we analyse the effects of sea level rise and potential bathymetric changes in the Wadden Sea on tidal current velocities. The model results indicate that tidal current velocities in the tidal inlets and channels of the Wadden Sea are increased in response to sea level rise. This is explained by the increased ratio of tidal prism to tidal inlet cross-sectional area, which is due to the characteristic hypsometry of tidal basins in the Wadden Sea including wide and shallow tidal flats and relatively narrow tidal channels. The results further indicate that sea level rise decreases ebb dominance and increases flood dominance in tidal channels. This is, amongst others, related to a decreased intertidal area again demonstrating the strong interaction between tidal wave and tidal basin hypsometry in the Wadden Sea. The bathymetry scenario defined in this study includes elevated tidal flats and deepened tidal channels, which is considered a potential future situation under accelerated sea level rise. Application of these bathymetric changes to the model mostly compensates the effects of sea level rise. Furthermore, changes in current velocity due to the altered bathymetry are in the same order of magnitude as changes due to mean sea level rise. This highlights the significance of considering potential bathymetric changes in the Wadden Sea for regional projections of the tidal response to sea level rise.

Sediment transport in a restored, river-influenced Pacific Northwest estuary

Predicting the success of future investments in coastal and estuarine ecosystem restorations is limited by scarce data quantifying sediment budgets and transport processes of prior restorations. This study provides detailed analyses of the hydrodynamics and sediment fluxes of a recently restored U.S. Pacific Northwest estuary, a 61 ha former agricultural area near the mouth of the Stillaguamish River in Washington, USA. Water level, flow velocity, and suspended-sediment concentration (SSC) were measured between 21 March 2014 and 1 June 2015 at breaches excavated in the former flood-protection levee to determine transport patterns and the net sediment budget of the restoration area. SSC within the restoration area was primarily controlled by SSC variability of the nearby main stem Stillaguamish, but coastal processes also played a major role in sediment delivery. Fluvial sediment loading was dominated by runoff events associated with rainfall that lasted hours to a few days. Additionally, the 22 March 2014 SR 530 (Oso) landslide elevated sediment supply to the restoration area and coastal region for several weeks, indicating the importance of distal geomorphic events to coastal sediment budgets in small mountainous river systems. Sediment fluxes were controlled by river SSC and tidal dynamics, which set the quantity of water transported into the restoration area. Peak water discharge at the restoration area was about 12% of the river discharge, and peak sediment flux at the restoration area was about 5% of the river sediment discharge, although net sediment import was <1% of the total river load. Although sediment was imported to the restoration area, and inferred rates of accretion appear sufficient to keep pace with present rates of local sea-level rise, full recovery is challenged by significant lost grade from historical subsidence and will likely take decades to centuries. These results have implications for estuary restoration planning globally and indicate the importance of understanding coupled fluvial–coastal processes.

Tidal Patterns and Sediment Dynamics in a Hypertidal Estuary Influenced by a Tidal Power Station

Rtimi, R.; Sottolichio, A. and Tassi, P., 2020. Tidal patterns and sediment dynamics in a hypertidal estuary influenced by a tidal power station. In: Malvarez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 1520–1524. Coconut Creek (Florida), ISSN 0749-0208.The Rance estuary is a relatively small low-inflow steep-sided ria, located in the Brittany coast of northern France, with a maximum spring tidal range of 13.5 m at the mouth. Taking advantage of this hypertidal regime, the first and largest operational tidal power station in the world was built at the estuary mouth and is in operation since the 1960s. Despite a well-known effect of the plant on the damping of estuarine water levels, little attention has been given to the propagation of the tidal wave along the estuary. Moreover, net siltation has been reported by several observations, but there is no specific knowledge on the role of the plant on sedimentation. In this study, tidal wave patterns and sediment dynamics are analyzed in this particular man-engineered system. A numerical model, based on a two-dimensional depth-averaged approach is implemented to predict the tide propagation and tidal currents along the estuary. Numerical results are analyzed and discussed to highlight the role of the plant on the transformation of the tidal wave in the estuary and its possible consequences on sediment dynamics and morphological evolution.

Short-term, linear, and non-linear local effects of the tides on the surface dynamics in a new, high-resolution model of the Mediterranean Sea circulation

The tides in the Mediterranean Sea are generally weaker than in other regions of the world ocean, but are locally intensified in passages with complex bathymetry, such as the Gibraltar Strait and the Sicily Channel. To date, a detailed understanding of their effects on the circulation, on the short time scales relevant to forecasting, is still missing, due to the lack of specific observations, and of basin-scale, numerical models explicitly accounting for the tidal forcing. The present investigation attempts to bridge this gap, using a newly developed forecasting model of the circulation of the Mediterranean Sea-Black Sea system that includes the effects of the main diurnal and semidiurnal astronomical tides. After validating the model barotropic tidal dynamics through comparison with the results of a tidal inversion software and historical data, a fully baroclinic run is analyzed, with focus on the Mediterranean Sea dynamics. The run covers the period 19 March–30 April 2018 and is initialized and forced with realistic fields produced by state-of-the-art operational models. After verifying that the modelled circulation and hydrology agree with available remote and in situ observations, attention is focused on short-time effects of the tidal forcing on the local circulation. Tides are found to significantly modulate transports not only across the Strait of Gibraltar and the Sicily Channel, as expected, but also across the Corsica Channel and the Otranto Strait. Tidal effects also modify relevant features of the circulation inside the basin, inducing local modulations of some of the main currents and exciting topographic waves that eventually get trapped over shallow bathymetric features, producing diurnal rotations of the flow patterns. Examples of the latter dynamics are found over the Adventure Bank and the Malta Plateau, in the Sicily Channel, and in the eastern portions of the Corsica Channel and the Otranto Strait. Furthermore, in several locations (Sicily Channel, Corsica Channel, Messina Strait, North Adriatic Sea), spectral analysis of the mean kinetic energy reveals the presence of spectral peaks corresponding to periods of about 8 and 6 h, which can only be interpreted as harmonics (overtides and compound tides) of the diurnal and semidiurnal tidal components, generated through non-linear interactions. This shows that, contrary to a widespread assumption, tidal effects in the Mediterranean cannot be linearly superimposed on the basin circulation.