contours). The strongest currents are identified in two areas: west of Ushant Island
and in the Fromveur Strait (locations A and B in Fig. 3). Bathymetry gradients and
the presence of islands cause flow acceleration, tend to tighten the streamlines and
provide the maximum velocities in these two sectors.
Figure 3 also shows a significant (up to 90°) misalignment in the current
direction during flood (red) and ebb tide (blue arrows) in the eastern and southern
sectors of Ushant Island. Such a deviation of the flow during a tidal cycle is known
as direction asymmetry. Sentchev et al. [22] documented the presence of transient
eddies in these areas which might explain such a large difference in the direction of
the ebb and flood flow. In the north-western sector of the domain, Δθ ranges from
0° to 10°, indicating that ebb and flood flows are mostly aligned.
Another interesting feature of the tidal dynamics around Ushant Island is the
velocity asymmetry a. The asymmetry varies in a wide range, from 0.5 to 2.5, and
shows a large spatial pattern with a > 1 in the west and a < 1 in the south (Fig. 3).
The strongest variation of asymmetry is observed in the Fromveur Strait. Here, the
asymmetry attains 2.5 in the onshore sector, indicating that flood flow velocities are
by far larger than the ebb flow velocities. The asymmetry decreases toward the
centre of the strait (point B in Fig. 3), where the tidal flow reaches a balance
(a = 1). Abrupt changes in the flow regime occur at very short distance in the strait.
The asymmetry values reveal very strong variation, from 1.9 to 0.8, between two
locations, B n and B s , separated by only 2.5 km at either side of the centre of strait
(location B). This gives flood flow velocities up to 2 m/s larger than ebb flow
velocities in B n , and ebb flow velocities larger by ∼1 m/s in B s . Such a strong
asymmetry variation, to the best of our knowledge, has never been documented
elsewhere.
Different properties of the tidal flow in nearshore environment can explain
asymmetry variations. Non-linear interactions between the principal semi-diurnal
constituents yield overtides and compound tides. It was established, that the phase
difference between semidiurnal and quarter-diurnal tidal constituents governs the
tidal flow asymmetry. In particular, the following relationship can be used to
diagnoze tidal asymmetry: γ = 2Φ M2 − Φ M4 , where Φ M2 and Φ M4 are the phases of
the M 2 and M 4 tidal current constituents [6]. Tidal current curve is symmetric when
γ = 90°, 270°. Maximum asymmetry is achieved at γ = 0°, 180°.
Harmonic analysis of velocity time series in the Fromveur Strait revealed the
presence of overtides and compound tides in the spectra. In the central point of the
strait (point B), the value of γ was ∼270°, thus indicating that the velocity curve is
balanced here (a ≈ 1). On the contrary, in the onshore and offshore sectors of the
Strait, the value of γ is close to 215° and 200° respectively, thus providing extreme
asymmetry values and a large imbalance of the current flow [29]. Non-linear
interactions between the principal semi-diurnal constituents generate residual currents which also contribute to tidal flow asymmetry.
Tidal currents in the Dover Strait The spatial variability of tidal currents in the
southern sector of the Dover Strait are quantified using parameters of synthesized
tidal current ellipses (Fig. 4) derived from the PCA. Spring tide period (May 16,
526
A. Sentchev et al.
and in the Fromveur Strait (locations A and B in Fig. 3). Bathymetry gradients and
the presence of islands cause flow acceleration, tend to tighten the streamlines and
provide the maximum velocities in these two sectors.
Figure 3 also shows a significant (up to 90°) misalignment in the current
direction during flood (red) and ebb tide (blue arrows) in the eastern and southern
sectors of Ushant Island. Such a deviation of the flow during a tidal cycle is known
as direction asymmetry. Sentchev et al. [22] documented the presence of transient
eddies in these areas which might explain such a large difference in the direction of
the ebb and flood flow. In the north-western sector of the domain, Δθ ranges from
0° to 10°, indicating that ebb and flood flows are mostly aligned.
Another interesting feature of the tidal dynamics around Ushant Island is the
velocity asymmetry a. The asymmetry varies in a wide range, from 0.5 to 2.5, and
shows a large spatial pattern with a > 1 in the west and a < 1 in the south (Fig. 3).
The strongest variation of asymmetry is observed in the Fromveur Strait. Here, the
asymmetry attains 2.5 in the onshore sector, indicating that flood flow velocities are
by far larger than the ebb flow velocities. The asymmetry decreases toward the
centre of the strait (point B in Fig. 3), where the tidal flow reaches a balance
(a = 1). Abrupt changes in the flow regime occur at very short distance in the strait.
The asymmetry values reveal very strong variation, from 1.9 to 0.8, between two
locations, B n and B s , separated by only 2.5 km at either side of the centre of strait
(location B). This gives flood flow velocities up to 2 m/s larger than ebb flow
velocities in B n , and ebb flow velocities larger by ∼1 m/s in B s . Such a strong
asymmetry variation, to the best of our knowledge, has never been documented
elsewhere.
Different properties of the tidal flow in nearshore environment can explain
asymmetry variations. Non-linear interactions between the principal semi-diurnal
constituents yield overtides and compound tides. It was established, that the phase
difference between semidiurnal and quarter-diurnal tidal constituents governs the
tidal flow asymmetry. In particular, the following relationship can be used to
diagnoze tidal asymmetry: γ = 2Φ M2 − Φ M4 , where Φ M2 and Φ M4 are the phases of
the M 2 and M 4 tidal current constituents [6]. Tidal current curve is symmetric when
γ = 90°, 270°. Maximum asymmetry is achieved at γ = 0°, 180°.
Harmonic analysis of velocity time series in the Fromveur Strait revealed the
presence of overtides and compound tides in the spectra. In the central point of the
strait (point B), the value of γ was ∼270°, thus indicating that the velocity curve is
balanced here (a ≈ 1). On the contrary, in the onshore and offshore sectors of the
Strait, the value of γ is close to 215° and 200° respectively, thus providing extreme
asymmetry values and a large imbalance of the current flow [29]. Non-linear
interactions between the principal semi-diurnal constituents generate residual currents which also contribute to tidal flow asymmetry.
Tidal currents in the Dover Strait The spatial variability of tidal currents in the
southern sector of the Dover Strait are quantified using parameters of synthesized
tidal current ellipses (Fig. 4) derived from the PCA. Spring tide period (May 16,
526
A. Sentchev et al.