The largest tides and fastest tidal currents
The largest tide ranges are found in bays that are close
to one fourth of a tidal wavelength long. Tide ranges
reach 15 m (50 ft) in Minas Basin in the Bay of Fundy
and in Ungava Bay (also in Canada). Tidal ranges
greater than 12 m (40 ft) occur at the northern end of
Cook Inlet near Anchorage in Alaska, in Bristol Bay
in the United Kingdom, in the Magellan Strait in Chile,
in the Gulf of Cambay in India, and along the Gulf of
St. Malo portion of the French coast bordering the
English Channel. In some bays, the very high tide range
at the head of the bay is due to a combination of both
a narrowing width and a near resonant situation (due
to the right length and depth). The highest tide ranges
may involve several amplifications, the bay being perhaps connected to a gulf which is perhaps connected
to a wide continental shelf, with amplifications of the
tide wave occurring in each basin. This is the case with
the Bay of Fundy tides, the tide wave being already
amplified by the continental shelf and the Gulf of Maine
prior to entering the Bay of Fundy. Huge tide ranges
are not restricted to bays. If the continental shelf is the
right combination of depth and width, a near resonant
situation can also result. This is the reason for the
12-m (40-ft) tidal ranges along the coast of southern
Argentina. The continental shelf there is over 965 km
(600 mi) wide and includes the Falkland Islands near
the edge of the shelf, where the tide range only reaches
2.0 m (6.5 ft). The distance from the Argentinean coast
to the edge of the shelf is fairly close to one fourth of
a tidal wavelength for that depth of water. Essentially,
that wide shelf has a natural period of oscillation that
is fairly close to the tidal period.
The largest tidal currents in estuaries tend to be near the
entrances. (Maximum tidal current speeds are zero at the
head of the bay, since there is no place for the water to
flow). As one moves down an estuary toward the ocean,
the maximum flood and maximum ebb tidal current
speeds increase, with the greatest speeds occurring at the
entrance, or, if the estuary is long enough, at the area of
smallest tide range (the nodal area). However, if the width
of the estuary decreases at any point, the current speeds
will be increased in that narrow region. (It must flow
faster, since the same volume of water is being forced to
flow through a smaller cross section). This can be especially dramatic if there is a sudden decrease in width and
depth. The largest tidal currents are found in narrow straits
in which the tides at either end have different ranges or
times of high water. Tidal current speeds greater than
7.7 m/s (15 knots) occur in Seymour Narrows, between
Vancouver Island and the mainland of British Columbia,
Canada. Tidal currents of 5.1 m/s (10 knots) are found in
South Indian Pass in Southeast Alaska and in Kanmon
Strait, Japan. Fast tidal currents in narrow straits that suddenly widen can also produce a tidal whirlpool, a violently
rotating funnel-shaped hole of water, which is explained
later in this entry.
Coriolis effects on tide ranges in wide estuaries
If an estuary is wide enough, one also sees larger tide
ranges on the right side of the bay (looking up the bay)
due to the Coriolis force. The Coriolis force is
a fictitious force due to our observing motion from the
rotating reference frame of the Earth (see Parker, 1998).
It acts perpendicular to the flow of the water, thus pushing
water currents to the right in the Northern Hemisphere
(and to the left in the Southern Hemisphere) as they flow.
For a pure progressive tide wave (no friction), at high
water the tidal current flows up the bay, so the tidal height
will be greater on the right-hand shore than on the lefthand shore. At low water, the tidal current flows down
the bay, so the tidal height will be lower on the right-hand
shore than on the left-hand shore. The result is that the tide
range (high water minus low water) will be greatest on the
right-hand shore (looking up the bay). For a pure standing
wave (no friction), the pattern of high water caused by the
Coriolis force is more complicated, as is shown in the
upper half of Figure 3. This figure shows lines of constant
tide range (corange lines), as well as lines of locations with
the same time of high water (cotidal lines), in an idealized
rectangular basin for the case where the effect of bottom
friction is ignored. A single point of zero tidal range
(a node) occurs in the center of the bay one fourth of
a tidal wavelength from the head of the bay.
This figure comes close to representing the corange and
cotidal lines in a bay that is very deep. A more typical case,
including the damping effect of bottom friction, is
shown in the bottom half of Figure 3. In this case the node
has moved to the left (when looking up the bay)
and becomes a virtual node since it is on land. (See
Parker, 2007, for derived mathematical formulas
describing this effect.) One can see some similarity
between the pattern of corange lines in the lower half of
Figure 3 and the M 2 corange lines in Figure 4 for the
Strait of Juan de Fuca–Strait of Georgia (although the
latter has many geographic variations not included in the
simple regular basin of Figure 3). In Figure 4 the pattern
of a quasinode is also seen to the southwest of Victoria.
Hydrodynamic effects on diurnal versus
semidiurnal tides in estuaries
In many estuaries it is common to have semidiurnal tides,
namely, two high waters (and two low waters) a day, with
the heights of the two high waters (and two low waters)
being approximately the same size. In some estuaries
and seas, there can be diurnal tides, namely, only one high
water (and one low water) per day. Most common, however, is to have mixed tides, namely, two high waters
(and two low waters) a day but where there is considerable
difference between the heights of two consecutive high
waters (and/or between the heights of the two consecutive
low waters), that difference being due to a strong diurnal
signal. It is tidal hydrodynamics that determines which
of these three types of tide one will see at particular
690
TIDAL HYDRODYNAMICS
The largest tide ranges are found in bays that are close
to one fourth of a tidal wavelength long. Tide ranges
reach 15 m (50 ft) in Minas Basin in the Bay of Fundy
and in Ungava Bay (also in Canada). Tidal ranges
greater than 12 m (40 ft) occur at the northern end of
Cook Inlet near Anchorage in Alaska, in Bristol Bay
in the United Kingdom, in the Magellan Strait in Chile,
in the Gulf of Cambay in India, and along the Gulf of
St. Malo portion of the French coast bordering the
English Channel. In some bays, the very high tide range
at the head of the bay is due to a combination of both
a narrowing width and a near resonant situation (due
to the right length and depth). The highest tide ranges
may involve several amplifications, the bay being perhaps connected to a gulf which is perhaps connected
to a wide continental shelf, with amplifications of the
tide wave occurring in each basin. This is the case with
the Bay of Fundy tides, the tide wave being already
amplified by the continental shelf and the Gulf of Maine
prior to entering the Bay of Fundy. Huge tide ranges
are not restricted to bays. If the continental shelf is the
right combination of depth and width, a near resonant
situation can also result. This is the reason for the
12-m (40-ft) tidal ranges along the coast of southern
Argentina. The continental shelf there is over 965 km
(600 mi) wide and includes the Falkland Islands near
the edge of the shelf, where the tide range only reaches
2.0 m (6.5 ft). The distance from the Argentinean coast
to the edge of the shelf is fairly close to one fourth of
a tidal wavelength for that depth of water. Essentially,
that wide shelf has a natural period of oscillation that
is fairly close to the tidal period.
The largest tidal currents in estuaries tend to be near the
entrances. (Maximum tidal current speeds are zero at the
head of the bay, since there is no place for the water to
flow). As one moves down an estuary toward the ocean,
the maximum flood and maximum ebb tidal current
speeds increase, with the greatest speeds occurring at the
entrance, or, if the estuary is long enough, at the area of
smallest tide range (the nodal area). However, if the width
of the estuary decreases at any point, the current speeds
will be increased in that narrow region. (It must flow
faster, since the same volume of water is being forced to
flow through a smaller cross section). This can be especially dramatic if there is a sudden decrease in width and
depth. The largest tidal currents are found in narrow straits
in which the tides at either end have different ranges or
times of high water. Tidal current speeds greater than
7.7 m/s (15 knots) occur in Seymour Narrows, between
Vancouver Island and the mainland of British Columbia,
Canada. Tidal currents of 5.1 m/s (10 knots) are found in
South Indian Pass in Southeast Alaska and in Kanmon
Strait, Japan. Fast tidal currents in narrow straits that suddenly widen can also produce a tidal whirlpool, a violently
rotating funnel-shaped hole of water, which is explained
later in this entry.
Coriolis effects on tide ranges in wide estuaries
If an estuary is wide enough, one also sees larger tide
ranges on the right side of the bay (looking up the bay)
due to the Coriolis force. The Coriolis force is
a fictitious force due to our observing motion from the
rotating reference frame of the Earth (see Parker, 1998).
It acts perpendicular to the flow of the water, thus pushing
water currents to the right in the Northern Hemisphere
(and to the left in the Southern Hemisphere) as they flow.
For a pure progressive tide wave (no friction), at high
water the tidal current flows up the bay, so the tidal height
will be greater on the right-hand shore than on the lefthand shore. At low water, the tidal current flows down
the bay, so the tidal height will be lower on the right-hand
shore than on the left-hand shore. The result is that the tide
range (high water minus low water) will be greatest on the
right-hand shore (looking up the bay). For a pure standing
wave (no friction), the pattern of high water caused by the
Coriolis force is more complicated, as is shown in the
upper half of Figure 3. This figure shows lines of constant
tide range (corange lines), as well as lines of locations with
the same time of high water (cotidal lines), in an idealized
rectangular basin for the case where the effect of bottom
friction is ignored. A single point of zero tidal range
(a node) occurs in the center of the bay one fourth of
a tidal wavelength from the head of the bay.
This figure comes close to representing the corange and
cotidal lines in a bay that is very deep. A more typical case,
including the damping effect of bottom friction, is
shown in the bottom half of Figure 3. In this case the node
has moved to the left (when looking up the bay)
and becomes a virtual node since it is on land. (See
Parker, 2007, for derived mathematical formulas
describing this effect.) One can see some similarity
between the pattern of corange lines in the lower half of
Figure 3 and the M 2 corange lines in Figure 4 for the
Strait of Juan de Fuca–Strait of Georgia (although the
latter has many geographic variations not included in the
simple regular basin of Figure 3). In Figure 4 the pattern
of a quasinode is also seen to the southwest of Victoria.
Hydrodynamic effects on diurnal versus
semidiurnal tides in estuaries
In many estuaries it is common to have semidiurnal tides,
namely, two high waters (and two low waters) a day, with
the heights of the two high waters (and two low waters)
being approximately the same size. In some estuaries
and seas, there can be diurnal tides, namely, only one high
water (and one low water) per day. Most common, however, is to have mixed tides, namely, two high waters
(and two low waters) a day but where there is considerable
difference between the heights of two consecutive high
waters (and/or between the heights of the two consecutive
low waters), that difference being due to a strong diurnal
signal. It is tidal hydrodynamics that determines which
of these three types of tide one will see at particular
690
TIDAL HYDRODYNAMICS