Geoscience Reference
In-Depth Information
tidal wave or
bore
up-estuary, steepening as it is opposed
by river flow. Several million people living around its
shores were relieved that calm, anticyclonic conditions
prevailed during the twentieth century's highest spring
tide in September 1993; a
storm surge
reinforced by
cyclonic low pressure and strong winds could have been
catastrophic.
High tides occur simultaneously at a number of places,
linked by
co-tidal lines
as they are drawn across the ocean.
The ocean surface tilts as it ebbs and flows, moving away
from and towards land, and is also tilted by the Coriolis
force. As a result, the water surface in enclosed oceans or
seas oscillates from side to side around
amphidromic
points
with zero or very low tidal range (
Figure 11.17
).
Coastline configuration also influences tidal range. Open
coasts capable of reflecting tides with little complication,
or enclosed seas like the Mediterranean with limited scope
for tide generation, experience
microtidal
ranges less than
2 m in amplitude. More indented coastlines and wider
continental shelves, enhancing wave reflection and
retardation, raise tides to
mesotidal
or
macrotidal
ranges
of 2-6 m and over 6 m respectively.
1
3
5
7
9
11
13
15
17
19
21
23
25
27
29
31
Immingham, England - Semidiurnal
6
4
2
0
San Francisco, USA - Mixed-dominantly semidiurnal
2
1
0
-1
Manila, Philippines - Mixed-dominantly diurnal
1
0
Do-son, Vietnam - Diurnal
4
3
2
1
0
13579 1 3 5
Days
17
19
21
23
25
27
29
31
Source: After King (1962)
Waves
Waves are the smallest mass disturbances, dependent on
the wind and therefore transient and irregular in strength
and direction, except in the sense that predominant wind
directions excite a similar response in wave direction. It
is important to note that waves transmit energy but very
little mass - i.e. a wave is an onward transmission of
energy from one water particle to the next in which the
wave form is created by the rotational rise and fall of each
particle in turn (
Figure 11.18
).
This is demonstrated by
the rise and fall of a beach ball as each wave passes but
otherwise remains in the same general position and spins.
In a given wind field, a series of waves, or
wave train
,are
separated by their
wavelength
(
L
) and
wave period
(
T
)
(
Plate 11.3
).
Wave velocity
V
=
L
/
T
. Wave height (
H
) is
determined by wind speed, duration and
fetch
, or distance
travelled over water. It is also the diameter of the orbital
path of individual water particles as they rotate and an
important determinant of wave energy,
E
.
E
= 1/8
it by variable rates when they are not, reaching a lowest
To understand why tidal levels and cycles are so
complex, consider what might happen on a featureless
Earth covered by a single ocean five to ten times deeper
than at present. The moon would draw a tidal wave 0·5
m high at the equator, travelling at 1,600 km hr
-1
in its
wake - the magnitude of the moon's pull being slight but
its rotation around Earth fast. This does
not
happen
because the global ocean is comparatively shallow, with
greater sea bed friction, and the moon's orbit 'wobbles'
between 28·5
S of an equatorial plane.
Moreover, it is interrupted by large continents and
indented coastlines. Thus tides which pass unobserved in
mid-ocean are stacked up disharmonically into confined
coastal spaces with varying shelf and shoreline slopes. This
is illustrated by the
Severn bore
in the Bristol Channel,
which has one of Earth's largest tidal ranges, 12·2 m at
Avonmouth. The Severn estuary is 220 km long, 150 km
wide at its mouth between Pembroke and Cornwall but
only 1·5 km wide near the Severn bridges. Tides lag by
three hours between its landward and seaward limit and
are still rising inland when they begin to fall at sea. Its
funnel shape creates a progressive rise of water, sending a
N and 28·5
H
2
(where
is water density).
L
/2, denoting
wave base
, is the
depth beyond which the declining oscillatory motion has
effectively ceased and is no longer able to disturb the sea
bed. As a wave enters water depths less than
L
/2 a circular
orbit of particles cannot be maintained. Bed friction slows
the forward part of the loop first, forcing the still
advancing rear to climb, steepening and elevating the
wave, and eventually causing it to break. In this way each
