Environmental Engineering Reference
In-Depth Information
mouths at high tidal levels and then releasing it to the sea through turbines or water wheels when
the tide is at its lowest level. Intermittent tidal power proved practical only where the tidal range
(the difference in elevation between high and low tides) exceeded a few meters. In recent years,
tidal plants have been constructed at the La Rance estuary on the Brittany coast of France and at
Annapolis Royal, Nova Scotia, in the Bay of Fundy, both of which generate electrical power.
The tidal rise and fall in the ocean is a consequence of the differential gravitational force
exerted on opposite sides of the earth primarily by the moon, and secondarily by the sun, which
gives rise to simultaneous higher ocean surface elevations on the earth's sides facing toward and
away from the moon (or sun) and lower elevations halfway in between.
22
At a given point in the
ocean, the period
T
of the lunar tide is one-half of the lunar day (
T
=
=
4.464E(4)
s); that of the sun is just 12 hours. The solar and lunar effects reinforce each other twice a month,
at the time of a full moon and a new moon, giving rise to maximum, or
spring,
tides. Halfway
between the spring tides, at the time of the first and last quarters of the moon's monthly cycle, the
solar effect is least, resulting in the minimum, or
neap,
tides.
23
The tidal range of the ocean at the continental margins is far from uniform worldwide. At a
few locations, such as the Bay of Fundy and Cook Inlet in North America and the Bristol channel
and Brittany coast in western Europe, resonance effects in the tidal motion lead to ranges as high
as 10 m, which contrasts with an average oceanic range of 0.5 m. This large amplification of the
oceanic tidal range is associated with a resonant motion of water movement across the shallow
waters of the continental shelf into restricted bays or estuaries. About two dozen sites in North and
South America, Europe, East and South Asia, and Australia have been identified as possessing this
tidal amplification.
The principle of tidal power is illustrated in Figure 7.21, showing in elevation a cross section
of a
tidal pool
separated from the ocean by a
dam
(or
barrage
). A
sluiceway
in the dam allows
water to flow quickly between the sea and the tidal pool at appropriate times in the tidal cycle, as
does also a turbine in a
power house
. If the tidal pool is filled to the high tide level and then isolated
from the sea until the l
at
ter reache
s
its low tide level, a volume of water is impounded within the
tidal pound of amount
AH
, where
A
is the average surface area of the tidal pond and
H
is the tidal
range. If the center of mass
CM
of this water is a distance
h
above the low tide level, work can be
extracted from this volume at the time of low tide by allowing it to flow through a turbine until the
12 h, 24 min
High tide
CM
Tidal pool
H
Ocean
h
Low tide
Dam
Figure 7.21
A cross section of a tidal power pool with dam.
22
The solar effect is about 40 % of that caused by the moon.
23
There are additional perturbations of the tidal range associated with the position of the moon relative to the
solar ecliptic plane and the variation of both the moon-earth and sun-earth distances.
