Environmental Engineering Reference
In-Depth Information
If an asteroid with the diameter of 400 m were to hit
the ocean at 20 km/s, it would lift the surface by about
2.1 km, and the maximum amplitude of the tsunami
generated by this impact would be about 50 m at the dis-
tance of 100 km, and nearly 250 m only 20 km away.
A near-shore impact off California or eastern Honsh¯
would thus instantly devastate a core region of one of
the world's two leading economies, and unlike a tsunami
generated by a distant earthquake, it would not give suf-
ficient time for mass evacuation of affected regions (Smil
2005b). Like tsunami, seiches are also generated by seis-
mic motions (as well as by other mechanisms), but they
develop primarily in closed or semiclosed bodies of
water, where their rhythmic sloshing (before the undis-
turbed level is regained) can exceed 3 m (Korgen 1995).
Waves generated by the gravitational power of the
Moon and the Sun have the great advantage of accurate
predictability because there are two tidal cycles roughly
every 24 hours. There is also considerable regularity
of tidal ranges, with variations ranging from differences
between the two successive cycles to monthly and an-
nual fluctuations. Average day lengthening of 1.5 ms/
century, caused by almost linear decrease of the Earth's
rotation since the Paleozoic era, represents an input of
about 3 TW into the tidal friction. Long-lasting debate
about the principal sites of this energy dissipation—in
oceanic tides or in imperfectly elastic mantle—has been
resolved overwhelmingly (@80% of the total flux) in favor
of the ocean. Satellite altimeter studies show that 25%-
30% of the total dissipation of about 2.5 TW occurs in
the deep ocean; the rest is due to bottom friction in shal-
low seas (Egbert and Ray 2000).
The maximum energy extractable in a tidal cycle goes
up with the square of the tidal range and linearly with the
impoundment's area. A 10-m tide filling a basin of 10
km 2 would deliver about 227 MW, or nearly 23 W/m 2 .
A quick formula to approximate this density is to multi-
ply the square of the tidal range by 0.2. Only combina-
tions of high tides (at least 5 m) and coastal features
allowing for suitable impoundments can provide sites for
commercial conversions of tidal energy (WEC 2001).
The Earth's highest tidal ranges are in Nova Scotia's Bay
of Fundy (6.47-11.71 m mean, 7.50-13.30 m spring
tides), in Alaska's Cook Inlet (average 7.5 m), in south-
ern Argentina (5.9 m), along the coast of Normandy
(5.0-8.4 m), and in the White Sea bays (up to 11.4 m).
The theoretical power of the 28 best potential sites is
360 GW (Merriam 1978).
The global water cycle is driven by annual evaporation
of some 430 Gt of water (fig. 2.6), but compared to its
latent heat, the kinetic energy of precipitation is quite
small. Even a fairly heavy rainfall of 2 cm/h, with rain-
drops falling with terminal velocity of 6 m/s, will have
impact energy of just 360 J/m 2 , that is, a power density
of only 100 mW/m 2 , but its effect may be large as it
breaks up topsoil unprotected by vegetation. Although
the resulting runoff may look far more damaging than
does the impact of raindrops, their kinetic energy and
hence their erosive power is far stronger. Terminal rain-
drop velocity is primarily a function of drop diameter (2
m/s for diameters of 0.5 mm, about 9 m/s for the larg-
est sizes about 5 mm), although strong driving winds,
boosting the drop velocity by the reciprocal of the cosine
of the rain's angle of inclination off the vertical, can make
a substantial difference (Wischmeier and Smith 1978).
Experimental measurements show that between 5% and
22% of a drop's impact energy is spent on cratering
and that the relation between crater volume and kinetic
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