Page 336 - Fundamentals of Geomorphology

COASTAL LANDSCAPES 319

( ) Spilling ( ) Plunging
a
b
Breaking position

Still water level
water
level
Still
() Collapsing ( ) Surging
d
c

Figure 13.3 Kinds of breaking waves derived from high-speed moving pictures.
Source: Adapted from Komar (1998, 210)

between rip currents. Even where waves approach a period 1990–99 eighty-two were reported, ten of which
coastline head on, a nearshore circulation of long- were generated by earthquakes associated with plate col-
shore currents, rip currents, and onshore currents may lisions around the Pacific Rim and killed more than
evolve. 4,000 people (Box 13.1).

Tsunamis Tides
Tsunamis are commonly produced by faulting of the sea Tides are the movement of water bodies set up by the
floor, and much less commonly by volcanic eruptions, gravitational interaction between celestial bodies, mainly
landslides or slumping, or by impacting asteroids and the Earth, the Moon, and the Sun. They cause changes
comets. They are also referred to as tidal waves, although of water levels along coasts. In most places, there are
they bear no relation to tides and are named after the semi-diurnal tides – two highs and two lows in a day.
Japanese word meaning ‘harbour wave’. The pushing up Spring tides, which are higher than normal high tides,
of water by sudden changes in the ocean floor gener- occur every 14–75 days when the Moon and the Sun are
ates a tsunami. From the site of generation, a tsunami in alignment. Neap tides, which are lower than normal
propagates across the deep ocean at up to 700 km/hr. low tides, alternate with spring tides and occur when
While in the deep ocean a tsunami is not perceptible as the Sun and the Moon are positioned at an angle of
it is at most a few metres high with a wavelength about 90 with respect to the Earth. The form of the ‘tidal
◦
600 times longer than its height. On approaching land, wave’ depends upon several factors, including the size
a tsunami slows down to around 100 km/hr and grows and shape of the sea or ocean basin, the shape of the
in height by a factor of about ten. It rushes ashore, either shoreline, and the weather. Much of the coastline around
as a tide-like flood, or, if wave refraction and shoaling the Pacific Ocean has mixed tides, with highs and lows
allow, a high wall of water. of differing magnitude in each 24-hour period. Antarctic
Tsunamis occur on a regular basis. The historical coasts have diurnal tides with just one high and one low
average is fifty-seven tsunamis per decade, but in the every 24 hours.