Geoscience Reference
In-Depth Information
(a)
LONGITUDE
60°
120°
180°
120°
60°
0°
80°
80°
Arctic Ocean
60°
60°
40°
40°
Pacific
Ocean
Atlantic
Ocean
20°
20°
0°
0°
20°
20°
Indian
Ocean
40°
40°
Macrotidal >6m
Mesotidal 2-6m
Microtidal <2m
0
5000
60°
60°
km
Southern Ocean
60°
120°
180°
120°
60°
0°
(c)
(b)
River deltas
Barrier islands
Tidal deltas
Spits
Funnel estuaries
Mud flats
Salt marshes
Mean neap range
Mean range
MICROTIDAL
MESOTIDAL
MACROTIDAL
Mid spring range
Figure 17.23
(a) Global pattern of microtidal, mesotidal and macrotidal ranges, (b) the variation of range during monthly tidal
cycles and (c) the occurrence of coastal landforms associated with tidal range. High frequency is indicated by solid lines. Sources:
(a) after Davies (1980), (b) after Hayes (1976), (c) after Hayes (1976).
basins. Tidal range is suppressed by coastal sea ice in polar
seas and wholly enclosed seas like the Mediterranean.
Wav e e n e r g y
patterns on a general global scale are
determined by wind speed, duration and fetch super-
imposed on the land-sea configuration. Two principal
storm belts within the general atmospheric circulation -
mid-latitude westerlies and tropical cyclone tracks -
generate high waves on affected coasts This contrasts with
low wave height and energy on the equatorial Doldrum
belt and circumpolar divergent coasts (
Figure 17.24
).
CONCLUSION
Of all dynamic geomorphic landsystems, those subject to
the work of the sea are among the most fragile. Coastal
materials may cross the threshold between terrestrial and
marine environments at time scales as short as a breaking
wave, tidal flow or storm surge. Coastlines, like flood
plains, offer a diverse range of economic opportunities
and aesthetic attractions for human occupation but at
a price.
Low-lying barrier coasts and estuaries are
