Environmental Engineering Reference
In-Depth Information
upset by the continents, and a lunar orbit varying by 28·5° either side of the equator, but
semidiurnal tides are still experienced over large sections of Atlantic, south Pacific and
Arctic coastlines. The tidal bulge of one hemisphere dwindles with increasing latitude in
the opposite hemisphere, leaving one diurnal tide around Antarctica and parts of the
Arctic basin. Mixed tides, predominant in the Pacific and Indian Oceans, show elements
of both patterns. Tidal range emphasizes the effect of coastlines and enclosed seas on the
global tide wave (Figure 17.14). The range is lowest on open coasts, which reflect the
wave, and increases with increasing width of continental shelf and partial enclosure of
marine basins. Tidal range is suppressed by coastal sea ice in polar seas and wholly
enclosed seas like the Mediterranean.
Wave energy patterns on a general global scale are determined by wind speed,
duration and fetch superimposed 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 17.15).
TECTONICS, CLIMATE AND SEA LEVELS
systems
Morphotectonic coastlines reflect convergent and divergent plate motion and its impact
on the continental slope-shelf system and orogens (see Figure 1). Leading - edge or
convergent-margin coasts (American Pacific, Sunda Arc and New Zealand) typically
have narrow shelves and are closely backed by emergent coastal orogens - providing a
steep slope continuum between orogen crest and continental slope. Rapid sediment
transfer from immature, disconnected terrestrial drainage systems through to submarine
canyons prevents the substantial coastal sedimentation which vigorous erosion of the
orogens might otherwise encourage. As a result, leading-edge coasts are dominated by
wave erosion and cliffs. In direct contrast trailing - edge or passive-margin coasts
(American Atlantic, Africa, western Australia and Arctic basin) are generally well served
by large, integrated river systems and broad shelves. River-fed wave deposition, with
large deltas in humid areas, is dominant. Epicontinental seas (Caribbean and east Asia)
accumulate terrigenous and biogenic sediments in relatively sheltered waters. Coastlines
are also shaped through their accordance or otherwise with orogenic structures. In
addition to island arcs, structurally accordant island archipelagoes and peninsulas are
found on the Cenozoic orogen coasts of the Alaska 'panhandle' and British Columbia,
southern Chile and Croatia and the Hercynian orogens of south-west Ireland and Brittany.
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