Page 335 - Fundamentals of Geomorphology

318 PROCESS AND FORM

Eventually, the waves lunge forward or break to form
surf. In breaking, waves of oscillation convert to waves
of translation and rush up the beach as swash. After
having attained its farthermost forward position, the
water runs down the seaward slope as backwash. Four
5 5
types of breaking wave are recognized: spilling, plunging,
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10 collapsing, and surging (Figure 13.3). Spilling breakers
give the appearance of foam cascading down from the
peaking wave crest. Plunging breakers have waves curl-
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ing over and a mass of water collapsing on to the sea
surface. Collapsing breakers have wave crests peaking
crest
ave
W
20 Wave crest
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as if about to plunge, but the base of the wave then rushes
up the shore as a thin layer of foaming water. Surging
breakers retain a smooth wave form with no promi-
nent crest as they slide up the shore, entraining little air
in the act. The occurrence of these waves depends upon
the deep-water wave height and the bottom slope. For a
5 5 given deep-water wave height, waves will spill, plunge,
collapse, and surge with increasing bottom slope. Spilling
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10
waves require a slope of less than about 11 , plung-
◦
15 ing waves up to 36 , collapsing waves up to 50 , and
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◦
◦
surging waves more than 50 .
◦
Breaking waves are either constructive or destructive,
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20
depending on whether they cause a net shoreward or a net
seaward movement of beach material. As a rule of thumb,
surging, spilling, and collapsing breakers create a strong
swash and gentle backwash and tend to be constructive,
washing sediment on to a beach. Plunging waves have a
relatively short swash and longer backwash, and tend to
5 5 be destructive, removing material from a beach.
Z Zone of
one
of
diffraction
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10 diffraction
Nearshore currents
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Currents are created in the nearshore zone that have
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a different origin from ocean currents, tidal currents,
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and wind-induced currents. Nearshore currents are pro-
duced by waves. They include longshore currents, rip
currents, and offshore currents. Longshore or littoral
Figure 13.2 Wave refraction approaching headlands and currents are created when waves approach a coastline
bays above differing offshore topography. obliquely. They dominate the surf zone and travel par-
Source: Adapted from Bird (2000, 11) allel to the coast. Rip currents,or rips, are fed by
longshore currents and develop at more or less regular
intervals perpendicularly to the beach and flow through
the breaker zone.They are strong currents and dangerous
to swimmers. Onshore currents are slower and develop