Environmental Engineering Reference
In-Depth Information
W
aves
Waves are produced by the friction of wind on open water. Wave height and power
depend on wind strength and fetch, the amount of unobstructed ocean over which the
wind has blown. In a wave, water travels in loops (essentially up-and-down move-
ments), with the diameter of the loops decreasing with depth. The diameter of loops
at the surface is equal to the wave height. Breakers are formed when waves come
into shallow water near the shore. The lower part of the wave is retarded by the ocean
bottom, and the top, having greater momentum, is hurled forward, causing the wave
to break. These breaking waves may do great damage to coastal property as they
race across coastal lowlands driven by high winds.
COASTAL EROSION, TRANSPORTATION, AND DEPOSITION
The geologic work of the sea consists of erosion, transportation, and deposition. The
sea accomplishes its work of coastal landform sculpting largely by means of waves
and wave-produced currents; their effect on the seacoast may be quite pronounced.
The coast and accompanying coastal deposits and landform development represent a
balance between wave energy and sediment supply.
W
ave
e
rosion
Waves attack shorelines and erode by a combination of several processes. The resis-
tance of the rocks composing the shoreline and the intensity of wave action to which
they are subjected are factors that determine how rapidly the shore will erode. Wave
erosion works chiefly by hydraulic action, corrosion, and attrition. As waves strike a
sea cliff,
hydraulic action
crams air into rock crevices, putting tremendous pressure
on the surrounding rock; as waves retreat, the explosively expanding air enlarges
cracks and breaks off chunks of rock (known as
scree
). Chunks hurled by waves
against the cliff wear off more scree in a process called
corrasion
. In a similar
process known as
attrition
, rocks collide with each other in the waves and become
smaller and smoother until they are reduced to pebbles and sand grains (Lambert,
2007). Several coastal features are formed by marine erosion due to various combi-
nations of wave action, rock types, and rock beds:
•
Sea cliffs
or
wave-cut platforms
are formed by wave erosion of underlying
rock followed by caving-in of the overhanging rocks. As waves eat farther
back inland, they leave a wave-cut beach or platform. Such cliffs are essen-
tially vertical and are common at certain localities along the New England
and Pacific coasts of North American.
•
Wave-cut benches
are the result of wave action not having enough time
to lower the coastline to sea level. Because of the resistance to erosion, a
relatively flat wave-cut bench develops. If subsequent uplift of the wave-cut
bench occurs, it may be preserved above sea level as a marine terrace.
•
Headlands
are finger-like projections of resistant rock extending out into
the water. Indentations between headlands are termed
coves
.
