The actual process of wave generation depends on several important factors, the most prominent
being wind, although pleasure craft and large vessels also cause significant wave activity in the form of
wakes. The height of wind-driven waves depends on the wind speed, duration, fetch length, and water
depth. Wind speed is obviously important, but duration (length of time the wind blows) must also be
considered because wind action must be sustained for wave growth. Fetch is the over-water distance
wind travels while generating waves. At a given site, the maximum fetch length, or longest over-water
distance, is generally the most important. Less important, but still critical, is the average water depth
along the fetch. Deeper water allows for somewhat larger waves because of decreased bottom friction.
Sediment Transport
The large variety of shoreline materials ranges from rock cliffs down to boulders, cobbles, gravel,
sand, silt, and clays. Geologists and engineers have developed several classification systems for these
materials and an example is given in Table 1.
Table 1
CLASSIFICATION OF SHORELINE MATERIALS
Size Description
Particle Size Range
(Inches)
(mm)
Boulder
greater than 10
greater than 256
Cobble
10 - 3
256 76
Gravel
3 - 0.18
76 -4.8
Sand
0.18 - 0.003
4.8 -0.07
Silt
0.003 - 0.00015
0.07 -0.004
Clay
smaller than 0.00015
smaller than 0.004
Rock characterizes cliff shorelines, such as the northern California shore. Boulders are often
present at the base of such cliffs because of rock fracturing and weathering. Cobbles and gravels are
prevalent beach materials in the Pacific Northwest, Alaska, and the Great Lakes area. Sand, the most
common shoreline material, is found in virtually all coastal areas. Silts and clays generally occur on
bluff shorelines or marshes, such as along the Great Lakes and various bays.
Littoral (shoreline) materials are derived from the deterioration and erosion of coastal bluffs and
cliffs, the weathering of rock materials found inland and transported to the shore by rivers and streams,
the disintegration of shells, coral or algae to form carbonate materials, and the production of organic
material (generally peat) by coastal marshes and wetlands.
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