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 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.
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.
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
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 and the production of organic material (generally peat) by
coastal wetlands.
Failure or erosion of a bluff causes material to be deposited at the base. Waves sort this material
and carry the fine-grained silts and clays far offshore where they settle to the bottom. The original
deposit is eventually reduced to sand and gravel, which form a beach. If no other littoral material is
carried to the sit by waves, even the sand and fine gravel will eventually disappear down the coast or
offshore, leaving only coarse gravel behind However, a new supply of material may be deposited on the
beach by a fresh failure of the bluff, and the process begins again. In most cases, littoral materials
comprising beaches are derived from erosion of the shoreline itself.
Rivers and streams carry sediments eroded from mountain forests, and fields, particularly during
floods. The sediment usually smaller than sand because coarser particles are not easily transported by
most streams. Except where streams traverse san drainage basins, the contribution to beach building
from this source is usually smaller than from the first source.
Coral reefs, shells, and other plant or animal matter are another material source. They gradually
break and weather in carbonate particles, which are, for instance, the primary component of beaches
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