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 fractions, which form a beach. Eventually, if no other
littoral material is carried to the site by waves, even the sand and fine gravel will 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 many cases,
therefore, littoral materials comprising beaches are often derived from erosion of the shoreline itself.
Rivers and streams that carry sediments eroded from mountains, forests, and fields are a second
source of littoral materials, particularly during heavy rains. This material is usually not coarser than sand
because gravel and cobbles are not easily transported by most streams. Once the material arrives at the
shore, the wave-sorting process begins; the sand particles move to the beaches while silts and clays move
offshore.
Coral reefs, shells, and other plant or animal matter are a third material source. They gradually
break and weather into carbonate sands, which are, for instance, the primary component of beaches south
of Palm Beach, Florida. Swamps, marshes, and coastal wetlands produce peats and other organic matter.
Too light to remain in place under continued wave action, they are ultimately washed offshore unless
stabilized.
These littoral materials are transported along the shore by waves (Figure 3). As waves approach
the shore, they move to progressively shallower water where they bend or refract until finally breaking at
an angle to the beach. The broken wave creates considerable turbulence, lifting bottom materials into
suspension and carrying them up the beach slope in the general direction of wave approach. A short
distance up the beach, the motion reverses direction back down the beach slope. In this case, the
downrush does not follow the path of the advancing wave but instead, moves down the slope in response
to gravity. The next wave again carries the material upslope, repeating the process, so that each
advancing wave and the resulting downrush move material along the beach. This is the downdrift
direction. As long as waves approach from the same direction, the alongshore transport direction
remains the same.
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