Littoral transport occurs not only by rolling bedload, as above, but also by the movement of
suspended sediment. The waves generate a longshore current that flows through the area where they
break (breaker zone). Alone, it is generally too weak to move appreciable quantities of sediment;
however, the turbulence from breaking waves suspends sediments that can then be moved downdrift by
the longshore current. This sediment generally settles out within a short distance, but the next wave
provides additional movement. Therefore, longshore transport is caused by the zig-zag movement of
bedload up and down the beach, and the turbulence and action of the wave-induced longshore current.
Water Level Variations
The water surface elevation itself constantly changes with time. The Stillwater level, or the water
level with no waves present, changes because of three processes; astronomical tides, storms, and periodic
lake level variations.
Astronomical Tides - Tides are generated by the gravitational attraction between the earth, moon,
and sun, and are classified as diurnal, semidiurnal, or mixed. Diurnal tides have only one high and one
low each lunar day. Semidiurnal tides have two approximately equal highs and two approximately equal
lows daily. Mixed tides are intermediate between them and typically have two highs and lows that occur
each day. However, in contrast to semidiurnal tides, there is a large inequality, or difference in height,
between either successive high or successive low waters (Figure 4). Most Atlantic coast tides are
semidiurnal and the heights of successive highs or lows are approximately equal. Gulf and Pacific coast
tides tend to be mixed, and in most cases, there is a distinct inequality between successive highs or lows.
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