13
Lesson N°2 :
The dynamic of shorelines
Breaking waves provide most of the energy that changes the shape and
texture of beach deposits.
The sediment of beaches and nearshore zones is continually being
moved by longshore currents and rip currents. As breakers stir up water
, sand grains , even large ones , are lifted off the bottom by wave
turbulence and get pushed along by the current before settling back
down to the sea bottom.
This up-and –down motion is repeated incessantly with each breaking
wave , transporting sand grains along the shore for remarkably long
distances. Even gravel can be rolled along the bottom by breaking
waves.Typically , sand particles , once they enter a beach system , are
transported parallel to the shore in the direction of the prevailing longshore current . when sand grains are swept into rip currents , they are
transported beyond the surf zone into the offshore. Strong rip currents ,
if they persist , can cause severe erosion of beaches.
Beach profiles :
An important technique for studying sand deposition and erosion is the
measurement of beach profile s , which delineate the shape of the beach
surface along a survey line or transect. By comparing profiles of the
same transect taken over time , it is possible to determine gains and
losses of sand.
Sand budgets :
Coastal engineers and geologists keep track of changes in the sediment
volume of a beach by the use of sand budgets , which are estimates of
the principal sand sources (credits) and sand losses (debits ) for a stretch
of shoreline.
Barrier islands :
Where coastal sand supplies are abundant and the sea floor slopes gently
, the mainland is commonly bordered by a series of barrier islands.