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Seas are dynamic in nature. They are one of the greatest weathering agents in nature along with rivers, glaciers and wind. The erosion of shores by sea waves, ingression of sea into land and consequent wasting of beach are known to man for ages. Apart from these, the man’s sea side activities like sunbathing, swimming and sea borne activities like marine transport require protection from dynamic action of sea.
Though there are many non-structural ways of dealing with the shore erosion like planting trees, beach nourishment, forced draining of ground water in shore bluffs, relocation etc., our interest here is the structural means of controlling or arresting shore erosion or ensuring shore protection. In modern days, many structural methods are available for sea shore protection.
Bulkheads
These are vertical retaining walls built parallel to the sea shore, primarily to hold or prevent the soil beneath it from sliding into sea. These are suitable only where the wave action is less.
Seawalls
These are massive walls built parallel to the sea shore, primarily to hold or prevent the sea waves from eroding backshore areas. These are suited well for protecting heavily built up or commercially developed areas from erosion by sea and to prevent sea ingression into land. These are suitable for areas with intense wave action to reduce the wave energy.
Revetments
These act as a cover or facing made of erosion resistant material laid directly on existing slopes, embankments or dykes to protect the surface from action of sea waves and currents. Revetments may be constructed of bituminous concrete or cement concrete or stones but the last is popular than the other two.
Dykes and Levees
Similar to the artificial riverside levees, the seaside dykes or levees built in the form of earthen embankments or walls to prevent elevated sea water levels or high tides from flooding low-lying areas inland. The extensive dyke system of the Netherlands is a fine example.
Groynes
Groynes are the oldest known and most popular shore-connected beach protection structures. These help to protect or control erosion an existing or restored beach. Built perpendicular to the shoreline for considerable length into sea, these help to trap the littoral drift and deposit the sediments they carry between the groyne walls. Groynes can also be built to protect the toes of sea-walls or bulkheads. In certain shore forms they may even be built inclined into sea. These may be built in different types such as high or low, long or short, permeable or impermeable, fixed or adjustable etc.
Visit http://www.usna.edu/NAOE/courses/en420/bonnette/Groins.html for an example.
Sills or Perched Beaches
There are certain locations on sea shores where sand loss is very rapid. In such situations, construction of a low retaining sill to trap the sand results in sand accumulation to form a "perched beach," which is elevated to above its original level. This also makes a good choice when a beach is necessary for recreational activities and beach nourishment or replacement are uneconomical.
Jetties
These are stone structures built perpendicular to shore, predominantly for training navigation channels and stabilising harbour and bay inlets. They help arresting shore transport deposits beyond the jetty, creating steady flow conditions in channel to encourage scouring of sea deposits, thereby ensuring required draft in the inlet channels.
Breakwaters
Breakwaters are the most important protection structure for ports and harbours, they are generally built as a system of two or three breakwaters to create an enclosed and safe area for ships. For shore protection they often run parallel to shore. These essentially work by reducing the wave energy reaching the protected area. Let us deal with them in detail in another article.
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