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A seawall is a form of hard coastal defense that are constructed on the inland part of a coast to reduce the effects of strong waves, typically to defend the coast around a town or harbour from erosion. The walls can be sloping, vertical or curved to reflect wave power. The term is typically used to describe walls built on the land parallel to the coast, but may also apply to breakwaters and groynes which are built in the water.
Seawalls are effective defenses over the course of many years, but may cause erosion in the long run. They cause the energy of the backwash to be reflected to the beach material beneath and in front of them, so the beach materials is gradually eroded. This problem may be reduced if coupled with beach nourishment (replacement of the eroded material) or rock aprons which reduce wave power by percolating the water slowly through gaps.
Seawalls may be constructed from a variety of materials, commonly reinforced concrete, boulders, steel or wire cages filled with pebbles. Seawalls require constant maintenance as the waves will constantly attack the base of the walls and cause them to be weakened. Modern concrete sea walls tend to be curved to deflect the wave energy back out to sea, reducing the force
Pondicherry saved by French-built seawall
On on December 26, 2004, when towering waves of the 2004 Indian Ocean earthquake crashed against India's south-eastern coastline killing thousands, the former French colonial enclave of Pondicherry escaped unscathed. During the city's nearly three centuries as a French colony, French engineers constructed and maintained a massive stone sea wall, which kept Pondicherry's historic center dry even though Tsunami waves drove water 24 feet above the normal high-tide mark.
The barrier was initially completed in 1735. Over the years, the French continued to fortify the wall, piling huge boulders along its 1.25-mile (2-kilometer) coastline to stop erosion by the waves pounding the harbor. At its height, the barrier running along the water's edge reaches about 27 feet above sea level. The boulders, some weighing up to a ton, are weathered black and brown. The sea wall is inspected every year. Whenever gaps appear or the stones sink into the sand, the government adds more boulders to keep it strong.
The Union Territory of Pondicherry recorded some 600 deaths from the huge Tsunami waves that struck India's coast after the mammoth underwater earthquake (with measured 9.0 on the Richter scale) off Indonesia, but most of those killed were fishermen who lived in villages beyond the man-made barrier.
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