Definitions

seawall

seawall

seawall: see coast protection.
A seawall is a form of hard and strong coastal defence constructed on the inland part of a coast to reduce the effects of strong waves. In Britain, "sea wall" also refers to an earthen bank used to create a polder – a dike. Seawalls may be constructed from a variety of materials: most commonly, reinforced concrete, boulders, steel, or wire cages filled with pebbles. Additional seawall construction materials may include vinyl, wood, aluminum and fiberglass composite. Seawalls can be expensive to build, costing between £4000 to £7000 per metre. Modern concrete seawalls tend to be curved to deflect the wave energy back out to sea. Poor designs require constant maintenance as waves erode the base of the seawall.

Design principles and types

A range of seawall types can be envisaged in relation to wave energy, resembling cliff and beach profiles. Vertical seawalls are built in particularly exposed situations. These reflect wave energy and under storm conditions standing waves (clapotis) will develop. In some cases piles are placed in front of the wall to lessen wave energy slightly. Curved or stepped seawalls are designed to enable waves to break and to dissipate wave energy. The curve can also prevent the wave overtopping the wall, and provide additional protection for the toe of the wall.

A series of rubble mound-type structures (revetments, riprap) are used in less demanding settings. The least exposed sites involve the lowest-cost bulkheads, or revetments of sand bags or geotextiles. These serve to armour the shore and minimize erosion. They may be either watertight, covering the slope completely, or porous, to allow water to filter through after the wave energy has been dissipated.

History

Pondicherry

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 (now Puducherry) escaped unscathed. During the city's nearly three centuries as a French colony, French engineers had constructed and maintained a massive stone seawall, 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 from the waves pounding the harbour. At its highest, 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 (which measured 9.0 on the moment magnitude scale) off Indonesia, but most of those killed were fishermen who lived in villages beyond the artificial barrier.

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