seawall

A seawall is a form of hard coastal defence constructed on the inland part of a coast to reduce the effects of strong waves and 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 in the short term, 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 are 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: most commonly, they are constructed of reinforced concrete, boulders, steel, or wire cages filled with pebbles. Additional seawall construction materials include: vinyl, wood, aluminum and fiberglass composite. Poorly designed seawalls require constant maintenance, as the waves can constantly attack the base of the seawall. Seawalls can be expensive to build, costing between £50 to £1000 per metre. Modern concrete sea walls tend to be curved to deflect the wave energy back out to sea, reducing the force.

Types of seawalls

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 mound-type structures are used in lesser energy settings.

The least exposed sites involve the lowest-cost technology, bulkheads or revetments of sand bags or geotextiles. These serve to armour the shore and impede 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.

Pondicherry saved by French-built seawall

Seawall in production in Galveston, TX, USA, 1905

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 Richter scale) off Indonesia, but most of those killed were fishermen who lived in villages beyond the man-made barrier.

Gold Coast Seawall

The Gold Coast seawall in Australia is contained within the Gold Coast's shoreline management plan. The original seawall was laid out following 11 cyclones in 1967 with assistance from coastal engineers from Delft University. The seawall alignment was selected to pick up as many of the older seawalls as possible. The seawall consists of three layers, armour boulders up to 4 tonnes, secondary armour around 360kg and a clay shale foundation layer. The seawall is 16m across and 6m high and has a front slope of 1:1.5. The seawall was tested in a wave tank to withstand attack from a 1:100 cyclone wave.

A Gold Coast Seawall costs around A$3000 per meter to construct in 2006. The seawall is constructed along a designated seawall alignment along urban sections of the Gold Coast coastline. Non-Urban sections of coastline including South Stradbroke Island and the Southport Spit are not licenced for the construction of a seawall. The Gold Coast Planning Scheme requires private property owners along the beach to construct the seawall at their property at the property owners expense prior to making any investment into their house. The Gold Coast City Council constructs sections of seawall that protect public land.

See also

• Retaining wall
• Dike (construction)
• Galveston Seawall (Galveston Island, Texas, USA)
• Smalls Lighthouse built on nine columns to let sea pass through.
• Seawall (Vancouver) (Vancouver, Canada)
• Constructing a new sea wall and defences at Lyme Regis in Dorset

External links

Wikimedia Commons has media related to:

Seawalls

• Channel Coastal Observatory - Seawalls
• Seawall Bulkhead Protection CMI

Gallery

Seawall (unknown location)	Seawall in Somers, Victoria	Seawall in Ventnor, UK	Seawall in Bembridge, UK
Seawall in Sicily	Seawall in Poland‎

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