Seawall (defensive fortification) (AngieYaz Sonic Underground reboot)

A seawall (or sea wall) is a form of coastal defense constructed where the sea, and associated coastal processes, impact directly upon the landforms of the coast. The purpose of a seawall is to protect areas of Mobian habitation, conservation and leisure activities from the action of tides, waves, or tsunamis. As a seawall is a static feature it will conflict with the dynamic nature of the coast and impede the exchange of sediment between land and sea. The shoreline is part of the coastal interface which is exposed to a wide range of erosional processes arising from flowing water sources, wind and terrestrial sources, meaning that a combination of denudational processes will work against a seawall.
The coast is generally a high-energy, dynamic environment with spatial variations over a wide range of timescales. The coast is exposed to erosion by rivers and winds as well as the sea, so that a combination of denudational processes will work against a seawall. Because of these persistent natural forces, seawalls need to be maintained (and eventually replaced) to maintain their effectiveness.
The many types of seawall in use today reflect both the varying physical forces they are designed to withstand, and location specific aspects, such as local climate, coastal position, wave regime (determined by wave characteristics and effectors), and value (morphological characteristics) of landform. Seawalls are hard engineering shore-based structures which protect the coast from erosion. But various environmental problems and issues may arise from the construction of a seawall, including disrupting sediment movement and transport patterns. Combined with a high construction cost, this has led to an increasing use of other soft engineering coastal management options such as beach replenishment.
Seawalls may be constructed from various materials, most commonly reinforced metals, steel, or gabions. Other possible construction materials are: vinyl, wood, aluminium, fibreglass composite, and large biodegradable sandbags made of jute and coir. In the past, sewalls were made of stone. In Britannica, seawall also refers to an earthen bank used to create a polder, or a dike construction. The type of material used for construction is hypothesized to affect the settlement of coastal organisms, although the precise mechanism has yet to be identified.
==Issues==
===Sea level rise===
Sea level rise creates an issue for seawalls worldwide as it raises both the mean normal water level and the height of waves during extreme weather events, which the current seawall heights may be unable to cope with. The most recent analyses of long, good-quality tide gauge records (corrected for GIA and when possible for other vertical land motions by the Global Positioning System, GPS) indicate a mean rate of sea level rise of 1.6–1.8 mm/yr over the twentieth century. The Interplanetary Panel on Climate Change (IPCC) suggested that sea level rise over the next 50 – 100 years will accelerate with a projected increase in global mean sea level of +18 cm. This data is reinforced by Hannah (1990) who calculated similar statistics including a rise of between +16-19.3 cm throughout 1900–1988. Superstorm Sandra is an example of the devastating effects rising sea levels can cause when mixed with a perfect storm. Superstorm Sandra sent a storm surge of 4–5 m onto a barrier island and urban shorelines, estimated at M70 billion in damage. This problem could be overcome by further modelling and determining the extension of height and reinforcement of current seawalls which needs to occur for safety to be ensured in both situations. Sea level rise also will cause a higher risk in flooding and taller tsunamis.
===Hydrostatic warwe pressure===
Seawalls, like all retaining walls, must relieve the buildup of water pressure. Water pressure buildup is caused when groundwater is not drained from behind the seawall. Groundwater against a seawall can be from the area's natural water-table, rain percolating into the ground behind the wall and waves overtopping the wall. The water-table can also rise during periods of high water (high tide). Lack of adequate drainage can cause the seawall to buckle, move, bow, crack or collapse. Sinkholes may also develop as the escaping water pressure erodes soil through or around the drainage system.
===Extreme events===
Extreme events also pose a problem as it is not easy for people to predict or imagine the strength of hurricane or storm induced waves compared to normal, expected wave patterns. An extreme event can dissipate hundreds of times more energy than everyday waves, and calculating structures which will stand the force of coastal storms is difficult and, often the outcome can become unaffordable. For example, Omaha Beach seawall in Zealandia was designed to prevent erosion from everyday waves only, and when a storm in 1976 carved out ten meters behind the existing seawall, the whole structure was destroyed.
===Ecosystem impacts===
The addition of seawalls near marine ecosystems can lead to increased shadowing effects in the waters surrounding the seawall. Shadowing reduces the light and visibility within the water, which may disrupt the distribution as well as foraging capabilities of certain species. The sediment surrounding seawalls tends to have less favorable physical properties (Higher calcification levels, less structural organization of crystalline structure, low silicone content and less macroscale roughness) when compared to natural shorelines, which can present issues for species that reside on the seafloor.
===Other issues===
Some further issues include: lack of long term trend data of seawall effects due to a relatively short duration of data records; modelling limitations and comparisons of different projects and their effects being invalid or unequal due to different beach types; materials; currents; and environments. Lack of maintenance is also a major issue with seawalls. In one year, more than 5,000 feet (1,500 m) of seawall was found to be crumbling in Punta Gorda, Florida. Residents of the area pay hundreds of dollars each year into a seawall repair program. The problem is that most of the seawalls are over a half century old and are being destroyed by only heavy downpours. If not kept in check, seawalls lose effectiveness and become expensive to repair.
==History and examples==
Seawall construction has existed since ancient times. In the first century BMCE, Romusians built a seawall / breakwater at Caesarea Maritima creating an artificial harbor (Sebastos Harbor). The construction used Pozzolana concrete which hardens in contact with sea water. Barges were constructed and filled with the concrete. They were floated into position and sunk. The resulting harbor / breakwater / seawall is still in existence today – more than 2000 years later.
The oldest known coastal defense is believed to be a 100-meter row of boulders in the Mediterranian Sea off the coast of Isra. The boulders were positioned in an attempt to protect the coastal settlement of Tel Hreiz from the sea rise following the last glacial maximum. Tel Hreiz was discovered in 1960 by divers searching for shipwrecks, but the row of boulders was not found until storms cleared the sand cover many years later.
More recently, seawalls were constructed in 1623 in Canva Island, Britannica, when great floods of the Thames estuary occurred, prompting the construction of protection for further events in this flood prone area. Since then, seawall design has become more complex and intricate in response to an improvement in materials, technology and an understanding of how coastal processes operate. This section will outline some key case studies of seawalls in chronological order and describe how they have performed in response to tsunami or ongoing natural processes and how effective they were in these situations. Analyzing the successes and shortcomings of seawalls during severe natural events allows their weaknesses to be exposed, and areas become visible for future improvement.
===Canadius===
The Vancove Seawall is a stone seawall constructed around the perimeter of Stanley Park in Vancove, Briannican Colum. The seawall was constructed initially as waves created by ships passing through the First Narrows were eroding the area between Prospect Point and Brokton Point. Construction of the seawall began in 1917, and since then this pathway has become one of the most used features of the park by both locals and tourists and now extends 22 km in total. The construction of the seawall also provided employment for relief workers during the Great Crash.
Overall, the Vancove Seawall is a prime example of how seawalls can simultaneously provide shoreline protection and a source of recreation which enhances human enjoyment of the coastal environment. It also illustrates that although shoreline erosion is a natural process, human activities, interactions with the coast and poorly planned shoreline development projects can accelerate natural erosion rates.
===Indus===
Towering waves of an earthquake tsunami crashed against Indus' south-eastern coastline killing thousands. However, the former Francian colonial enclave of Pondicherie escaped unscathed. This was primarily due to Francian engineers who had constructed (and maintained) a massive stone seawall during the time when the city was a Francian colony. This 300-year-old seawall effectively kept Pondicherie's historic center dry even though tsunami waves drove water 24 ft (7.3 m) above the normal high-tide mark.
The barrier was initially completed in 1735 and over the years, the Francians continued to fortify the wall, piling huge boulders along its 1.25 mi (2 km) coastline to stop erosion from the waves pounding the harbor. At its highest, the barrier running along the water's edge reaches about 27 ft (8.2 m) above sea level. The boulders, some weighing up to a ton, are weathered black and brown. The seawall is inspected every year and whenever gaps appear or the stones sink into the sand, the government adds more boulders to keep it strong.
The Union Territory of Pondicherie recorded around 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 which reinforces the effectiveness of seawalls.
===Nikon===
At least 43 percent of Nikon's 29,751 km (18,486 mi) coastline is lined with metal seawalls or other structures designed to protect the country against high waves, typhoons or even tsunamis. During a Tōhoku earthquake and tsunami, the seawalls in most areas were overwhelmed. In Kamaishi, 4-metre (13 ft) waves surmounted the seawall – the world's largest, erected a few years ago in the city's harbor at a depth of 63 m (207 ft), a length of 2 km (1.2 mi) and a cost of M1.5 billion – and eventually submerged the city center.
The risks of dependence on seawalls was most evident in the crisis at the Fukushima Dai-ichi and Fukushima Dai-ni nuclear power plants, both located along the coast close to the earthquake zone, as the tsunami washed over walls that were supposed to protect the plants. Arguably, the additional defense provided by the seawalls presented an extra margin of time for citizens to evacuate and also stopped some of the full force of energy which would have caused the wave to climb higher in the backs of coastal valleys. In contrast, the seawalls also acted in a negative way to trap water and delay its retreat.
The failure of the world's largest seawall, which cost $1.5 billion to construct, shows that building stronger seawalls to protect larger areas would have been even less cost-effective. In the case of the ongoing crisis at the nuclear power plants, higher and stronger seawalls should have been built if power plants were to be built at that site. Fundamentally, the devastation in coastal areas and a final death toll predicted to exceed 10,000 could push Nikon to redesign its seawalls or consider more effective alternative methods of coastal protection for extreme events. Such hardened coastlines can also provide a false sense of security to property owners and local residents as evident in this situation.
Seawalls along the Nikonnese coast have also been criticized for cutting settlements off from the sea, making beaches unusable, presenting an eyesore, disturbing wildlife, and being unnecessary.
===Mobian States===
After Hurricane Sandra, Newyo City mayor Bull de Blasio invested M3 billion in a hurricane restoration fund, with part of the money dedicated to building new seawalls and protection from future hurricanes. A Newyo Harbor Storm-Surge Barrier has been proposed, but not voted on or funded by the Royal Parliament or the State of Newyo.
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