A seawall (also written as sea wall) is a form of coastal defence 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 human habitation, conservation and leisure activities from the action of tides and waves.[1] 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.[2]
The coast is generally a high-energy, dynamic environment with spatial variations occurring over a wide range of temporal scales.[3] The shoreline is part of the coastal interface which is exposed to a wide range of erosional processes arising from fluvial, aoelian and terrestrial sources, meaning that a combination of denudational processes will work against a seawall.[4] Given the natural forces that seawalls are constantly subjected to, maintenance (and eventually replacement) is an ongoing requirement if they are to provide an effective long term solution.
The many types of seawall in use today reflects both the varying physical forces they are designed to withstand, and location specific aspects, such as: local climate, coastal position, wave regime, and value of landform. Seawalls are classified as a hard engineering shore based structure used to provide protection and to lessen coastal erosion. However, a range of environmental problems and issues may arise from the construction of a seawall, including disrupting sediment movement and transport patterns, which are discussed in more detail below.[5] 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 a variety of materials, most commonly: reinforced concrete, boulders, steel, or gabions. Additional seawall construction materials may include: vinyl, wood, aluminium, fibreglass composite, and with large biodegrable sandbags made of jute and coir.[6] In the UK, sea wall also refers to an earthen bank used to create a polder, or a dike.
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A seawall works by reflecting incident wave energy back into the sea, therefore reducing the energy and erosion which the coastline would otherwise be subjected to.[7] In addition to their unsightly visual appearance, two specific weaknesses of seawalls exist. Firstly, wave reflection induced by the wall may result in scour and subsequent lowering of the sand level of the fronting beach.[8] Secondly, seawalls may accelerate erosion of the adjacent, unprotected coastal properties because they affect the littoral drift process.[9] Fundamentally, a cost-benefit approach is an effective way to determine whether a seawall is appropriate or not and if the negative effects are worth the protection of threatened property.
The design and type of a seawall varies depending on unique aspects specific to each location, and the erosional processes and environment which they are placed in.[10] There are three main types of seawalls: vertical; curved or stepped; and mounds. These are described more comprehensively below and summarised with positive and negative aspects and global examples within Table 1.
Seawall type | Advantage | Disadvantage | Global example |
Vertical |
|
|
Vancouver Seawall |
Curved |
|
|
Torcross UK and Seagrove Bay UK. |
Mound |
|
|
Central Waterfront, Seattle |
Furthermore, Figures 1 and 2 below provide a visual illustration of the structure and design of these three types of seawall.
Figure one: Vertical and curved-type seawalls. Figure two: Mound-type seawall.
Source: University of Barcelona (2007) [13]
Seawall construction was first documented in 1623 in Canvey Island, UK, when great floods of the Thames estuary occurred, prompting the construction of protection for further events in this flood prone area (Council of Europe, 1999).[14] 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. Analysing the successes and shortcomings of seawalls during severe natural events allows their weaknesses to be exposed, and areas become visible for future improvement and reassessment.
On December 26, 2004, towering waves of the 2004 Indian Ocean earthquake tsunami crashed against India's south-eastern coastline killing thousands. However, the former French colonial enclave of Pondicherry (now Pondicherry) escaped unscathed. This was primarily due to French engineers who had constructed (and maintained) a massive stone seawall during the time which the city was a French colony. irul This 300 year old seawall effectively kept Pondicherry's historic centre dry even though tsunami waves drove water 24 feet above the normal high-tide mark.
The barrier was initially completed in 1735 and over the years, the French continued to fortify the wall, piling huge boulders along its 1.25 mile (2 km) 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 and whenever gaps appear or the stones sink into the sand, the government adds more boulders to keep it strong (Allsop, 2002).[15]
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 which reinforces the effectiveness of seawalls.
The Vancouver Seawall is a stone seawall constructed around the perimeter of Stanley Park in Vancouver. The seawall was constructed initially as waves created by ships passing through the First Narrows were eroding the area between Prospect Point and Brockton Point. The Vancouver Seawall also exemplifies how seawalls can be utilised and valued for recreational activities and coastal sightseeing. A pedestrian, cycling and roller blading pathway exists on the seawall and has been extended far outside the parameters of Stanley Park. Construction of the seawall began in 1914, 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 (Belyea & Ross, 1992).[16] The construction of the seawall also provided employment for relief workers during the Great Depression and seamen from the HMCS Discovery on Deadman's Island who were facing punishment detail in the 1950s (Steele, 1985).[17]
Overall, the Vancouver 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.
At least 40 per cent of Japanâs 35,000 kilometre coastline is lined with concrete seawalls or other structures designed to protect the country against high waves, typhoons or even tsunamis (New York Times, 2011).[18] When a Tsunami struck in 2011 following a magnitude 9 offshore earthquake, the seawalls in most areas were overwhelmed. In Kamaishi, 4-metre waves surmounted the seawall âthe worldâs largest, erected a few years ago in the cityâs harbour at a depth of 63 metres, a length of 2 kilometres and a cost of $1.5 billion â and eventually submerged the city centre (Musubi, 2011).[19]
The risks of dependence on seawalls was most evident in the crisis at the Daiichi and Daini 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 defence 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 sea walls to protect larger areas would have been too costly to be effective. In the case of the ongoing crisis at the nuclear power plants, higher and stronger sea walls 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 Japan 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 (Msubi, 2011).[20]
The Maritime Engineering Division at the University of Salerno (MEDUS) developed a new procedure to study, with a more detailed and innovative approach, the interactions between maritime breakwaters (submerged or emerged) and the waves, by an integrated use of CAD and CFD software (MEDUS, 2011).[21]
In the numerical simulations the filtration motion of the fluid within the interstices, which normally exist in a breakwater, is estimated by integrating the RANS equations, coupled with a RNG turbulence model, inside the voids, not using a classical equations for porous media. The breakwaters were modeled, as it happens in the full size construction or in physical laboratory test, by overlapping three-dimensional elements and the numerical grid was thickened in such a way to have some computational nodes along the flow paths among the breakwaterâs blocks.[22]
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 (Allan et al. 1999).[23] The International Panel on Climate Change (IPCC) (1997)[24] suggested that sea level rise over the next 50 â 100 years will accelerate with a projected increase in global mean sea level of +18 cm by 2050 AD. This data is reinforced by Hannah (1990)[25] who calculated similar statistics including a rise of between +16-19.3 cm throughout 1900â1988. 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.
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 New Zealand was designed to prevent erosion from everyday waves only, and when a storm in 1976 carved out 10m behind the existing seawall the whole structure was destroyed (GeoResources, 2001).[26]
Some further limitations 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 (Christchurch City Council, 2009).[27]
In conclusion, a cost benefit approach is an effective way to determine whether a seawall is appropriate and if the benefits are worth the expense. Besides controlling erosion, consideration must be given to the effects of hardening a shoreline upon natural coastal ecosystems and human property or activities. Overall, a holistic approach to planning is ideal. It is important to remember that 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. Table 2 summarises the overall positive and negative effects of seawalls which is useful when comparing their effectiveness to other coastal management options such as beach nourishment.
Advantages of Seawalls | Disadvantages of Seawalls |
---|---|
|
|
After Short (1999).[28]
Fundamentally, seawalls are generally a successful way to control coastal erosion but only if they are constructed well and out of materials which can withstand the force of ongoing wave energy. Likewise, a thorough understanding of coastal processes and location specific morphodynamics is imperative to enhance the successful implementation and lifespan of a seawall. Seawalls are highly useful as they are more long term than other soft engineering options and they can simultaneously provide recreation opportunities and protection from not everyday erosion but that of extreme events. Analysing the successes and shortcomings of seawalls during severe natural events allows their weaknesses to be exposed and areas become visible for future improvement and reassessment.
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