Seawall
Overview
A seawall is a form of coastal defence
Coastal management
In some jurisdictions the terms sea defense and coastal protection are used to mean, respectively, defense against flooding and erosion...

 constructed where the sea, and associated coastal processes, impact directly upon the landforms of the coast
Coast
A coastline or seashore is the area where land meets the sea or ocean. A precise line that can be called a coastline cannot be determined due to the dynamic nature of tides. The term "coastal zone" can be used instead, which is a spatial zone where interaction of the sea and land processes occurs...

. The purpose of a seawall is to protect areas of human habitation, conservation and leisure activities from the action of tides and wave
Wave
In physics, a wave is a disturbance that travels through space and time, accompanied by the transfer of energy.Waves travel and the wave motion transfers energy from one point to another, often with no permanent displacement of the particles of the medium—that is, with little or no associated mass...

s. 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 coast is generally a high-energy, dynamic environment with spatial variations occurring over a wide range of temporal scales.
Encyclopedia
A seawall is a form of coastal defence
Coastal management
In some jurisdictions the terms sea defense and coastal protection are used to mean, respectively, defense against flooding and erosion...

 constructed where the sea, and associated coastal processes, impact directly upon the landforms of the coast
Coast
A coastline or seashore is the area where land meets the sea or ocean. A precise line that can be called a coastline cannot be determined due to the dynamic nature of tides. The term "coastal zone" can be used instead, which is a spatial zone where interaction of the sea and land processes occurs...

. The purpose of a seawall is to protect areas of human habitation, conservation and leisure activities from the action of tides and wave
Wave
In physics, a wave is a disturbance that travels through space and time, accompanied by the transfer of energy.Waves travel and the wave motion transfers energy from one point to another, often with no permanent displacement of the particles of the medium—that is, with little or no associated mass...

s. 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 coast is generally a high-energy, dynamic environment with spatial variations occurring over a wide range of temporal scales. 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.
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
Hard engineering
In civil engineering of shorelines, hard engineering is generally defined as controlled disruption of natural processes by using man-made structures.-Examples:...

 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. Combined with a high construction cost, this has led to an increasing use of other soft engineering
Soft engineering
In civil engineering of shorelines, soft engineering is the use of ecological principles and practices to reduce erosion and achieve the stabilization and safety of shorelines and the area surrounding rivers, while enhancing habitat, improving aesthetics, and saving money...

 coastal management options such as beach replenishment.

Seawalls may be constructed from a variety of materials, most commonly: reinforced concrete
Reinforced concrete
Reinforced concrete is concrete in which reinforcement bars , reinforcement grids, plates or fibers have been incorporated to strengthen the concrete in tension. It was invented by French gardener Joseph Monier in 1849 and patented in 1867. The term Ferro Concrete refers only to concrete that is...

, boulders, steel, or gabion
Gabion
Gabions are cages, cylinders, or boxes filled with soil or sand that are used in civil engineering, road building, and military applications. For erosion control caged riprap is used. For dams or foundation construction, cylindrical metal structures are used...

s. Additional seawall construction materials may include: vinyl, wood, aluminium, fibreglass composite, and with large biodegrable sandbags made of jute and coir. In the UK, sea wall also refers to an earthen bank used to create a polder
Polder
A polder is a low-lying tract of land enclosed by embankments known as dikes, that forms an artificial hydrological entity, meaning it has no connection with outside water other than through manually-operated devices...

, or a dike.

Design principles and types

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. 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. Secondly, seawalls may accelerate erosion of the adjacent, unprotected coastal properties because they affect the littoral drift process. 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. 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.

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 seawalls: are designed to enable waves to break to dissipate wave energy and to repel waves back to the sea. The curve can also prevent the wave overtopping the wall and provides additional protection for the toe of the wall.
Mound-type structures: (revetments, riprap) are used in less demanding settings where lower energy erosional processes operate. The least exposed sites involve the lowest-cost bulkheads and revetments of sand bags or geotextiles. These serve to armour the shore and minimise erosion and may be either watertight or porous, which allows water to filter through after the wave energy has been dissipated.

Table 1: Comparison of the positive and negative effects of vertical, curved and mound type seawalls with a global example inclusive for each type.
Seawall type Advantage Disadvantage Global example
Vertical
  • The first implemented, most easily designed and constructed type of seawall.
  • Vertical sea walls deflect wave energy away from the coast.
  • Loose rubble can absorb wave energy.
  • These are partial to a lot of expensive damage in a short period of time.
  • Vertical design can be undercut by high-wave energy environments over a long period of time.
  • Vancouver Seawall
    Curved
  • Concave structure introduces a dissipative element.
  • The curve can prevent waves from overtopping the wall and provides extra protection for the toe of the wall
  • Curved seawalls aim to re-direct most of the incident energy, resulting in low reflected waves and much reduced turbulence.
  • More complex engineering and design process.
  • The deflected waves can scour material at the base of the wall causing them to become undermined.
  • Torcross
    Torcross
    Torcross is a village in the South Hams district of Devon in England. It stands at grid reference at the southern end of Slapton Sands, a narrow strip of land and shingle beach which separates the freshwater lake of Slapton Ley from Start Bay and carries the A379 coastal road north to Dartmouth.-...

     UK and Seagrove Bay
    Seagrove Bay
    Seagrove Bay is a bay on the north east coast of the Isle of Wight, England. It lies to the east of the village of Seaview. It faces east towards Selsey Bill and its shoreline is 1 km in length. It stretches from Nettlestone Point in the north to Horestone Point in the south.Roughly at the...

     UK.
    Mound
  • Current designs use porous designs of rock, concrete armour.
  • Slope and loose material ensure maximum dissipation of wave energy.
  • Lower cost option.
  • Less durable.
  • Shorter life expectancy.
  • Cannot withstand or protect from high-energy conditions effectively.
  • Central Waterfront, Seattle
    Central Waterfront, Seattle
    The Central Waterfront of Seattle, in the state of Washington, USA, is the most urbanized portion of the Elliott Bay shore. It runs from the Pioneer Square shore roughly northwest past Downtown Seattle and Belltown, ending at the Broad Street site of the Olympic Sculpture Park.The Central...


    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)

    History

    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). 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.

    Pondicherry

    On December 26, 2004, towering waves of the 2004 Indian Ocean earthquake
    2004 Indian Ocean earthquake
    The 2004 Indian Ocean earthquake was an undersea megathrust earthquake that occurred at 00:58:53 UTC on Sunday, December 26, 2004, with an epicentre off the west coast of Sumatra, Indonesia. The quake itself is known by the scientific community as the Sumatra-Andaman earthquake...

     tsunami crashed against India's south-eastern coastline killing thousands. However, the former French colonial enclave of Pondicherry (now Puducherry) 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. This 300 year old seawall effectively kept Pondicherry's historic centre dry even though tsunami
    Tsunami
    A tsunami is a series of water waves caused by the displacement of a large volume of a body of water, typically an ocean or a large lake...

     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).

    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
    Moment magnitude scale
    The moment magnitude scale is used by seismologists to measure the size of earthquakes in terms of the energy released. The magnitude is based on the seismic moment of the earthquake, which is equal to the rigidity of the Earth multiplied by the average amount of slip on the fault and the size of...

    ) off Indonesia, but most of those killed were fishermen who lived in villages beyond the artificial barrier which reinforces the effectiveness of seawalls.

    Vancouver Seawall

    The Vancouver Seawall
    Seawall (Vancouver)
    The seawall in Vancouver, Canada is a stone wall that was constructed around the perimeter of Stanley Park to prevent the erosion of the park's foreshore. Colloquially, the term also denotes the pedestrian, bicycle, and roller blading pathway on the seawall, and which has been extended far outside...

     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). 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).

    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.

    Japan

    At least 40 per cent of Japan’s 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). 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 —the world’s largest, erected a few years ago in the city’s harbour at a depth of 63 metres, a length of 2 kilometres and a cost of $1.5 billion — and eventually submerged the city centre (Musubi, 2011).

    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
    Japan
    Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...

     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).

    Advanced numerical study

    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
    Computer-aided design
    Computer-aided design , also known as computer-aided design and drafting , is the use of computer technology for the process of design and design-documentation. Computer Aided Drafting describes the process of drafting with a computer...

     and CFD
    Computational fluid dynamics
    Computational fluid dynamics, usually abbreviated as CFD, is a branch of fluid mechanics that uses numerical methods and algorithms to solve and analyze problems that involve fluid flows. Computers are used to perform the calculations required to simulate the interaction of liquids and gases with...

     software (MEDUS, 2011).

    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 breakwater’s blocks.

    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 (Allan et al. 1999). The International Panel on Climate Change (IPCC) (1997) 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 by 2050 AD. This data is reinforced by Hannah (1990) who calculated similar statistics including a rise of between +16-19.3 cm throughout 1900–1988. 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

    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).

    Other limitations

    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).

    Efficacy and trade-offs

    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.
    Table 2: Advantaged and Disadvantages of Seawalls.
    Advantages of Seawalls Disadvantages of Seawalls
    • Long term solution in comparison to soft beach nourishment.
    • Effectively minimizes loss of life in extreme events and damage to property caused by erosion.
    • Can exist longer in high energy environments in comparison to ‘soft’ engineering methods.
    • Can be used for recreation and sightseeing.
    • Forms a hard and strong coastal defence.
  • Very expensive to construct.
  • Can cause beaches to dissipate rendering them useless for beach goers.
  • Scars the very landscape that they are trying to save and provides an ‘eyesore.’
  • Reflected energy of waves leading to scour at base.
  • Can disrupt natural shoreline processes and destroy shoreline habitats such as wetlands and intertidal beaches.
  • Altered sediment transport processes can disrupt sand movement that can lead to increased erosion down drift from the structure.


  • After Short (1999).

    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
    Soft engineering
    In civil engineering of shorelines, soft engineering is the use of ecological principles and practices to reduce erosion and achieve the stabilization and safety of shorelines and the area surrounding rivers, while enhancing habitat, improving aesthetics, and saving money...

     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.

    See also

    • Constantinople Sea Walls
    • Breakwater
      Breakwater (structure)
      Breakwaters are structures constructed on coasts as part of coastal defence or to protect an anchorage from the effects of weather and longshore drift.-Purposes of breakwaters:...

    • Retaining wall
      Retaining wall
      Retaining walls are built in order to hold back earth which would otherwise move downwards. Their purpose is to stabilize slopes and provide useful areas at different elevations, e.g...

    • Accropode
    • Dike
    • Alaskan Way Seawall
      Alaskan Way Seawall
      The Alaskan Way Seawall is a seawall which runs for along the Elliott Bay waterfront southwest of downtown Seattle from Bay Street to S. Washington Street. It was built to provide level access to Seattle's piers and supports the Alaskan Way Viaduct and Alaskan Way itself, which is a surface street...

    • Galveston Seawall
      Galveston Seawall
      The Galveston Seawall is a seawall in Galveston, Texas, USA that was built after the Galveston Hurricane of 1900 for protection from future hurricanes. Construction began in September, 1902, and the initial segment was completed on July 29, 1904. From 1904 to 1963, the seawall was extended from ...

    • Georgetown Seawall
      Georgetown Seawall
      The most famous stretch of seawall in Guyana is the Georgetown Seawall.Seawall is the name given to the wall of concrete built along the foreshore with the sea in Guyana, mostly in Demerara. Earth walls are called sea-dams....

    • Gold Coast Seawall
    • Saemangeum Seawall
      Saemangeum Seawall
      The Saemangeum Seawall, located on the southwest coast of the Korean peninsula, is the world's longest man-made dyke, measuring 33 kilometres. It runs between two headlands, and separates the Yellow Sea and the former Saemangeum estuary....

    • Vancouver Seawall
      Seawall (Vancouver)
      The seawall in Vancouver, Canada is a stone wall that was constructed around the perimeter of Stanley Park to prevent the erosion of the park's foreshore. Colloquially, the term also denotes the pedestrian, bicycle, and roller blading pathway on the seawall, and which has been extended far outside...

    • The Embarcadero (San Francisco)
      The Embarcadero (San Francisco)
      The Embarcadero is the eastern waterfront and roadway of the Port of San Francisco, San Francisco, California, along San Francisco Bay, constructed atop an engineered seawall on reclaimed land, and derives its name from the Spanish verb embarcar, meaning "to embark"...

    • Hard Engineering
      Hard engineering
      In civil engineering of shorelines, hard engineering is generally defined as controlled disruption of natural processes by using man-made structures.-Examples:...

    • Coastal Management
      Coastal management
      In some jurisdictions the terms sea defense and coastal protection are used to mean, respectively, defense against flooding and erosion...


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