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 human habitation, conservation, and leisure activities from the action of tides, waves, or tsunamis.Kamphuis, W J. (2010) Introduction to Coastal Engineering and Management. World Scientific Publishing Co Ltd. Singapore. 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.
Seawall design factors in 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 that protect the coast from erosion. Various environmental issues may arise from the construction of a seawall, including the disruption of sediment movement and transport patterns.Kraus, N & McDougal. (1996) The Effects of Seawalls on the Beach: Part I: An Updated Literature Review in Journal of Coastal Research. Vol. 12, No. 3. Combined with a high construction cost, this has led to increasing use of other soft engineering coastal management options such as beach replenishment.
Seawalls are constructed from various materials, most commonly reinforced concrete, boulders, steel, or gabions. Other possible construction materials include vinyl, wood, aluminum, fiberglass composite, and biodegradable sandbags made of jute and coir.Clarke, J R. 1994. Integrated Management of Coastal Zones. Fao Corporate Document Repository, USA. In the UK, 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.Hsiung AR, Tan WT, Loke LHL, Firth LB and others (2020) Little evidence that lowering the pH of concrete supports greater biodiversity on tropical and temperate seawalls. Mar Ecol Prog Ser 656:193-205
Types A seawall works by reflecting incident wave energy back into the sea, thus reducing the energy available to cause erosion.Kajendra, R. (2011) Seawalls have two specific weaknesses. Wave reflection from the wall may result in hydrodynamic scour and subsequent lowering of the sand level of the fronting beach.Masselink, G and Hughes, M J. (2003) Introduction to Coastal Processes and Geomorphology. Oxford University Press. New York. Ch 11. Seawalls may also accelerate the erosion of adjacent, unprotected coastal areas by affecting the littoral drift process.NOAA. (2007) Shoreline Management: Alternatives to Hardening the Shore. Retrieved online 15 April 2011 from: http://coastalmanagement.noaa.gov/shoreline.html
Different designs of man-made tsunami barriers include building reefs and forests to above-ground and submerged seawalls. Starting just weeks after the disaster, in January 2005, India began planting Casuarina and coconut saplings on its coast as a natural barrier against future disasters like the 2004 Indian Ocean earthquake. Studies have found that an offshore tsunami wall could reduce tsunami wave heights by up to 83%.
The appropriate seawall design relies on location-specific aspects, including surrounding erosion processes.GeoResources. (2001) Coastal management. Retrieved online 18 April 2011 from: There are three main types of seawalls: vertical, curved or stepped, and mounds (see table below).
+ Seawall types Type Illustration Advantages Disadvantages Example Vertical Vertical seawalls are built in particularly exposed situations. These reflect wave energy. Under storm conditions a non-breaking standing wave pattern can form, resulting in a stationary clapotic wave which moves up and down but does not travel horizontally. These waves promote erosion at the toe of the wall and can cause severe damage to the seawall. In some cases, piles are placed in front of the wall to lessen wave energy slightly. The first implemented, most easily designed and constructed type of seawall. Vertical seawalls deflect wave energy away from the coast. Loose rubble can absorb wave energy. These can suffer 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. Curved Curved or stepped 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. 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. Mound Mound type seawalls, using revetments or 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. Current designs use porous designs of rock, concrete armour. Slope and loose material ensure maximum dissipation of wave energy. Lower cost option. Shorter life expectancy. Cannot withstand or protect from high-energy conditions effectively.
Natural barriers A report published by the United Nations Environment Programme (UNEP) suggests that the tsunami of 26 December 2004 caused less damage in the areas where natural barriers were present, such as mangroves, coral reefs or coastal vegetation. A Japanese study of this tsunami in Sri Lanka used satellite imagery modelling to establish the parameters of coastal resistance as a function of different types of trees. Satellite imagery and modelling show how forests cushion the impact of tsunamis Natural barriers, such as coral reefs and mangrove forests, prevent the spread of tsunamis and the flow of coastal waters and mitigated the flood and surge of water.
Trade-offs A cost-benefit approach is an effective way to determine whether a seawall is appropriate and whether the benefits are worth the expense. Besides controlling erosion, consideration must be given to the effects of hardening a shoreline on natural coastal ecosystems and human property or activities. A seawall is a static feature which can conflict with the dynamic nature of the coast and impede the exchange of sediment between land and sea. The table below summarizes some positive and negative effects of seawalls which can be used when comparing their effectiveness with other coastal management options, such as beach nourishment.
+ Advantages and disadvantages of seawalls according to Short (1999)Short, A. (1999) Handbook of Beach and Shoreface Morphodynamics. John Wiley and Sons Ltd. Ch 7. Advantages Disadvantages 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 defense. Expensive to construct. May be considered aesthetically unattractive. 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. This can cause beaches to dissipate, rendering them useless for beach goers.
Generally, seawalls can be a successful way to control coastal erosion, but only if they are constructed well and out of materials that can withstand the force of ongoing wave energy. Some understanding is needed of the coastal processes and morphodynamics specific to the seawall location. Seawalls can be very helpful; they can offer a more long-term solution than soft engineering options, additionally providing recreation opportunities and protection from extreme events as well as everyday erosion. Extreme natural events expose weaknesses in the performance of seawalls, and analyses of these can lead to future improvements and reassessment.
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, with which the current seawall heights may be unable to cope.Allan, J C, Kirk, R M, Hemmingsen, M & Hart, D. (1999) Coastal Processes in Southern Pegasus Bay: A Review – A Report to Woodward-Clyde New Zealand Ltd. and the Christchurch City Council. Land and Water Studies Ltd. Christchurch. 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 Intergovernmental Panel on Climate Change (IPCC) (1997)Intergovernmental Panel on Climate Change. 2007. IPCC Fourth Assessment Report: Climate Change 2007. Retrieved Online 15 April 2011 from: www.ipcc.ch/publications_and_data/publications_and_data_reports 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)Hannah, J. (1990) "The Analysis of Mean Sea Level Data from New Zealand for the Period 1899–1988" in The Journal of Geophysical Research, 95, No. 88. who calculated similar statistics including a rise of between +16-19.3 cm throughout 1900–1988. Superstorm Sandy of 2012 is an example of the devastating effects rising sea levels can cause when mixed with a perfect storm. Superstorm Sandy sent a storm surge of 4–5 m onto New Jersey's and New York's barrier island and urban shorelines, estimated at $70 billion in damage.Link test, "Sea-Level Rise Implications for Coastal Protection from Southern Mediterranean to the USA Atlantic Coast." EGU General Assembly Conference Abstracts. Vol. 15. 2013. This problem could be overcome by further modeling 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 of flooding and taller tsunamis.
Hydrostatic water 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