Seawall
A seawall 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. 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, and value 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. 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. 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.
Types
A seawall works by reflecting incident wave energy back into the sea, thus reducing the energy available to cause erosion. 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. Seawalls may also accelerate the erosion of adjacent, unprotected coastal areas by affecting the littoral drift process.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. There are three main types of seawalls: vertical, curved, stepped, and mounds.
Natural barriers
A report published by the United Nations Environment Programme 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. 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 | Disadvantages |
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Issues
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. The most recent analyses of long, good-quality tide gauge records 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 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 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. 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. 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 that will stand the force of coastal storms is difficult and, often the outcome can become unaffordable. For example, the Omaha Beach seawall in New Zealand 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 when compared to natural shorelines, which can present issues for species that reside on the seafloor.The Living Seawalls project, which was launched in Sydney, Australia, in 2018, aims to help many of the marine species in Sydney Harbour to flourish, thus enhancing its biodiversity, by modifying the design of its seawalls. It entails covering parts of the seawalls with specially designed tiles that mimic natural microhabitats – with crevices and other features that more closely resemble natural rocks. In September 2021, the Living Seawalls project was announced as a finalist for the international environment award the Earthshot Prize. Since 2022 it has become part of Project Restore, under the auspices of the Sydney Institute of Marine Science.
Other issues
Some further issues include a lack of long-term trend data of seawall effects due to a relatively short duration of data records; modeling 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 2013, more than 5,000 feet of seawall was found to be crumbling in Punta Gorda, Florida. Residents of the area pay hundreds of dollars each year for 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 BCE, Romans built a seawall or breakwater at Caesarea Maritimacreating an artificial harbor. The construction used Pozzolana concrete which hardens in contact with seawater. 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 Mediterranean Sea off the coast of Israel. Boulders were positioned in an attempt to protect the coastal settlement of Tel Hreiz from 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 a sand cover in 2012.
More recently, seawalls were constructed 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. 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 tsunamis 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.