Ecosystems and inhabitants of coastal zones, low-lying areas, and small island nations are exposed to a variety of natural hazards such as coastal erosion, coastal flooding, submergence, saltwater intrusion and tsunamis as a result of storm surges and extreme weather events. Due to climate change induced effects, the likelihood of these naturally hazardous events will increase, making coastal areas more vulnerable to land loss, infrastructure damages and displacement.  According to the United Nations, approximately 40% of the world's population lives within 100 kilometres of a coast and 10% live in coastal areas that are less than 10 metres above sea level (United Nations, 2017).

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Sea level rise is regarded as the most significant risk to both human and natural systems in coastal low-lying areas due to climate change (Nurse et al., 2014; Wong et al., 2014). Increasing greenhouse gases (GHGs) in the atmosphere produce changes in the climate system on a range of time scales that impact the coastal physical environment through sea level rise. Sea level rise and the resulting extreme sea levels in coastal areas are mainly driven by ocean thermal expansion and the loss of ice mass from glaciers and ice sheets in Greenland and Antarctica (Vousdoukas et al., 2018). On shorter time scales, physical coastal impacts such as inundation, erosion, and coastal flooding arise from severe storm surges, wave overtopping and rainfall runoff (Wong et al., 2014). A storm surge is an episodic increase in sea level driven by shoreward wind-driven water circulation and atmospheric pressure (Vousdoukas et al., 2018).  On longer time scales, climate change impacts on wind and waves can cause changes in sediment transport at the coast and associated changes in erosion or accretion (Wong et al., 2018).

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Throughout the past century the global mean sea level has risen at approximate rates of 1.3mm to 1.7mm per year (Nurse et al., 2014). Since 1993, it has accelerated to rates between 2.8 and 3.6mm per year (Nurse et al., 2014). Future climate projections suggest sea level rise on the order of 0.5 to 2.0m by 2100 (Vousdoukas et al., 2018; Wong et al., 2014; Storlazzi et al., 2011). Climatic and sea level projections are essential for future risk assessment, coastal planning and the design of coastal protection.

CURRENT AND PROJECTED IMPACTS

Extreme Weather Events

• Increasing seawater temperatures provide more energy for storms that develop at sea, leading to fewer but more intense tropical cyclones globally (United Nations, 2017).

• 90% of natural disasters are classed as climate related, cost the world economy US$520 billion per year, and push 26 million people into poverty each year (United Nations, 2017).

• Since 2008, an annual average of 21.5 million people have been forcibly internally displaced by sudden weather-related hazards (United Nations, 2017).

Impacts on Human Settlements

• For 136 port cities over 1 million inhabitants, the number of people exposed to extreme sea level is expected to increase from 39 million in 2005 to 59 million by 2070 given an increase in global mean sea level rise of 0.5 metres. This number increases to 148 million if socio-economic development is considered (Wong et al., 2014).

• 72 to 187 million people are expected to be displaced due to land loss and erosion by 2100 assuming global mean sea level rise of 0.5 to 2.0 m. Upgrading coastal defences would reduce these impacts roughly by three orders of magnitude (Wong et al., 2014)

• Sea level rise of half a metre could displace 1.2 million people from low-lying islands in the Caribbean Sea and the Indian and Pacific Oceans (United Nations, 2017).

Impacts on Infrastructure, Industry, and Transport

• Hurricanes like Katrina (2005) caused US$100 million of damage to Mississippi’s ports and Sandy (2012) led to a week-long shut-down of the Port of New York, generating economic damages reaching US$50 billion (Wong et al., 2014)

• The potential costs associated with damage to harbours and ports due to sea level rise could be as high as $US111.6 billion by 2050 and $US367.2 billion by the end of the century (United Nations, 2017).

• By 2070, asset exposure is estimated to increase to approximately 9% of projected global GDP given a sea level rise of 0.5m, a 10% increase in extreme water levels, and a fixed subsidence rate in susceptible cities (Wong et al., 2014).

• Rebuilding costs of tourist resorts in Caribbean countries are estimated to reach US$10 to 23.3 billion in 2050 (Wong et al., 2014).

• Sea level rise of 1 metre would result in the loss or damage of 21 airports, inundation of land surrounding 35 ports, and damages and losses of at least 149 multi-million dollar tourism resorts (Wong et al., 2014).

Impacts on Ecosystems

• The effects of sea level rise will impact coastal ecosystems through coastal erosion, wetland and coastal plain flooding, salinization of aquifers and soils, and a loss of habitats for fish, birds, and other wildlife and plants (Plag & Jules-Plag, 2013).

• The IPCC estimates that a continuation of the current global sea level rise would likely lead to a loss of as much as 33% of coastal land and wetland habitats in the next hundred years (Plag & Jules-Plag, 2013).

COASTAL PROTECTION AND ADAPTATION

Without adaptation, hundreds of millions of people will be affected by coastal flooding and will be displaced due to land loss by year 2100. The majority of those affected are from East, Southeast, and South Asia. The most economically vulnerable countries are low-lying developing countries like Bangladesh, Vietnam and Small Island Development States. They are expected to face associated annual damage and adaptation costs of up to 10% of their GDP. However, the benefits of protecting against increased coasting flooding and land loss at the global scale are larger than the social and economic costs of inaction (Wong et al., 2014).

Although the potential global impacts of coastal flood damage and land loss on human settlements in the 21st century are substantial, these impacts can be reduced considerably through coastal protection. Natural storm surge barriers exist, such as sand dunes, mangroves and coral reefs, but with massive global coral bleaching and loss of exposed fringing reef flats (Storlazzi et al., 2011), new approaches and engineered solutions are required.

NATURAL REEFS

Coral reefs are submerged geologic marine formations. Although typically located in shallow, tropical waters, there are also deep-sea corals that are located around the globe. By definition, a reef refers to any submerged marine navigation barrier that is 18m in depth or less (David and Brannet, 2010). Shallow reef-building corals live in strict environmental conditions of warm, relatively clear and saline waters. Their ecosystems are broken into 3 sections; the fore-reef, reef crest, and reef flat, also known as a lagoon. The fore-reef is the seaward side of the reef, which drops deeper than the suitable habitat for corals. Corals build reef structures known as reef crests that are built up by ramparts. Latypov (2016) describes these coral ramparts as massive rounded coral heads and robust branching corals, which build a rigid cavernous palisade of intergrown coral skeletons. These major features reach up towards the surface of the water, growing vertically and, depending on sea water levels, horizontally. They have the ability to span tens to thousands of metres wide parallel to coastlines, therefore providing structural habitats for vertebrate and invertebrate species. Crests may touch the shoreline but usually enclose reef flats and sandy lagoons (Latypov, 2016). The shallow reef flats, protected by the crests, are home to corals forming massive buttresses separated by narrow sandy channels, which pass a steady flow of sediment made from dead coral, calcareous algae and other organisms (Latypov, 2016). The crest and canyon pattern create a rugged terrain, which helps sediment flow and dissipation of wave energy through friction.

REEFS AS BREAKWATERS

Coral reefs are a natural form of defence from erosion and flooding of coastal communities. Through wave attenuation and the production and retention of sand, natural reef crests function similar to breakwaters by dissipating wave energy and protecting the shoreline (Borja et al., 2018). The friction caused by the rigid terrain of the reef flat and the initial break over the reef crest significantly reduces the energy of offshore waves (Elliff and Silva, 2017).  Reefs can significantly reduce wave energy across 3 reef environments, the reef crest, the reef flat and the reef as a whole. Studies by Ferrario et alia (2014) have shown that reef crests dissipate on average 86% of incident wave energy, with the reef flats dissipating 65% of the remaining wave energy, making the whole reef have a total energy reduction of 97%. It should be noted that the whole reef dissipating 97% of wave energy is consistent from small daily wave events to sea swells and hurricane level waves. Along with wave energy, wave height is significantly reduced along all 3 reef environments. Wave height is reduced 64% of the incident height by the reef crest, 43% of the remaining height by the reef flat and 84% by the whole reef. Reduction of wave energy and height occurs mostly within 150m of the reef crest (Ferrario et al, 2014).

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Natural habitats are currently helping to protect coastlines, yet climate change and human interference are destroying coral reef ecosystems. The restoration and protection of coral reefs is necessary, however current efforts may be lacking in comparison to the current rates of climate change. As a mitigation effort, artificial reefs are being constructed as submerged breakwaters in an effort to reduce offshore sediment transport by rip currents, sediment loss to the continental shelf, and promote slow beach recovery, creating a buffer zone for coastal infrastructure. Borja et alia (2018) use a 3 phase approach to study the role of reefs in shoreline protection. First is an analysis of historical changes in the shoreline, sea levels, wave climate, etc. Secondly is an assessment of the role of coral reefs in shoreline positioning. Lastly, a reef-based solution is designed specific for the local coastlines, whether restoration-based or artificial design.