Sea level rise has occurred throughout the 20th and 21st centuries and will continue to rise due to climate change [1]. The magnitude of future sea level rise is uncertain and depends on differing climate adaptation pathways. However, we do know that the level of rise will be substantial [1]. By 2100, global sea levels could rise by more than one meter when the loss of glaciers is considered [2,3]. In some cases, sea level rise could be even more extreme [4]. In Australia, sea levels have risen at an average rate of 1.6 mm annually over the past four decades, and it is projected to be 0.52–0.98 m by 2100 for the highest greenhouse gas emissions scenario [5]. According to the IPCC’s sixth report, the average rate of sea level rise increased about three times between 2006 and 2018 [3].

Worldwide, there are 680 million people living in low-lying coastal zones that are threatened by rising sea levels. With current urbanization trends, three times this population is expected to live and be located in coastal areas by 2050. In total, 50% of the world’s population will be within 100 km of coastal areas. As a coastal country, Australia conforms to these trends, with more than 50% of its population living within 7 km of the coast [6]. In the next forty years, another 6.8 million people are expected to inhabit coastal areas due to migration and population growth [7]. Within this group, approximately six per cent of Australian addresses are within three kilometers of the shoreline and in areas that are less than five meters above mean sea levels [6].

To support coastal settlements, extensive infrastructure is also located in coastal zones. Such coastally located infrastructures are particularly susceptible to sea level rise and its associated risks, including coastal flooding, seawater intrusion and storm surges. With future sea level rise, such events will be intensified [1,8]. Due to sea level rise and the related risks and impacts on infrastructure systems, coastal societies face multiple evolving hazards, including the immobilization of transportation, blackouts, saltwater intrusion of water supplies and cascading failures that can reverberate throughout the whole settlement system [9,10,11,12]. Insensitive urban development can interact with climate-related risks to exacerbate environmental vulnerability and intensify socio-ecological inequity [13,14].

Wastewater treatment plants (WWTPs) are particularly vulnerable when compared with other coastal infrastructures due to their inherent characteristics. For example, in order to minimize the need for energy and cost, coastal WWTPs are typically located at low elevations to collect the consumed water before discharging it through pipelines to adjacent water bodies under the action of gravity [15]. WWTPs are also particularly centralized [16,17] when compared with other infrastructure, such as transport and power generation systems, which have in-built redundancy. Therefore, WWTPs are more vulnerable due to their centralized structure [18]. Lastly, as WWTPs depend on transport and energy systems to function and be maintained, the failure of such systems can also have knock-on effects, leading treatment systems to fail [18]. If WWTPs services are interrupted or damaged due to sea level rise, extensive populations can be affected well beyond the zones directly inundated [18].

There is a range of threats and risks related to coastally located WWTS. According to such current research, the foremost threat to WWTPs is flooding caused by climate-change-related sea level rise [18]. As sea levels rise, WWTPs situated in coastal areas with low elevation may be subjected to permanent flooding or frequent nuisance flooding due to high tide levels, other extreme weather events, such as storm surges, or a combination of these. In addition to the flooding exposure of coastal WWTPs, increased sea levels can also block outfalls from the system or reduce the efficiency of discharge [18]. Without the assistance of additional or larger pumps and pipelines, the flow rates will cease or decrease, causing siltation and effluent backflow, leading to further maintenance and repair costs [15]. Furthermore, the debris left by the flooding may also block the inlets or outlets of pipelines and cause major damage [15].

In addition to flooding, coastal storms are one of the most serious threats to coastal communities, resulting in huge human and economic losses every year [37,38,39]. The risk to WTTS due to flooding caused by coastal storms has been recognized as a worldwide problem. For example, in September 2004, Hurricane Ivan produced a 4-m-plus storm surge in Pensacola, USA, which resulted in the local WWTP experiencing four days of significant flooding and power outages [40]. More recently, a storm hit Colorado in 2013. The WWTP was breached, and massive quantities of untreated wastewater polluted the sea [41]. In recent decades, extreme coastal storms and related hazards have intensified due to SLR and show an upward trend. In Australia, about 87% of the total economic damage each year is caused by weather-related factors, mostly due to floods, storms and tropical cyclones [42]. With future climate change and SLR expected to continue, extreme events will continue to intensify and become more frequent. In the next 100 years, Australia will experience increasing coastal vulnerability to its many WWTS (Figure 1) and other infrastructure due to climate-change-related events [43]. As summarized in Table 1, sea level rise poses a number of direct threats and impacts on WWTPs.

Within South Australia, wastewater treatment systems are frequently in close proximity to coastal ecosystems. Such wetlands typically include seagrasses, intertidal mangroves and supratidal samphire or salt marsh vegetation. Coastal ecosystems, especially mangroves, are well known for their ecological importance, providing breeding and nursery sites for crustaceans, shellfish, fish, birds and mammals. In addition, they contribute a range of significant economic and social services as a form of green–blue infrastructure. For example, they help ameliorate greenhouse gas emissions through carbon sequestration. According to Danielsen [50], although mangroves account for only 0.5% of coastal areas worldwide, they contribute 10–15% in coastal sediment carbon storage and export 10–11% of terrestrial particulate carbon to the ocean. They also help to significantly attenuate wave energy to mitigate coastal storm surge hazards. Experimental models have demonstrated that 30 mangrove trees per 100 m² in a 100 m wide belt have the potential to attenuate the maximum tsunami flow pressure by more than 90% [51]. More recently, Sun and Carson [39] quantified the disaster cost avoidance savings delivered by mangroves and coastal wetlands. However, coastal ecosystems are under threat globally due to climate change and other human-related impacts, such as land use change. Alongi [52] indicates that one-third of mangrove forests have disappeared during the last 50 years. Such environmental degradation processes will exacerbate sea level rise and increase the number of threats related to coastal WWTPs and other infrastructure.

One of the most serious factors associated with sea level rise resulting in coastal wetland system loss has been described as ‘coastal squeeze’ [53,54,55]. Even though mangroves and other coastal ecosystems often have the capacity to migrate as sea levels rise, such inland movement is often blocked or “squeezed” by natural or built topographic conditions [55,56]. For example, the inland movement of coastal ecosystems can be blocked by topographic features, such as embankments, roads, seawalls or natural dune systems [56,57,58]. We, therefore, consider such coastal ecosystems as a critical element in developing and shaping future coastal landscapes.