Sea-Level Rise Driving Increasingly Predictable Coastal Inundation in Sydney, Australia

Abstract

As global mean sea level continues to rise, thresholds corresponding to coastal inundation impacts are exceeded more frequently. This paper aims to relate sea level rise (SLR) observations and projections to their physical on-the-ground impacts. Using a large coastal city as an example, we show that in Sydney, Australia, frequencies of minor coastal inundation have increased from 1.6 to 7.8 days per year between 1914 and present day. We attribute over 80% of the observed coastal inundation events between 1970 and 2015 to the predominantly anthropogenic increases in global mean sea level. Further, we find that impact-producing coastal inundation will occur weekly by 2050 under high-and medium-emission/SLR scenarios and daily by 2100 under high emissions. The proportion of tide-only coastal inundation events (i.e., where no storm surge is required to exceed flood thresholds) will increase with SLR, such that most coastal inundation events, including those considered historically severe, will become a predictable consequence of SLR and astronomical tides. These findings are important for coastal managers as frequency, severity, and predictability of inundation impacts can all now be related to the amount of SLR (e.g., a planning allowance or SLR projection). By incorporating known historical inundation events, this allows contextualization, visualization, and localization of global SLR and the changing nature of future coastal inundation risk.Plain Language Summary As sea levels rise, the daily highest tide reaches higher and further inland, and as a result, we see coastal flooding more frequently. Coastal flooding is when roads, carparks, walking paths, gardens, and, in more extreme cases, homes and businesses are impacted by high sea levels. Using Sydney, Australia, as an example, we find that most coastal flooding events we observe today would not have happened without human-caused sea level rise. Further, coastal flooding is expected to occur in Sydney on average once per week by 2050 and every day by 2100 if high greenhouse gas emissions continue. In the past, the most severe coastal flooding impacts, such as flooding of main roads and private property, only occurred with large coastal storm events. However, we find that these severe floods will occur much more frequently as sea levels continue to rise, as they will eventually occur on the daily high tides. As the timing and heights of daily high tides are driven by the Sun, Moon, and the seasons, these severe coastal floods will become very predictable. This will have implications for the coastal and emergency managers tasked to deal with this changing risk.

Australian Coastal Flooding Trends and Forcing Factors

“…This means that as sea level rises, not only will the frequency of minor coastal floods increase, in a matter of decades moderate floods could occur as frequently as minor floods do today. Hence, this also highlights the importance of defining coastal flood tipping points (e.g., Thompson et al., 2021) and emergence times of frequent flooding (e.g., Hague et al., 2020). We know, for example, that areas subject to regular riverine flooding rapidly become uninsurable (Box et al., 2013).…”

“…(2018), the difference between these two definitions is typically negligible on results, as Case 4 occurrences are often much less common than Case 3 occurrences. However, in locations where tides’ influence on flooding is most pronounced (e.g., Sydney, Australia) Case 4 events can account for one‐third of all tide‐only (i.e., Cases 3 and 4) threshold exceedances (Hague et al., 2020). Given our focus is on determine the cause of specific floods, the benefits of having mutually exclusive and exhaustive subsets allow for more robust findings.…”

“…The degree to which flood frequency will increase is dependent on greenhouse gas emission pathways (Fox‐Kemper et al., 2021) and whether actions can be taken to defend assets, such as raising the elevations of assets and hence, increasing flood thresholds. Under high emissions and without flood risk mitigation actions, flooding in Sydney will become regular and predictable (i.e., chronic) by mid‐century (Hague et al., 2020). In Australia's most populous state, New South Wales, Hanslow et al.…”

“…Our interest is in examining the impact of these rising sea levels on changes to coastal flood frequency around Australia, especially minor flooding for which the most rapid frequency changes are currently occurring (Hague et al., 2020; Thompson et al., 2021). In this context, minor, or nuisance (e.g., Moftakhari et al., 2018), coastal flooding is when still water levels (i.e., the water levels measured by tide gauges in sheltered bays, harbors, and estuaries) cause inundation of low‐lying assets such as streets or roads, paths, gardens or parks, or buildings below the floor level (Table S1 in Supporting Information S1; Hague et al., 2019).…”

Climate Change and Changes in Compound Coastal‐Riverine Flooding Hazard Along the U.S. Coasts

“…To explore high flood levels due to the concurrence of high SLs and river flows at a large scale, metrics should be based on consequences rather than just probability (Hague et al. 2020). In many situations, there is no exact correspondence between the extremeness of flood events based on return periods (i.e., probability of occurrences) and the impact they cause.…”