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.

Research Priorities for Coastal Geoscience and Engineering: A Collaborative Exercise in Priority Setting From Australia

“…Australia's coast is also particularly vulnerable to the effects of climate change with increases in sea level, changes in wave climate and storminess, and ocean warming all expected to have significant and, in some areas, above global average impacts in Australia (Department of Climate Change, 2009;Gurran et al, 2011;Ranasinghe, 2016;IPCC, 2019). Indeed, these impacts are beginning to emerge (e.g., Harley et al, 2017;Hanslow et al, 2019;Hague et al, 2020) and are forecast to escalate in coming decades (e.g., Kinsela et al, 2017;Hanslow et al, 2018;Holper et al, 2018;Seashore Engineering, 2019;Ware et al, 2020). Consequently, it is essential to identify the key research priorities that will allow societies to address these major global challenges.…”

Identifying oceanographic conditions conducive to coastal impacts on temperate open coastal beaches and the importance of empirical impact data

“…Studies have further shown that the height of ESLs could increase further than that which will occur due to global mean sea level (GMSL) rise in some locations (Pickering et al 2017;Devlin et al 2017;Harker et al 2019;Muis et al 2020;Meucci et al 2020). For example, research has noted the importance of understanding the contribution of storms (Stephens et al 2020), astronomical tides (Hague et al 2020) and wave climate conditions (O'Grady et al 2019a) to the total water level and resulting coastal impacts (Church et al 2006;Ranasinghe 2016;Hague et al 2020). Individually, each factor may not be considered extreme but can compound to an ESL event which causes greater impact to the coast (Church et al 2006;Hague et al 2020).…”

“…McInnes et al 2016;O'Grady et al 2019a;Meucci et al 2020;Kirezci et al 2020) or their impact (e.g. Hague et al 2019Hague et al , 2020, with both approaches giving unique insights into the spatial and temporal scales of events. Climate change is likely to change ESL drivers, which, particularly in concurrence with rising mean sea level, will result in an increase in adverse/damaging coastal impacts (Church et al 2006).…”

“…For example, research has noted the importance of understanding the contribution of storms (Stephens et al 2020), astronomical tides (Hague et al 2020) and wave climate conditions (O'Grady et al 2019a) to the total water level and resulting coastal impacts (Church et al 2006;Ranasinghe 2016;Hague et al 2020). Individually, each factor may not be considered extreme but can compound to an ESL event which causes greater impact to the coast (Church et al 2006;Hague et al 2020).…”

“…A similar pattern is found in modelling tsunami impact in Australia (Allen and Greenslade 2016) and inundation in the Marshall Islands (Smith and Juria 2019). De ning impact-based thresholds (Hague et al, 2019(Hague et al, , 2020Smith and Juria 2019) provides the potential for quantifying the collective effects of waves, storm surges and tides in a uni ed impact-based framework, even if the individual effects of these drivers are minor (McInnes et al 2016). It would also allow for site speci c warning approaches to be developed, not unlike those currently in place for riverine ooding.…”