Baseline minor, moderate and major flood rates at Australian coastal gauges

Minor, moderate, and major floods have occurred episodically in Australian estuaries over the last two decades. Half of the locations experience minor flooding no more than once every 4.5 years on average. The top 10% minor-flood-prone locations experienced minor flooding at least every other year on average. Moderate flooding occurs once a decade on average across the 75 gauges. In other words, in an average year, 7 or 8 of the 75 gauges expect to see moderate flooding. Major flooding has occurred at 31 of 75 gauges over their observational records (Supplementary Table S1). Minor, moderate, and major flooding occurs at twice the frequency at NSW gauges than at Queensland gauges. A previous study argued that NSW has lower coastal flood hazards than other states as storm surges and extreme rainfall are largely independent there (Wu et al., Reference Wu, McInnes, O’Grady, Hoeke, Leonard and Westra2018). This is not true based on observed flood rates. Other more impact-based approaches may be better suited for identifying spatial patterns in flood hazards across national and regional domains than metrics based solely on water levels or drivers.

In years where minor flooding occurs, it generally only occurs on a small number of days in that year. Half of all locations average no more than 4.2 days of minor flood threshold exceedance in years when flooding occurs. These floods are predominantly driven by fluvial factors, with compounding coastal influences in some circumstances, as reflected in the Bureau’s Service Level Specification for the warning services thresholds were defined for. The rareness of minor, moderate and major floods in the present climate adds further weight to our argument that the influence of riverine factors on the total number of coastal floods will be negligible once sufficient SLR occurs for these thresholds to be exceeded by tides alone (e.g., Figure 2d).

Emergence of tide-only flooding

Analysis of freeboards (Supplementary Table S1) indicates that many locations that presently only experience compound riverine-coastal flooding will experience flooding due to tides alone this century, based on current SLR planning guidelines (McInnes et al., Reference McInnes, Church, Monselesan and Zhang2015; Norman and Gurran, Reference Norman and Gurran2018). Minor-HAT freeboard is the SLR required for the emergence of tide-only flooding (Hague and Taylor, Reference Hague and Taylor2021). Thirty locations have minor-HAT freeboards of less than 0.7 m. Of these, 11 have minor-HAT freeboards of less than 0.2 m. These locations are expected to see the emergence of tide-only flooding in the coming decades (Fox-Kemper et al., Reference Fox-Kemper, Hewitt, Xiao and Yu2021). Four gauges expect tide-only minor flooding with less than 0.1 m additional SLR (Figure 3a) – Mooloolaba (Sunshine Coast, QLD), Stockton (Newcastle, NSW), Tempe Bridge (Sydney, NSW) and Brisbane Port Office (QLD). The emergence of moderate and major tidal floods is also expected under 0.7 m SLR. 12 locations have moderate-HAT freeboards of less than 0.7 m (Figure 3b). Four locations expect major tide-only flooding to emerge with SLR amounts less than 0.7 m (Figure 3c) – Mooloolaba (0.42 m), Stockton (0.54 m), Picnic Point (Maroochy River, QLD) (0.61 m), and Lakes Entrance (Victoria) (0.62 m).

Figure 3. HAT-freeboards associated with (a) minor, (b) moderate, and (c) major flood thresholds. These equal the SLR for tidal flooding to emerge at each severity. Differences between (d) HAT and AHAT, and (e) AHAT and MHAT. These equal the additional SLR for tidal flooding to occur on average once per year and once per month, respectively, once tidal flooding has emerged. Results are also provided in Supplementary Table S1. Barrack St (Perth, Western Australia) is not shown.

The 30 locations where 0.7 m SLR leads to the emergence of tide-only flooding represent 40% of all studied locations. Given not all tidal estuaries in Australia have official flood thresholds it is likely that other locations not identified here may also be expected to see the emergence of tidal flood regimes this century. Further, offsets between flood thresholds are less than 0.3 m in several locations (Supplementary Table S1). This is less than the Australian standard for DEM vertical accuracy over bare ground (Wilson and Power, Reference Wilson and Power2018). Verification of DEM-derived flood thresholds may be required to ensure the resultant flood hazard assessments are reliable (e.g., Habel et al., Reference Habel, Fletcher, Anderson and Thompson2020). The freeboard-based methods demonstrated in this study provide a method to identify locations where higher-accuracy local-scale modelling may be required to inform adaptation pathways (Hanslow et al., Reference Hanslow, Fitzhenry, Hughes, Kinsela and Power2023). For example, digital elevation models based on LiDAR data from unmanned aerial vehicles (UAVs) can achieve vertical errors less than 0.1 m (Chen et al., Reference Chen, Johnson, Drummond and Glamore2020). These local-scale models can assess if adaptation triggers may be reached before flooding becomes chronic or tidal (Buchanan et al., Reference Buchanan, Oppenheimer and Parris2019; Allison et al., Reference Allison, Stephens, Blackett, Lawrence, Dickson and Matthews2023).

Establishment of tidal and chronic flood regimes

Once tide-only flooding emerges, flood frequencies increase rapidly and predominantly tidal flood regimes become established (Sweet and Park, Reference Sweet and Park2014; Thompson et al., Reference Thompson, Widlansky, Hamlington and Sweet2021; Hague et al., Reference Hague, McGregor, Jones, Reef, Jakob and Murphy2023b). We show this is also true for Australian estuaries, as there are small differences between HAT-, AHAT- and MHAT-freeboards (Figure 3d,e and Supplementary Table S1). Across all 75 locations, none have more than 0.07 m offset between HAT and AHAT, with an average of less than 0.04 m. This means it takes only an additional 0.07 m SLR for tide-only flooding to occur annually following its first occurrence. The 12 locations that have moderate-HAT freeboards less than 0.7 m, also have moderate-AHAT freeboards less than 0.7 m. Only 0.11 m further SLR leads to tide-only flooding occurring monthly on average. Therefore, less than 0.2 m SLR is enough for tidal flooding to go from being non-existent to chronic. Following from the freeboard and SLR equivalence shown in Figure 1, DEMs that conform to the Australian standard of 0.3 m vertical accuracy likely cannot distinguish between areas with no tidal flooding and those with chronic tidal flooding. Further, 0.2 m is only two decades’ worth of SLR (under a higher SLR scenario) if tidal flooding emerges after 2050 (Fox-Kemper et al., Reference Fox-Kemper, Hewitt, Xiao and Yu2021). This result applies regardless of flood severity as the difference between major-MHAT and major-HAT freeboards is the same as the difference between minor-MHAT and minor-HAT freeboards (Supplementary Table S1).

Observed exceedances of AHAT and MHAT are much higher than their expected frequency of exceedance under tides alone. For example, AHAT and MHAT are exceeded on average 28 and 61 days per year. These annual flood rates are comparable to accepted definitions of how frequently flooding needs to occur to be considered chronic (Sweet and Park, Reference Sweet and Park2014; Thompson et al., Reference Thompson, Widlansky, Hamlington and Sweet2021; Gold et al., Reference Gold, Anarde, Grimley and Hino2023). Freeboard metrics provide robust estimates of the maximum possible SLR amount that will lead to the emergence of tide-only flooding of some frequency. However, it is likely that less SLR will be sufficient for the establishment of chronic flood regimes. First, storm surges lead to higher numbers of coastal floods than expected under the average weather conditions implied in harmonic tidal analysis (Hague et al., Reference Hague, Jones, Jakob, McGregor and Reef2022, Reference Hague, McGregor, Jones, Reef, Jakob and Murphy2023b). Second, compound floods will continue to occur in addition to coastal floods due to tides alone, and coastal floods due to the combination of tides and surges. Third, increases in tidal range may have led to larger increases in HAT than expected due to mean SLR alone.

The establishment of chronic minor flood regimes in the lowest reaches of many estuaries is inevitable without flood risk mitigation that eliminates all flood impacts associated with exceedances of the present-day minor flood threshold. Global mean sea level rises exceeding 1 m are inevitable due to past emissions (DeConto et al., Reference DeConto, Pollard, Alley and Dutton2021; Box et al., Reference Box, Hubbard, Bahr and Fausto2022). Mooloolaba, Stockton (Newcastle), Picnic Point (Maroochy River), Lakes Entrance, Settlement Point (Port Macquarie) and Belmont (Lake Macquarie) all have Major-AHAT freeboards less than 1 m (Supplementary Table S1). This means that a future state where tides alone exceed historically significant flood peaks is a matter of when rather than if for these locations. The main sources of uncertainty over the timing of emergence of frequent flooding are how rapidly SLR responds to past and future emissions, and how flood thresholds relate to high water levels (i.e., freeboard). Metrics that do not consider how high water levels must be to cause flooding, such as sea level allowances and amplification factors (Buchanan et al., Reference Buchanan, Oppenheimer and Kopp2017; Woodworth et al., Reference Woodworth, Hunter, Marcos and Hughes2021), are therefore unlikely to be useful for assessing the potential for flood regime transitions in estuaries. The only locations with minor-AHAT freeboards less than 1 m are those several kilometres upstream, as flood thresholds increase upstream (Figure 3a). Figure 4 shows this phenomenon applies even more to moderate and major flood thresholds.

Figure 4. Flood thresholds decrease in height the closer to the river mouth a location is. Tidal ranges tend to increase in height the closer to the river mouth a location is. Examples are shown from (a) Brisbane River, (b) Richmond and Wilsons Rivers, and (c) Hunter River. All flood thresholds were reported in metres AHD in (a) and (c), and Richmond River Valley Datum in (b). Base map from Open Street Map.

Identifying characteristics of locations most at-risk of flood regime transition

Locations with smaller tidal ranges and lower freeboards are most at-risk of flood regime transition. First, the 12 locations that expect moderate tidal flooding with less than 0.7 m SLR (Figure 3b) are those where water level variability is smallest. The mean HAT–LAT tidal range of these “at risk” locations is 1.4 m, compared to 1.56 m for the overall mean. Both of these numbers are less than typical coastal tidal ranges (Hague et al., Reference Hague, Jones, Jakob, McGregor and Reef2022, Reference Hague, McGregor, Jones, Reef, Jakob and Murphy2023b), suggesting Australian estuaries may be in general at greater risk of chronic flooding than other coastal locations globally. Second, the same 12 locations all have minor-HAT freeboards of less than 0.4 m. This is much less than the average of all 75 locations, 1.34 m. This is also true for major-HAT freeboards. The average across all sites is 3.33 m, whilst the average across the 12 most at-risk sites is 0.86 m. Small tidal ranges and lower freeboards are known characteristics of locations identified as most at risk of chronic flooding (Hague et al., Reference Hague, McGregor, Jones, Reef, Jakob and Murphy2023b). Locations that experience the most frequent or severe flooding due to storm surges are not necessarily the same locations at risk of frequent future flooding. Analogously, our results suggest that whether a location experiences predominantly riverine or predominantly coastal flooding at present has little bearing on whether it will transition to experience chronic tidal flooding in the future. The locations most at risk in future are not necessarily the most at risk now. This means that assessments of present-day compound flood risk may be of limited use for assessing future compound flood risk.

Being situated immediately inside the mouth of a tidal river or lake may be another good indicator for locations’ potential for highest future compound flood risk. Heights of minor, moderate and major flood thresholds decrease with decreasing distance a gauge is from the estuary mouth in the Brisbane River (Figure 4a), Richmond and Wilsons Rivers (Figure 4b), and Hunter River (Figure 4c). This is also true in other rivers and estuaries where two or more gauges are installed (Supplementary Table S1). Concerningly, the locations most immediately inside the estuary mouth generally do not have official flood thresholds defined. This is also evident in Figure 4 – Newcastle, Ballina, and Brisbane’s airport, port area, and surrounding suburbs do not have defined flood thresholds. These are locations that could be the most at risk but could not be included in this study’s assessment. The lack of flood thresholds in these locations is due to flood warning services being targeted towards fluvial floods (Bureau of Meteorology, 2013), which have historically led to the largest impacts and losses (Callaghan and Power, Reference Callaghan and Power2014). These impacts have generally occurred further upstream where flood peaks (and hence, flood thresholds) are higher. We suggest that identifying large population centres just inside estuaries may be a useful first step for identifying possible gaps in water level datasets and impact databases. An example of how this can be done is shown for Ballina in the next section.

Previous studies have identified present-day flood hotspots as locations where large storm surges and large riverine discharges coincide temporally (Wu et al., Reference Wu, McInnes, O’Grady, Hoeke, Leonard and Westra2018; Couasnon et al., Reference Couasnon, Eilander, Muis and Ward2020; Nasr et al., Reference Nasr, Wahl, Rashid, Camus and Haigh2021). Several studies have argued that future changes in storm surges and riverine discharges will be the most important factors determining future flood hazards in these areas (Zscheischler et al., Reference Zscheischler, Westra, Van Den Hurk and Zhang2018; Bevacqua et al., Reference Bevacqua, Vousdoukas, Zappa, Hodges, Shepherd, Maraun, Mentaschi, Feyen and Feyen2020). Our results suggest otherwise. First, the emergence of tide-only flood cannot be captured if only considering storm surges and riverine discharges without astronomical tides (Ray and Foster, Reference Ray and Foster2016; Hague and Taylor, Reference Hague and Taylor2021). Second, estimating the emergence of tide-only flooding, or indeed any changes in flood frequency, depends on knowing the flood threshold (Rasmussen et al., Reference Rasmussen, Kulp, Kopp, Oppenheimer and Strauss2022; Hague et al., Reference Hague, McGregor, Jones, Reef, Jakob and Murphy2023b; Sun et al., Reference Sun, Dangendorf, Wahl and Thompson2023). The large variations in flood thresholds within the same river system (Figure 4) suggest that more factors than simply storm surge and riverine flow peak values determine flood thresholds. For example, the existence of assets that could be inundated during high water level events is an important factor in determining whether a flood hazard of some severity exists. Third, the locations which are most likely to experience future frequent flooding in Australia are not the same locations where large storm surges and large riverine discharges coincide. It could be expected that this is also true globally, given that small tidal range and lower freeboards are reliable global indicators for emerging chronic flood hazards (Hague et al., Reference Hague, McGregor, Jones, Reef, Jakob and Murphy2023b).

Assessing the potential for flood regime change without official flood thresholds: Ballina, NSW

The previous section showed that locations with large population centres just inside estuaries are likely most at risk of flood regime change with SLR. Ballina is a town of 46,000 people located at the mouth of the Richmond River (Figure 4b). It has no official flood thresholds defined, but experiences frequent nuisance tide-driven flooding due to high tides and storm surges (Hague et al., Reference Hague, Jones, Jakob, McGregor and Reef2022). One such coastal flood event occurring during king tides in January 2014 (Figure 5). Ballina experiences much more severe flooding during periods of high streamflow in the Richmond River, especially when these coincide with high tides. One such compound flood event occurred over 1–4 March 2022 (Bureau of Meteorology, 2022b; Dakin, Reference Dakin2022; Lerat et al., Reference Lerat, Vaze, Marvanek and Ticehurst2022; Figure 5). Impacts during this flood included loss of communications and electricity, damage to thousands of homes and businesses despite precautionary sandbagging, hundreds of emergency rescues despite an evacuation order, and relocation of patients to a makeshift hospital on higher ground (Ciccarelli et al., Reference Ciccarelli, Swain and Gilbert2023; Wahlquist and Tondorf, Reference Wahlquist and Tondorf2023). We use the March 2022 and January 2014 events to demonstrate that freeboard metrics can be applied robustly to “event thresholds” (i.e., the highest level recorded over a multi-day flood event), to assess the potential for flood regime changes in locations without official flood thresholds.

Figure 5. Images of flood impacts in Ballina with the location (or extents if aerial photograph) depicted in the image shown on the map. Upper images labelled with numbers from the 1–4 March 2022 compound flood event, reproduced with permission of local photographers Hover Images (i–iii), Aimee Keenan (1–6) and Frank Coughlan (7–9). Lower images labelled with letters are from 2 to 3 January 2014 king tide event, reproduced from Witness King Tides flickr, under CC-BY-2.0. Credit Tom Coster (A) and Garry Owers (B–E). Water level gauge locations are shown as stars – Byrnes Point (black), Missingham Bridge (red) and Ballina Breakwall (yellow).

Estimating freeboards requires estimates of HAT and a flood threshold. Flood thresholds for Ballina are estimated by identifying water levels at gauge where water levels are representative of reported impacts at the time impacts are reported (Hague et al., Reference Hague, Murphy, Jones and Taylor2019, Reference Hague, Grayson, Talke, Black and Jakob2023a). Estimating freeboard for the 2022 compound flood event was complicated by a strong hydraulic gradient throughout the lower Richmond River and no gauges in the immediate vicinity of the CBD where most impacts were reported (Figures 5 and 6). The Breakwall recorded 2.23 m, whilst just over 1.5 km upstream Missingham Bridge recorded 2.47 m. Byrnes Point, in West Ballina 5.5 km upstream from the Breakwall, recorded 3.05 m. (Note, these water levels are relative to a low-water datum, rather than AHD). Assuming a linear hydraulic gradient along the length of estuary is justified based on the observed linear relationship between flood peaks of each of the gauges (Figure 5). Applying this to estimate water levels at the western edge of the CBD suggests a flood threshold of 2.67 m is an appropriately conservative estimate for the water level that led to the impacts in Figure 5. Similarly, a HAT of 1.97 m can be estimated. This yields an event freeboard of 0.7 m.

Figure 6. Water levels (in m) along the lower Richmond River based on observations at Breakwall, Missingham Bridge and Byrnes Point. For locations refer stars in Figure 5. Observations from 2014 coastal flood are shown in orange and 2022 compound flood is shown in red with corresponding values reported in m RRVD. Estimates of HAT from harmonic tidal analysis are shown in black, with estimate of HAT under 0.7 m SLR dotted. The approximate location of Ballina CBD is shown as shaded blue area, with estimate of flood peak indicated by number with black arrow, as described in the text.

Several other lines of evidence also suggest that 2.65 m is an appropriate estimate for the 2022 flood threshold in the Ballina CBD. First, based on apparent flood depths in the photographs, the water levels in 2014 coastally forced event and the 2022 compound-forced event depicted in Figure 5 (images i–iii and 1–9) are unlikely to differ by more than 0.6 m. The 2014 event recorded water levels between 2.03 m and 2.07 m along the estuary (Figure 6). For example, higher parts of footpaths remain visible in image i, despite the road also experiencing flooding during the 2014 event (images C and D in Figure 5). Second, DEM data indicates heights of between 1.3 m and 1.8 m AHD in the region where flooding is pictured in Figure 5. For example, the bus bay (location E, curved roof in iii) is at 1.3 m elevation but the intersection of Crane and Moon St (the non-flooded intersection in the centre of ii) is at 1.8 m. 1.8 m AHD is equivalent to 2.64 m RRVD (Modra, Reference Modra2013).

Estimating freeboard for the coastal-driven 2014 flood is more straightforward as water levels at all three gauges (Breakwall, Missingham Bridge, Byrnes Point – see Figure 4b) in the Ballina area recorded similar values (Figure 6). Estimates of event freeboards were identical for all gauges, at 0.08 m as HAT similarly varies between the three gauges (Figure 6). Hence the event freeboard for CBD impacts can be assumed to be 0.08 m. The constant freeboard along the Richmond River during this event (Figure 6) suggests that it is appropriate to assume that SLR will lead to all three gauges experiencing the same increases in water levels for future coastal driven flood events (Hanslow et al., Reference Hanslow, Morris, Foulsham and Kinsela2018; Fox-Kemper et al., Reference Fox-Kemper, Hewitt, Xiao and Yu2021). HAT has been exceeded on average 4.2 days per year by storm surges at the Breakwall over the 20-year epoch 2003–2022. It follows from the difference between 2.67 m and 1.97 m being 0.7 m that 4.2 days per year is the expected frequency of March 2022 flood impacts under 0.70 m SLR (Taherkhani et al., Reference Taherkhani, Vitousek, Barnard, Frazer, Anderson and Fletcher2020; Hague et al., Reference Hague, McGregor, Jones, Reef, Jakob and Murphy2023b). This 0.7 m is also an estimate of the SLR required for tides alone to be sufficient to lead to the impacts experienced in March 2022. Whilst tidal flooding may be primarily “nuisance” severity at present, it could lead to very damaging floods by late-century based on current SLR projections (McInnes et al., Reference McInnes, Church, Monselesan and Zhang2015; Fox-Kemper et al., Reference Fox-Kemper, Hewitt, Xiao and Yu2021). This finding assumes no successful future measures that ameliorates the impacts presently associated with a 2.67 m water level.

International relevance of the results

This study focuses on eastern Australian, but our findings have global implications. For example, we find that the defining feature of locations most at-risk of the emergence of tidal flooding is having lower freeboards. This metric is known to be important globally for identifying future coastal flood hazard hotspots (e.g., Hague et al., Reference Hague, McGregor, Jones, Reef, Jakob and Murphy2023b). This means that it is likely that locations with flood thresholds close or below the heights of the highest high tides (i.e., small or negative freeboards) are likely to be at risk of transitioning to tidal flood regimes, regardless of where they are in the world. It further indicates the importance of defining, or accurately estimating flood thresholds for all coastal locations, and continuing to develop methods to do this (Hague et al., Reference Hague, Murphy, Jones and Taylor2019; Moore and Obradovich, Reference Moore and Obradovich2020; Mahmoudi et al., Reference Mahmoudi, Moftakhari, Sweet, Moradkhani and Muñoz2024). This is necessary to define the freeboard metric and hence, identify future flood hotspots in estuaries where populations may be at risk. These may be locations where undertaking detailed hydrodynamic modelling may need to be prioritised in a resource-constrained environment to best inform coastal adaptation policy.