Swan River Coastal Flooding 2025
- Matt Eliot
- Apr 8
- 3 min read
Swan River Coastal Flooding Plots 2025
A preliminary evaluation of day-to-day coastal flooding hazard for 2025 has been developed from analysis of the Department of Transport Barrack Street tide gauge data, combined with tidal predictions for 2025.
Please note: This analysis does not include sustained residuals developed by oceanographic processes, including the 2025 marine heat wave. This has caused roughly 0.1-0.2m higher water levels so far in 2025!
5minute observations from 2001-2022 were analysed harmonically, to separate tides and tidal residuals. Tidal fitting included the annual and semi-annual constituents, which provide an approximation to the seasonal mean sea level cycle, much of which is the result of overall changes in weather conditions, particularly storminess.
Seasonal variation of the tidal residual was determined. It is apparent that this has elevated extreme (0.1%) residuals for the transitional months in May and September – these do not necessarily indicate more severe storms but represent the opportunity for a severe storm when the background level of storminess is relatively low.
Predicted tide for Barrack Street was obtained from Department of Transport predictions for 2025. The daily maximum predicted tide illustrates the highly seasonal nature of high tides for the Swan River region, which is a combined result of the mean sea level cycle, and twice-annual tidal modulation, with daily tide range peaking in June and December.
Predicted high tide and tidal residual levels have been added to present the relative hazard of coastal flooding if a storm is expected to occur on that day. Consequently, Bureau of Meteorology predictions can be used as a guide to expected water levels:
- If no storm is expected, the predicted high tide level is likely to be reached;
- For a strong, moderate or severe storm, the corresponding threshold can be used as an estimate of expected flood level.
This simplified assessment does not incorporate the effects of waves or river flooding and therefore should be interpreted with care. Actual water level experienced may vary, depending on whether the storm passage is simultaneous to high tide, or occurs at a lower tide.
Some Things to Note
Perth’s tidal and weather climate includes two phases of high tide (June and December), a seasonal cycle of mean sea level (peaking in May-June) and a seasonal pattern of storms. This creates a rather unusual setting where high water levels (and low water levels) are largely restricted to relatively short seasons of the year, although it is possible for storm surges to occur in all months.
In addition to the seasonal pattern, we have also experienced longer-term sea level rise, cycles of mean sea level variability and long-term tidal cycles. Coming off a sustained period of rising sea levels, with 5.4mm/year since 1990 (0.20m rise), we’re experiencing another marine heat wave, which raises sea level, and are at the peak of the tidal cycle, with the highest predicted tide in 2025 being 7cm higher than in an ‘average’ year.
2025 possibly represents the highest risk of Swan River coastal flooding in the modern record.
However… we’re not talking devastation! It’s only marginally worse than last year, and it’s likely to be comparable to the marine heat wave phase of 2011-2013.
(a) Influence of Mean Water Level & Storm Surges. (b) Influence of Inter-annual Tidal Modulation
Barrack Street observations from 1990 have seen an accelerated rise of mean water level with 5.4mm/y rise. However, this is partly exaggerated by the transition from El Niño to La Niña prevalence, with the preceding period of 1955-1990 having a standstill in the Fremantle long-term record, preceded by a rise of 3mm/y from 1915-1955. The overall rate of 1.4mm/y over the 20th Century is consistent with estimates of global mean sea level rise, with accelerated sea level rise inferred over recent decades.
Coastal Flooding Risk Plots 2025
Prediction doesn’t include the effects of long-term oceanographic processes (e.g. 2025 marine heat wave), river flooding, or waves.
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