A coastal road can sit above the expected sea level and still be exposed to flooding. Waves may wash over nearby barriers, while low sections of shoreline give water a route inland. That is why coastal flood modelling needs to look beyond how much the sea might rise.
A study featured by the Engineering Institute of Technology (EIT) examines this problem along Perth’s coast. Mohamed M. Abouelnaser, Dr Kumari Gamage and Dr Shasha Wang investigated potential wave heights and flooding, including at Fremantle and Mullaloo, by combining past wave conditions with estimates of future sea levels.
For engineers working on coastal infrastructure, the research raises a practical question: how will the water behave when it reaches the site?
Where Sea-Level Estimates Fall Short
Sea-level rise changes the height of the water beneath the waves. The waves themselves can carry water higher, pushing it up a beach or over a seawall.
Two processes help explain this. Breaking waves can raise the average water level close to shore, known as wave setup. The rush of water up a beach or structure is wave runup. If water crosses the top of a barrier, it is called overtopping. The Queensland Government’s coastal glossary describes these distinctions.
A seawall’s height therefore tells only part of the story. Its performance also depends on the waves reaching it and the water level at the time. A design based on sea-level change alone could miss conditions that allow water to cross it.
Bringing Perth’s Coast into the Model
The Perth researchers used 45 years of reconstructed offshore wave conditions, seabed information and sea-level projections drawn from the Intergovernmental Panel on Climate Change’s Sixth Assessment Report.
They used DHI MIKE 21 software to simulate coastal conditions, alongside an analysis of the historical wave record to estimate rare, severe events. The paper describes 75 simulations combining different wave directions, event frequencies and sea-level scenarios.
There is a reason to test those combinations. As waves move into shallower water, the depth and shape of the seabed change their behavior. Their approach direction matters too. DHI’s wave modelling documentation explains how the software represents these changes.
An offshore wave-height estimate needs that local context before it can inform a coastal design. What reaches a particular beach or harbor will depend on the conditions along the way.
The ground above water matters just as much. A dune may form a barrier; a low section may allow water through. CoastAdapt’s modelling guidance highlights the need for detailed local data when assessing where coastal flooding could occur.
Reading the 9.2-Metre Result
EIT’s summary reports significant wave heights exceeding 9.2 meters under an extreme high-emissions scenario. It is a striking figure, but it needs an explanation.
Significant wave height describes the sea state. It is commonly explained as the average height of the largest third of waves, according to the US National Weather Service. Individual waves may be higher.
It is not a measurement of floodwater on land. A 9.2-meter significant wave height does not mean a nearby street would be under 9.2 meters of water.
For a design team, the useful details are where that wave height applies, the conditions used to produce it and how the waves behave closer to shore. Without those details, a large number can give a misleading impression of local risk.
Looking Behind the Flood Map
A flood map can make a complicated study look quite straightforward: water reaches the shaded area and stops at its edge. In practice, that edge depends on the model’s inputs.
The Perth paper uses sea-level estimates from the upper end of the assessed range. Different choices could change the area shown as flooded.
The researchers also compared simulated wave heights with measurements from Hillarys buoy over a short period in June 2025. That comparison helps check the wave modelling under those conditions. It does not establish how accurately the model will reproduce flooding on land during a future extreme event.
Before using a footprint for design, engineers need to know how much it changes when key inputs change. A boundary that moves substantially with a different sea-level estimate or ground elevation needs particular care.
Using the Research on the Ground
The IPCC describes climate scenarios as possible futures built on stated assumptions. They help examine consequences, rather than provide certain forecasts.
For a coastal road, modelling could help identify sections that warrant closer investigation. For a seawall, it could help compare performance under different water and wave conditions. These are practical uses of the Perth study’s approach, rather than design recommendations for either location.
EIT’s research record offers a starting point for that work. At a particular site, the next decision will depend on local surveys, checks of the model and a clear explanation of which assumptions most influence where the water goes.
References
Engineering Institute of Technology — Perth Wave Heights and Coastal Inundation Research
Abouelnaser, M. M., Gamage, K. and Wang, S. — Assessing Climate Change Impacts on Potential Wave Heights and Coastal Inundation in Perth, Australia: A Numerical Modelling Approach. Author-uploaded full text
Queensland Government — Storm Tide Monitoring Glossary
DHI — MIKE 21 Spectral Waves
CoastAdapt — Modelling Tools to Assess Local Scale Inundation and Erosion Risk
US National Weather Service — Significant Wave Height
IPCC — AR6 Working Group II, Annex II: Glossary
This article was published October 21st, 2026 and the content is current as at the date of publication.
