Coastal floods can be devastating. They are also complicated. This article breaks down 7 processes that cause coastal floods. If you enjoy living near the beach, keep reading.

Australians love living near the beach… but floods are devastating

As Australians, we are blessed with bright sunshine, golden beach and blue oceans. Most of us enjoy living near the beach and a lot of us do. Many people see living near the beach as a privilege. But it has its downside: We are more vulnerable to coastal floods.

Coastal floods are extremely dangerous. In June 2016, a high astronomical tide (a ‘King Tide’) caused widespread destruction in NSW. In combination with an extreme storm surge caused by east coast lows, the tide caused $38 million damage.[1],[2]

Coastal floods hit Australia. Beachside homes damaged by a storm at Collaroy in Sydney's north. Source: ABC Online. Photo by Peter Rae, Fairfax Media.

Beachside homes damaged by a storm at Collaroy in Sydney’s north. Source: ABC Online. Photo by Peter Rae, Fairfax Media. Photo links to story and image.

Flood composition is complex

The composition of floods in coastal regions is complex. It involves the intersection of the hydrosphere across oceanic, atmospheric and terrestrial domains. As a result, extreme water levels depend on the interaction of many factors, such as:

  • the bathymetry of the basin
  • regional climate patterns
  • astronomical tides
  • wind-setup
  • wave-setup
  • run-up, and
  • the inverse barometric effect.

7 Processes that cause coastal floods

The figure below shows many factors that can cause flooding in coastal, urban areas. They include:

  • marine processes, such as high astronomical tide, storm surge and large waves
  • intense rainfall falling on urban catchments
  • large river discharge from upper catchments.



Processes causing coastal floods. It shows, in this order: 1 - High astronomical tide, 2 - High storm surge, 3 - Large waves, 4 intense rainfall, and 5 large river discharge.

Processes causing coastal floods

1. High astronomical tide

High astronomical tide is caused by normal variations in the astronomical tide cycle. The cycle is influenced by the alignment of the Sun and Moon with the Earth. High astronomical tide increases the risk of floods during a storm.

2. Storm surge

Storm surge is a rise above normal coastal water levels. It’s caused by a combination of wind action and low pressure acting on the ocean’s surface.

Storm surge is often caused by severe weather conditions. These may include:

  • tropical cyclones (such as hurricanes)
  • mid-latitude extratropical storms (such east coast lows in Australia)
  • fast-moving frontal systems that bring both strong winds and heavy rainfall.

3. Large waves

Local winds or swells from storms far away cause large waves. A ‘large wave’ rises above normal coastal water levels. They often occur under the severe weather conditions mentioned above.

4. Intense rainfall

Intense rainfall is a large amount of rainfall falling within a short time. Because of the short time in which rain falls on every part of the catchment, it can cause flash flooding. This is especially the case in urban areas.

Intense rainfall is often caused the same severe weather conditions mentioned above.

5. Large river discharge

Severe rainfall in upper catchment(s) cause river flooding. It can inundate estuarine areas, especially if it coincides with any of the coastal flood drivers above.[2]

6. A combination of the factors above

Flood risk in coastal and estuarine regions is traditionally assessed by individually considering points 1-5. But, a relationship may exist between many of the flood drivers (e.g. storm surge, large waves and intense rainfall). Combined, they significantly increase the risk of flood in coastal and estuarine regions.[3]

Previous research[4] tells us that, in Australia, extreme storm surge and intense rainfall are dependent. This dependence can lead to up to 35-fold increase in flood risk in these regions.[5]

7. Human intervention

Humans are a factor that people may not always consider when it comes to coastal flooding. But they are, especially when there are engineering structures within or upstream of the catchment. Such structures might include reservoirs and dams.

The design, construction, operation and maintenance of these structures may have an impact on the intensity and timing of floods within the catchment. In turn, this will have an impact on flooding downstream, in the estuarine regions. Developments within, and upstream of, the catchment may reduce the time needed for flood water to reach affected areas. This may also increase flood risk.

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1. 3. Marks, L. (2016). “NSW weather: Another king tide, large swell hits coast, insurance bill at $38m.” <>. (16 Dec., 2016).

2. Bursten, J., Garber, S., and Taylor, D. (2016). “Contextualising the Return Period of the June 2016 East Coast Low: Waves, Water Levels and Erosion.” 25th NSW Coastal Conference, Coffs Harbour, NSW, Australia.

3. Zheng, F., Leonard, M., and Westra, S. (2015). “Application of the design variable method to estimate coastal flood risk.” Journal of Flood Risk Management, n/a-n/a.

4. Leonard, M., Westra, S., Phatak, A., Lambert, M., van den Hurk, B., McInnes, K., Risbey, J., Schuster, S., Jakob, D., and Stafford-Smith, M. (2014). “A compound event framework for understanding extreme impacts.” Wiley Interdisciplinary Reviews: Climate Change, 5(1), 113-128.

5. Wu, W., Westra, S., Leonard, M., and McInnes, K. L. (2017). “Mapping dependence between extreme rainfall and storm surge across the Australian coastline using ROMS.” European Geosciences Union (EGU) General Assembly 2017, EGU, Vienna, Austria.

6. Zheng, F., Westra, S., and Sisson, S. A. (2013). “Quantifying the dependence between extreme rainfall and storm surge in the coastal zone.” Journal of Hydrology, 505, 172-187.