Coastal flooding (inundation)

Coastal flooding, also known as coastal inundation, can be caused by high tides, storm surges and tsunami. It can also be caused or exacerbated by sea level rise and subsidence:

High tides

High tides, spring tides, and particularly the highest high tides, known as king tides can cause coastal flooding in low lying areas. Tide forecasts are available from NIWA. 

Storm surges

Storm surges are the main cause of coastal flooding and occur when there is a low-pressure weather system and/or strong winds pushing water onshore. The biggest floods occur when a king tide and storm surge occur at the same time, known as a storm tide.

Sea level rise

Climate change is driving sea level rise as the warming temperatures cause the melting of ice and an expansion of ocean water. As well as gradually exposing more areas to the sea, sea level rise means that storm surges and storm tides occur more often and with greater impacts. 

Sea level rise is already occurring (New Zealand tide records show an average rise of 1.7 mm per year over the 20th century) and the rate of sea level rise is already increasing. Both the amount and rate of sea level rise is projected to continue to increase in the future.

Subsidence

New Zealand is a very dynamic country, meaning that some areas of land are moving up and others down over time. Where areas are moving down (subsidence) this causes a relatively higher sea level, increasing coastal flood risk in these areas. Subsidence can occur quickly, for example due to an earthquake, or slowly, for example due to shrinkage of drained peat soils.

In the Waikato region, we know that some areas such as the Hauraki Plains are subsiding. This is largely because of peat shrinkage, but may also be from deformation movement associated with the Kerepehi Fault. 

The relative ground movement of New Zealand’s coastline is not yet well understood. NZ SeaRise is a research programme that aims to address this knowledge gap.

In the Waikato region coastal flooding is more likely to occur in:

• the low-lying areas around the Firth of Thames (including State Highway 25)
• some eastern Coromandel Peninsula settlements
• large, intensively farmed flood plains in the Hauraki district which are below or near sea-level.

High sea levels can also affect river flooding, increasing water levels some way up a river from where it flows into the sea.

We have developed the Coastal Inundation Tool to show which areas of our region’s coastline are susceptible to coastal inundation (flooding), both now and in the future with projected sea level rise. 

You can search for your property or holiday destination and explore the flooding that may occur at different sea levels. Sea level scenario tables tell you what sea levels are expected to occur in each coastal area, from high tides, storms and sea level rise.

We have sea level recorders which provide long term records of tides, storm surge and sea level rise. Visit our Monitoring coastal hazards page for more information.

This information also helps us warn the public of potential coastal flooding, and keep people updated during a flood event.