Coastal hazards
When natural coastal processes, like storms or high tide events, pose a threat to human life, property or infrastructure they become a coastal hazard. Learn more about the different types of coastal hazards we experience in Waitaha/Canterbury.
There is a history of damaging coastal erosion and seawater flooding along parts of the Waitaha coast. These events are expected to occur more often and will impact more locations due to climate change and accelerated sea level rise.
If we understand coastal hazards, we can make informed decisions about where to build or not build, or what activities to undertake within the coastal environment.
Coastal hazard types
Coastal erosion
Coastal erosion is where waves, currents and tides gradually eat away coastal land. Beach sands and gravels are transported away from the shoreline, and are not replaced by enough new material to keep the shoreline stable. Coastal erosion can occur over a very short period (hours or days) during storms, or gradually over much longer timeframes (years, decades, centuries). Erosion can threaten infrastructure and property located along the coastline.
Protecting and restoring natural systems, like dunes and wetlands, improve the resilience of our coasts against the effects of storms, floods and sea level rise. This includes protecting buildings, infrastructure and cultural sites.
Find out more about coastal erosion and how we are planning for the future.
Tsunami
A tsunami is a series of powerful waves that move through the ocean. They are created by the sudden movement of the sea floor, usually from a large underwater earthquake, an underwater volcanic eruption or landslide under or into the sea.
The Waitaha coast is exposed to tsunamis from several different sources, including large earthquakes in the tectonic plate collision zones off the coasts of the North Island and Fiordland, as well as earthquakes off the Central and South American coasts.
Find out more about tsunamis and evacuation zones in Waitaha.
Coastal flooding
Coastal flooding, also called seawater flooding, is when seawater floods normally dry land in low-lying coastal areas behind the beach. Coastal flooding occurs when a combination of high tides, low-pressure systems, strong onshore winds, and elevated swells drive the sea up and over the natural beach crest, or coastal stopbank.
Many parts of Waitaha's coastline are vulnerable to coastal flooding. It can threaten coastal communities, endanger stock, and cause long-term damage to productive land. Vulnerable locations include low-lying areas between Waitarakao/Washdyke Lagoon and the Orari River, between the Rakaia River and Taumutu, Leithfield and Amberley Beach, and estuary margins.
Coastal flooding, when combined with heavy sea conditions, can cause river mouths to block, which can cause or worsen river flooding near the coast.
Factors that impact coastal hazards
While issues like sea level rise and land subsidence (sinking) are not coastal hazards, they can exacerbate the effects of coastal hazards either independently or in combination.
Sea level rise
Sea levels are rising due to the warming of our atmosphere and oceans. The sea level around Waitaha has risen by around 20 cm over the past 100 years. More than half of this has occurred over the past 30 years, indicating an acceleration in the rate of sea level rise.
Because of sea level rise, high tides and storm events will be able to reach further inland, exposing larger areas of land to coastal erosion and flooding hazards. There will also be gradual, progressive, and permanent flooding as the high tide level moves landward.
Rising groundwater
In low-lying areas close to the coast, groundwater levels can be influenced by the level of the sea. Sea level rise will bring groundwater levels closer to the ground surface which may cause permanent, or more extensive periods of water ponding. This will have implications for natural and built environments. Higher water tables at the coast will also make river and coastal flooding worse.
Land subsidence
Aotearoa/New Zealand is a geologically active country and our land can move up and down, either very quickly due to earthquakes, or gradually from slower movements in the earth’s crust. Waitaha is no exception, and in many places, the land is sinking at rates of several millimetres per year. Sinking land will, in effect, increase the rate of local sea level rise. This means the impacts of sea level rise – flooding, coastal erosion and rising groundwater – will happen sooner than in locations where the land is not sinking.
Find out more about the latest estimates of sea level rise and the impacts of land subsidence on the NZ SeaRise website or contact us.
Monitoring coastal hazards
Monitoring coastal hazards and what drives them allows us to assess the level of risk to coastal communities, infrastructure, and ecosystems. By understanding the frequency, intensity, and potential impacts of coastal hazards we can make informed decisions to mitigate risks and enhance resilience against these hazards.
Shoreline change
Our natural hazards team undertakes surveys of our coast every six to twelve months using GPS survey equipment. We monitor over 250 cross-sections along our coast to measure changes in the position of shorelines and the volume of sand and gravel on a beach.
For more detailed technical information on coastal erosion, inundation and coastal erosion planning zones, visit our coastal monitoring web app.
Sea level
Sea level recorders are installed in several locations around Waitaha. The Lyttelton and Timaru Port Companies, NIWA and Christchurch City Council manage the recorders. We use the data they provide to understand long-term effects of sea level rise and short-term changes in sea levels from high tide events and storm surges.
Map showing the current state of Waitaha coastline stability
View sea level data for Lyttelton, Sumner, Timaru and Akaroa.
Waves
Monitoring waves helps to track coastal storms and improve wave forecasts and coastal inundation warnings for coastal communities. Measuring waves over the long-term allows us to see how waves and storms change over time and determine the impacts of climate change on wave conditions.
A network of wave buoys from Dunedin to Kaikōura collects data on wave conditions every 30–60 minutes, and captures the variability in wave conditions along the Otago and Waitaha coastline.
View wave data for Kaikōura, Pegasus Bay, Banks Peninsula, Timaru and Waitaki (Pipeline buoy).