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Clifton to Tangoio Coastal Hazard Strategy 2120

HB Coast

Hazard Information

What are the coastal hazards?

The Clifton to Tangoio Coastal Hazards Strategy 2120 will consider three main hazards along this part of the Hawke’s Bay coast through to 2120:

  • Coastal erosion
  • Coastal inundation (sea water flooding)
  • Tsunami.

Influence of Climate Change

All hazards, and their severity, will be influenced by climate change which will bring increased ‘storminess’ and sea level rise.  

The latest information from the Parliamentary Commissioner for the Environment in their November 2014 report ‘Changing climate and rising seas: Understanding the science’ says: 

“Over the last century, the average sea level around the world has risen by about 20 centimetres. The International Panel on Climate Change (IPCC) expects it to rise another 30 centimetres or so by the middle of the century and up to a metre or more by the end of the century.”

New research tells us what our highly valued coastline will look like due to sea level rise in the 100 years to 2120. The good news is that our communities and local authorities have time to plan and to make appropriate decisions

Three coastal hazards

COASTAL EROSION

People in Hawke’s Bay are familiar with images of homes wrecked by coastal swells at Te Awanga or the stretches of gravel beach at Westshore seasonally eroded by wave action. This is life at the coast as we know it. But jump forward to 2065 or 2120 and these effects will be more pronounced.

There are a few parts of the coastline between Clifton and Tangoio where shoreline movement is relatively stable, such as Marine Parade Beach.

However, large parts of the coastline has recorded an erosion trend which is likely to increase. Over the next century, sea level rise in combination with increased wave heights and storm intensities is expected to significantly impact on the gravel barrier ridge protecting the Tangoio to Clifton coastline.

Future decisions also need to take into account that:

  • Erosion rates will continue to accelerate with sea level rise.
  • Erosion will not stop.
  • The local extent of erosion (or accretion) also depends on the ‘budget’ of beach sediments, ie the net gains or losses in the total volumes of sand and gravel contained within that beach.

In preparing the Strategy, the areas at risk from coastal erosion will be reassessed out to the year 2120.  This will update earlier assessments and incorporate current understanding of shoreline movements, recent investigations and improved projections of sea level rise.  The reassessed hazard extents will inform decision making as we develop responses to these risks.

COASTAL INUNDATION

The Hawke’s Bay coastline between Clifton and Tangoio is defined by a gravel barrier ridge which provides a vital defence from the sea.  Without it, large areas of Napier City and some of Hastings District would be regularly inundated (flooded by the sea) and potentially uninhabitable.

As a result of the changing climate, sea/wave/swell levels could be higher than the gravel barrier ridge. 

We will also need to consider the changing nature of the barrier ridge over time as climate change drives sea level rise, and the extent to which the barrier ridge will continue to provide protection from inundation.

We can calculate the extent of extreme inundation by taking into account storm surge, wave set-up and wave run-up.  If wave run-up levels exceed the barrier ridge crest (which will be likely for future climate change), a zone of influence of significant run-up effects will need to be established, ie a defined area where inundation could affect land and property.

TSUNAMI

The power of tsunami was shown to be horrific on Boxing Day 2004 in Sumatra and Thailand, and in the event which devastated the Japanese coast in April 2011.

Hawke's Bay Regional Council (HBRC) has already developed tsunami hazard maps to provide information to our communities.

Much of this work was initiated by the Hawke's Bay Civil Defence Emergency Management Group and is based on tsunami wave height research from GNS Science. HBRC has completed two-dimensional tsunami hazard mapping for the coastline between Tangoio and Clifton.

Under the Clifton to Tangoio Coastal Hazards Strategy, this work has been reassessed and confirmed, and based on this, an analysis of the risk presented by tsunami has been developed.

There are two main types of tsunami:

Distant tsunami

This is a tsunami that starts somewhere across the Pacific Ocean, eg. South America, Tonga, Japan, Alaska.  There will be time for an official warning and evacuation.

Near source tsunami

This tsunami would start near the coast, probably in the tectonic plate boundary 120 kilometre offshore to the east, known as the Hikurangi Trough.  Because this tsunami is generated so close to shore, there will be no time for an official warning, and people need to be aware of the natural signs of earthquakes, as promoted by Hawke’s Bay Civil Defence Emergency Management.

Other Hazard information

East Coast LAB (Life at the Boundary)

A new project called East Coast LAB began in 2015. It’s goal is to inform the public of the potential hazards from the Hikurangi plate boundary 120 kilometres offshore of Hawke’s Bay, and provides information about natural hazards, the large amount of science research underway in this area and how to get involved - www.eastcoastlab.org.nz

Civil Defence

For more information about hazards you may wish to go to the Hawke's Bay Civil Defence Emergency Management website - www.hbemergency.govt.nz

 

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