Matata Debris Flow Disaster

The Institute of Geological and Nuclear Sciences (IGNS) has published two reports on the devastating debris flow disaster at Matata in the Bay of Plenty in May last year.

The disaster was caused by an intense rainfall event on the 18th of May 2005, when rainfall rates exceeded 2 mm per minute as a result of a severe thunderstorm. The rain fell in a narrow band only a few kilometres wide in steep catchments above and inland from the coastal settlement. It is suspected that, had the rain fallen closer to Matata, a more devastating outcome might have occurred.

This event appears to be a 500-year recurrence event, and historical records indicate smaller events have occurred since 1860. There is also evidence that equally as large or larger debris flows have occurred in the distant past.

The intense rainfall triggered many landslips in the catchments behind the township, resulting in debris flows in the Awatarariki and Waitepuru Streams. These debris flows are dense mixtures of debris and water which move more rapidly than water and, in this event, moved boulders up to 7 metres in diameter. Up to 10% of the flow at Matata was organic matter stripped from the forest in the catchment area.

Although the disaster occurred during a swarm of more then 55 small earthquakes that occurred within 5 km of the township between February 12th and August 25th 2005, the IGNS team of McSaveney, Beetham and Leonard concluded that the earthquakes were not a contributing factor. The largest quake in the swarm was magnitude 4.2.

The team from IGNS noted that huge boulders from past debris flows had been used as landscape features in the area, and that parts of Matata have always been at risk. One conclusion stated “There are areas around Matata where it is unsafe to live.”

Although many residents have returned to their homes, there are still a number who have not owing to their houses being severely or totally damaged.

With the benefit of improved analysis and modern science, we are now learning that certain areas in New Zealand are not safe places to live. This creates a quandary for residents, insurers and local authorities as they try to find common grounds for protecting life, investment and lifestyle.

Of the two reports (available via the “News” link on the Geonet website which you can access from the link on this site), the smaller, two-page “The 18 May 2005 Debris-flow Disaster at Matata” provides concise, factual reading. It will be a handy fact sheet for many New Zealand residents, and will likely prove to be an excellent classroom resource.

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