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Matata Debris Flow

Please note that this desktop assessment was undertaken in 2013 using the best available information and knowledge at the time.  It did not include any community engagement, which  would be required to inform the levels of risk.

In this section the Matata debris flow that occurred in March 2005 is tested using the risk based approach presented in the report. The debris flow resulted from a high intensity rainfall event. To undertake this assessment, information that has been derived from the following sources has been used:

  • Statistics New Zealand – population and GDP figures;
  • Journal articles (Bull et al., 2010) (return period and damage from the debris flow); and
  • Cost of deaths and injuries (Ministry of Transport, 2012).

Step 1 - Know your hazard

The keys inputs that are relevant to the consideration of the decris flow for the Matata area using a risk-based approach are as follows:

  • Number of properties in the hazard zone – 122 properties;
  • Number of buildings the hazard zone – 114 dwellings;
  • Likely number of occupants – 300 (based on an average occupancy of 2.64 per dwelling) (Statistics New Zealand, 2012);
  • Regional GDP – $4.318 billion (2003) (Statistics New Zealand, 2003);
  • Lifelines – road, rail, power, telecommunications, water;
  • Critical buildings – none in the hazard zone;
  • Social cultural buildings – none in the hazard zone that we are aware of;
  • Building value – (based on average value of a dwelling in Matata – 114 x $250,000) $28.5 million;
  • Debris flow return period - 1:500; and
  • Cost of a death $3.77 million and cost of an injury $207,000 (Ministry of Transport, 2012).

Step 2 - Consequence analysis

Using the consequence table above, the severity of the impact for the identified consequences is as follows:

Buildings of Social and Cultural Value

Insignificant – none affected


All buildings in hazard zone had their functionality compromised

Critical Buildings

Insignificant – none affected


Significant damage to road and rail which was out of service for greater than a week. Due to the importance of these links, greater than 20% of the population was affected.


= (28,500,000 / 4,318,000,000) x 100 = 0.66% of regional GDP (there were no deaths or injuries which were required to be factored into the equation)

Health and safety

Insignificant – no deaths or injuries

Using ‘first past the post’, the event is categorised as catastrophic, with a severity of impact of V.

Step 3 - Likelihood analysis

Using the likelihood table below, the debris flow is considered to be unlikely (Level 3) as the return period is estimated to be once every 500 years.


Step 4 - Take a risk-based approach

Using the severity of impact calculated in Step 2 and the likelihood number calculated in Step 3, the debris flow is considered to be a Discretionary Activity (tolerable level of risk).

The risk-based planning framework (adapted from Saunders, 2012)


Bull, J.M.; Miller, H.; Gravley, D.M.; Costello, D.; Hikuroa, D.C.H.; Dix, J.K (2010) Assessing debris flows using LIDAR differencing : 18 May 2005 Matata event, New Zealand, Geomorphology. 124(1-2).

Ministry of Transport (2012). Social cost of road crashes and injuries June 2012 update. Wellington, Financial, Economic and Statistical Analysis Tean, Ministry of Transport.

Saunders, W. S. A. (2012). Innovative land use planning for natural hazard risk reduction in New Zealand. People, Environment and Planning. Palmerston North, Massey. Doctor of Philosophy: 229.

Statistics New Zealand 2012 http://www.stats.govt.nz/Census/2006CensusHomePage/QuickStats/AboutAPlace/SnapShot.aspx?type=region&ParentID=1000006&tab=Households&id=1000006

Statistics New Zealand 2003 http://www.stats.govt.nz/browse_for_stats/economic_indicators/GDP/regional-gross-domestic-product.aspx.