Fatalities forecast if planned dams fail

19:59, May 19 2014

Three people will die and more than 100 will be at risk if two proposed storage ponds in Canterbury breach embankments, releasing 8.2 million cubic metres of water, according to expert modelling.

Waimakariri Irrigation Ltd (WIL) plans to construct two storage ponds, or dams, on the southwest side of Wrights Rd and Dixon Rd in Oxford, North Canterbury.

The two storage ponds would contain enough water to fill more than 3000 Olympic swimming pools.

A Damwatch Services report said its modelling of the "worst-case scenario", an eastern bank breach with "no warning", indicated the likely loss of life would be three plus 107 people put "at risk" and about 40 houses "rendered uninhabitable".

The resource consent hearing for the WIL project will be held next month, over two weeks from June 3.

Plans show the ponds will be partially above the ground with built-up embankments of heights between 4.5 metres and 12.5m.


Controls and pipes will deliver water to WIL shareholders.

Households in the path of a possible "inland tsunami" are angry the project is being planned in spite of projections of loss of life and devastation to properties by Damwatch.

With the earthquake events of 2010 and 2011 still raw, residents are worried seismic activity could cause a breach.

Damwatch modelling shows scenarios for western, northern, southern and eastern breaches, with the flood plain being identified as stretching about 22 kilometres east.

The "high risk" area where people and buildings are in danger stretches almost as far but significantly diminishes after about 6.5km.

Damwatch said a dam break was unlikely.

"A dam breach is a very low likelihood event and the results of the dam breach analysis should be considered in this context."

Environment Canterbury received 159 submissions - 44 in support, 115 against and 57 submitters requesting to speak before the commissioners.

The submissions against the plan include some from families who fear a loss of life.

"In the light of very real and devastating [2011] earthquake . . . it is simply an insanity or demonic plan to sacrifice the lives and homes of an entire currently safe community," said a submission from Leo-lo-Shiva and Ekaterini Kovani.

Brentworth Dairy Farm operates directly across from the site of the proposed dam and although it is a shareholder of WIL, it has concerns over "personal safety" and the design of the ponds.

Three Brentworth houses are proposed to be relocated because of the risk of being between 66m and 140m from the dam wall.

". . . if there was a dam break, then the result for the occupants of houses on the Brentworth property would be catastrophic, regardless of the likelihood of it occurring," said the submission.


Eastern breach (flood zone)

Area of flood zone (square km): 73 Households in the flood zone: 176 Households at risk*: 40 Population at risk: 107

Southern breach (flood zone)

Area of flood zone (km2): 59 Households in the flood zone: 75 Households at risk*: 19 Population at risk: 54

Northern breach (flood zone) Area of flood zone (km2): 66 Households in the flood zone: 198 Households at risk*: 35 Population at risk: 95

Western breach (flood zone) Area of flood zone (km2): 15 Households in the flood zone: 50 Households at risk*: 1 Population at risk: 4 * At risk is defined as a property in the dam breach flood zone and inundated by 0.5m or greater.

Source: Damwatch Services 

The Press