Environment Canterbury today reported a significant reduction in the estimated costs of recovery from the 2010 and 2011 earthquakes for the Halswell Drainage District.
Commissioner Donald Couch says this result has come about from a better understanding of the way lateral spread (movement of banks towards the river) has affected the Halswell River.
"An aerial survey (Lidar) allowed us to develop hydraulic models of the river so the effects on flood capacity could be better understood," Mr Couch said.
The investigation, approved by the Halswell Drainage District Liaison Committee, revealed the flood capacity to be less affected by lateral spreading than first thought and the cracked river banks to be generally more stable.
"This has allowed a substantial reduction in the scope of works required to restore the river, with a reduction in the rates required to fund the work," Mr Couch said.
Estimates of the sum required for restoration of the system have been revised from $3.44 million to $1.35 million. A proportion of the cost would be met by government and there would be a substantial reduction in the Halswell Drainage District Earthquake Recovery rate over the next 10 years.
This additional rate was set up by Environment Canterbury to restore the drainage network to its pre-quake condition.
Donald Couch says about 2 kilometres of the 114-kilometre drainage network was affected by liquefied sand in September 2010, with less damage from the February, June and December 2011 quakes.
"This was quickly restored by Environment Canterbury contractors following each of the quakes.
"The more significant effect on the system was liquefied sand and lateral spread affecting 25 kilometres of the 42-kilometre long Halswell River, causing water levels to rise about half a metre in many areas.
"Engineers initially anticipated much river-widening and bank-stabilising work would be required to restore flood capacity, but dredging of the river largely restored normal river levels in most areas."
A first round of dredging was completed and the least stable banks were stabilised in 2010/11. Some minor bank work is continuing.
A second round of river dredging and further minor bank stability work is still required. Works will probably be completed in the summer of 2012/13.
A public meeting is being held in the Lincoln Events Centre on Tuesday 27 March at 7.30 pm to report in more detail the ongoing and planned work and elect Liaison Committee representatives from different parts of the catchment for a 2-year period. The committee also has representatives from the Selwyn District and Christchurch City councils.
A 10-year loan repayment period had been programmed in 2010/11 to fund 3 years of works, but the meeting will be able to recommend either a rates reduction or a decrease in the time needed to fund the loan.
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