'Tonnes of water drained from unstable hill

More than 20,000 litres of water has been drained from a large, unstable hill posing a potential threat to Kaitaia township.

 

Contractors working for the Northland Regional Council recently installed 10 horizontal fan drains at Bell’s Hill, which over a number of years has been slowly slumping towards the Awanui River, which runs beneath it.

 

Joe Camuso, Rivers Programme Manager for the Northland Regional Council says there have been concerns in some quarters that the hill could collapse, posing a risk to Kaitaia, especially if the river was in flood at the time.

 

Mr Camuso says geotechnical engineers hired by the Regional Council had suggested installing the drains could boost the hill’s stability by about 30 percent.

 

That work had been carried out recently (subs: April) and had already drained about 24,000 litres of water – weighing about 24 tonnes – from the hill and also lowered its water table by about one metre.

 

Mr Camuso says water can cause slips both through its sheer weight and the way it effectively lubricates slip-prone soils.

 

“In layman’s terms, that means that by lowering the water table, we’ve reduced two of the main drivers that could cause the area to slip.”

 

Far North-based Regional Councillor Joe Carr, Chairman of the Awanui River Management Liaison Committee, says even with the significant improvement the drains have offered, the hill was still not completely stable.

 

“However, this latest work is a real improvement and has to be regarded as a positive first step by the local community. Having said that, it’s fair to say more significant work will probably now be required at a cost of several hundred thousand dollars to realistically reduce the risk of slipping much more.”

Cr Carr says in the meantime, the latest work should effectively enable a bit more breathing space while the Regional Council, the Far North District Council and local community investigate longer term solutions, what level of risk they jointly feel is acceptable and how any work should be funded.

 

Meanwhile, Mr Camuso says the roughly 30mm diameter drains vary in length from 60 to 70 metres and draw water from deep within the hill into a run-off area.

 

The approximately $20,000 cost of installing the drains had been met by the Regional Council via its targeted Awanui River Management Rate. Cr Carr says the Awanui River Flood Management Liaison Committee had recommended funding the work from that river management rate in the interim on the basis that the money would eventually be reimbursed.

 

The Council’s Environmental Management Committee is expected to consider the reimbursement issue at its May meeting.