Extra drains to protect Bell’s Hill slip

5 Jan 2016, 10:11 AM

Tens of thousands of dollars' worth of extra horizontal drainage has been installed to help better protect the Bell's Hill slip site in Kaitaia.

The Northland Regional Council has been monitoring and draining the area for a number of years since a large ground movement occurred above the nearby Awanui River.

Monty Knight, the Northland Regional Council's Kaitaia-based Te Hiku constituency representative, says the slip covers an area of about 4000 square metres and has been moving for more than 15 years.

"A network of horizontal drains has been used for some time to keep groundwater levels low and reduce the likelihood of landslip movement into the nearby Awanui River because if it was big enough, a slip could potentially dam and divert the river, causing significant flooding issues for urban Kaitaia."

He says the management strategy appears to be working, with monitoring showing that over the past four years the hill has moved a total of just 10mm.

"However, to be able to stop movement completely, engineers have advised us the drainage must be able to rapidly drain the slip area after heavy rain to prevent short-term elevations in the water table."

Councillor Knight says to that end, the council took advantage of the dry weather in the lead up to Christmas to install additional horizontal drainage.

"A network of new drains has been drilled up to 60 metres into the slip and about three metres lower than the existing drains."

The $40,000 cost is being shared between the regional and Far North District Council as some of the slip is on abandoned land under FNDC's jurisdiction and below this is a district council 'paper' road.

"While the slip is not directly connected to the Awanui Flood Scheme, the river, and the town, would be directly affected if the slip was to 'let go'," Cr Knight says.

He says the regional council and the Awanui River Liaison Committee will continue to keep a close watch on the slip site.