Solid progress made on $3m Helensville dam upgrade

We are making steady progress on a major upgrade to the dam that holds most of Helensville’s drinking water.
The stabilisation of the Mangakura 1 dam face and strengthening works on its concrete spillway will bring it up to New Zealand dam safety standards and increase its resilience in the event of an earthquake or flood.
Dam safety manager Shreesh Basnyat says: “This is the biggest dam upgrade we’ve undertaken since we strengthened Cosseys Dam in the Hūnua Ranges about 20 years ago.
“Stage one involved increasing the footprint of the earth-filled dam to stabilise it. Over summer we built a buttress to support the downstream slope of the dam and backfilled to make the dam structure wider and more stable. More than 2500 cubic metres of backfill has been added to the dam.”
A sand filter layer was also installed in the downstream dam face to prevent internal erosion.

: Water conditioning and compaction of the completed fill layer at Mangakura 1 Dam
Photo: Water conditioning and compaction of the completed fill layer
Basnyat says a huge amount of effort was needed before the upgrade could begin.
“The sand and fill material needed to be a certain grade – it wasn’t readily available from a quarry. We had to make our own recipe, ask the quarry to make it for us and carry out compaction trials to prove it would perform as we need it to.”
The dam structural safety surveillance system has also been upgraded to enhance ongoing monitoring.

Placing and levelling of the Mangakura Dam 1 embankment fill layer
Photo: Placing and levelling of the Mangakura Dam 1 embankment fill layer
Work began on stage two – strengthening works on the dam’s concrete spillway – in April.
“We’re going to be installing anchors and carrying out other strengthening work to make the spillway better able to withstand seismic loads,” Basnyat says.
“One of the biggest challenges we’ll face in this stage is the weather. We need to make sure the dam is about a metre from spilling at all times, so we may have to draw down the upstream dam and keep it within a metre from spilling too.”
The dam upgrade is expected to be completed in Spring.
We have carried out several smaller upgrades to protect the dam and its connecting infrastructure and upgraded its instrumentation and monitoring systems since it became responsible for the dam when the SuperCity formed in 2010.
Update on Helensville Wastewater Treatment Plant
Meanwhile, Helensville’s wastewater treatment plant is performing extremely well after an extensive $17m upgrade was completed last year.
The plant – which was the first in New Zealand to adopt membrane aerated biofilm reactor treatment technology – has surpassed expectations, with the average ammonia level in the discharge at just 1.6mg/L, compared to the current consent limit of 15mg/L.
The upgrades have also made the plant easier for staff to monitor and operate. With a standby generator, it can continue to operate during power outages, which saves considerable time.