Vivid imagery captured by Watercare on Tuesday reveals the scale of devastation caused by the Tasman Tempest weather event in the Hūnua Ranges.
Massive slips – hundreds of metres long – have torn through areas with commercial forestry; erosion is also apparent in areas of dense native bush.
Watercare chief executive Raveen Jaduram says the video footage is very concerning: “This footage really highlights the vast scale of the slips, which are widespread throughout the Hūnua Ranges.
“With large areas of exposed soil and water-laden earth, further rainfall – as forecast by the MetService for this coming weekend – elevates the risk of more silt washing into our water supply dams.
“We are proactively working with Auckland Council to identify practicable measures that can be implemented to manage the slips and protect our dams.”
One of the slips caused by the Tasman Tempest.
We are currently having the rainfall data captured at the dams during the five-day period from 7-12 March externally validated. However, current information suggests that the volume of rain that fell in a 24-hour period during the Tasman Tempest – when the slips occurred – was significantly higher than the volume that fell in a 24-hour period during Cyclone Bola in March 1988.
“Our current data suggests the Tasman Tempest delivered up to two and a half times more rain to the Hūnua Ranges than they received during Cyclone Bola. The nature of this storm and the damage it caused was unprecedented,” says Jaduram.
Elevated silts levels in the water storage dams are having a significant impact on the treatment capacity at Ardmore Water Treatment Plant, which supplies up to two thirds of Auckland’s water.
In the days following the 7-8 March downpour, Auckland’s water supply situation was reported as fragile – with the Ardmore plant treating half its normal volume of water and the other four metropolitan plants running at maximum. At this time, we began asking Aucklanders to reduce their water use by 20 litres per day, saying that otherwise we may be forced to release partially treated water into the region’s supply – requiring a boil water notice to be issued.
By Wednesday 15 March, the water supply situation was in a stable and sustainable position, and we were able to begin to slowly increase the volume of water being treated at the Ardmore plant.
“Over the past week, we have increased production at our Ardmore plant by 30 million litres a day,” says Jaduram. “This is a real achievement by our water treatment team as the silt levels at the dams remain extremely high.
“However, we are still producing significantly less water than normal, and with another rainfall event on its way, we’re not intending to increase the output of our Ardmore plant further this week. Maintaining a stable and sustainable position is our priority.
“This means the need for everyone to reduce their water use by 20 litres of water per day is still as important as ever.
“We are currently asking for Aucklanders to make voluntary savings. Voluntary savings are about people finding simple ways to reduce their water use every day – such as shorter showers.”
Over the past seven days, Aucklanders have used an average of 404 million litres of water per day. While this is slightly over the target the company set – of 400 million litres or less - it compares very favourably to the March average prior to this rainfall event, of around 450 million litres of water per day.
“The response we’re seeing from Aucklanders to our Save 20 message is heart-warming. We’re a big city and yet we’re proving that in times of need, we can come together and do our bit to preserve the health and wellbeing of our diverse community,” says Jaduram.