Worst water in history
Auckland’s water issues could last months longer if rain continues to hit.
Watercare water operations manager Priyan Perera said rain could add tonnes more super-fine silt into Watercare’s four Hunua Ranges dams.
The silt, generated from scores of landslips scarring the Hunuas, is suspended in the catchment’s water which flows down to the Ardmore water treatment plant.
‘‘You’d expect dirt particles to sink out, but because it’s so fine it could well take months,’’ Perera said.
Heading into winter rains, Perera is pessimistic Watercare will get breathing space for the existing silt buildup to dissipate.
‘‘This is the worst water quality we’ve ever had in the dams,’’ he said.
A journey in a Watercare ute convoy along catchment roads shows the slip’s extent. Leaden clay has smashed its way down steep slopes, bulldozing native bush and pine plantations alike.
Recently-milled pine copses seemed especially vulnerable to slips, already dead root systems offering flimsy protection against the eroding floodwaters.
Watercare is considering conducting an aerial survey to map catchment landslips to try and calculate how many tonnes of earth tumbled into its Hunua reservoirs.
Flooding in the Hunua Ranges during the storm was worse than during one of the costliest cyclones in New Zealand history.
‘‘The Tasman Tempest delivered up to two and a half times more rain to the Hunua Ranges than during Cyclone Bola,’’ Watercare chief executive Raveen Jaduram said on Wednesday.
The analysis compared rainfalls between the first 24 hours of both the Tasman Tempest and Cyclone Bola and was based on data captured at the dams during the five-day period between March 7 and 12.
A Watercare maintenance worker said at the height of the Tasman Tempest, one Hunua Ranges rainfall level collector measured 40cm of rain falling in one hour.
New footage from the ranges shows large slips through areas of commercial forestry and erosion in native bush.
Production at Ardmore had increased by 30 million litres this week but it was still producing ‘‘significantly less water than normal’’.
‘‘With another rainfall event on its way, we’re not intending to increase the output of our Ardmore plant further this week.’’
Watercare still maintains every Aucklander needs to reduce their water usage by 20 litres per day.
According to Jaduram, Aucklanders used an average of 404 million litres of water per day in the past week, four million above the target but well below the usual usage in March of 450 million litres of water per day.