Photo: Murky water in Upper Huia Dam and a large slip.
The torrential downpours that triggered widespread flooding in west Auckland have also caused landslips that have blocked access to some of the city’s western dams and seriously muddied the raw water.
Rain gauges at our higher western catchments, which feed into Upper Huia, Upper Nihotupu and Waitakere dams, recorded between 220mm and 270mm of rain in a 12-hour period on Monday and Tuesday. These catchments normally receive between 150mm and 170mm of rain in the entire month of August.
With both private and public accessways to higher dams blocked by slips, our staff chartered a helicopter to carry out an aerial survey of its infrastructure and water supply lakes this week.
Chief operations officer Mark Bourne says this provided valuable insight into the extent of the storm damage.
“Our online turbidity instruments had already told us the storm had impacted raw water quality but with access roads blocked by landslips, we haven’t yet been able to get to our higher dams.
“By flying over our catchments, we could see significant slips that in some cases have taken out mature trees and are sending clay-coloured water into the water supply lakes.
“Upper Huia Dam appears to have fared the worst in terms of water quality, and we suspect the raw watermain may have been moved by a slip. Given we can’t access this dam at the moment to inspect it, we have taken it out of service.”
This means the Upper Huia Dam is currently spilling into Lower Huia Dam.
Bourne says turbidity readings in Lower Huia Dam are “off the charts”.
“Before the storm, our turbidity readings were about 2 or 3 NTU (Nephelometric Turbidity
Units). Our headworks team has since sampled turbidity at different depths of our Lower Huia storage lake and found the water at the bottom of the lake is the dirtiest, with turbidity readings up to 500 times greater than normal.
“The good news is, Auckland’s resilient water network means there is no impact to our customers.
“Even though the raw water quality in our western dams has deteriorated, we are still able to treat it at our Huia Water Treatment Plant to produce top quality drinking water for our customers.
“Our diverse range of water sources and increased treatment capacity at our Waikato River plants mean we are able to continue running our Huia plant at low production levels while still meeting Auckland’s water needs.”
The rain also meant Auckland’s wastewater network was inundated with stormwater, which caused wastewater overflows across the city at some pump stations and manholes.
Our maintenance crews cleaned and disinfected the impacted areas as soon as the overflows had stopped and it was safe to do so.