Photo: Muddy water in a stream below Lower Huia Dam.
Last week’s torrential downpours caused large-scale landslides in Auckland’s Waitakere Ranges, pushing sediment into the water storage dams. In response, we will open valves on the lower dams for a short period later today to release some of the murkiest water.
Chief operations officer Mark Bourne says opening the free discharge valves at Lower Huia and Lower Nihotupu dams should help to improve the quality of the water within the water supply lakes.
“When we inspected the area last week, we identified significant landslides in our water catchments, some of which have blocked roads, taken out mature trees and sent clay-coloured run-off into our water supply lakes.
“This means turbidity readings in all of our western supply lakes are much higher than normal. Before the storm, levels were about 2 or 3 NTU (nephelometric turbidity units). Samples taken at different depths of Lower Huia’s lake have found the water at the bottom is the dirtiest, with turbidity readings up to 500 times greater than normal.”
Bourne says the operations will be carried out under existing consent conditions, adhering to limits on flow rates and duration. We normally time the free discharge operation for spring when lake storage levels are traditionally highest.
“Currently, the western dams are full and have excess water flowing down their spillways. We’d prefer the excess water to be released from the bottom of the dams than the top, because the water at the top is of better quality.”
Water will be released from each dam for no longer than four hours, with a team on site to monitor water quality and safety during the operation.
Huia Water Treatment Plant is currently treating water from Upper Nihotupu, Lower Nihotupu and Lower Huia dams. Upper Huia Dam has fared the worst in terms of water quality, and with concerns that its watermain may have been shifted by a landslip, Watercare has taken this dam out of service.
“While the water coming into the Huia Water Treatment Plant is not as clear as it normally would be, our robust treatment processes mean there is no change to the quality of the treated water that leaves the plant. It is absolutely safe to drink,” Bourne says.
“Our staff are actively monitoring changes in the quality of the water flowing into the plant to ascertain if our treatment processes need to be adapted accordingly.”
Had there not been dams in the area, the material from the slips would have washed directly into the Manukau Harbour, instead of being released through a controlled operation.
Before the storm struck, we had been running the Huia plant at minimum flow – producing between 50 and 55 million litres a day. Given the deterioration in water quality, it is continuing to run the plant at its minimum output.
The Waitākere Water Treatment Plant was taken out of service for 24 hours immediately after the storm but has since been treating water as normal from Waitākere Dam.
Click here to see how we improve the clarity of water in our dams.