Unique opportunity to add resilience to Dannevirke’s water supply

What lies hidden under the roof of this impounded supply?

At an extra Tararua District Council meeting on March 15 councillors received an update on the Dannevirke Impounded Supply from its senior programme manager, Roger Earp, and Three Waters manager, Jean de Villiers.

They were receiving a summary of progress since the council voted in September 2022 for just over $1 million to progress the planning of leak repairs, along with investigating water pre-treatment and other water source solutions.

Project manager Earp explained to councillors that the situation presents a unique opportunity to add resilience to Dannevirke’s water supply with a long-term fix.

He and Three Waters manager, Jean de Villiers, explained that the task was not a simple matter of fixing the leaks but also involved finding a second water source as the council could not take water from the Tamaki in high turbidity (floods), nor could it take as much as it needed at low river levels in summer.

Three Waters manager Jean de Villiers and impounded supply senior programme manager Roger Earp after talking to councillors on March 15.

In addition, only half the stored water can be extracted from the dam currently due to an engineering oversight and currently can only hold 9.5m of water instead of 12.5m to minimise the chance of a wall collapse (unlikely).

At that point the presenters confirmed that investigations are progressing into the different issues but even assessing the extent of the leaks is a problem due to the dam cover, infrastructure manager Chris Chapman said a submersible device is being employed to investigate further.

Councillors were keen to ask questions, Councillor Johns leading with the question “Is the dam a dead duck?”

De Villiers pointed out that the Impounded Supply had served Dannevirke well during Cyclone Gabrielle and that a tank farm as an alternative had its own risks and difficulties. Referring to the Impounded Supply he said “It’s a huge piece of infrastructure that has massive value.”

Councillor Sutherland asked if there was a way to extract more water from the supply to which Chapman said floating pumps could remove a further quantity but not the amount Dannevirke requires.

Focusing on the turbidity issue at the source of water in the Tamaki, Sutherland asked if the infiltration galleries could be modified and Johns asked if the council has systems elsewhere in Tararua which could be copied. Chapman replied that Pahiatua had a system which was expensive and a new unit would need to be designed.

Seeking a new source was another issue raised by de Villiers, saying that analysis of previous aquifer investigations so far suggests that it is unlikely that another single source will be identified locally that can provide the 20 litres per second required and the council is looking further afield.

Councillors will have to wait until May to start making decisions.

An Optioneering Workshop is scheduled for mid-April with all related parties including iwi to decide possible options and determine a recommended course of action.

Councillor Wallace wished them well, saying it is vital for Dannevirke as “water is becoming like liquid gold”.