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Wastewater outfall pipe repairs no easy or quick fix, says NCC

Napier City Council expects to have leaks in the Awatoto Wastewater Outfall pipe repaired by the end of October, while work gets underway on the design and cost of the outfall replacement.

A leak at a fibreglass join in the pipe about 700m offshore and approximately two metres under the seabed surface was discovered in August 2018 and NCC had been monitoring the situation while working on a solution. Then a second smaller leak was found at approximately 630m from shore in April this year. Divers inspected the second site shortly after Alert Level 4 was lifted, and found that this was the site of an old repair that had been damaged by something, such as an anchor. There are two smaller leaks at this site.

At the Council meeting yesterday, Council resolved to: apply for a variation to the existing wastewater discharge consent for the leakage at 700m to enable an ongoing discharge at this point until the joint is fully repaired or the outfall is replaced; develop an emergency response plan to manage additional damage or failure of the pipeline; engage a specialist dive team to undertake the lowest risk repairs possible for both sites, and start planning the early replacement of the outfall to minimise risks, increase levels of service and tie in with improvements to the Wastewater Treatment Plant.

Today an abatement notice has been issued by the HBRC. Council still intends to work through the repair options with the HBRC over the coming months, as mentioned at yesterday’s meeting, in order to address the requirements of the notice.

Monitoring had not shown any serious adverse environmental impact from the leaks to date.

The HBRC, which issues NCC with the consent to discharge treated wastewater and will need to approve the consent variation, has been kept informed since the discovery of the first leak in 2018. The repairs need to be finished by 30 October.

“The preferred option for repair is the quickest, and lowest risk we can manage at this point in time, as able to be completed by the 30th of October. No repair will be easy or comes without risk, both to the divers who will be carrying out the work, and to the environment should something go wrong. An emergency response plan, together with a detailed breakdown of the work to be undertaken to ensure risks are adequately managed, will be in place when the work gets underway,” says Jon Kingsford, Director Infrastructure Services.

The outfall can currently be shut down for up to three hours at a time while the repair is carried out, as there is some wastewater storage available, but this will depend on a number of factors, such as network flow rates, and weather.

The Wastewater Treatment and Outfall Masterplan that is currently being developed and expected to be finished next month will help to determine future requirements for the full replacement of the outfall, and whether there are other options, such as discharging treated wastewater to land.

This replacement could cost between $20 million and 40 million, and a detailed investigation needs to be carried out to both make sure the preferred option is the right one, and Council has a more accurate cost estimate. This information is likely to be available by the end of the year as the Masterplan targets are worked through.

Further information about the project will be part of the Annual Plan 2020/21 consultation, and the next Long Term Plan, in 2021. Council is proposing to set aside $2 million for the repair and $2 million to create storage for wastewater at the treatment plant to allow for future shutdowns for maintenance and repairs, and aid management of stormwater events.

The costs for the repair and the new storage will be funded from reserves so there will be no effect on rates.

The 1.54km long outfall pipe has been in place since 1972. The first leak occurred at the point where the two sections of pipe were joined. The fibreglass joint was installed at this point in 1984.

Council has spent close to a million dollars on proactive maintenance of the outfall in the past two financial years.

As part of recent improvements to planned maintenance, divers inspect the pipeline and diffusers annually. During these inspections, ports of the diffusers are cleaned. However, not all the diffuser ports are functioning due to blockages, missing diffuser parts - damaged by causes unknown - or they are buried under an everchanging seabed.

NCC is also working with the Hastings District Council and Pan Pac to apply for protection of all pipelines under the Submarine Cables and Pipelines Protection Act 1996.

8 June 2020

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