Connected residents will be supplied primarily from North Pine Water Treatment Plant which draws water from North Pine Dam's storage 2.JPG

North Pine Dam upgrade

Planning work for the dam upgrade is underway.

The upgrade of the dam will increase the dam’s capacity to withstand very extreme floods.

The required upgrades reflect changes in good practice in dam design over the years.

Engineering analysis methods and standards for dams; improved rain and flood data; and scientific understanding of dam safety risks have continually improved since North Pine Dam was constructed in 1976.

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Modern Practices

Modern good practices for dams include detailed risk assessment methods that are recommended in National and Queensland dam safety guidelines.

The existing dam can withstand rare and extreme floods including possible inflows much higher than the January 2011 flood. The largest flood we can possibly estimate is the probable maximum flood and it would exceed the capacity of the dam.

With a large population living downstream of North Pine Dam, the risk to downstream communities means the dam needs greater capacity to withstand the most extreme floods.

Reduced Full Supply Level

Lowering the full supply level for the dam is a safety precaution to further improve the capacity to withstand extreme floods as an interim measure until an upgrade is completed.

This is an accepted and long-established way of reducing safety risks associated with rare and extreme flood events.

The North Pine Dam reduced full supply level is 36.0 metres Australian Height Datum. This level is equivalent to a storage volume of approximately 68% of the original design storage capacity.

There will be no immediate impacts or changes to recreation restrictions caused by the decision to reduce the Full Supply Level of North Pine Dam.

With the reduced full supply level, it is possible that releases from North Pine Dam (minor flow events, much smaller than floods which impact houses) may occur more frequently. This will depend on future rainfall causing inflows to the dam and only if inflows are sufficient to require releases so that the reduced full supply level can be maintained.

The flood operations for North Pine Dam with the reduced full supply level are not expected to impact downstream flood levels for large and rare floods that can impact houses.

Any future flood releases will be managed in accordance with the 2019 Flood Mitigation Manual for North Pine Dam.

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Downstream view of North Pine Dam. Photo credit Peter Hansler

Investigative work

26 Feb 2020

Geotechnical drilling work is being undertaken downstream of the North Pine Dam spillway.

Seqwater is conducting geotechnical investigations downstream of the main dam spillway at North Pine Dam.

This work is part of Seqwater's Dam Improvement Program to help to assess what upgrade work North Pine Dam may require in the future.

Seqwater contractors will be undertaking this investigation work from Monday 10 February 2020 for approximately five weeks, subject to any wet weather or unforseen project delays.

What to expect:

Work hours are from 6.30am-6.30pm Monday to Saturday
Work may be required on Sunday, from 9am-5pm
Equipment will include a drilling rig, trucks and utility vehicles
There may be a minor increase in noise and traffic during this work.

Access to the lake and recreation areas will not be affected during this work.

Recreation at Lake Samsonvale, North Pine Dam

Seqwater water supply operators at North Pine Water Treatment Plant.jpg

Birds eye view of North Pine Dam wall. Photo credit Peter Hansler

Sun setting at North Pine Dam. Photo credit Peter Hansler

Why was the Full Supply Level changed at North Pine Dam?

Seqwater enacted a Reduced Full Supply Level for North Pine Dam following further assessment of potential safety risks for very extreme flood events.

Lowering the full supply level for the dam until an upgrade is completed is a precautionary measure which will help reduce safety risks associated for rare and extreme flood events, beyond which the dam has experienced to date.

North Pine Dam is in good structural condition and continues to safely operate.

How much water has been released to reduce the full supply level?

Due to the current dry conditions and low lake level at the time the reduced full supply level was enacted in December 2019 there was no requirement to release any of the stored water that is conserved for drinking water supply.

What does lowering the storage level do?

Lowering the storage level (full supply level) of dams is an accepted and long-established way of reducing safety risks associated with rare and extreme flood events. The lower full supply level assists to reduce the peak lake level in the dam for potential future flood events.

Several other dams have previously been lowered as part of our Dam Improvement Program including Sideling Creek, Cooloolabin and Leslie Harrison dams, all of which have now been upgraded.

If the dam is unsafe why aren’t improvements being made?

North Pine Dam is in good structural condition and continues to safely operate. The dam is currently able to manage large and extreme floods including events greater than the 2011 flood.

The planned upgrade is to enable the dam to continue to operate safely in the future and improve the capacity of the dam to manage rare and very extreme flood events into the future.

North Pine Dam is one of a number of dams across South East Queensland identified as requiring an upgrade as part of Seqwater’s Dam Improvement Program. Other dams across South East Queensland currently scheduled for future upgrades include Lake Macdonald and Ewen Maddock dams on the Sunshine Coast as well as Wivenhoe and Somerset dams.

What are the upgrades required?

The upgrade of the dam will increase the dam's capacity to withstand very extreme floods.

With a large population living downstream of the dam, the risk to downstream communities means North Pine Dam needs greater capacity to withstand the most extreme floods. The potential options to increase the capacity of the dam for extreme floods may require a selection, or combination of, spillway modification, dam raising, and dam strengthening.

No options have been finalised or decided at this time and further work is required. Seqwater is undertaking assessments and investigations to develop and review feasible options for further consideration.

What is the likelihood of the dam failing?

North Pine Dam is in good structural condition and continues to safely operate. With the reduced full supply level, it is estimated that North Pine Dam can withstand a flood approximately two times larger than the rare extreme flood that occurred in January 2011.

Data collected from significant rainfall and flood events since the dam was designed and constructed, and the developments in flood estimation methods, have improved the ability to estimate the magnitude of potential rare and extreme flood events.

The probability of a flood sufficient to cause dam failure is low. Based on National and State guidelines for dams, Seqwater considers the very low probability of dam failure to be an unacceptable risk due to the potential consequences for a large population downstream of the dam.

The planned upgrade will aim to provide capacity for very extreme floods up to three times larger than the January 2011 flood.

How much will the levels be reduced?

The Reduced Full Supply Level for North Pine Dam is 36 metres AHD which is a storage volume of approximately 145,500 ML and represents 68 per cent of the original water supply storage capacity. The lake level in January 2020 was below 60 per cent capacity and as such there was no immediate requirement to release water.

How will the reduced levels impact water supply?

The reduced dam level is not expected to impact on the long-term water supply security for SEQ.

When is the dam planned to be upgraded?

Planning for the North Pine Dam upgrade is underway. Seqwater will provide further advice to the community on the timing and details of improvement work as the project progresses.

Are homes more at risk of flooding during floods requiring dam releases now that Full Supply Level has been reduced?

Lowering the full supply level for the dam until an upgrade is completed will help reduce safety risks associated with rare and extreme flood events, beyond which the dam has experienced to date (i.e. floods considerably larger than the 2011 flood).