Leslie Harrison Dam is set for a $24 million upgrade as part of Seqwater’s Dam Improvement Program.

Seqwater Chief Executive Officer Jim Pruss said the Dam Improvement Program would enable the region’s drinking water storages to continue to operate safely into the future.

Leslie Harrison Dam is one of number dams across the region to be upgraded over the next six years along with Lake Macdonald, Cooloolabin and Ewen Maddock dams on the Sunshine Coast and Sideling Creek Dam (Lake Kurwongbah) near Petrie, north of Brisbane.

Final detailed design for the Leslie Harrison Dam upgrade is expected to be completed before the end of 2017, with construction expected to begin mid-2018, subject to final approvals. The upgrade will involve strengthening the dam’s main embankment and spillway to improve performance and better withstand more extreme weather events.

The dam was built in 1968 and has not received a major upgrade since its spillway gates were installed in 1984 to increase the drinking water storage level. Seqwater lowered the dam to its current level in 2014 and removed the gates in 2015 as part of its investigations and to improve the safety of the dam.

“Dams are long-life assets and require continual assessment, monitoring and maintenance,’’ Mr Pruss said.

“Many of South East Queensland’s dams were built in the 1960s and the 1970s. Over the years there have been significant advances in dam design and better understanding of extreme weather events.’’

Mr Pruss said once the upgrade was complete, Leslie Harrison Dam would remain at its current lowered full supply level and the gates would not be reinstated.

“To reinstate the gates would require additional upgrade work that would need at least a further $18 million investment to a total of $42 million,” Mr Pruss said. “The spillway gates were originally installed by Redland City Council for additional water supply and any decision to reinstate the gates needed to be based on water supply requirements.

“We have completed extensive modelling and our assessments indicated the additional water supply was not required and that the lowered dam level would not affect the future water security for the Redland region.

“Based on that assessment, we were not able to justify that additional investment, which would have been added to the cost of bulk drinking water.

“Most of Redlands’ drinking water is sourced from North Stradbroke Island. The region is also connected to the SEQ Water Grid via the Eastern Interconnector Pipeline, which provides additional water security.’’

Mr Pruss said it was important to note that the upgraded dam would continue to provide vital flood mitigation for the community downstream.

“There is no loss in the level of flood mitigation provided by Leslie Harrison Dam as a result of the spillway gates being removed. The dam was originally constructed without spillway gates and the upgrade will maintain it without the gates,” he said.

“The gates were added for water supply and not to improve flood mitigation.”

Mr Pruss said dam upgrades were significant and critical investments and Seqwater was prioritising its Dam Improvement Program to deliver a staged capital program, which safeguarded the community and delivered best value for money.

“Our dam assessments and planned upgrades to date have been endorsed by the Queensland Dam Safety Regulator and reviewed by independent dam experts,’’ he said.

In recent years Seqwater has upgraded a range of dams across South East Queensland including Lake Manchester (2008), Borumba (2008), Hinze (2011), North Pine and Ewen Maddock Stage One (2012), Maroon and Moogerah (2014) and Wappa (2017).

The Leslie Harrison Dam upgrade will take about two years to complete and will be undertaken over 2018 and 2019.