The 90-year-old dam is built on Cabbage Tree creek, not far above its confluence with the Brisbane river, approximately 25km west of Brisbane.
The report, commissioned by SEQWater and prepared by HydroTasmania, says the integrity of the dam has also been affected by leaks from several cracks in the dam wall. The report investigates several options to repair the dam, including the possibility of raising full supply level by 3m, increasing reservoir capacity. The cost of this option is estimated at US$15.3M.
The report said other solutions, which involved overtopping and/or increasing the spillway capacity, have costs ranging from US$9.7M to US$10.1M.
The repair work would also include anchoring the sides of the dam to the rockface with 39 cables.
The report further recommended that the vertical cracking in the dam's wall should be sealed with PVC and stainless steel strips to reduce water loss and remove maintenance requirements.
Construction of Lake Manchester dam commenced in 1912 and was completed in 1916.