Potts Hill Water Reservoirs #1 and #2 are located in Canterbury-Bankstown, New South Wales, which is part of the greater Sydney metropolitan area in Australia. The reservoirs store potable water and are operated by the Sydney Water Authority, which is owned by the state government. Originally open lagoons with a capacity of 900 million liters, the storages were re-engineered reducing the total capacity and introducing geosynthetic lining materials with fully sealed floating covers to minimize evaporation, reduce temperature fluctuations, and eliminate UV effects on water quality and the ingress of contaminants. Reservoir #2 is quadrilateral shaped, located inside Reservoir #1 (see Figure 1), and confined by concrete walls and a concrete floor. The design includes an under-cover baffle curtain to ensure effective mixing of chlorine treated in-flow with the stored water as the inlet and outlet penstocks are adjacent. Reservoir #1 is a rectangular shaped enclosure, confined by earthen banks, the concrete walls of Reservoir #2, and a concrete floor. Each reservoir contains about 300,000,000 liters (300 mega liters) of water for a total of storage about 600 mega liters. As a result, this is the largest water reservoir with a floating cover system in the Southern Hemisphere.

The original bottom liner and floating cover systems were comprised of a 1.14 mm thick reinforced Polypropylene (RPP) geomembrane with accompanying geosynthetics. Having performed reliably for 16 years, aging of the floating cover system increased the required maintenance activities and reduced its mechanical strength properties. The cover replacement project commenced with an assessment of geomembrane material developments that had occurred over the 20 years operation of this asset. An extensive materials test program was undertaken to ensure that the selected materials, in particular, the geomembrane, would provide a high service life.

The bottom liner system for each reservoir consists of a field fabricated EIA geomembrane underlain by a conductive geotextile and geonet, which were installed on top of the existing 1.14 mm thick RPP geomembrane (see Figure 2). A conductive geotextile was included below the RPP geomembrane so electrical leak location testing could be performed. The staging area for installation of the geonet and conductive geotextile is shown in Figure 3. Project timing considerations resulted in the EIA geomembrane being field fabricated to allow simultaneous removal of the old floating cover and installation of the lining system to compensate the aggressive project schedule. The entire EIA geomembrane was arc tested using ASTM D7953 to ensure no leaks in the completed bottom liner system geomembrane.

The floating cover system, ballast tubes, associated access ladders, access hatches (see Figure 4),and vents were factory fabricated because: (1) the intricate geometry of the defined sump with a central plate floating cover system that was selected for the reservoirs and (2) the narrow roll width of the selected material to reduce field welds and to reduce the installation time.

The relining and covering of Water Reservoir #2 were completed between April 2022 and February 2023. It was critical to complete the installation on time so potable water did not have to be produced using an expensive desalination process. The relining and covering of Reservoir #1 were completed between March 2023 and January 2024. Even though it took about the same time to re-line and cover, relining and covering Reservoir #1 was more challenging than Reservoir #2 because the bottom geomembrane had to be connected to the 8m high and 600m long vertical concrete wall separating the two reservoirs.

The main lessons learned from this project are summarized below:

- Use of a baffle curtain with a ballast line is helpful in maintaining a defined sump in the  floating cover system.
- Well-constructed geomembranes exhibit low seepage levels so groundwater seepage through the cracked concrete vertical wall surrounding Water Reservoir #2 had to be repaired to reduce overall reservoir leakage.
- A rigorous Construction Quality Control process resulted in zero geomembrane failed welds during construction.
- Installation of ballast at various locations on the floating cover can help direct surface water to the defined sump of the floating cover.
- CAD modeling of the reservoirs allowed potential geomembrane stress concentration points to be eliminated.
- A rigorous safety in design process lead to designing off-cover storm water removal pumps to assist with removing storm water from the floating cover system.
- An innovative vertical wall ballasting system was developed to control wind up-lift and cover location control.

Use of factory fabricated geomembranes, floating cover system hatches, vents, ballast tubes, and other accessories aided the project in a variety of ways. For example, both reservoir projects were completed on schedule, with no injuries, and no quality issues. Use of factory fabricated geomembranes also allowed installation to proceed 1.5 to 2.0 times faster than a field fabricated geomembrane. In particular, 1,500 to 3,000 square meters of geomembrane were installed per day. Factory fabrication also allowed the highest possible weld quality to be produced in a controlled environment. Large prefabricated geomembrane panels reduced construction duration. Most importantly, the tight schedule for returning the water reservoirs back to service prevented use of the expensive desalination plant to provide potable water to Sydney, Australia.