Upgrading Hinze Dam18 January 2013
The upgraded Hinze Dam has increased water capacity on Australia’s Gold Coast, writes Richard Davidson and Christopher Dann.
The Hinze Dam, located on the Nerang River, about 18 miles west of Australia's Gold Coast, was initially completed in 1976 and upgraded in 1989, but it had again become outdated. With a rapidly growing population and widespread drought, the pressure on water supply continued to grow.
To remedy the situation, the Gold Coast City Council (GCCC) formed the Hinze Dam Alliance (HDA) to design and deliver the Hinze Dam Stage 3 project (HDS3). The HDA team consisted of the owner, GCCC (Seqwater, the Queensland Bulk Water Supply Authority, took over the project from GCCC following Queensland Government water reform initiatives in 2008); and three non-owners, URS, design; Sinclair Knight Merz (SKM), approvals and stakeholder management; and Thiess, construction.
The goal of the Stage 3 project was to reduce the impact of potential flooding in the Nerang River catchment and increase the dam's water storage capacity, ultimately improving security and reliability for the region's water supply. This required the dam wall to be raised from 93.5m to 108.15m.
Key design features
The design of the project required a broad range of field investigations and studies. These included:
- Geotechnical Investigation - The geology of the site is complex and site investigations included drilling more than 80 boreholes and over 200 test pits and trenches.
- Spillway Design - URS used 3-D computer modeling to develop a spillway design. It also constructed a physical scale model to develop the design while ensuring that the full range of floods could safely pass through the spillway.
- Embankment Design - The embankment wall was raised almost 15 meters to 108.15 meters and extended across the saddle of the dam by 800 meters. The complex foundation conditions at the right abutment was a major challenge for the design team and the foundation treatment included the construction of a plastic concrete cutoff wall to depths of just over 50 meters, the largest wall of its kind in Australia.
- Intake Towers - Two intake towers are located within the reservoir to draw water from the storage to supply nearby water treatment plants. A seismic hazard study was conducted to confirm the design earthquake loads for the raised towers and the results of the study allowed for a reduction in the scope of work required, leading to significant cost savings.
- Fishway - The dam had long been a barrier to fish movement. An innovative "trap and haul" fish transfer system was designed near the downstream toe of the dam to allow fish to move upstream, beyond the embankment, for the first time in more than 40 years. The design of the system was conducted with the cooperation of the Department of Primary Industries & Fisheries, creating a new benchmark for that kind of system.
The project entailed two distinct phases. The first was the Optimization Phase. The project was awarded before it was defined. All that existed were the key project objectives of flood mitigation, increased water supply, and regulatory compliance. The Alliance performed a detailed optimization study that focused primarily on the storage configuration by investigating yield versus dam height, acceptable flood capacity for the dam, and economic considerations.
The final configuration of the dam was determined from the optimization study following a study process that included stakeholder workshops with representatives from the Alliance design, environmental, communication, and construction teams, the Gold Coast City Council, Gold Coast water officers, members of the Council's independent expert review team, and the Queensland Dam Safety Regulator.
A probabilistic quantitative cost benefit analysis was developed to facilitate selection of the optimal upgrade solution, which included economic, social, and environmental considerations. The cost benefit analysis included capital costs for project delivery, operating costs, and risk costs for project delivery.
Most of the environmental and social impact costs were contained in the risk cost component of the evaluation, with the understanding that a full evaluation of the impacts would be included in the Environmental Impact Statement. Cost estimates were developed using a unit-rates cost estimating approach by the Alliance construction team. A range analysis with probabilistic cost estimating was undertaken using best estimates of construction quantities and cost rates with 50 percent confidence limit and high estimates of construction quantities and cost rates with 95 percent confidence limit. An 80 percentile confidence limit was adopted to define the estimated cost of each option. The economic benefits of the broad range options were evaluated and categorized to include the flood mitigation and water supply benefits of the project.
The second phase was the Works Delivery Phase. The Alliance delivery method facilitated the implementation of a number of innovations that significantly benefited the project. Since the project involved the renovation of an existing structure and the uncertainties associated with previous works completed in Stages 1 and 2, close cooperation among designers, regulators, construction operators, and owners was essential.
The Alliance developed a basic strategy for constructing the upgrade from the outset, along with strategies to manage potential flooding, clay supply shortfalls, and revised inundation clearing requirements. The early planning allowed teams to transition into different areas of work and modify the work sequence as new issues arose.
One of the principal geotechnical issues for the project was the potential for internal erosion and piping within the complex geology at the right abutment. A plastic concrete cutoff wall was selected as the best solution to reduce the risk of piping to acceptable levels and planning was required to manage a range of key risks including complex technical challenges; potential risks to dam safety, the environment and surrounding community; as well as delivering the project on a tight construction schedule and within budget.
A critical element of the construction works, regarding stability and dam safety, was the excavation of the foundation at the right abutment. The excavation was undertaken while the reservoir was close to full supply level and required removal of the existing rockfill stability berm to expose the foundation and Stage 2 filters that interfaced with the Stage 3 works. The required excavation works reduced the stability of the embankment and it took careful staging and construction monitoring to manage the dam safety during construction. A comprehensive dam safety management plan was developed to meet safety standards.
While the downstream rockfill berm was under construction, significant deformations were recorded on the main embankment. The deformation was not related to stability as the embankment slopes were constructed from high quality rockfill and founded on a slightly weathered greywacke with safety factors above 1.3.
Throughout the course of the project, a collaborative inquiry-by-design method of engagement was implemented to educate the community about sustainable design and best practice principles, and community/stakeholders were invited to participate in the design development process. This resulted in community acceptance of some initially unpopular ideas, such as closing public vehicle access across the crest of the dam. Local residents decided that the inconvenience was outweighed by averting the potential risks to the water supply.
Good for 50 years
Thirty-five years after it first opened, the Gold Coast's Hinze Dam has come of age. The Stage 3 upgrade raised the embankment, increased the dam's surface area, and almost doubled the dam's capacity, reducing flood risks in the Nerang River catchment and securing a safe and reliable water supply for the next 50 years.
The upgrade also included large "baffle blocks" to dissipate energy from the water as it leaves the spillway. The spillway is designed to cut peak flood outflow in half in a 1-in-100-year flood event, reducing the risk of downstream damage.
Finally, the Hinze Dam has the distinction of being the highest central clay and rock dam in Queensland.
Richard Davidson, Senior Principal and Vice President, URS Corporation, Denver; and Christopher Dann, Senior Principal, URS Australia, Brisbane. www.urscorp.com