It was clear to all parties – owner, operator and design consultant – that the spillway capacity at Dartmouth Dam in Victoria, Australia, would have to be increased to achieve an acceptable level of risk against contemporary dam safety guidelines for an extreme hazard dam. However, it was not immediately evident that a traditional solution would not suffice.

Standing 600ft (180m) high, Dartmouth Dam is the highest dam in Australia with a storage capacity of nearly 3.1M acre-feet (3856GL), nearly seven times that of Sydney Harbor. Located on the Mitta Mitta River in Northeastern Victoria, Dartmouth Dam is one of the iconic dams of Australia and as such receives unique attention and scrutiny in providing essential water supply to this dry continent. Three states that grow a large proportion of the food supply in the country rely on water stored in the dam to support irrigation and urban demands in the area.

The dam is a popular recreational trout fishery, with the Mitta Mitta River upstream of Lake Dartmouth regularly stocked by the Victorian Department of Primary Industries. Since its completion in 1979, the dam has spilled only four times – 1990, 1992, 1993 and the largest spill event in 1996. Overflows from the spillway enter a concrete chute then flow over excavated rock cascade steps that were formed when the rock for the dam was quarried from the valley walls.

Australian dam safety practice

A dam safety risk assessment completed in 2007 identified that the largest contributor to dam safety risk was flood overtopping. Based on the Australian National Committee on Large Dams (ANCOLD) Acceptable Flood Capacity Guidelines (1998), extreme hazard category dams should target the flood capacity equivalent to the Probable Maximum Flood (PMF) as the long-term dam safety standard. Currently the flood capacity at Dartmouth is less than half of this requirement.

The more recent ANCOLD Risk Assessment Guidelines which are followed in some jurisdictions provide for an alternative approach based on risk assessment. Although the dam safety risk at Dartmouth achieves the ANCOLD tolerable risk criteria through application of the ALARP principle, the owner considers that works to further reduce dam safety risk is warranted. Based on the outcomes of the risk assessment and considering the portfolio of the owner’s dams, it was identified that the design of an upgrade to the dam and spillway to achieve PMF flood capacity and reduce the risk of overtopping during an extreme flood event should proceed.

Innovative approach

URS extended work by others to conduct a rigorous study and evaluate a range of spillway and dam upgrade options that would pass the PMF inflow of about 710,000ft3/sec (20,200m3/sec). Several traditional spillway upgrade options to increase the flood capacity of the dam were developed, but a number of construction and operational risks were identified for each of these traditional options. To minimize risk and associated costs, an innovative approach was needed.

As part of the design process, a physical model study was conducted by SMEC Australia for several upgrade options, with a 1:60 scale physical model measuring 92ft (28m) long, 40ft (12m) wide and 12ft (3.7m) high being constructed. In an effort to keep the spillway crest at its current location, and utilize a passive (i.e. no moving components) spillway structure, an innovative weir design called the Piano Key Weir was tested in the physical model. The Piano Key Weir is a relatively new design; only a few have ever been built worldwide, the highest being 13ft (4m). Similar to a labyrinth weir structure, the Piano Key Weir consists of single or multiple cycles, but unlike a labyrinth weir, it has a rectangular plan shape, and sloped apex bays (not vertical walls) that give the Piano Key Weir its greater efficiency.

Prior to testing the Piano Key Weir in the physical model, a literature review of published design and physical model testing data of Piano Key Weirs was conducted but yielded minimal information, in particular on their application to a weir of this size (labyrinth weirs of 30ft (9m) and higher were already being considered as upgrade options for Dartmouth). Given the dearth of previous examples, the model study evolved from a pure design function to a combination research and development with design purpose. The physical model study was used to confirm that this unique design would:

1) Satisfy current structural design standards including large dynamic pressure fluctuations.

2) Not create excessively high cavitation pressures on the spillway chute.

3) Not increase the already high erosive forces on the excavated rock cascade spillway downstream.

4) Be cost competitive compared to other upgrade options.

A successful preliminary test of the Piano Key Weir in the physical model was conducted based on design parameters from the limited published data and design experience from traditional labyrinth weirs. Then, Computational Fluid Dynamics (CFD) modelling was used to optimize the design shape of the Piano Key Weir to increase the overall hydraulic efficiency of the structure. CFD is the study of fluid mechanics that uses numerical methods and algorithms to solve and analyze problems involving fluid flows. High-speed computers were used to perform the millions of calculations required to simulate the interaction of liquids and gases with surfaces designed by boundary conditions. The structure was then retested in the physical model for further optimization and to obtain detailed measurements for design.

The physical model study results showed that the Piano Key Weir would nearly triple the existing spillway capacity and safely pass the PMF by attenuating the inflow peak from 710,000ft3/sec (20,200m3/sec) to an outflow of almost 410,000ft3/sec (11,500m3/sec).

Piano key weir recommended

When the studies for Dartmouth Dam were completed, URS recommended the Piano Key Weir combined with a parapet wall raise of the dam embankment to progress forward to detailed design. The decision took into account a number of factors, including construction risk, reduction in dam safety risk, capital and life-cycle costs, and potential construction staging. Due to the greater efficiency of the Piano Key Weir, less excavation and less concrete than a traditional labyrinth spillway will be required for construction. Also, as a fixed crest reinforced concrete structure maintenance effort and costs are minimized.

The choice of the Piano Key Weir resulted in a socially and economically viable solution that also satisfied sustainability principles. Its application allows for the spillway to be modified in its existing location, reduces environmental and visual impacts, and eliminates the need to relocate or close a popular visitor center.

As tested, the prototype Piano Key Weir design has seven-cycles, is 30ft high (9m), 300ft wide (91m) and 118ft (36m) long, with a total weir wall length of nearly 1970ft (600m). If constructed, it will be the largest Piano Key Weir of its kind in the world. The physical model testing demonstrated that the weir would function as predicted and confirmed the results of the CFD modelling. Additionally, the Dartmouth physical model study was performed in a rigorous manner and documented so that its findings can be applied to future projects. The study included hydraulic measurements in the CFD and physical models, head ranges over the Piano Key Weir up to 37ft (1.3 times the height of the structure, or 11.2m), and still and video photography.

The selected Piano Key Weir design combined with a parapet wall raise of the embankment meets the flood capacity requirements with a secure, low maintenance operating regime, provides a new spillway at the existing spillway location minimizing the impact to other dam infrastructure and the environs, and allows for staged construction.

The fixed crest of the Piano Key Weir is attractive to the owner in comparison to alternative fusegate or fuseplug options as it minimizes maintenance requirements and eliminates the possibility of incorrect operation causing unintended downstream flooding and the potential large loss of valuable stored water. No decision has been made on proceeding with the plan.

By Michael A. Phillips, P.E., Associate Water Engineer, URS Corporation, Brisbane, Australia; Kelly Maslin, Principal Water Engineer, URS Corporation, Melbourne, Australia; and Andrew Reynolds, Manager Major Projects, Goulburn-Murray Water, Tatura, Australia