Richard Herweynen, Colleen Stratford
Assessing the potential for erosion of foundation rock downstream of a spillway is a problem faced on many dams, whether new or existing. The problem is made particularly difficult not only due to the uncertainty in determining the erosion potential of the rock, but also due to the variable hydrologic characteristics of flood events.
The selected spillway option for Wyaralong Dam comprises a centrally located primary spillway with a secondary spillway located on the left abutment. A stilling basin energy dissipater is provided at the toe of the primary spillway. Downstream of the secondary spillway, an apron channel will direct flows back to the stilling basin. However, for flood events larger than the 1 in 2000 AEP event, the capacity of the secondary spillway apron is exceeded and flows spill out across the left abutment of the dam towards the river channel. Erosion of this left abutment was viewed to be a potential dam safety issue, and as such, careful consideration was required during the design stage to determine the acceptability of this spillway arrangement.
In order to provide structure to a problem which often relies solely on engineering judgment, a decision process was developed, taking into consideration some of the more definable aspects of the problem. These aspects included the geological characteristics, the initial hydraulic characteristics, the flood duration, the nature of erosion should it occur and the stability of the dam. This paper describes the decision process and methodology used at Wyaralong Dam to
determine the acceptability of erosion. This paper will present the process in a way that it can be used by others in future dam projects, both new and upgrades.
A Unique and Holistic Approach to the Erodibility Assessment of Dam Foundations
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An essential criterion for any new dam project in Australia is to provide for passage of fish past the structure in both the upstream and downstream direction. In recent projects with a relatively high barrier this has been provided by mechanised systems such as locks, lifts or a combination of both.
A nature-like fishway provides for passage of fish past a barrier by applying some of the features of natural streams. The concept has been increasingly applied to fishway designs in North America and Europe. A nature-like fishway will provide variable flow depths, velocities and turbulence across its width and along its length and is constructed using natural materials to simulate the natural stream characteristics. The variable flow conditions coupled with the use of natural materials inherently result in different channel substrates that support the passage of a large range and size of fish species as well as other aquatic species. Where fish habitat has been depleted, a nature-like fishway can also supplement and enhance aquatic habitat.
The performance of nature-like fishways can be difficult to quantify due the very nature of the system. However, qualitative assessments in North America are indicating that a wide range of species are using such fishways and that fish species that were previously extirpated from rivers are again migrating.
The nature-like fishway concept has been applied to in-stream structures up to four metres high in the eastern states of Australia. However, the substantial progress made with this design in North America and Europe has not as yet been applied in this country.
This paper analyses the advantages and disadvantages of nature-like fishways over mechanised systems, such as locks and lifts, and makes an assessment of the suitability of the concept to dams in Australia with relatively high walls.
M. Tooley, N. Anderson, N. Vitharana, G. McNally, C. Johnson and D. Moore
There is a significant stock of aging concrete dams in Australia which would not meet the requirements of the current recognised dam safety practices applicable to concrete gravity dams.
In this paper, field and laboratory investigations undertaken for two concrete gravity dams are presented, these being Middle River Dam and Warren Dam both owned and operated by the South Australian Water Corporation. The field investigations included a comprehensive drilling program recovering core samples ranging in diameter from 61mm (HQ) to 95mm (4C), continuous imaging (RAAX) of the drilled holes and installation of piezometers. Geological logging of the holes and mapping of the unlined spillway were also undertaken. The laboratory program included the testing of concrete lift joints and concrete samples in direct tension, shear and compression.
Concrete in Middle River Dam is suffering from extensive Alkali Aggregate Reaction (AAR), and consequently a suite of laboratory testing is being undertaken to determine the current level of deterioration and residual reactivity so that potential future AAR-induced expansion can be incorporated into any upgrade design solution.
The main purpose of the study is to determine whether site-specific parameters can be used to re-assess the stability of these two dams as calculations, based on the current standards, have shown that the dams have exceeded the allowable factors of safety values at the storage water levels experienced to date.
The findings may be useful to dam designers and owners faced with the upgrading of concrete dams, where traditional assumptions can result in no upgrade or an upgrade costing several million dollars.
Hamish Smith, Graeme Maher
In order to achieve environmental sustainability it has become standard engineering practice to include a fishway on all new or refurbished large dams in Australia.
As regulators expand their understanding of fishways, project approval conditions associated with these complex engineering structures are changing. Regulators now increasingly wish to participate in the development and selection of the final fishway to be adopted.
This paper describes the process developed and implemented at Queensland’s most recent dam under construction, the Wyaralong Dam, to ensure that the views and opinions of regulators and stakeholders were sought and considered during the fishway selection and design process.
With no written guidelines available on “how to select and design a suitable fishway”, all associated parties entered into the process without a full knowledge of how it would unfold and what the final outcome would be.
This paper demonstrates that in an increasingly regulated environment it is possible to have regulators, proponents and stakeholders work cooperatively together to achieve a result that provides for sustainable development and is acceptable to all parties.
This paper will provide a model that could be adopted for the development of new fishways or the refurbishment of existing fishways on large dams in Australasia.
Changing Regulatory Environment – Large Dams and Fishways
C.Johnson, D.Stephens, M.Arnold and N.Vitharana
As part of Melbourne Water’s dam safety upgrade program, emergency release capacity is being investigated at a number of dams. Recent work undertaken by the Water Resources Alliance (WRA) for Melbourne Water has highlighted the lack of current Australian guidelines for appropriate emergency release capacity. With no relevant ANCOLD Guidelines, current practice still references the 1990 USBR guidelines which relate the length of time to empty a reservoir to the hazard and risk associated with dam failure. As hazard category assessment criteria has been improved since and dam design and safety standards are more stringent, the applicability of the USBR criteria in today’s environment is under consideration.
With the prevailing climatic conditions requiring the augmentation of Melbourne’s water supplies, the Tarago Reservoir was recently brought back into service. However, the dam lacked adequate emergency and environmental release capacity, with this being critical to manage construction flood risk for a pending filter raising project. Through an analysis of recorded inflow data, it was evident the existing scour facility had insufficient capacity to handle the recorded inflows, and would not be able to maintain the reservoir at an appropriate level during the proposed works. The length of time to empty the reservoir for the existing scour facility and the preferred scour upgrade option were calculated and it was found that by providing a new 1200mm scour facility, USBR emptying times were met or exceeded. The enlarged outlet capacity was also required to meet the new environmental flow requirements for the dam.
The paper will review international guidelines, share the experience of several Australian water authorities in assigning emergency release capacity for their dams, and discuss the specific work undertaken to provide suitable emergency release capacity at Tarago Reservoir for Melbourne Water.
Jared Deible, Richard Herweynen, Gary Dow
The foundation is an important element in the stability of any dam. Understanding the foundation and the potential failure mechanisms associated with the dam foundation is critical to developing the final dam design. This paper will discuss the challenges encountered with the foundation at the Taum Sauk Upper Reservoir Dam and the Wyaralong Dam.
The Upper Reservoir of the Taum Sauk project is a 2.3 million cubic metre roller compacted concrete (RCC) dam located near Ironton, Missouri, USA. The RCC dam was constructed in accordance with United States Federal Energy Regulatory Commission (FERC) guidelines to replace a rockfill dike that failed abruptly on December 14, 2005. Wyaralong Dam is a new RCC dam, for water supply, located on the Teviot Brook near the township of Beaudesert in south-east Queensland.
Wyaralong and Taum Sauk each had challenges associated with identifying potential failure mechanisms in the foundation and with analysing the stability of the dam for these potential failure mechanisms. The geology at the projects was very different, but challenges for each project were quantifying the amount of reliance that was placed on the rock mass at the toe of the dam, developing the shear strength parameters, and developing the associated failure mechanisms that would be analysed.
The design of Wyaralong and the rebuilt Taum Sauk Upper Reservoir, including the geometry of the dam sections, were developed based on the foundation features at each project. Foundation treatments and excavation designs were developed based on the stability analyses conducted during the design phase. These foundation treatments included removal of weak layers or defects where necessary, but features were left in place in the foundation at selected locations at each project. Where features were left in place, stability analyses concluded the dam was stable. The stability analyses at each project considered three dimensional effects along features in the foundation where appropriate.
As the foundation was uncovered during the construction phase of each project, the parameters used in the stability analysis conducted during the design phase were confirmed or adjusted. The excavation and foundation preparation activities were adjusted as necessary based on actual conditions during the construction phase.
Challenges Associated with Identifying and Analysing Potential Failure Mechanisms in Dam Foundations – Taum Sauk Upper Reservoir Dam & Wyaralong Dam Case Studies