The Ross River Dam, designed in the early seventies, does not meet current dam safety criteria for overtopping and piping within the embankment or the foundation. The dam comprises a 40m long concrete overflow spillway flanked by a central core rockfill embankment of 130 m on the right bank and 170m on the left bank with a 7620 m long left bank earth fill embankment, which has no internal filter zones for piping protection. The embankment was extensively assessed and treated forfoundation deficiencies in 1982, and further assessed in 20002002 for appropriate upgrade options.
This paper describes the process of validation of the detailed design using Risk Based Design Criteria. This process included data mining for historical performance and original design intention,
comparison of the original design against current and historical investigations and assessment of the upgrades using the large volume of data available from previous work. A design team comprising specialist hydrologists, hydrogeologists, geologists, geotechnical and dams engineers worked within a risk assessment framework at all stages of the design to ensure the design was validated using the design Validation Model. This process incorporated assessment of crest level based on flood risk and wave overtopping, review of 2D and 3D seepage models to assess piping and foundation erosion potential, assessment of fissured soils within the embankment foundation for structural stability and evaluation of spillway model testing for potential spillway failure modes.
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N. Vitharana and S. Terzaghi
There is a large stock of embankment dams throughout the world needing the assessment of their
safety as required by modern dam safety regulations. Due mainly to economic and site constraints
associated with potential dam upgrading work, it is imperative that a rational approach be adopted in
assessing their safety and in designing the remedial works. One of the most important criteria is the
selection of appropriate geotechnical parameters under different conditions. Predominant loading
conditions in a dam are much different from those in other structures such as bridge and building
foundations and therefore the direct adoption of traditional approaches may not always be valid. This
paper presents the various aspects of issues associated with the stability assessment of dams including
the rational selection of the parameters and numerical codes available to dan/geotechnical engineers
to assess their safety.
Deryk Forster and Manoj Laxman
The Stage I construction of the Ross River Dam was completed in December 1973. The reservoir
reached full supply level (FSL) and then spilled in January 1974. In 1976, the left embankment was
raised to Stage II level. Spillway gates were installed in February 1978 with full supply level for
Stage 1A (FSL).
In the years following the first filling of the reservoir after the raising of FSL, salt scalding
downstream of the northern portion of the left embankment occurred. This was attributed to
foundation seepage. Investigations started in 1978 to define what remedial measures were required to ensure the safety of the left embankment. Fissured clays were first discovered in the foundations of the Ross River Dam during these investigations.
Fissures could substantially reduce the overall strength of the soil foundations. Therefore the effect of these fissures needs to be considered when evaluating the acceptable levels of reliability against
embankment failure. More extensive fissuring was discovered during the current investigations and a
cataloguing system was employed to characterise the foundation conditions.
A simplified layer model was adopted early on in the design but did not fully demonstrate the
complexity of the subsurface conditions. Extensive use was made of historical geological data,
current investigation data and the application of GIS systems. The resulting model more clearly
represents the foundation conditions and high degree of variability and was used in subsequent risk
assessments for the upgrade design.
Peter Hill, Kristen Sih, Rory Nathan, Phillip Jordan
This paper presents a number of innovative hydrologic investigations undertaken for the recent detailed design of upgrades for Ross River Dam in North Queensland. A key issue for estimating extreme floods in the tropics is the estimation of flood events of long critical durations. The implication is that there is an increased focus on estimating the correct volume (not only the peakflow). This paper describes the regional analysis of flow volumes that was used to validate the estimated flood volumes.
Another issue of considerable importance is the assumed relationship between inflows and initial reservoir level. The analyses described in this paper showed that inflows are independent of reservoir levels for the more frequent events but for more extreme events they are correlated. This has important implication on how the initial reservoir level is incorporated in the hydrologic analysis. The final aspect covered by the paper is the derivation of seasonal flood frequency curves. This is particularly important given the highly seasonal nature of rainfalls in the tropics and the results are important for assessing risks during construction and scheduling the upgrade works
Changes to the Regulatory and legal environment have resulted in an increased focus on the
importance of proficient management of dams. Operation and maintenance manuals are now a
Regulatory requirement in Tasmania for all but very low hazard dams and are also required to ensure that dams are managed efficiently and safely. To meet these requirements Hydro Tasmania has developed the ‘Smart’ operations and maintenance manual.
Hydro Tasmania has a large portfolio of dams and as a result requires a large number of operations and maintenance manuals. This would result in an overwhelming array of information that is subject to evolving change if the traditional approach to the manual was adopted. To overcome this burden, a controlled electronic manual was developed to enable:
• Critical operation and maintenance information to be collated with minimal effort;
• Electronic hyperlinks to key existing operation and maintenance documents, reference
materials, and portals into operational data bases; and
• A means of updating and controlling information that is subject to change.
This paper will discuss how Hydro Tasmania developed its user-friendly operation and maintenance manuals in an innovative, unique and controlled manner to ensure prudent management of dams and to comply with Regulatory change.
Sonny Connors, Shaun Nugent, Brett Taylor, Brian Walford
The Tarong coal-fired power station near Kingaroy in southern Queensland discharges ash to a storage facility of 42,000 ML capacity, impounded by a 48 m high-zoned earth and rockfill dam embankment. The embankment was constructed in 1980–81. In recent years, Tarong Energy Corporation (TEC) has investigated a number of options for a new storage facility as the remaining capacity of the existing ash dam storage diminishes. TEC determined that the existing facility should be upgraded to provide additional storage capacity for the short term. At the same time, there emerged a requirement to improve the long-term seismic resistance of the embankment. Enlarging the existing spillway cut provided the material for a 400,000 m3 weighting zone and, by reducing the design flood freeboard, extended the ash disposal capacity by several years without a need to raise the embankment. Challenges included significant foundation seepage and deteriorated riprap. The paper describes the issues, risks, adopted criteria, investigation undertaken, and implementation of the upgrading works. Innovative approaches to the provision of future storage capacity are outlined.