N. Vitharana, G. Bell, J. Jensen and J. Sinha
When the storage was enlarged in 1971, Wyangala Dam provided a storage of 1220Gl. The original concrete gravity dam was completed in 1936 with an initial storage of 37.5Gl. The enlargement comprised the construction of a central core earth and rockfill dam utilising the existing concrete gravity as an upstream “toe” dam. At its deepest section, the toe (concrete gravity) dam is 60m high with a base length of 40m. The rockfill dam is 85m and the full supply level is at 75m. Two cylindrical reinforced concrete intake towers were constructed utilising the crest of the toe dam as their bases.
Screening level analyses commissioned by The NSW Department of Land and Water Conservation have recommended that detailed seismic assessment of the toe dam and intake towers be undertaken. In 2001, GHD Pty Ltd undertook inelastic time-history analysis using site-specific seismic loadings. Toe dam was modelled together with the rockfill dam using a 2-dimensional model. Intake towers were modelled incorporating the composite behaviour of concrete and reinforcing steel with limited concrete strains to prevent the loss of cover concrete and the buckling of longitudinal steel. Time-history analyses supplements by conventional pseudo-dynamic analysis procedures.
This paper described the constitutive modelling, structural analysis criteria, evaluation of hydrodynamic and dynamic earth pressures and the findings.
C.F. Wan, R. Fell, M.A. Foster
This paper presents the findings of experimental investigation of the rate of piping erosion of soils conducted at the University of New South Wales.
Two tests, namely the Slot Erosion Test and the Hole Erosion Test, have been developed to study the erosion characteristics of a soil. The erosion characteristics are described by the Erosion Rate Index, which indicates the rate of erosion due to fluid traction, and the Critical Shear Stress, which represents the minimum shear stress when erosion starts. Results of the two laboratory erosion tests are strongly correlated. Values of the Erosion Rate Index span from 0 to 6, indicating that two soils can differ in their rates of erosion by up to 106 times. Coarse-grained soils, in general, are less erosion-resistant than fine-grained soils. The Erosion Rate Indices of coarse-grained cohesionless soils show good correlation with the fines and clay contents, and the degree of saturation of the soils, whereas the Erosion Rate Indices of fine-grained cohesive soils show moderately good correlation with the degree of saturation. The absence of smectites and vermiculites, and apparently the presence of cementing materials, such as iron oxides, improves the erosion resistance of a fine-grained soil.
The Hole Erosion Test is proposed as a simple index test for quantifying the rate of piping erosion in a soil, and for finding the approximate Critical Shear Stress corresponding to initiation of piping erosion. Knowledge of these erosion characteristics of the core soil of an embankment dam aids assessment of the likelihood of dam failure due to piping erosion in a risk assessment process.
Tony McCormick, John Grimston, Robin Dawson
Project Aqua is a proposed hydroelectric and irrigation resource sharing development on the Lower Waitaki River in New Zealand’s South Island. The NZ $1 billion project aims to deliver approximately 540 MW peak power at an economically viable price, while minimising environmental and social impacts. Application of traditional hydro concepts in historical studies for the same reach has not provided an economic solution. The current proposal challenges conventional thinking in many areas with innovative concepts allowing a significantly lower cost while not sacrificing safety or flexibility.
Development of storage may involve high social and environmental impacts. No significant storage is needed for Project Aqua as the operation of existing upstream dams can be modified to provide for peaking demand and maintenance of minimum flows. The river intake offers innovative features with its very low profile structure. The concept allows a departure from the traditional barrage or dam diversion and maintains an open braid for jet boat and fish passage. This concept has proven to be a major feature in the overall project progression to the current stage.
The largest impact component of the scheme is the eight canals designed to carry 340 cumecs over 63 km through six power stations. Cuts and fills form the canals with locally derived materials used for the embankments and lining. Expensive lining has been minimised by balancing flow exchange with groundwater through the cut and fill sections.
Feasibility design has been completed and resource consents are currently being sought. This paper will cover the significant design features and impacts.
Mike Taylor, Paul Maisano and Rod Conway
Daylesford Dam forms an ornamental lake, known locally as Lake Daylesford, situated on Wombat Creek within the heart of Daylesford in Victoria. It is a focus of the local tourism industry and is vitally important to the Daylesford community as a recreational, social and environmental asset, with important heritage value.
On 24 October 2000, the 12m high embankment was overtopped following heavy rainfall and was in danger of breaching. This could have resulted in loss of the dam and lake, downstream damage to roads and the environment and possible loss of life. The overtopping of the dam prompted the Hepburn Shire Council, land manager for the dam, to initiate a safety review of the dam as well as the commissioning of a Dam Surveillance Program and a Dam Safety Emergency Plan.
The spillway is of the side-channel type with a 30m long concrete sill at the entrance discharging into a 5m wide unlined trough and chute. The existing spillway can only accommodate a peak flow of 24m3/s, which represents an AEP of less than 1 in 20. The required flood capacity in terms of the latest ANCOLD guidelines on spillway adequacy is for an AEP of 1 in 1 000 which equates to 120m3/s.
Following discussions with Hepburn Shire Council, and an evaluation of public usage of the Lake Daylesford area, it was assessed that the following constraints apply when considering options for increasing spillway capacity:
The proposed solution includes the following:
J.H. Green, P.E. Weinmann, G.A. Kuczera, R. J. Nathan and E.M. Laurenson
Assigning an Annual Exceedance Probability (AEP) to the Probable Maximum Precipitation (PMP), and subsequently to the PMP Design Flood, is an integral part of the risk assessment process for large dams. Laurenson and Kuczera (1998) conducted a review of existing PMP risk estimation practices in Australia and concluded that, in the absence of any better information, the work by Kennedy and Hart (1984) provided the most appropriate estimates to adopt but with the proviso that the method should be viewed as interim pending the outcomes of ongoing research.
This paper gives an overview of a joint research project that is working towards obtaining credible estimates of exceedance probabilities of extreme rainfalls using the concept of storm arrival probability and storm transposition probability. It also outlines the work to be carried out over the next 12 months that will culminate in the combining of the outcomes of the two components and the application to test catchments. Finally, the paper discusses desirable follow-up action to promote the adoption of the research results by practitioners.
R.J. Nathan, P.E. Weinmann and P.I. Hill
Current practice for estimation of design floods is typically based on the “design event” approach, in which all parameters other than rainfall are input as fixed, single values. Considerable effort is made to ensure that the single values of the adopted parameters are “AEP-neutral”, that is, they are selected with the objective of ensuring that the resulting flood has the same annual exceedance probability as its causative rainfall. While this approach represents current best practice in Australia (and overseas), it does suffer from a number of limitations.
This paper describes the development and application of a Monte Carlo (or joint probability) framework which offer an alternative to the design event method. This technique recognises that any design flood characteristics (e.g. peakflow) could result from a variety of combinations of flood producing factors, rather than from a single combination. The approach mimics “mother nature” in that the influence of all probability distributed inputs are explicitly considered, thereby providing a more realistic representation of the flood generation processes.
The advantages of the technique are illustrated by application to a hypothetical dam located on a real catchment. The manner in which standard design inputs are incorporated are discussed, as is the relationship of the approach to current guidelines.