Murray Gillon, Robin Fell, Harry Keys, M Foster
Volcanic eruptions at Mt Ruapehu in 1995-96 resulted in the deposition of about 7m of tephra over the rock rim overflow of Crater Lake. There is a long history of lahars (debris flows) associated with releases of water from Crater Lake. The 1995-96 eruptions emptied the lake and it has slowly been refilling from rainfall runoff and snow melt. When the lake level rises above the rock rim the tephra layer will act as a “barrier” or dam. Breaching of the barrier will release water and generate a lahar. The magnitude of the lahar flow will be a function of the lake level at the time of breaching
Extensive studies of the effects of the lahar that would be generated by the failure of the tephra barrier have been undertaken. The studies included a failure modes and likelihood analysis to provide information on the relative likelihood of failure as the lake level rises for the different failure modes applicable to this situation. The paper describes the failure modes considered and the results of the analysis.
The Ross River Dam was constructed in 1974 following design by the State Government, including
hydraulic model testing, by SMEC. The maximum spillway discharge at that time was 1100 m3/s.
Latterly, the dam and spillway have come up for a comprehensive review given that the dam is in an extreme hazard category because of its location only a short distance upstream of the city of
Townsville. The revised hydrology has produced outflow hydrographs peaking at over 4 000 m3/s – more than three and a half times the original – to be passed through the 130 ft (39.62 m) wide
The paper describes the hydraulic modelling planned and carried out to determine changes needed to handle such high discharges. The modelling was to provide for the installation of radial gates and piers, and study of the water level, pressure and dissipation conditions in the dissipator for several key discharges through the range to PMF. Pressure measurements included transients, consideration of the potential for uplift of the basin floor slabs, the integrity of the walls to handle the differential loads, and, as a major consideration, the energy conditions in the flow exiting the dissipator and the integrity of the rock downstream to avoid erosion. Each of these aspects will be addressed in the paper both from the modelling and interpretation standpoint and from the civil structural analysis standpoint, together with a description of the strengthening works required to achieve a satisfactory outcome.
Philip Styles, Brett Stephens, Stephen Perrett
The Wivenhoe Dam Spillway Augmentation Project involved the construction of an additional spill way on the right abutment of the main dam. The right abutment is located in massive sandstones and siltstones of Jurassic and Upper Triassic age.
Seismic refraction surveys and borehole drilling conducted at the design stage for the project indicated that part of the spillway area was likely to be marginally rippable to unrippable using a Caterpillar D9 bulldozer or equivalent. Further assessment and rock strength testing was conducted during the initial stages of excavation where D9 and D10 bulldozers were in operation. The results from this further work indicated that a section of the spillway extending from the proposed position of the ogee crest to approximately 100m further upstream were unlikely to be unrippable for a D9 dozer and marginally rippable for a D10.
Excavation options considered for this section included full scale blasting and load out, limited small scale ‘popping’ combined with ripping or the use of larger ripping equipment. Based on an assessment of cost-benefit, and given the availability of larger ripping equipment, it was decided touse a combination of D10 dozers and a Komatsu 475A bulldozer (D11 equivalent) equipped with single tine ripping tools. The use of this equipment proved successful with better than anticipated production rates being achieved. This resulted in significant cost and time savings for the project and reduced the likelihood of potential adverse impacts on the existing dam grout curtain, environment,travelling public and residents that may have occurred during blasting.
What happens when the people that undertake early environmental investigation stay on as part of the delivery team throughout the design and construction phases of a major project such as the Wivenhoe Alliance?
Often, the early investigation for projects, particularly in the case of environmental impact assessments and approvals processes, is carried out independently of the construction team. In the case of the Wivenhoe Alliance, these issues were set out in the scope of the project itself and delivered by the same team during construction.
The benefits and outcomes have been impressive not only for the project, but for SEQWater and the local community into the future. Improved biodiversity values, increased water quality protection, safety improvements, and value for money are only some of the key benefits experienced.
Individuals within the team also benefit. Environmental professionals are able to implement their
knowledge ‘on-ground’ and progressively improve practices in an area of constant change due to
construction initiatives and timeframes.
This paper explores the specific areas in which the involvement of environmental professionals throughout early investigation and planning, design and construction have benefited the Wivenhoe Alliance and the outcomes that have resulted from this innovative approach.
Trevor Allen, Phil R. Cummins, Trevor Dhu, John Schneider
Ground-motion attenuation models have been derived for the Australian crust. These models
employ both empirical and stochastic methods and are the first spectral ground-motion models to be
based entirely on Australian ground-motion data. In the past, these studies have been hampered by a lack of quality ground-motion data given Australia’s relatively low levels of seismicity.
Two key datasets have been employed to derive empirical ground-motion attenuation models for
Australia; one from data recorded in the Palaeozoic crust of southeastern Australia (SEA) and the
other from the Archean shield region of southwestern Western Australia (WA). Empirical ground motion models are derived for each of these datasets. In general, long-period (e.g. 1 sec) ground motion energy appears to attenuate less in WA than in SEA. These empirical models suggest that SEA has similar near-source attenuation with eastern North America. Because of the limited and spatially clustered nature of the WA dataset, however, we could not constrain attenuation models as well as in SEA.
Stochastic methods are employed to simulate ground-motions for larger earthquakes in regions
where recordings from real events are not available. These models are largely based upon source and attenuation parameters derived from empirical studies. Stochastic models are derived for SEA only.
Spectral ground-motion predicted by these models are generally lower than ground-motion predicted
by both eastern and western North American models, particularly at short-periods (T < 0.5 sec).
Results from this study have significant implications for earthquake hazard and risk in Australia. They
suggest that we are currently overestimating earthquake hazard in SEA. Furthermore, they suggest
that we cannot simply rely on North American ground-motion models to predict earthquake ground
motions in Australia.
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