Malcolm Barker, Toby Loxton
The Gladstone Area Water Board (GAWB) owns and operates Awoonga Dam, which is a concrete-faced rock fill embankment with a fixed crest concrete spillway on the left bank impounding a storage volume of 770,000 ML.
The current arrangement can accommodate the Probable Maximum Flood, allowing for flow over Saddles 3, 4 and 6 on the left abutment. A comprehensive study was carried out to evaluate the erosion potential downstream from Saddles 3 and 6 as well as other spillway options adjacent to the existing dam. One option was a radical approach including the removal of the Saddle Dam 3 and provision of downstream erosion protection works. This reduced the PAR and improved the overall dam flood capacity; however concerns were expressed about the environmental impact of possible erosion downstream from Saddle 3 for relatively frequent events.
A risk assessment showed that the erosion protection works downstream from the Saddle 3 or 6 were not cost effective and the preferred option for the upgrade was the closure of the Saddle Dam 3 with an auxiliary spillway created in Saddle 6,
This paper summarises the methods used and the outcomes from this study.
2011 – Awoonga Dam Acceptable Flood Capacity design – the anguish of erosion risk and implications for design
The enlargement of the Cotter Dam is being undertaken by ACTEW to provide a greater security of water supply to Canberra. The project involves constructing a larger, higher new dam wall immediately downstream of the existing Cotter Dam, to allow the present dam to continue functioning and supplying water while construction is underway. The project raised a number of environmental issues partly because the Cotter Dam currently supports a self-sustaining population of (endangered) Macquarie Perch, and because the Bendora Dam, upstream of Cotter Dam, contains a breeding population of (endangered) Trout Cod. Bendora Dam will not be physically affected by the works on Cotter Dam, but its operations may be altered. An ecological risk analysis was conducted to identify critical environmental risks that would need to be investigated and managed or ameliorated and management strategies were put in place to reduce risks. ACTEW have adopted an adaptive management approach to the project, but in order to implement that approach it is necessary to conduct effective monitoring of the fish populations of concern. These potentially include the two endangered species, as well as potential predators (such as cormorants) and competitors (such as trout). Power analysis has been used as a tool to evaluate whether it is feasible to monitor key populations sufficiently rigorously to be able to confidently detect a change (either an increase or decrease in a population). For Macquarie Perch and trout it should be possible to detect population changes statistically with a logistically feasible monitoring program.
2011 – Using risk analysis, power analysis and adaptive management to minimise ecological impacts of the Cotter Dam enlargement
Paul Somerville, Hong Kie Thio
There is a large degree of uncertainty as to the true state of nature (i.e. epistemic uncertainty) regarding many aspects of seismic hazard analysis. Such differences are often highlighted by differences between alternative models put forth by different model proponents. This epistemic uncertainty is treated by giving weight to all viable alternative models through the use of logic trees in probabilistic seismic hazard analysis, rather than just using a preferred model. This paper reviews epistemic uncertainties that arise from alternative distributed earthquake source models; alternative models for the recurrence of earthquakes on those sources; alternative approaches to including active faults; alternative models for the recurrence of earthquakes on active faults; alternative ground motion prediction models for Australia; and alternative methods for incorporating site response. It also reviews alternative representations of the design response spectrum for the development of ground motion time histories.
2011 – Recent Developments in Seismic Hazard Analysis
Dan Forster, Murray Gillon
A robust and defensible dam surveillance process is considered to be the ‘front-line of defence’ in ensuring dams do not present an unacceptable risk to people, property and the environment. The concept of a ‘Quality Chain of Dam Surveillance’ describes the surveillance process as a multi-linked chain where each step in the process forms a critical link. Without rigorous attention given to quality assurance links in the chain can become tenuous or broken and thus compromise the integrity of the whole chain. Hydro Tasmania is currently re-engineering its existing surveillance process using the Quality Chain of Dam Surveillance as a basis.
This paper presents the concept of the quality chain and uses the Hydro Tasmania improvement initiative as an example application of the concept. The paper is intended to provide a fresh perspective on what is sometimes considered a stale topic and reinforces the need for a considered approach to dam surveillance.
2011 – The Quality Chain of Dam Surveillance
Nanda Nandakumar, Janice Green, Rory Nathan, Kristen Sih& Robert Wilson
A detailed assessment of hydrologic risk was undertaken for Hume Dam. Data available and relevant to the hydrologic risk assessment were collated and assessed. The catchment was divided into 35 different sub-catchments, each with its own set of parameters that characterised the local hydrologic response. Recorded streamflow was used to calibrate the flood response of selected gauged sub-catchments, and a combination of historic and synthetically-derived data was used to validate the model and loss parameters. The 35 models were combined into a single catchment-wide model. A Monte Carlo approach was adopted for the validation of the models and the derivation of Hume Dam inflow and outflow frequency curves. A range of PMFs which satisfy ANCOLD’s definition of the PMF were also estimated. The PMPDF outflow was estimated to be 7,600 m3/s which can be passed by the dam. Depending upon the assumptions made, the peak PMF outflow was estimated to be in the range from 10,300 m3/s to 14,900 m3/s
2011 – Assessment of Hydrologic Risk for Hume Dam
Krey Price, Mike Harvey, Bob Mussetter, Stuart Trabant
The California Department of Water Resources, Division of Dam Safety (DWR-DSD), has determined that San Clemente Dam on the Carmel River in Monterey County, California, does not meet seismic safety standards. Several alternatives have been considered to decommission the dam and eliminate the hazard, including thickening of the 25-m-high, concrete arch structure, lowering the dam, and complete removal. At the present time, the upstream reservoir that had an original storage capacity of about 1.8 GL, is essentially filled with sediment. The 29-km reach of the Carmel River between the dam and the Pacific Ocean passes through urbanised areas within the upscale Carmel Valley; flooding and channel stability in these areas are significant concerns. The Carmel River also contains habitat for the endangered steelhead and red-legged frog that could be positively or negatively affected by the decommissioning.
After an extensive series of hydraulic and sediment transport modelling studies, two actions remain under consideration: (1) dam thickening, which will require reconstruction of the existing fish ladder and construction of an adjacent, 3-metre diameter sluice gate to prevent sediment build-up from blocking the ladder outlet, and (2) removal of the dam and rerouting the river into a tributary branch of the reservoir, which would isolate approximately 65 percent of the existing sediment deposits from future river flows and eliminate a significant fish-passage problem. Both options were modelled extensively in hydrologic, hydraulic, and sediment transport applications. Since available models do not adequately represent sediment dynamics at the sluice gate, a special sediment routing model was formulated to evaluate this aspect of Option 1. Option 2 is currently preferred by the resource agencies, since it would optimise endangered species habitat; however, this option would be three to four times more expensive than Option 1, and funding limitations may impact the alternative selection. Evaluation efforts are ongoing, along with approaches to address liability issues associated with the decommissioning actions for the privately owned facility, while optimising the benefits and costs of the selected action.
Modelling Studies to Design and Assess Decommissioning Actions for a Seismically Unsafe, Concrete Arch Dam