Monique de Moel and Gamini Adikari
Parks Victoria manages over 4 million hectares of parkland and a portfolio of over $1.9 billion worth of infrastructure assets. Within this portfolio, Parks Victoria is responsible for a large number of dams and their associated structures. Consequence category of these dams varies from Extreme to Very Low. Parks Victoria recognised that these assets required a dam safety management and monitoring program. The development of a program commenced with a portfolio risk assessment in 1998 which progressed to detailed design reviews of a selected number of dams and the initiation of an ongoing dam safety and surveillance program. This initial work identified the need for dam safety upgrade works within this asset portfolio which Parks Victoria has been progressively addressing. In 2012 Parks Victoria identified that a review of the risk profile of the dams was warranted. The review included consideration of alternative options such as staging of works, reducing storage volume and decommissioning, as well as non-technical considerations such as increasing the recreational use and the environmental value of these assets. This paper outlines the approach adopted by Parks Victoria in developing and improving its dam safety program and how it has assisted in minimising dam safety risks. Specifically, Parks Victoria’s approach of adopting measures that recognize the purpose and benefits of the individual storages, whilst being sympathetic to the requirements of the other infrastructure within its diverse portfolio of assets is highlighted. Since this work commenced in 1998, Parks Victoria have been successful in the development of an effective dam safety and management program which has resulted in the reduction of risks associated with this portfolio of assets.
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Simon Lang, David Stephens, Peter Hill, Mark Arnold and Tommie Conway
Considerable thought has been given in recent years to managing the risks associated with floods during the construction of new dams and dam upgrades. Both ANCOLD and the NSW DSC provide some limited advice on how this risk should be managed, with many dam owners aiming for societal risk during construction to be no higher than pre-construction. One approach to do this is to draw down the reservoir such that sufficient airspace is created to reduce the probability of overtopping the construction works to be equal to that of overtopping the dam crest pre-construction. However, this frequently leads to very large releases of valuable water resource being required. This approach also fails to consider that the conditional probabilities of failure may be quite different during construction than during normal operation. A risk-based approach was applied for the recent upgrade of Tarago Reservoir. Existing event trees from a failure modes analysis were adjusted to reflect the construction conditions. In some cases, the event probabilities increased (for example as a result of excavation of the dam embankment), however some also decreased (for example as a result of more rapid means of detecting and intervening in breach formation during construction). The conditional probabilities of failure during construction were then used to estimate the overall seasonal probability of failure, and it was found that a limited draw down of the reservoir would be sufficient to ensure that risks were no higher during construction than pre-construction. To reinforce this, the cost-to-save-a-statistical life was estimated for further drawdown of the reservoir and used to demonstrate that the risks were as low as reasonably practicable.
Matthew Sentry and Darren Loidl
To triple Yass’ water storage capacity, Yass Valley Council was required to increase the height of their existing concrete weir by 3.0 m. The 100 m wide weir was originally constructed back in the 1920’s. Upgrade works to the weir included raising the height of the existing concrete weir by 3.0 m with reinforced concrete; install 33 number 27 strand post-tensioned ground anchors vertically into the crest; construct a new outlet structure; upgrade existing mechanical pipe works; and replace the existing pedestrian bridge with a concrete bridge capable of vehicle traffic.
The key project constraints during construction were to maintain constant water to the town’s water treatment plant and maintain minimum 70% reservoir storage.
The original weir had no auxiliary means of flow diversion and the construction constraints meant that the water storage could only be reduced by 1.0 m from the existing crest during construction, resulting in the construction work being carried out in an active water course with minimal means of flow diversion. These key project constraints meant that there was a high risk of flooding during construction work.
Geotechnical Engineering was engaged by Yass Valley Council to carry out the required upgrade work at Yass Dam. Prior to construction work commencing, risk workshops with client and designers clarified the flood risks during construction. To minimise the impact of flood events during construction, Geotech implemented several flood mitigation measures which were controlled by a detailed construction flood management plan. These control measures included construction of two temporary diversion slots cut into the existing concrete weir capable of supporting a 1 in 2 year rain event whilst allowing construction work to continue; re-design of concrete works to minimise the volume of concrete which was to be cut from the existing wall’s downstream face; detailed construction sequencing to minimise impact to existing and new wall during construction work; and the early installation and stressing of anchors.
Although a detailed construction flood management plan was developed and implemented, the Yass Dam site was impacted by 13 floods during the 20 month construction period. Several floods recorded water levels between 1.5 m and 1.9 m above the existing crest, resulting in work ceasing for weeks if not months at a time. As a result of the consistent flooding, Geotech was able to develop stronger and more resilient methods to be able to effectively work within an active watercourse on dam structures where minimal flow diversions are available. This paper presents the unique techniques implemented through the Yass Dam Upgrade project and discusses the effectiveness of these techniques and lessons learnt through the 13 flood events experienced.
Kinchant Dam is a zoned earth and rockfill embankment situated on the north branch of Sandy Creek, approximately 30 km southwest of Mackay in central Queensland. Kinchant Dam was constructed in stages. The ‘Initial Development Stage’ which consisted of an embankment length of approximately 3.3 km and full supply level (FSL) of EL 49.21 m AHD was completed in 1977. Further development completed in 1986 (Stage I) increased the FSL to EL 57.21 m AHD with an embankment length of 5.5 km and a maximum embankment height of 22.3 m. The dam has a storage capacity of 62,800 Ml and a 60 m wide emergency spillway with a fixed crest level of EL 58.21 m AHD, one metre higher than the FSL.
A series of investigations have been carried out since its construction as a consequence of both regulatory safety reviews and observed excessive pore pressures within the foundation that have led to wet patches developing at the toe of the dam. In one area at the toe, pore pressures were such that artesian conditions developed. This paper outlines the history of various stages of construction of the dam, the foundation investigations since construction and the safety review and comprehensive risk assessment process that lead to the upgrade design and construction of remedial works. The remedial works include the extension of the downstream filter material adjacent to the clay core and the provision of additional pressure relief wells at the downstream toe of the dam.
Peter Mulvihill and Ian Walsh
The Falls Dam was constructed in the 1930’s to provide storage for several irrigation schemes in the Manuherikia Valley situated in New Zealand’s South Island region of Central Otago.
The opportunity to retrofit a small hydropower plant to the concrete faced rock fill dam was taken in 2003, utilising existing tunnels complemented by an innovative syphonic penstock system. The key design and construction features of this integrated scheme are described, along with experience from the first 10 years of the generation performance.
Looking ahead, there may be further integration challenges as current investigation of irrigation storage requirements leads to major redevelopment at this dam site and substantial changes to generation parameters.
Jamie Campbell, Gregg Barker, Paul Southcott and Michael Wallis
The assessment of consequences of dambreak is used as input to the design parameters of dams, dam safety requirements and dam risk assessments. For many low consequence category dams, the consequences of failure can be dominated by itinerants, in particular vehicles on roads within the dambreak inundation area. Estimating the population at risk (PAR) and potential loss of life (PLL) rigorously is mathematically complex, requires significant user judgment and can be very sensitive to input assumptions. This paper presents a simple, practical tool that has been developed to assist engineers and analysts in assessing the PLL of itinerant road users within a dambreak inundation zone. The tool allows for a logical and defensible analysis based on an event tree approach and provides guidance on appropriate factors to be used in calculating the overall fatality rate of people exposed to the dambreak hazard. This paper details the tool and how to apply it to typical dambreak problems, providing the reader with the information required to estimate the consequences on itinerant road users; the paper also details how the concepts discussed can be applied to other itinerants.