Rachel Jensen, Adam Broit, Chriselyn Kavanagh
Downstream emergency response is a critical driver in the consequences and potential life loss associated with dam flooding and failure. This response is highly varied between stakeholders, communities and the nature of the flooding or dam threat. As assessments on dam failure consequence and potential loss of life become increasingly important in understanding holistic dam risk, they are also becoming increasingly complex.
As part of a portfolio wide Comprehensive Risk Assessments, Sunwater have undertaken workshops with a wide range of stakeholders to better understand downstream emergency response and the warning timeline. The workshops have been aimed at facilitating better downstream stakeholder engagement, obtaining key data for consequence assessments and developing consistency in assumptions for potential life loss.
This paper presents the standardised methodology undertaken for warning time workshops, the outcomes for a range of downstream stakeholders and correlations between stakeholder groups which influence warning time response. These outcomes may be used by practitioners in the absence of catchment specific warning time data and provide a counterpoint to international standard warning time assumptions.
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Zara Bostock, Helena Sutherland
Ewen Maddock Dam is located approximately 12.0 km west of Caloundra, in the Sunshine Coast area of Southern Queensland. The dam is a homogeneous earthfill embankment dam 10.5 m high and 724 m long. The dam was originally built between 1973 and 1976 and later upgraded in 1982 to raise the ogee spillway crest by 2.44 m to the current Full Supply Level (FSL) of 25.38 m AHD.
Seqwater is undertaking a staged upgrade of Ewen Maddock Dam to address deficiencies identified during the Acceptable Flood Capacity (AFC) Review (GHD, 2010). The consequence category assigned to Ewen Maddock Dam is ‘Extreme’ with a downstream Population at Risk greater than 1000.
Stage 1 construction was completed in 2012 to manage the seepage underneath the dam to reduce the risk of piping and improve embankment stability. Stage 2A involved retrofitting a filter in the existing embankment and raising the dam 1.61 m to 30.11 m AHD using a reinforced concrete parapet wall. Stage 2B involves spillway upgrade works and was split from 2A due to approval constraints.
Stage 2A construction was completed in April 2021, navigating various project and dam safety challenges. This paper presents some practical ways dam safety and risk was managed on the ground from the perspective of both the designer and owner.
Sonel Reynolds, Alex Gower, Bob Wark
During the outlet works upgrade in 2017 it was found that the valve pit and stilling basin at Mundaring Weir were not founded on rock. Based on these observations and the arrangement of the spillway and outlet works, it was considered that during significant spillway overflow events, a high velocity jet could displace the stilling basin slabs, erode the underlying material, and progress to failure of the outlet pipe and valve pit. A comprehensive risk assessment was conducted to estimate the likelihood of stilling basin slab uplift, erosion of the underlying material, and failure of the outlet works. A geotechnical investigation was undertaken comprising drilling nine boreholes and a program of geophysical downhole logging. Computational Fluid Dynamic (CFD) modelling was used to determine the pressure fluctuations and turbulence intensity over the spillway slab which could lead to uplift. The erodibility of the rock mass material below the stilling basin slabs was assessed using the outcomes of the geotechnical investigations and CFD output, with analyses based on the Kirsten Index and eGSI. A net benefit analysis was conducted to assess whether preventative remedial works were justified. Through this process it was demonstrated that the business risk was low and risk reduction measures were not justified.
Christopher Dann, Chad Martin, Garry Fyfe, Nigel Rutherford
This paper presents a case study on remedial works that were undertaken at Lock and Weir One
along the River Murray, that to our knowledge are the first of their kind in Australia.
The weir structure’s left abutment is comprised of a stepped concrete structure founded on timber
piles, with timber sheet piles extending beneath the structure to cut off seepage through underlying
alluvium. A piping incident occurred at the left abutment in late 2014 and a filter blanket was
installed as an emergency response measure. A detailed review of historic construction documents
showed that there was a “missing” timber sheet pile upstream of the piping boil. Geotechnical
investigations, including piezometer installation confirmed the missing timber sheet pile was the
likely cause of the piping incident. A piping risk assessment showed the residual risk of further piping
was reasonably high.
A range of remedial works was considered as permanent risk reduction works. However, these
solutions required extensive temporary works to expose the missing timber sheet pile including a
cofferdam to access the defect and partial demolition of a recently constructed fishway structure.
An alternate Secant ‘Grout Column’ solution was developed that comprised targeted drilling and
backfill grouting to close the gap where the sheet pile was not installed and to grout an inferred void
under the abutment structure. This solution was successful at reducing seepage through the
abutment structure, as indicated by monitoring piezometers.
Chris Nielsen, Ron Guppy, Gary Hargraves, Robert Fowden
Dam safety upgrade projects of major dams typically involve a large capital investment. It is important that expenditure decisions are based on sound criteria, both technical and non-technical. Independent peer review of technical matters plays a key role in meeting design, construction and safety objectives within practical financial constraints and assuring robust, resilient and reliable project outcomes.
An independent technical review is recommended for all dam projects.
The Queensland dam safety regulator has developed guidelines associated with technical review for dam safety projects that considers scope and limitations, expertise and governance. The guidelines are informed by literature, recent projects, a commission of inquiry, internal and external review and industry feedback. The guidelines are being implemented across major dam safety upgrade business cases through preparation of terms of reference by the Queensland Government’s business planning and implementation entities, who maintain the responsibility of providing assurance to state government projects, as well as the state’s major dam owners.
The terms of reference, supported by the underlying principles in the guidelines, provide a platform for consistent and appropriate application of technical assurance to dam projects in Queensland. Among other matters, governance is highlighted as a critical factor for success as well as clarity of the roles, responsibilities and reporting lines of all parties. The application of both guidelines and terms of reference to recent projects is discussed.
Vicent Espert, Peter Buchanan, Colleen Baker, Malcolm Barker, Mark Locke
Mangrove Creek dam is an 80 m high CFRD constructed between 1976 and 1982 for water supply to the NSW Central Coast area, and is currently operated by Central Coast Council (CCC). The dam is classified as a ‘High A’ Consequence Category dam for both Sunny Day and Flood breach in accordance with ANCOLD guidelines.
Previous assessments of the dam identified that it would not be able to safely pass the ANCOLD Fallback flood capacity of the PMP flood in its current configuration. As such, the dam has been operated at a restricted full supply level for many years.
In 2020, GHD was engaged by CCC to develop a concept and detailed design to increase the spillway capacity using a standards-based approach to achieve the flood capacity fallback position. The first phases of this contract also required GHD to undertake additional investigations and analyses of various aspects of the dam and spillway to confirm the scope of works for the upgrade. During this review, it became evident that although the spillway capacity does not meet the ANCOLD fallback position, the Annual Exceedance Probability (AEP) of the existing capacity was relatively low and could potentially be deemed acceptable from a risk-based position.
A Risk Assessment was subsequently undertaken, with a SFAIRP assessment developed based on the new Dam Safety NSW guidelines. This assessment may be the first one to be completed for a major dam using the Dam Safety NSW guidelines. This paper discusses the different outcomes for a standards-based ‘Fallback’/’Simplified’ criteria and risk criteria based on DS NSW regulations, as well as the investigations developed to maintain confidence in the assessment. In addition, it describes a practical case for the application of SFAIRP criteria to a major dam.
In the case of Mangrove Creek Dam, the application of the new DS NSW Guidelines resulted in the dam being assessed as acceptable in its current state, with the FSL returned to the original design level. The outcome provided significant savings to the client, by avoiding costly upgrade works and avoiding disruption to the operation of the storage – a real success story.