Phillip Kennedy, Robert Murphy, Pat Russell, Chi Fai Wan
Central Highlands Water (CHW) owns thirty four dams varying significantly in size, age, and condition. Thirty of the dams are used for water supply purposes with the remainder providing storage for wastewater reuse schemes. Out of the thirty-four dams, eighteen are more than one hundred years old. They are zoned earthfill embankments, some with a puddle clay core. Fourteen of the dams have been assessed as having potentially high to extreme consequences if the dam fails. The key safety issues among these high consequence dams are inadequate flood capacity, slope instability, and high potential for piping.
CHW’s management policy includes a commitment to identify, assess, prioritise improvements to, and periodically review the safety of its dams, and implement a dam safety upgrade works program. CHW’s Water Plan 3 (2013 – 2018 economic regulatory period) includes nine dam safety upgrade projects, which were identified from risk assessments and investigations carried out over several years.
In 2013, CHW and MWH formed a Delivery and Operational Efficiency Review (DOER) Group to refine and confirm priorities for the proposed dam safety upgrades. The main objectives of the DOER Group were to identify solutions to meet current ANCOLD guidelines and any opportunities to achieve 10% – 20% reduction in capital expenditure costs during planning or delivery of the works for Water Plan 3, while achieving the intended risk reduction. The key elements of the DOER were to (1) form a working group to cover operational, planning and executive management considerations together with dam safety consultants and Victorian dam management experience; (2) closely scrutinise previous assessments; (3) challenge the justification for the project; (4) understand the priorities whilst aiming to deliver a major works program; and (5) identify additional investigations.
Initial investigations of the DOER Group developed a revised program of works allowing confirmed capital works to proceed while investigations into other projects were carried out. The follow-up investigations have identified optimal outcomes through a program of cost-effective solutions for CHW.
This paper aims to share the experience from planning the DOER, and the further investigations that resulted in the development of an optimised delivery strategy for the upgrade projects.
Keywords: Delivery and Operational Efficiency Review, Risk.
Mark Arnold, Chris Topham, Phil Cummins
A central tenet of the ANCOLD Guidelines on Dam Safety Management (2003) is that the higher the consequence of failure of a dam, the more stringent the surveillance scope, frequency, and safety criteria that should be applied to that dam. This concept has generally served the industry well to date in assisting regulators and dam owners to focus on the dams that could have the highest impacts if they failed. ANCOLD 2003 does also suggest that risk may be taken into consideration, however it is the experience of the authors that for dam surveillance and monitoring programmes, the majority of owners and consultants are reluctant to stray too far from the tables provided in the Guideline. However, two owners have recently embarked on a formal process to apply a risk based approach to the specification of surveillance and monitoring for their dams. This paper outlines how sub-optimal outcomes that can arise when the guideline tables are applied exclusively, presents the process undertaken by two owners of large portfolios of high consequence dams, and demonstrates the benefits achieved when a risk based approach is used. The paper concludes that any update or rewrite of the 2003 Dam Safety Management Guidelines should promote a risk based, rather than a consequence based approach to surveillance and monitoring.
Keywords: Risk, risk-based surveillance programme, instrumentation, monitoring.
Colleen Baker, Sean Ladiges, Peter Buchanan, James Willey, Malcolm Barker
Dam Owners and Designers are often posed with the question “what is an acceptable flood risk to adopt during the construction of dam upgrade works?” Both the current ANCOLD Guidelines on Acceptable Flood Capacity (2000) and the draft Guidelines on Acceptable Flood Capacity (2016) provide guidance on the acceptability of flood risk during the construction phase. The overarching principle in both the current and draft documents is that the dam safety risk should be no greater than prior to the works, unless it can be shown that this cannot reasonably be achieved.Typically with dam upgrade projects it is not feasible to take reservoirs off-line during upgrade works, with commercial and societal considerations taking precedent. It is therefore often necessary to operate the reservoir at normal levels or with only limited drawdown. The implementation of measures to maintain the risk at or below that of the pre-upgraded dam can have significant financial and program impacts on projects, such as through the construction of elaborate cofferdam arrangements and/or staging of works. This is particularly the case where upgrade works involve modifications to the dam’s spillway.The use of risk assessment has provided a reasonable basis for evaluating the existing and incremental risks associated with the works, such as the requirement for implementation of critical construction works during periods where floods are less likely, in order to justify the As Low As Reasonably Practicable (ALARP) position. This paper explores the ANCOLD guidelines addressing flood risk, and compares against international practice. The paper also presents a number of case studies of construction flood risk mitigation adopted for dam upgrades on some of Australia’s High and Extreme consequence dams, as well as international examples. The case studies demonstrate a range of construction approaches which enable compliance with the ANCOLD Acceptable Flood Capacity guidelines
Richard Herweynen, Tim Griggs, Alan White
The Ministry of Public Utilities, Sarawak, Malaysia used an independent dam safety consultant to advise them on whether the Murum Dam was ready for impoundment. They were looking for a holistic assessment of the dam from a dam safety perspective. As a result, a risk framework was adopted to identify the key issues that needed to be addressed prior to impoundment of the Murum Dam. The process adopted which is presented in this paper, was transparent and defensible; and provided a reasoned approach for which items must be completed prior to the commencement of impoundment. As a result effort was focused on the key activities required prior to impoundment – whether this was the completion of specific works, the availability of key instrumentation to monitor the dams performance, the availability and operation of key dam safety systems, or the appropriate emergency preparedness should a dam safety incident occur during first filling. This systematic process based on a risk based approach, was a useful method of determining the dam’s readiness for impoundment, and provided an excellent way of communicating the importance of activities to the key stakeholders. The authors believe that this method is transferable to other dam projects, for an assessment of a dam’s readiness for impoundment.
Keywords: Dam safety, risk, impoundment, reservoir filling.
The Ross River Dam, designed in the early seventies, does not meet current dam safety criteria for overtopping and piping within the embankment or the foundation. The dam comprises a 40m long concrete overflow spillway flanked by a central core rockfill embankment of 130 m on the right bank and 170m on the left bank with a 7620 m long left bank earth fill embankment, which has no internal filter zones for piping protection. The embankment was extensively assessed and treated forfoundation deficiencies in 1982, and further assessed in 20002002 for appropriate upgrade options.
This paper describes the process of validation of the detailed design using Risk Based Design Criteria. This process included data mining for historical performance and original design intention,
comparison of the original design against current and historical investigations and assessment of the upgrades using the large volume of data available from previous work. A design team comprising specialist hydrologists, hydrogeologists, geologists, geotechnical and dams engineers worked within a risk assessment framework at all stages of the design to ensure the design was validated using the design Validation Model. This process incorporated assessment of crest level based on flood risk and wave overtopping, review of 2D and 3D seepage models to assess piping and foundation erosion potential, assessment of fissured soils within the embankment foundation for structural stability and evaluation of spillway model testing for potential spillway failure modes.