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
A common concern for large spillways is erosion of the receiving plunge pool and potential impacts on the stability of the dam.Devils Gate Dam is an 84m high, double curvature arch concrete dam, located in northern Tasmania and constructed between 1968 and 1970.The full 134m long crest is designed as a free-overflow spillway and spill flows impact the downstream valley sides and plunge pool below, where energy is dissipated to reduce riverbank erosion downstream.To protect foundation rock,the plunge pool and large portions of the valley sides were concrete lined with 450mm thick reinforced and anchored concrete. During spill events the area is inundated by up to 12m of tail-water.In 2016 damage to the plunge pool concrete was discovered by divers during a special inspection of the impact areas, but poor visibility limited the understanding of the extent and severity. Subsequent investigations, including detailed sonar scanning, improved the understanding but it was not until the plunge pool was fully dewatered that the full extent of the damage was quantified.The damage commenced around 35m downstream of the dam arch and consisted of approximately 330 square metres of moderately to severely eroded concrete and exposed, deformed, and in some areas completely removed reinforcing bars. The most significant feature was a penetration through the concrete up to 2.5m into the foundation rock.A number of stressed anchor heads were also damaged or destroyed.A full appreciation of the damage necessitated the decision for immediate repairs given the impending power station refurbishment (commencing January 2018) which will subject the plunge pool to nine months of constant spill.This paper outlines the diving and sonar investigations undertaken in 2016, discusses the challenging tasks of dewatering the plunge pool and gaining access through the narrow canyon, and presents the physical works to strengthen the damaged areas.It discusses the difficulty of identifying and treating such damage, and serves as a cautionary tale for other owners who have fully submerged plunge pools downstream of spillways.
Tian Sing Ng, David Gardiner
Spillway structures play an important part in regulating the designed reservoir water level and are paramount to protect the structural integrity of the dam structure. Impermeability and tight crack control are prime importance in the design and construction of the spillway lining in order to minimise the potential failure modes of cavitation damage and stagnation pressure related failure. A spillway chute is essentially continuously restrained by the roughness of the rock surface and the ground anchors. The provision of control joints, i.e. expansion, contraction and movement joints,are therefore of little benefit due to the restraint as open cracks will still occur. Steel fibre reinforced concrete has been used for resisting erosion of the surface due to abrasion and/or cavitation. Steel fibres combined with conventional reinforcement also provide an amazing synergy to effectively reinforce concrete due to their ability to provide an effective restraining tensile force across open cracks. For the spillway chute,this means any concrete panel size or shape can be considered, even when the chute is fully restrained. Most importantly, this cost effective solution can be constructed joint free while maintaining watertightness. This paper presents some basic principles governing the design of joint free dam spillways employing steel fibre combined with conventional reinforcement. The focus of this paper describes the design and construction of the 400 m long Happy Valley Dam Outfall Channel together with overseas project examples.
Steven E Pells, Philip J N Pells
Junction reefs dam was designed in 1895 and constructed by 1897 as a multiple arch brick structure which was the first of its kind in Australia, and one of the earliest in the world. The dam was envisioned to provide mechanical and electrical power for gold mining. This paper provides an historical overview of the unique structure, and reassesses some of its engineering characteristics, such as the stress conditions in its unusual arches and reverse concrete gravity wing walls. The hydrology of the dam is re-assessed from the viewpoint of evaluating its potential as a mini hydro scheme. Commentary is also provided on the performance of its unlined spillway, which has been subject to regular spills for 120 years.
There is increased pressure from stakeholders for projects to include evaluation of emerging broader development issues within the environmental assessment process. These emerging issues are not well documented or understood and at the forefront of untested preliminary government policy positions.
Agencies expect proponents to invest in evaluating these matters outside of typical assessment practices. Requests are made late in the evaluation and approval process.Assessmen involves matters not directly related to the project or within the proponent’s control and occurs late in the project development cycle.
The Lower Fitzroy River Infrastructure Project (LFRIP) was identified through the Central Queensland Regional Water Supply Study in 2006, as a solution to secure future water supplies for the Rockhampton, Capricorn Coast and Gladstone regions. The Gladstone Area Water Board and SunWater Limited, as proponents, propose to raise the existing Eden Bann Weir and construct a new weir at Rookwood on the Fitzroy River in Central Queensland.
The LFRIP environmental impact statement (EIS) was approved, subject to conditions, by the Queensland Coordinator-General in December 2016 and the Commonwealth Minister for the Environment and Energy in February 2017. Achieving conditions that will realise positive environmental outcomes while simultaneously achieving project objectives, particularly with regard to timeframes and costs, was not without its challenges.
The EIS was developed in accordance with the requirements of the State Development Public Works Organisation Act 1971 (Qld) and the Commonwealth’s Environment Protection and Biodiversity Conservation Act 1999, including an extensive stakeholder consultation programme. These regulatory requirements are well understood and applied to projects as normal accepted practice. They ensured that potential project impacts and benefits were identified, that appropriate levels of effort were applied to investigations to establish baseline conditions and that risks to and impacts on environmental (including social and cultural) matters were adequately mitigated and managed.
The environment is not static. Emerging issues and perceptions results in regulation and policy changes in response to political and social drivers. During the development of the EIS both new legislation and new policies were imposed on the project.New legislation resulted in additional assessment around matters previously considered mitigated and managed (fish passage). New legislation introduced new matters for assessment (connectivity). Collaboration and engagement with stakeholders were key to understanding the applicability of these elements to the project and for developing an approach to address the legislative requirements late in the project’s development and assessment process.
In Queensland,policy is emerging to mitigate and manage impacts of development on the Great Barrier Reef World Heritage Area’s universal values. The EIS was required to address the direct project impacts on water quality and the impacts arising because of the LFRIP (facilitated development). Water secured by the LFRIP is for urban, industrial and agricultural purposes. Urban and industrial developments are well regulated and subject to specific environmental approvals processes. Use of water for agricultural purposes, intensive irrigated agriculture in particular,is less regulated. Policies developed are reactive and require individual projects to address these impacts.In the absence of regulatory guidelines for assessment of consequential impacts, the project adopted a collaborative approach. The proponents established a working group, including State and Commonwealth technical agencies. This allowed for robust and scientifically defendable methodologies to be developed and agreed upfront. Streamlining the approach by including key decision makers assisted in managing expectations and focused the assessment on realistic and achievable outcomes relative to the project. The result was defendable outcomes allowing timely decision making and avoided rework as much as possible.
This paper describes developments in environmental assessment relating to new and augmented weirs.
Peter Allen and Mark Rhimes
Recent tropical cyclones have had significant impacts on coastal Queensland and produced significant inflows into a large number of major dams with the triggering of a number of Emergency Action Plans for downstream release hazards. While there were several floods of record, there were no significant dam safety incidents. The dams seemed to have been blamed for a lot of this flooding even though they provided significant flood mitigation. This paper will cover the emergency responses to these events, the public perceptions and the associated third party reviews of these events. Community expectations and the ability to undertake post flood event assessments of dam operations is also driving such investigations.This paper will also discuss the consequential updates being made to Queensland Emergency Action Planning Guidelines to encourage effective engagement with local emergency planners and other stakeholders in the development of these guidelines.