Elaine Pang, Robert Fowden
There are numerous established methods available for assessing the consequences of failure for earthen water dams.The estimation of breach dimensions and failure times remains the greatest common area of uncertainty, particularly for dams under 10m in height, where the number of historic records behind the established methods reduces considerably.Also, various factors can have a significant impact on the strength of small dam embankments, potentially contributing to the likelihood of failure.Consequently, failure impact assessments for smaller dams may rely more heavily on the engineering judgement of the responsible engineer. Although the consequences of failure may indeed be lower for smaller dams, the large number of unknown or unregulated dams in some locations means that it can be difficult to quantify their overall contribution in terms of dam safety risk. This paper presents an on-going project to compile and analyse observed small earthen dam failures with the intent of refining existing statistical breach relationships for smaller dams.Context is provided through an overview of DEWS’ investigative program, including the presentation of several case studies which highlight field data collected throughout the program.
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Monique Eggenhuizen, Peter Buchanan, Reena Ram, Tusitha Karunaratne
The Department of Environment, Land, Water and Planning (DELWP) has a regulatory role for the safety of dams under the Water Act 1989 (Act) and is the control agency for dam related emergencies. Local Government in Victoria is divided up between 79 LocalGovernment Authorities (LGAs), each responsible for administering local infrastructure and community services such as roads, drainage, parks etc. Current records indicate that 42 of the 79 LGAs own or manage up to 435 dams and retarding basins.Many of these assets, which include a mix of old water supply dams, ornamental lakes and retarding basins, have been accumulated by LGAs over many years as a result of asset transfers and conversions, land development projects, flood mitigation programs and opportunistic acquisitions by the transfer of land. DELWP engaged GHD to assist and provide advice to the LGAs to significantly improve and update knowledge on LGA dams and retarding basins. The objective of this project is to ascertain where the State’s LGA dams and retarding basins are located, what risks they might pose to communities and infrastructure, what to consider during emergency management planning and response, and to provide owners with the essential management tools and procedures to effectively manage these assets, if these are not in place already.The outcome of this project was to support LGAs to improve management of their dams and retarding basins. It aimed to do this by assisting LGAs with the development of basic dam safety programs that will enable LGAs to more effectively manage their portfolios of dams and retarding basins in terms of ongoing maintenance, dam surveillance and emergency planning and response, and demonstrate due care.This project had a number of key challenges. These included the requirement to process and assess a large number of sites within a small timeframe whilst achieving good value for money,without compromising DELWP’s objectives. A number of efficient methods were adopted during this project particularly during the initial data gathering process, identifying those dams which needed to be inspected based on embankment heights, reservoir capacity and consequences, rapid preliminary assessment of consequences, the development of effective templates for the site inspections, and a method of applying qualitative risk assessments, applicable to the majority of the dams, allowing a consistent assessment of the status of each dam and damsafety documentation.The methods discussed(although developed specifically for the Victorian LGA dams portfolio)provide a sound basis for a screening tool to assess a large number of smaller dams in an efficient manner and quickly identify higher consequence category dams requiring attention. This method could easily be modified and adapted to be applied to similar portfolios of dams.
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.
Petros Armenis, Malcolm Barker, Peter Christensen, Graham Harrington
The Canterbury Earthquake Sequence in September 2010 and February 2011 caused large areas of land to change by differing amounts throughout Christchurch, New Zealand. Land levels fell by more than 300 mm in some areas. This increased flood risk in the tidal reaches of the Avon River. Urgent repairs were completed with the objective to restore the tidal river defences to a crest level equivalent to a 1% AEP tide level. This work needed to be completed prior to impeding spring tides.
The levees will be required for up to 20 years and then probably be rebuilt on a new alignment. To better understand the risks associated with the ongoing reliance of the levees for flood protection in the interim, a risk assessment was undertaken using conventional Australian National Committee on Large Dams (ANCOLD) practices and levee design procedures. Careful consideration was made to the performance of the existing levees under seismic, flood and tidal loading from which the societal and individual risk profiles were derived. The work included the following:
This paper will present the levee design and the process applied for the analysis of the levee and the upgrade options selection
Richard Herweynen, Jamie Campbell, Mohsen Moeini
Hydropower storage plays an expanding role in integrated power systems internationally and can enable increased use of intermittent renewable energy sources such as wind and solar.With an increased amount of renewable energy within the Australian grid, pumped storage has gained increased focus in the past 2years. Entura have been working with Genex Power Ltd. to investigate, evaluate, optimise and design the Kidston Pumped Storage Project, located at the old Kidston gold mine in Northern Queensland. Through this design process, the final arrangement developed included an upper reservoir turkey’s nest dam to be built on the existing waste rock dump on the northern side of the Eldridge Pit, using the existing waste rock dump material and lining it with an HDPE liner. The original waste rock dump was formed during the mining operation by progressively dumping the waste rock predominantly from the Eldridge Pit excavation, with the haul truck traffic being the only compaction that occurred. Since the closure of the mine about 20 years ago, some consolidation of the waste rock dump has occurred.As a result, the key risks identified for the construction of the turkey’s nest dam on top of the waste rock dump were: (1) the stability of the slopes of the waste rock dump, which were generally at the angle of repose for the rockfill material; (2) the absolute settlement of the waste rock dump as the final dam crest level requires a settlement allowance in excess of the flood freeboard requirements; and (3) the differential settlement as excess differential settlement could cause fatigue stress cracking within the liner.This paper presents the investigation and modelling undertaken to confirm the feasibility of constructing this turkey’s nest dam on top of the existing rock waste dump, utilising the historical data on dumped rockfill dams. The paper also presents the feasibility design developed for the upper storage.
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