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
Gavan Hunter, David Jeffery and Stephen Chia
The Main Embankment at Tullaroop Dam in central Victoria is a 43 m high earthfill embankment with a very broad earthfill zone and rockfill zones at the outer toe regions. There has been an extensive history of cracking within the Main Embankment since formalisation of visual inspections in 1987.Widespread cracking has been observed on the crest and downstream shoulder. Cracking on the crest has mainly been longitudinal, but transverse cracks have also been observed. Cracking on the downstream shoulder has comprised longitudinal, diagonal and transverse cracking. In April 2004, a 60 mm wide diagonal crack opened on the downstream shoulder of the left abutment (from crest to toe) and Goulburn-Murray Water constructed a local filter buttress in 2005/06 on the left abutment. In 2011/12 a longitudinal crack opened up on the upper downstream berm toward the right abutment. The crack was initially 15m long and 10 to 215 mm wide, then propagated several months later to 70 m in length, 40 to 50 mm width and greater than 3 m in depth.In May 2011 three piezometers within the earth fill core recorded a very rapid rise in pore water pressure equivalent to 12 to 13 m pressure head above their previous readings. The piezometers were located on the same alignment (upstream to downstream) and were located below the crest and downstream shoulder, and the rise was to levels close to and above the embankment surface. The piezometers then showed a steady fall with time returning to the pre rise levels after 4 to 6 weeks.In 2015/16 Goulburn-Murray Water undertook dam safety upgrade works to reduce the risk of piping through the Main Embankment by extension of the filter buttress across the full width of the embankment. During these upgrade works, very deep (greater than 5 m) and extensive transverse cracks were observed in the embankment over relatively subtle slope changes on the right abutment.Thecracking and pore water pressure behaviour in the Main Embankment at Tullaroop Reservoir present an important case study. The paper provides details on the cracking and postulated crack mechanisms, and the rapid pore water pressure rise and postulated mechanisms. A summary of the upgrade works is also provided.
John Harris, James Robinson, Ron Fleming
Haldon Dam Remediation: A Case Study of Earthquake Damage and RestorationJohn Harris, James Robinson, Ron FlemingAECOM New Zealand LimitedAECOM New Zealand Limited, Fleming Project Services Limited Haldon Dam is a 15m high zoned earth-fill embankment irrigation dam, located approximately 10 km south-west of Seddon, in the Awatere Valley, New Zealand. The crest and upstream shoulder of the embankment suffered serious damage during the 2013 Cook Strait earthquakes, and the Regulator enforced emergency lowering of the reservoir by 5.5m to reduce the risk of flooding to Seddon Township from a potential dam failure. AECOM was engaged by the owner to carry out a forensic analysis of the damaged dam and subsequently the design of the 2-Stage remedial works. The remedial works addressed the existing dam deficiencies and earthquake damage in order to restore the dam to full operational capacity and gain code compliance certification. Key features oft he approach included holding a design workshop with the owner prior to undertaking detailed design, careful rationalisation of the upstream shoulder to optimise the competing interests of strength and permeability, contractor and regulator involvement in the design and construction process, and balancing risk and constructability with the chimney filter retrofit. This paper presents a description of, and approach to, remedial works solution undertaken to remediate a substandard and earthquake-damaged dam to fully operational status in an area of high seismicity. Applying this approach, the objective of achieving a robust, safe, economical design that was acceptable to the regulators and the owner was achieved.
Peyman Andaroodi, Barton Maher
Seqwater is a statutory authority of the Government of Queensland that provides bulk water storage, transport and treatment, water grid management and planning, catchment management and flood mitigation services to the South East Queensland region of Australia. Seqwater also provides irrigation services to about 1,200 rural customers in the region that are not connected to the grid and provides recreation facilities. Seqwater owns and operates 26 referable dams regulated under Queensland dam safety legislation.
Leslie Harrison Dam is an Extreme Hazard category dam located in the Redland Bay area of Brisbane.A significant portion of Population at Risk is located within a short distance downstream of the dam, reducing the available warning time in the event of a dam safety issue and impacting on the estimated loss of life used to assess risk. Following the Portfolio Risk Assessment undertaken by Seqwater in 2013, a series of detailed investigations were undertaken to confirm the assessed risk and the scope and urgency of the upgrade works.
Before a final decision on the scope and timing of the dam upgrade is made, Seqwater has completed a detailed review of the downstream consequences. This review was intended to update the Population at Risk(PAR) and Potential Loss of Life(PLL) estimates using the latest estimation methods for a range of scenarios. Three life loss estimation methods were used including empirical and dynamic simulation models and the results were compared.
This paper discusses the updated consequences assessment and the impact on the assessed risks, for Leslie Harrison Dam for both the current dam and the proposed upgrade scenarios using the revised Potential Loss of Life estimates.
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.
Paul Somerville, Andreas Skarlatoudis and Don Macfarlane
The 2017 draft ANCOLD Guidelines for Design of Dams and Appurtenant Structures for Earthquake specify that active faults (with movement in the last 11,000 to 35,000 years) and neotectonic faults (with movement in the current crustal stress regime, in the past 5 to 10 million years) which could significantly contribute to the ground motion for the dam should be identified, and be accounted for in the seismic hazard assessment. The purpose of this paper is to provide guidance on the conditions under which these contributions could be significant in a probabilistic seismic hazard analysis (PSHA)and a deterministic seismic hazard analysis (DSHA).We consider five primary conditions under which identified faults can contribute significantly to the hazard: proximity, probability of activity, rate of activity, magnitude distribution, and return period under consideration