Makeena Kiugu, Siraj Perera
Dam owners are influenced by drivers such as ensuring economic efficiency, achieving industry good practice, and meeting regulatory or due diligence obligations when making decisions on how to manage their dams. While these drivers can be inter-related, the decisions finally made by dam owners are reflected in planned and completed dam safety activities.
In Victoria, dam owners update the regulator on the status of their dam safety management programs every year. Victorian dam safety regulation is underpinned by risk management principles. Benchmarking of dam safety management practices is also promoted within the industry. The information provided to the regulator includes risk levels of dams, scheduled upgrades and associated cost estimates, interim risk reduction measures, and details of surveillance, emergency management and operation and maintenance programs. A considerable amount of information has been collected over the past few years allowing trends in dam safety management activities to be examined at a State-wide level.
This paper will consider how dam safety management decisions, and the drivers behind those decisions, are reflected in the dam safety practices of Victorian dam owners. Trends in dam safety activities will be observed and linkages made to prevailing industry-wide challenges.
Dam owners are increasingly being required to address a wider range of issues in an environment of limited resources. Ensuring due diligence and improving emergency preparedness are some current challenges facing dam owners. This paper also examines how these emerging drivers may influence dam safety activities into the future.
Keywords: Dam safety management
Richard R. Davidson, P.E., CPEng Kenneth B. Hansen, P.E.
Early in the twentieth century, placing concrete core walls within embankment dams was a popular construction technique for small to medium height dams. It became in vogue as a replacement for the popular British dam construction technology of puddle clay core dams which were used between the 1860’s and 1920’s. It avoided the many problems with semi-hydraulic / manned placement methods of the puddle clay cores within narrow trenches. However, after the mid 1930’s this concrete core wall construction fell out of favour because of the improvements made in embankment compaction methods and the difficulties in building reinforced concrete core walls to more significant heights.
Today concrete core wall embankment dams are now reaching an age where their continued performance is being questioned. This dam building technology has become extinct and is unknown to the last few generations of dam engineers. Therefore, it is relevant to re-examine this dam building technology in a modern context and work on answering the following questions. How have these dams performed after almost a century of service? Are there unanticipated performance features that have produced positive results when subjected to extreme flood and seismic events? Does the concrete provide enhanced performance over time? What role does steel reinforcement play in the performance of the core wall? Are there lessons here that can be applied to the more common concrete cutoff wall solutions being applied to embankment dams with seepage problems? This paper examines these questions with a number of illustrative case histories to provide a retrospective illumination of this forgotten dam building technology.
Keywords: Embankment dams, Concrete core walls, Dam construction history.
Bronson L McPherson, Eric J Lesleighter, David C Scriven, Erik F R Bollaert
A number of medium to major floods in Queensland caused substantial scour around spillway structures. This included the Paradise Dam primary spillway which experienced significant scour of the rock body below the spillway during flooding in January 2013. The occurrence has led to a series of evaluations of the geology, and the prevailing hydraulics behaviour as part of a process to determine the scour mechanism, and to determine the response of the spillway and areas downstream to future floods of larger magnitude. Part of the process has been to utilise a large-scale physical model to obtain transient data which together with the detailed geologic assessment would be incorporated into the comprehensive scour modelling procedures developed by Dr Erik Bollaert, AquaVision Engineering, Switzerland.
The paper will describe the design and construction of the physical model with special features to obtain pressure transients from more than 60 transducers, and velocity transients in more than 40 locations using Acoustic Doppler Velocimeter (ADV) instrumentation. The features of the rock scour will be discussed and the geology of the area below the spillway apron will be described. The range of discharges, and the model’s results including the pressure and velocity characteristics will be described in detail to illustrate the violent nature of the turbulence in the energy dissipation zone. The paper will go on to describe the computational scour modelling procedures of calibration and application, demonstrating a “system” approach to spillway scour analysis for plunge pools and similar situations with energy dissipation on natural materials.
Keywords: Spillways, flood hydraulics, hydraulic modelling, rock scour, transients, numerical analysis, energy dissipation.
Gavan Hunter, Andrew Pattle and Mark Foster
A piping incident occurred during first filling of Rowallan Dam, Tasmania in 1968. The incident occurred at the interface of the embankment with the spillway wall, a 15 m high near vertical wall, where the contact earthfill eroded into the single stage downstream filter. Repairs were undertaken in 1968/1969 and the reservoir has operated largely without incident since.
A risk assessment in 2009 identified that piping through the embankment at the spillway wall interface remained a significant dam safety risk. Investigations in 2010 encountered cracking within the earthfill core at the spillway wall interface.
Dam safety upgrade works were undertaken in 2014/15 to address the piping failure mode at the spillway walls and also within the upper portion of the embankment. The works required excavation down to a rock foundation at depths up to 18 m adjacent to the spillway walls and this excavation provided an unusual opportunity to closely examine active piping features that had been preserved when interim repairs in 1968/69 had arrested the progression of piping. The repair comprised reconstruction of a significant portion of the embankment at the spillway and the reconstruction of the upper 7 m of the crest, which included dual filters downstream of the earthfill core.
The findings from the forensic investigations of the deep excavations adjacent to the right spillway wall are described in this paper along with a summary of finding from the 1968/69 repair works and a discussion of the piping mechanism at the spillway wall. The paper also covers the design and construction of the repair work. The focus of this paper is on advancements in our understanding of piping risk arising from the Rowallan Dam work.
In conclusion, (i) the upgrade works successfully reduced the dam safety risk of Rowallan Dam; (ii) the findings support the methodologies of the piping toolbox; (iii) the case study provides insight into filtering and crack filling mechanisms that have a broader implication for estimating the risks of internal erosion within existing dams; and (iv) the findings support the assessment of the low residual risks for piping through the embankment away from the upgrade work areas (crest reconstruction and spillway walls).
Keywords: Earth and rockfill embankment, piping incident, piping mechanism, dam safety upgrade.
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.
Paul Somerville, Andreas Skarlatoudis, and Hong Kie Thio
Engineers need ground motion time histories for the analysis of the response of structures to earthquake ground shaking. In current practice, these time histories are usually spectrally matched to a uniform hazard response spectrum. At low probabilities, this spectrum is too “broadband” (i.e. large over an unrealistically broad range of periods), and envelopes a set of more appropriate design response spectra, termed conditional mean spectra. These concepts are illustrated using a site-specific probabilistic seismic hazard analysis of ground shaking in which ground motion time histories are spectrally matched to conditional mean spectra that were derived from the uniform hazard spectrum.
Keywords: Ground motion time histories, Conditional mean spectrum.