Chriselyn Meneses, Simon Lang, Peter Hill, Mark Arnold
Risk is the product of likelihood and consequences. Much effort is put into the risk assessment process for large dams to ensure there is a consistent approach to estimating failure likelihoods across an owner’s portfolio. For example, the use of common peer review teams and methods like the ‘piping toolbox’ allow the risk assessment team to apply repeatable logic and processes when estimating failure likelihoods. However, the methods for estimating life safety consequences are often not applied consistently. This inconsistency leads to estimates of potential loss of life (PLL) that vary between dams in unexpected ways, because results from the most commonly applied method (Graham, 1999) are sensitive to threshold changes in flood severity and dam failure warning time.
The recently released Reclamation Consequence Estimating Methodology (RCEM) is intended to supersede Graham (1999). RCEM varies fatality rates continuously with DV, and is therefore less sensitive to changes in flood severity. In this paper, estimates of PLL from RCEM are compared with results from Graham (1999) for five dams. Results from the latest US Army Corps of Engineers model for estimating the consequences of dam failure (HEC-FIA 3.0) are also compared with RCEM and Graham (1999) for one dam. Comment is then made about the important considerations for applying RCEM consistently across a portfolio of dams.
Keywords: potential loss of life, dam safety, risk analysis
A. Scuero, G. Vaschetti, J. Cowland, B. Cai , L. Xuan
Nam Ou VI rockfill dam is part of the Nam Ou VI Hydropower Project under construction in Laos. The scheme includes an 88 metres high rockfill dam, designed as a Geomembrane Face Rockfill Dam (GFRD), which when completed will be the highest GFRD in Laos. The only element providing watertightness to the dam is an exposed composite PVC geomembrane, installed according to an innovative design now being increasingly adopted to construct safe rockfill dams at lower costs. The same system will shortly be installed on a water retaining embankment for a coal mine in NSW, Australia, and has been approved for a tailings dam in Queensland, Australia. At Nam Ou VI the geomembrane system is being installed in three separate stages, following construction of the dam. The first two stages have been completed, and the last stage will start in November 2015. The paper, after a brief discussion of the adopted system’s concept, advantages and precedents, focuses on the construction aspects.
Keywords: GFRD, PVC geomembrane, waterproofing, rockfill dam.
P C Blersch, W van Wyk , R Steenkamp
Construction of the partially completed Calueque Dam on the Cunene River in Angola was abandoned in 1976 due to the hostilities in Angola. In 1988 the dam was bombed, causing significant damage to the bridge deck, other structures and equipment. Work to complete and rehabilitate the dam commenced in late 2012 and included major earthworks, extensive concrete repairs and refurbishment and installation of mechanical equipment, including ten spillway radial gates and two outlet gates with lifting equipment, emergency gates and cranes, including electrical and control systems. A number of challenges were encountered in planning and executing the project but were overcome largely as a result of detailed historical project information having been retained well beyond the norm and through the involvement of a key member of the original project team in the current project.
Keywords: Dam rehabilitation, radial gates, zoned earthfill embankment
Chahnimeh reservoirs with 1.4 billion cubic metres storage capacity have a critical role in water supply for both drinking water and agricultural purposes for the whole Sistan region in eastern Iran. Sistan river used to be the only source for agricultural purposes, so that several gated diversion weirs were constructed on the river in the past 50 years. Because of climate change and upstream development causing flow fluctuations, the river alone is no longer a reliable source for irrigation purposes. So the idea of storing water in Chahnimeh reservoirs and optimised operation of reservoirs have become a necessity. In order to achieve this, development of structures to have efficient operational plan of the river and reservoirs system is underway.
Several projects have been built for more efficient use of the reservoirs, some projects still being designed. One of the latest is the project of “Development of Operational Infrastructures for Chahnimeh Reservoirs” designing a structure to regulate flow between Chahnimeh I and III reservoirs. This kind of structure operating between two connecting reservoirs is so rare, so that innovation is needed to design a cost effective structure covering different operational conditions. Different structures were investigated and the summary of selection of structure types are presented. The paper illustrates challenging design of the project, useful for engineers who might be or will be dealing with such a project. By designing gates with pre-compressed rubber sealing, huge amount of costs associated with having two different gates for different directions of flow are avoided. Because of saturated foundation, by designing a diversion system between two reservoirs, it is possible to undertake pre-consolidation of foundation soil and to drain saturated foundation water. This would reduce settlement of the foundation of the structure after construction to the extent that by construction of a pile group, the gated structure will perform with high reliability for gates function. This type of structure is so rare and the methods and experiences of the presented design can be used by other engineers and consultants in similar projects. The estimated cost of the project is 15 million dollars and with construction under way, completion is expected in 2017.
Keywords: regulating structure, gates, reservoirs, reservoir operation
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.
Maz Mahzari and Chi-Fai Wan
Upgrading of an existing dam often faces challenges in both static and seismic safety assessment. The use of new hydrological and seismological data and improved design methods often mean more severe loading which outdates the original design and demands expensive upgrade works. Establishing the design criteria for checking the structural adequacy of an existing dam for multiple unusual load events occurring within a relatively short time frame presents another challenge.
A probabilistic approach is presented to rigorously address the effects of multiple load events while maintaining a consistent risk of failure for the structure. This is based on a probabilistic conditional combination where probability of each event is defined and used to develop a joint probability distribution. For instance if an earthquake occurs following a severe flood, the seismic hazard curve of the site can be used to adjust the seismic loading with shorter average recurrence interval to be used in conjunction with the pre-earthquake flood when assessing the structural adequacy of the dam. With this method of adjustment, the design can benefit from the choice of a reduced seismic design loading and hence a more cost effective design solution.
The proposed method is straightforward and can be effectively used in most engineering practices, including the design of hydraulic structures such as dams.
Keywords: Dams, Seismic Hazard, Post-earthquake, Risk analysis