Michael Bassett-Foss , David Bouma , Dewi Knappstein
The Wairarapa Water Use Project (WWUP) in the southern North Island, New Zealand, is investigating new water storage schemes involving large dams that will allow the community to make use of the water resources that are currently available, but not necessarily available at the time they are needed. It is estimated that the 12,000 hectares currently irrigated in the Wairarapa could be increased to about 42,000 hectares depending on actual demand. The WWUP provides for a range of possible needs, such as supply of new areas of irrigation, increased reliability for existing irrigation and frost fighting, environmental augmentation of low summer river flows, environmental flushing flows, stock drinking water, power generation, municipal water supply, and recreational use.
WWUP objectives include early engagement of stakeholders, early integration of financial, social, cultural and environmental factors in decision-making, management of uncertainty associated with the preliminary level of investigation and evolving regulatory framework, development of an equitable framework for efficiently comparing options, and balancing long and short-term considerations.
A large number of dam options were identified, storing 3 to 80 million m3 of water, and progressively narrowed to a shortlist of 2 sites through a complex process of concept development, desktop studies, site visits, hydrological analyses, cost estimates and multi-criteria analyses.
The WWUP demonstrates how sustainable new major water storage schemes can be promoted in a highly regulated environment of a developed nation.
Keywords: Dams, water storage, stakeholder engagement, environment, water allocation, multi-criteria analysis
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
Robert Kingsland, Michelle Black, Andrew Russell
Managing the vibration impacts associated with blasting is a challenge for mine planners and operators. In an open cut mining environment production blasting is often an integral part of operations. The management of surface water is a key operational requirement for open cut pits and mine water dams are often a part of the water management infrastructure. Consequently, mine water dams are often subject to blasting impacts.
For the mine operator the foremost questions are, “how close can mine blasting progress towards the dam?” and “what is the maximum vibration that the structure can be safely subjected to?” For the dam safety regulator the key concerns are around potential modes of failure, consequence of failure, the likelihood of failure and the management of risk.
With reference to case studies, this paper will discuss the acceptable blasting limits for earth dams, impacts on various dam elements and failure mode analysis. Failures modes discussed include embankment cracking, slope failure and deformation, foundation cracking and outlet structure cracking. Risk mitigation measures will be presented including design, operation and monitoring controls.
Keywords: blasting impacts, embankment dams, coal mine.
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.
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.
Kim Robinson, Andrew Pattle and Thomas Shurvell
Rowallan Dam is a 43m high clay core rock fill dam located in Northern Tasmania. The dam impounds 121GL used for hydro power generation and has a High A consequence category.
Over the summer of 2014/15 major reconstruction works were carried out on the dam to repair a piping incident from 1968. The work entailed reconstructing two sections of the dam down to foundation level and the upper 7m of the 568m dam crest. During the work, the dam was temporarily exposed to a significantly increased flood overtopping risk.
A range of measures were taken to manage the overtopping risk; such as increasing the dewatering capacity of the dam, lake draw down, installation of a sheetpile wall, development of emergency backfill procedures and a flood forecasting system.
The focus of this paper is on the flood forecasting system and how this was integrated into the overall management of overtopping risk during construction. The forecast models were run automatically on a 2 hour schedule using the latest BoM forecast, telemetered lake levels and rainfall from 7 gauges surrounding the catchment. The system provided a continuous 7 day lake level forecast which guided the site team on when to release water to manage the storage.
In the event that the lake level forecast reached a predetermined trigger level, the dam safety team would have been automatically notified and various emergency procedures would have been triggered in response to the flood warning.
This paper discusses the measures that were taken to manage the flood risk, how it worked in practice and conclusions which are applicable more generally to managing overtopping risk during dam works.
Keywords: dam construction flood risk, flood forecasting