Eric Lesleighter, Bronson McPherson, Karen Riddette, Jon Williams
Abstract: The paper describes in part the hydraulics investigations required for the major upgrade works for Lake Manchester Dam; investigations which utilised two modelling procedures. Following a brief outline of the dam rehabilitation program and works, the inadequacy of the former spillway for the revised hydrology is described. The urgency of the construction program led to the use of CFD modelling following preliminary desk studies of the hydraulics, in order to delineate certain features of the new spillway and the plunge pool area and allow the construction program to be fast-tracked. As part of the overall program to refine the spillway design, and due to the evident complexity of the flows over the spillway, physical hydraulic model studies were also carried out.
The investigations provided a very good opportunity to compare the results from two modelling approaches carried out within the same design activity. Specific results of aspects such as flow patterns, velocities, pressures, and wave action will be compared. A prominent purpose of the paper will be to acknowledge the advantages and limitations of both approaches, and seek to provide the guidelines and advice that designers and dam owners should follow and adopt to ensure the hydraulics requirements of projects are soundly engineered.
Keywords: spillways, physical modelling, numerical modelling, CFD.
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Matthew Pollard, John Vitkovsky, Richard Priman
Abstract: South East Queensland (SEQ) currently has severe (Target 140) restrictions imposed to help secure supplies during the current drought which is the worst on record. Additionally, a $9 billion water infrastructure program is being fast-tracked to increase the climate resilience of the region and provide for long term sustainable growth.
The Draft South East Queensland Water Strategy (SEQWS) released in March 2008 was prepared by the Queensland Water Commission to reduce the likelihood of ever experiencing such severe restrictions again and to ensure water security into the future. The Strategy includes a Water Supply Guarantee underpinned by advanced analytical techniques for estimating the system yield from surface water, groundwater and manufactured water supplies connected by the SEQ Water Grid. This approach builds on the Level of Service (LOS) Objectives methodology, originally promulgated by the Water Services Association of Australia in their June 2005 paper entitled “Framework for Urban Water Resource Planning”. The approach has led to a significant improvement in our understanding of water supply risks and the associated planning implications for assessed yields/water availability.
To assess yield using the LOS objective methodology and to determine the benefits of the new water infrastructure, a regional water balance model for the connected SEQ Water Grid simulating the water sharing arrangements of all of the SEQ water sources was established. The model uses a logic tree to allocate water using a “proportional storage rule” from multiple sources to meet competing demands. This approach enabled quantification of the increase in system yield resulting from the construction of the SEQ water grid which allows demands from previously disconnected areas to be met by more efficient allocation of water from supply sources. Stochastically generated dam inflow data was used to facilitate a more comprehensive assessment of climate variability and water supply risk than is possible with historic data alone.
The paper discusses the managed hydrologic risk approach adopted in developing the regional water balance model which implements the LOS Objectives approach and improves the understanding of the relationship between the level of service, supply yield and risks associated with climate variability. The result is a far more thorough approach to planning for future water service delivery and water infrastructure.
Keywords: level of service objectives, South East Queensland Water Strategy, SEQ Water Grid, yield, water security, regional water balance model, stochastic modelling, climate variability.
Jiri Herza, Graeme Maher, Nihal Vitharana, Megan Evers, Michael Somerford
Abstract: Drakesbrook Dam is a 17m high earthfill embankment constructed in 1931 to provide water for the Waroona Irrigation District, 80km south of Perth. The dam has a storage capacity of 2.30 GL and is classified as a “High A” hazard dam according to ANCOLD Guidelines on Dam Safety Management.
A Dam Safety Review, undertaken in 2001 identified a number of deficiencies associated with main embankment, outlet works and the main spillway requiring remedial works. Detailed design of these remedial works is currently underway. Challenging features of this project are the design of a liner for the existing curved conduit and the design of a new spillway with an usual drop-type stilling basin.
This paper presents the salient aspects of the remedial works design along with the various design and construction criteria adopted to achieve an economical design of a new spillway and outlet conduit sleeve without compromising the safety of the dam.
Keywords: remedial works, earth embankment, conduit lining, spillway replacement
David S Bowles, Sanjay S Chauhan, Loren R Anderson, Terry F Glover
Abstract: A nested model is presented for considering variability and knowledge uncertainty in a dam safety risk assessment of an existing dam and interim risk-reduction alternatives (operating restrictions) during the staged implementation of a permanent structural risk reduction measure. The effects of some important aspects of natural variabilities on estimated risks are represented as cumulative distributions of probability of failure, annualised life loss, economic risk cost, and an F-N representation of life loss. Many cumulative distributions are generated to represent the effects of some important aspects of knowledge uncertainties.
An important aspect of the knowledge uncertainty is the current level of development of an already-initiated piping failure mode. Also, an approach to conditioning the system response probabilities (SRPs) for the piping failure mode on the duration of reservoir pool exceedance is included in the failure event tree risk model.
ANCOLD and Reclamation tolerable risk guidelines are evaluated at selected percentiles of variability and percentiles (levels of confidence) of knowledge uncertainty. The incremental cost-per-statistical-life saved and benefit-cost ratio for interim risk-reduction alternatives are estimated and evaluated to examine the case for more-severe levels of operating restriction than the least-severe operating restriction that is estimated to satisfy, at a selected percentile of variability and a desired level of confidence, the limit values in all of the tolerable risk guidelines that were considered.
Keywords: Dam safety, risk analysis, risk assessment, uncertainty analysis, aleatory uncertainty, epistemic uncertainty.
G. Shams Ghahfarokhi, PHAJM van Gelder, JK Vrijling
Abstract: Risk and reliability analysis is presently being performed in almost all fields of engineering depending upon the specific field and its particular area. Probabilistic risk analysis (PRA), also called quantitative risk analysis (QRA) is a central feature of hydraulic engineering structural design.
Actually, probabilistic methods, which consider resistance and load parameters as random variables, are more suitable than conventional deterministic methods to determine the safety level of a hydraulic structure. In fact, hydraulic variables involved in plunge pools, such as discharge, flow depth, and velocity, are stochastic in nature, which may be represented by relevant probability distributions. Therefore, the optimal design of a plunge pool needs to be modelled by probabilistic methods.
The main topic of this paper is concerned with the reliability-based assessment of the geometry of the plunge pool downstream of a ski jump bucket. Experimental data obtained from a model of a flip bucket spillway has been used to develop a number of equations for the prediction of scour geometry downstream from a flip bucket spillway of a large dam structure. The accuracy of the developed equations was examined both through statistical and experimental procedures with satisfactory results. In addition, reliability computations have been carried out using the Monte Carlo technique.
The main conclusions are that structural reliability analysis can be used as a tool in the dam safety risk management process and that the most important factors for further analysis are erosion, friction coefficient, uplift and self-weight.
Keywords: risk analysis, reliability, plunge pool, Monte Carlo simulation, flip bucket, large dams
Peter Hill, Phillip Jordan, Rory Nathan, Emily Payne
Abstract: There are a number of issues that need to be considered when deriving estimates of floods used to assess construction flood risk. This paper outlines the derivation of seasonal flood frequency curves and highlights the important differences in seasonality across Australia and the variation with the exceedance probability. Examples are provided as to how these seasonal frequency curves are used to estimate the construction flood risk during a particular construction activity in a safety upgrade for an existing dam or construction of a new dam. The paper also touches on the issues associated with estimating consequences for assessing construction flood risk.
Keywords: construction flood, risk, seasonal hydrology, hydrologic loading