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
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Abstract: Selection of suitable dam and associated structure sites for run-of-river hydropower projects in the Himalayan region requires a thorough assessment of the hazards and the consequences of their occurrence. Typical projects in this environment of deep ridge and ravine terrain comprise concrete gravity dams with steel flood or sluice gates. The steep abutment slopes generally require spillways to be over the dam crest and not as separate structures.
Hazards to be considered for such structures are linked mainly to geological and hydrological factors and include landslides both at the dam and in the reservoir, outwash floods from a variety of initiating causes (landslide dams or glacial melt), floods from storm events, weak or unstable foundations, seepage and pore pressures and earthquakes. Each hazard is related to a risk scenario and a matrix of events is evaluated for potential cause, consequence and impact. A measure of consequence to the project at various stages of design, construction and operation is obtained and provides a basis for preparing risk mitigation measures. Risk management plans can then be prepared using a forum process with stakeholders to achieve a satisfactory outcome.
Examples are provided from specific studies carried out for hydropower projects in the Indian Himalaya. They include a risk assessment of a floodwave overtopping a dam from debris torrent after breaching of a landslide formed dam, terrain studies and geomorphological assessment to locate landslides in dam abutments and design and construction issues relating to a project site astride a major tectonic thrust zone.
Keywords: geotechnical risk, hazard and risk matrix, risk management plan, landslides, landslide dams, hydropower dams and tunnel, Himalayas
Brunner Wolfgang G, Bi Arthur, Chang William, Zong Dung Feng
ABSTRACT: In the South of the Sichuan province the 240 MW Yeleh hydroelectric power project is under construction by Sichuan Nanya River Basin Hydraulic Power Development Company Limited, a state-owned enterprise, and China Gezhou Ba Water & Power Group Company Ltd.. This project sets out to develop the mountain cascade of the Nanya river, a tributary of the Dadu River, provide superior electricity and adjust flood peak and frequency. The specialist contractor Foundation Engineering Company of China Water Resource and Hydropower (FEC) was awarded the contract of foundation treatment at the right bank, which included the construction of a 75 m deep concrete cut-off wall inside a 6.0 m x 6.5 m tunnel. The requirement for 19 317 m3 cut-off wall to be constructed in permeable and very dense gravel/cobble formations and a demanding project program led FEC to adopt the BAUER Low Headroom Cutter CBC25/MBC30 in conjunction with the overlap cutter joint.
Keywords: hydroelectric project, dam, dam tunnel, cut-off wall, trench cutter
Mark Hore, Joseph Matthews
Abstract: Substantial flooding occurred in the Gippsland region in late June 2007, following the severe bushfires experienced during the previous summer. Major damage was sustained to the regions infrastructure, which included the gated storages at Glenmaggie Dam and Cowwarr Weir, located in the neighbouring Macalister and Thomson river catchments respectively. The flood event at Glenmaggie Dam produced a record peak inflow of well over 250,000 ML/day, which was more than twice the previous record.
The magnitude and intensity of the flood event created a number of issues for the staff at Southern Rural Water (SRW), who are engaged in the safe operation of the facilities. The event tested the organisations emergency management systems and the ability of the organisation to effectively manage events at multiple sites. Some of challenges faced included: a rapid rise in storage volumes; the loss of upstream warning gauges; the accumulation of large volumes of debris; the development of suitable release strategies and the communication and engagement with the local community.
The event caused a significant amount of damage to key infrastructure at both sites, with the Cowwarr Weir storage being the worst affected. A number of high priority projects have been completed since the event, with spending to date totalling $3 million. The unusual nature of the flood event provided the opportunity to review previous flood assessments and to identify deficiencies with elements of the existing infrastructure. Of particular interest was the subsequent hydrology review which provided a comparison with previous modelling assessments. The review included a flood frequency assessment which showed that the magnitude of the Glenmaggie flood was in the order of a 1:200 AEP(Annual Exceedance Probability) event, which was disproportionate to the rainfall event frequency assessed as having a 1:50 AEP, when averaged across the catchment. The aim of the paper is provide a case study for other dam engineers who may be preparing for future flood events in similarly affected catchments.
Keywords: flood, gated storage, dam performance, remedial works.
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.
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