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
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Chris Topham, Paul Southcott, Tim Cubit
Abstract: Dee Dam is a 15 m high and 270 m long central core earth and rockfill dam on the upper reaches of Hydro Tasmania’s Tungatinah Power Scheme. The dam is assessed to have a High A hazard category. Hydro Tasmania’s portfolio risk assessment found that a risk based upgrade was warranted to protect against both piping and flood overtopping failure modes.
A $4.2M modernisation project was implemented in 2008 comprising the installation of a full height downstream filter with rockfill buttress, repairs to cracking in the diversion conduit and raising of the core for improved flood capacity. Lowering of the Dee Lagoon to facilitate full height excavation of the downstream shoulder of the dam was impractical to Hydro Tasmania for production, environment, cost and stakeholder reasons. Hence, careful analysis and risk management was required to ensure the safety of the dam during the construction of the above works against a full storage.
This paper presents the risk objective for the upgrade work, modelling and analysis undertaken to assess dam safety during the works. A wide range of construction risk mitigation measures were employed prior to and during the upgrade works. The response to latent conditions and potential dam safety incidents that occurred during construction are described. The continuous adaptation of the construction methodology to suit site conditions encountered during the project is also presented. The project was successfully completed in June 2008. The approach of detailed investigations and design coupled with a strong risk based approach during the construction proved to be effective in managing the dam safety risks of construction work with a full reservoir.
Keywords: earth and rockfill dam, filters, construction risk mitigation, dam safety, dam safety incidents.
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
Abstract: Queensland’s regions need additional water supply infrastructure to underpin the future economic growth of the state, and secure the water supplies of existing communities in light of climate variability.
Queensland’s economy has consistently been the fastest growing of all the States for the last decade. The major component of Queensland’s growth is underpinned by resources and industry, in regional areas of the State. If the enabling infrastructure is absent, or if timeframes for development do not align with the needs of the industry, this growth will be constrained, or will go elsewhere. Future investments that support growth cannot and will not occur without certainty around the availability of future water supply.
Queensland’s regions are emerging from the worst drought in recorded history, this has revealed serious deficiencies in the resilience of our regional water supplies. For the first time in living memory, major regional centres have been forced to face the prospect of running out of water.
The Queensland Government has been proactive in identifying these issues and has responded resolutely through the release of Statewide Water Policy which supports a $451 million investment in regional water infrastructure, which aims to meet the needs of urban and rural users.
As part of this policy SunWater has been named proponent for a range of projects considered essential for Queensland’s future economic sustainability which include the Nathan Dam on the Dawson River, Water for Bowen & Proserpine Pipeline Projects, Rookwood Weir & Eden Bann Weir Raising (with RRC & GAWB), Connors River Dam & Pipelines, Nullinga Dam and Kinchant Dam Raising. SunWater is also investigating the raising of Queensland’s largest dam, Burdekin Dam. This paper will outline the need for water supply infrastructure within regional Queensland, and the challenges faced by the projects being progressed by SunWater including the technical issues, environmental sustainability and approvals, and commercial development challenges.
Keywords: Queensland, Water Supply Infrastructure, Dams
Erik Bollaert, Nicola Bretz
Abstract: Gebidem Dam is a 122 m high double curvature arch dam situated in the canton of Wallis, Switzerland. The dam reservoir collects glacier water of the longest glacier in Europe, i.e. the Aletsch glacier (25 km length). The water is very rich in sediment. This results in huge amounts of fine sediments depositing into the reservoir. The 55 m3/s turbined by the Bitsch power station contain between 10 and 13 kg of sand, or an average of almost 40 tons per hour.
Hence, a yearly flushing is organized allowing to release about 400’000 m3 of solid material from the bottom outlets of the dam towards downstream. Moreover, the crest spillway of the dam is frequently used during summer months to release water from the reservoir, resulting in high-velocity jets impacting the downstream plunge pool. The latter consists of a series of flat concrete plates tied together by means of steel anchors and supported by a layer of underlying mass concrete.
Intensive jet impact on this concrete apron within the last 40 years has led to severe damage and, as such, replacement of the apron becomes necessary. The present paper describes the methodology applied for design of a new concrete apron. Especially the interaction between the hydrodynamic pressures exerted by overtopping jets and the steel anchorage necessary to prevent uplift of the new concrete apron is pointed out.
Keywords: scour, concrete apron design, anchorage
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.