David Hilyard, William Ziegler, Heather Middleton
New South Wales has a significant number of dams, including major water supply dams, located over or near mines. Mining near dams imposes dam safety risks including: mine subsidence, mine blast vibration, presence of mine personnel downstream, rapid changes in consequence during mining, and loss of stored waters. The NSW Dams Safety Committee(DSC) regulates mining near dams, using risk assessment to review applications to mine near dams. A structured approach allows rational, evidence-based decision making by stepping through a procedure involving: initial consultations, screening risk assessment, evaluation of technical arguments, risk assessment, and development of risk management strategies. The risk assessment for dam walls develops acceptance criteria, reviews 19 possible risks to dam walls, and site-specific hazards. For potential for loss of stored waters, four possible groups of flow paths from storage to underground mine are reviewed; flows are evaluated with Monte Carlo simulation in terms of tolerable loss. Risks are assessed from a dam engineering viewpoint, which may be more conservative than the perception of risk in the mining industry, considering both tolerable risks and operational time frames. Case studies include: a tailings dam 100 m upstream of an active open cut and underground portal was undermined by longwall mining, with about 1.5 m subsidence of parts of the embankment as each of four longwall panels was extracted; longwall mining beneath a major Sydney water reservoir, with no observed impact on the stored waters; and open cut mining immediately downstream of a mine water dam. Risk-based methodology has provided the DSC with increased confidence in reviewing applications to mine near dams.
Keywords: Mining, dams, risk assessment, New South Wales, Dam Safety Committee
Robert Wark, R.N.M. Nixon
Sediment inflows to Lake Argyle, the reservoir formed by the construction of the Ord River Dam, were seen as a significant threat to the Ord Irrigation Project when the scheme was being developed through the 1960s. Sediment monitoring was built into the operation of Lake Argyle when the Ord River Dam was completed in 1971. The paper describes the strategies that have been in place to assess sediment loads and monitor sediment build up in the reservoir.
Spectacular reduction in sediment flows has been achieved through developing a comprehensive catchment management program. The program commenced in the early 1960s and was adapted and modified as progress was made. The paper describes the steps taken to identify the areas of the catchment at risk, the measures implemented and the current status of the catchment.
A key feature of the catchment management program has been the willingness to critically review progress and adapt the program. A variety of sediment tracing techniques have been used to help confirm the sources of sediment in the catchment, and the paper describes these, and the broad range of results and how they have helped direct the work on catchment management.
Keywords: Sediment, monitoring, catchment management, Lake Argyle, Ord River
Eric Lesleighter, Peyman Andaroodi, Colleen Stratford
In January 2011 major flooding was experienced across a large part of Southern Queensland. The flood discharges through the Wivenhoe Dam spillway caused extensive erosion of the rock in the plunge pool. While not an issue in relation to the spillway structure’s security, the rock erosion experience was dramatic for a number of reasons. The paper presents details of the extent of erosion under head conditions that can be classed as moderate only when compared with many taller dams. The discharges over several days resulted in a pile of huge rock blocks downstream of the plunge pool.
The paper describes the plunge pool design dimensions, the geology, the hydrology of the releases, the hydraulics of the plunge pool, the surveys of the pool and rock mound, and moves on to discuss the mechanism of the fracturing and transport of the rock. Similar relevant experiences will be cross referenced, especially from details of recent experiences at the Kariba Dam and the study of remedies in the context of the dam’s actual safety.
From an actual major experience of erosion, and the sheer volume of rock that was lifted up and out of the plunge pool, the occurrence stands as a timely demonstration of what can happen in similar spillway situations, and suggests the type of awareness that spillway design needs to accommodate for energy dissipation facilities in unlined spillways plunge pool.
Keywords: Spillways, plunge pools, rock erosion, scour, plunging jets, pressure transients.
Simon Lang, Peter Hill, Wayne Graham
The empirical method developed by Graham (1999) is the most widely used in Australia to estimate potential loss of life from dam failure. It is likely to remain that way while spatially based dynamic simulation models are not publicly available (e.g. LIFESim, HEC-FIA and LSM). When the Graham (1999) approach was first developed the prevalence of spatial data and the speed of computers was much less. In addition, most people did not have mobile phones, social media was in its infancy, and automatic emergency alert telephone systems were 10 years from being used in Australia. Graham (1999) was intended to be applied to populations at risk (PAR) lumped into a discrete number of reaches. The selection of fatality rates for the PAR in each reach was based on average flood severity and dam failure warning times. Today, there is typically much more spatially distributed data available to those doing dam failure consequence assessments. Often a property database is available that identifies the location of each individual building where PAR may be, along with estimates of flood depths and velocities at those buildings. News of severe flooding is likely to be circulated by Facebook, Twitter and e-mail, in conjunction with official warnings provided by emergency agencies through radio and television and emergency alert telephone systems.
This raises the question of how Graham (1999) is best applied in today’s digital age. This paper explores some of the issues, including the estimation of dam failure warning time, using Graham (1999) to estimate loss of life in individual buildings and the suitability of Graham (1999) for estimating loss of life for very large PAR.
Keywords: loss of life, dam safety, risk analysis.
John Grimston, David Leong, Robin Dawson
The Angat Multipurpose Project, originally constructed in the 1960’s, is located 60 km north-east of Manila, and provides power, irrigation and domestic water supply and flood mitigation. The major water-retaining structures of the scheme are a 131 m high main rockfill dam and a 55 m high rockfill saddle dam.
Previous seismology studies have identified the presence of a possible branch of the West Valley Fault crossing under the saddle dam. If the fault dislocated, the branch under the saddle dam could produce horizontal and vertical shear displacements. Further, earthquake shaking poses a risk outside the fault zone. If the main dam/saddle dam were to fail in such an event, there would be major consequences in respect to both the water supply (serves a population of approximately 10 million) and the large population living below the dams. The dams are thus in the highest hazard category under any internationally accepted standard.
A study to investigate the dam safety aspects and identify remediation works which would bring the seismic performance of the main dam/saddle dam system up to an acceptable level was undertaken and included:
The main conclusions were:
Keywords: Dam, Remedial, Seismic, Fault, Spillway.
David Stephens, Kristen Sih, Peter Hill, Rory Nathan, David Dole
The spring and summer of 2010-11 were characterised by severe flooding affecting much of Victoria. In a number of cases, communities downstream of large dams developed to supply water for irrigation and critical human and stock needs were significantly impacted. Following the floods, the Victorian Government commissioned the Victorian Floods Review (VFR) to consider the total warning and response to these floods. Whilst dam operations were not specifically included in the terms of reference, overwhelming community interest lead to the VFR commissioning a high level review of the way a number of key dams were operated during the floods. This review identified some of the inherent tensions in the legislative framework for water harvesting, storage and dam safety in Victoria. These tensions were often matched by the conflicting expectations of the public living immediately downstream of the dams versus those dependent on the water resource stored in the dams. The final report of the VFR was handed down in December 2011 and contained a number of recommendations specifically for dam owners. These recommendations are reviewed and discussed in light of both the legal and public relations ramifications for owners and operators of large water supply dams. An overview is also given of the operational constraints to downstream flood mitigation facing many dam owners. Such constraints are typically imposed by the type of dam (i.e. fixed crest), relatively small storage and outlet capacities when compared to flood volumes and limitations on the reliability of forecast rainfall information. Some possible ways of overcoming these constraints are identified and discussed.
Keywords: Flood, mitigation, Victorian Floods Review