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.
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Gurmeet Singh, Nanda Nandakumar, Md Atiquzzaman, Andrew Richardson
This paper describes the flood modelling of the Lachlan River floodplain and highlights the impacts of complex terrain and concurrent tributary flows on river hydraulics for extreme design flood event. This study was undertaken as part of Portfolio Risk Assessment of dams operated by State Water Corporation.
It is demonstrated how a one dimensional/two dimensional coupled model can provide realistic spatial distribution of hydraulic parameters for consequence assessment and emergency flood risk management.
Keywords: 1D/2D coupled hydraulic model, time to peak, duration of flooding, rate of rise.
David S. Bowles, Sanjay S. Chauhan, Loren R. Anderson, Ryan C. Grove
A risk assessment (RA) was conducted for 27 miles of Herbert Hoover Dike to better understand and estimate the Baseline failure risk. Unique aspects of this risk assessment included the following: high stillwater levels persisting for almost a year; highly dynamic and spatially variable wind loading; short-duration wind setup that reduces likelihood of piping; dike length that increases probability of failure; and multiple breaches with overlapping inundation areas that affect failure probability and consequences and the risk evaluations.
A wide range of stillwater and wind loading combinations were considered. Following a potential failure modes analysis (PFMA), failure modes included were: piping through foundation, embankment piping, piping along conduits, piping along structures through embankment, embankment and flood wall instability, and overwash and overtopping. System response probabilities (SRPs) were estimated using toolboxes, analyses and expert judgment. Life-loss consequences were estimated using LIFESim. RA calculations were performed using DAMRAE-HHD, which includes length effects. Estimated risks were evaluated against the US Army Corps of Engineers (USACE) tolerable risk guidelines (TRG). Uncertainties were explored using sensitivity analyses.
Penelope Shaw, Brian Walford, Daniel Yates
The Bowen Basin is one of Australia’s economic mining powerhouses. Key to the mining of coal is the efficient management of the available water resources. For many years drought has meant that coal mines have had to manage operations with low quantities of water. For a new mine, this involved sophisticated hydrologic modelling as part of staying in business. In recent years a turnaround in weather conditions and changes to discharge permissions have meant that mines have too much saline water!
The paper describes how one mine augmented its small system of storages with a new dam and pumping system to meet regulatory requirements. Although not a large dam, the challenges confronted are shown to be similar to those involved in the delivery of much larger projects, all related to the power of water.
Keywords: Design storage allowance, water management, challenges, environmental protection, mining dams, automated pumping systems, spillway upgrading.
Robert Kingsland, Jamie Anderson, Andrew Russell, David Brooke
This paper presents the methods, observations and results from a programme of No-Erosion Filter (NEF) testing for the evaluation of a manufactured filter aggregate product that did not conform to normally accepted D15F grading limits. Base materials tested include both dispersive and non-dispersive soils. The results are compared against published no-erosion, excessive erosion and continuing erosion thresholds. The paper comments on the validity of the adopted thresholds and the effectiveness of the NEF test as a filter evaluation method.
Keywords: dam, filter, test, no-erosion
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