Simon Lang, David Stephens, Peter Hill, Mark Arnold and Tommie Conway
Considerable thought has been given in recent years to managing the risks associated with floods during the construction of new dams and dam upgrades. Both ANCOLD and the NSW DSC provide some limited advice on how this risk should be managed, with many dam owners aiming for societal risk during construction to be no higher than pre-construction. One approach to do this is to draw down the reservoir such that sufficient airspace is created to reduce the probability of overtopping the construction works to be equal to that of overtopping the dam crest pre-construction. However, this frequently leads to very large releases of valuable water resource being required. This approach also fails to consider that the conditional probabilities of failure may be quite different during construction than during normal operation. A risk-based approach was applied for the recent upgrade of Tarago Reservoir. Existing event trees from a failure modes analysis were adjusted to reflect the construction conditions. In some cases, the event probabilities increased (for example as a result of excavation of the dam embankment), however some also decreased (for example as a result of more rapid means of detecting and intervening in breach formation during construction). The conditional probabilities of failure during construction were then used to estimate the overall seasonal probability of failure, and it was found that a limited draw down of the reservoir would be sufficient to ensure that risks were no higher during construction than pre-construction. To reinforce this, the cost-to-save-a-statistical life was estimated for further drawdown of the reservoir and used to demonstrate that the risks were as low as reasonably practicable.
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Lelio Mejia and Ethan Dawson
The 26 km long Tekapo Canal is a major component of the Upper Waitaki Power Scheme in the Mackenzie Basin in New Zealand’s South Island. The canal, commissioned in 1977, conveys water from a power plant at Lake Tekapo to a power plant at Lake Pukaki. To support a re-lining and repair works project along sections of the canal, seismic deformation analyses were performed. Earthquake-induced settlements and deformations for three critical embankment sections were estimated. Two dimensional, nonlinear, dynamic numerical analyses were performed with the computer code FLAC. Analyses were performed for the Maximum Design Earthquake (MDE), the Serviceability Level Earthquake (SLE) and for aftershocks of various magnitudes. A critical feature in the repairs and upgrades was a geosynthetic liner to be placed along portions of the canal. Seismic performance of this liner would be affected by cracking in the underlying embankment. Crack size estimates (width and depth) were developed based on evaluation of the computed deformations and the empirical correlations of Fong and Bennett (1995) and Pells and Fell (2002). Calculated deformations were generally small, and indicative of adequate seismic stability. Recommended design crack widths for the MDE ranged from 20 to 50 mm, while recommended design crack depths for the MDE ranged from 1.0 to 2.5 m.
The dam surveillance industry relies on deformation survey data to assist in understanding and monitoring dam performance. My paper presents an overview of New Zealand dam deformation surveying. The fundamentals and best practice of deformation surveying are discussed, along with accuracies achieved and developments in automated measurements in real time. The key to achieving high accuracy in the results is using precise well calibrated survey instruments, many redundant measurements, quality survey marks and rigorous computational routines.
Nanda Nandakumar and Stephen Farrelly
Fuseplug auxiliary spillways are used to increase the discharge capacity in dam upgrades for flood security. Hydrologic level-pool routings are used to determine the size and trigger levels for fuseplugs. In the level-pool routing, the water surface from the body of the storage to fuse bays is generally assumed to be horizontal and any drawdown effects on the water level are neglected. This paper assesses the validity of this assumption using the CFD model results for Keepit Dam. It is shown that equal spacing of trigger levels can result in premature activation, and the drawdown effects need to be taken into account in determining spacing of trigger levels. It was also shown that the design water levels for the intermediate AEPs are underestimated.
A comparison of inflow and outflow frequency curves showed that peak outflows can exceed the peak inflows due to fuseplug operations, but the downstream impact is expected to be negligible due the size of the flood in which the peak outflow will exceed the peak inflow.
Richard Davidson, Jennifer Williams, Roger Raeburn and Jason Boomer
Ashton Dam is a 20-m high embankment dam located on the Henry’s Fork River in Eastern Idaho. It is a high hazard structure licensed with the FERC. The dam was completed in 1916 as a zoned earth and rockfill dam utilizing a low plasticity silt core. Ashton Dam is located approximately 13 km north of Teton Dam and is the sole remaining structure of four similarly designed dams. Over the years, the dam’s condition deteriorated, evidenced by periodic recurrence of sinkholes, sediment plumes and settlement.
PacifiCorp initiated a major 3-year rehabilitation project for the structure. Based on a risk-based design process, a new zoned embankment was reconstructed. Significant structural upgrades were also required for the powerhouse, training walls and gated spillway. To facilitate this construction, a new diversion tunnel and gated outlet structure were built to divert the river and manage flood flows. Cofferdams were required for both the upstream and downstream construction works.
Several challenges were encountered during construction, which were managed with a risk-based process. These included addressing the uncertainties that were known during design and the unknowns that were discovered during construction. Some of the construction challenges covered in the paper include utilization and processing of low plasticity silty material for embankment reconstruction, tunnel construction through fractured basalt with a major shear zone, a lake tap excavation in the wet, dewatering of the embankment excavation, left abutment treatment, real-time redesign of structural features, and fill placement in a constrained excavation.
This paper provides a synopsis of how these design and construction challenges were addressed and overcome on a “blue ribbon” trout stream with high public visibility and interest. Of particular concern was the need for cold weather concrete work, managing flood flows, lake tap and embankment excavation during the very limited construction seasons, and maintaining environmental river controls for the sensitive downstream ecosystem.
Keywords: Risk-based design, Embankment Reconstruction, Piping, Aged Concrete Repair
Lyndon Johnson, Alan White and Chris Topham
The integrity of foundation drainage systems is a key factor in minimising uplift pressures under concrete gravity dams. Contemporary industry practice for foundation drainage systems (and modern criteria presented in the imminent release of the concrete gravity dam guidelines) will lead owners with older concrete dams to consider enlarging foundation drainage systems via borehole drilling in the foundation. This paper presents the cautionary tale of a dam owner that undertook foundation drilling works in the gallery of a 67-m high concrete gravity arch dam and experienced borehole “blowout” in one of the drilled holes. Water under 90% of full reservoir head issued from the borehole and needed to be controlled. The context of the works is presented, followed by a description of the blowout, the risk mitigation measures that were planned prior to the work, and which ultimately had to be initiated. Management of the incident is discussed, including the use of blowout protection collars and valves, subsequent investigatory drilling, and pressure grouting programme. Dam safety concerns associated with the incident and their management are presented. The paper concludes with some recommendations to manage these risks for other owners considering a drilling programme in a concrete dam foundation.