Fault displacement can occur due to primary faulting on a main fault intersecting a dam foundation or rim, as well as by secondary faulting. This secondary faulting may be triggered locally by the occurrence of primary faulting on a main fault; its occurrence is conditional on the occurrence of an earthquake on the main fault. A probabilistic approach is most viable for fault displacement hazard analysis. Unlike the case of probabilistic ground motion hazard, which is nonzero even for short return periods due to the occurrence of a broad range of earthquake magnitudes in a wide region around the site, probabilistic fault displacement hazard is zero for return periods less than the recurrence interval of surface faulting earthquakes on the fault. In Australia, these recurrence intervals typically lie in the range of 10,000 to 100,000 years.
Consequently, the fault displacement hazard due to primary faulting may be zero or negligible for return periods shorter than 10,000 or 100,000 years. For longer return periods, the hazard is best evaluated using a risk-based approach, as recommended by ANCOLD (2018); the alternative of using a deterministic approach, which disregards return period, could potentially yield a large fault displacement. The probability of triggered secondary faulting, conditional on the occurrence of a large earthquake on the main fault, is typically one or two orders of magnitude lower than that on the main fault, and so is even more likely to be zero or negligible for return periods shorter than 10,000 to 100,000 years
Design Review Boards or Panels play an important role in supporting owners and designers in creating resilient design of water storage and tailings dams. Their essential roles are to constructively challenge the project team to deliver on the project objectives through a design which meets the 3R’s of resilience, robustness and reliability, and to provide assurance to potentially non-technical owner / project management. This can sometimes create an uncomfortable situation if one or more of the project team is not aligned with the agreed criteria. Time and cost pressures can often push a project or execution team to undertake insufficient analysis or to consider non-justifiable construction processes or shortcuts.
Regardless, the Review Board must remain steadfast in their advice and guidance with a strong focus on “data-supported decisions”. Finding and maintaining an effective board requires commitment at the highest levels. This paper will examine some of the challenges in addressing governance, membership and turnover, and conflict resolution.
Dams and levees within the U.S. Army Corps of Engineers (USACE) inventory were constructed for a variety of purposes including flood control, navigation, hydropower, recreation, and fish and wildlife conservation. USACE transitioned to using life safety risk as a key input to all dam and levee safety decisions in 2006. This was implemented for many reasons, paramount among them is forming a consistent basis to evaluate the safety of dams and levees and prioritize the implementation of risk reduction measures in a consistent manner across the agency to best utilize available resources. This requires knowledge of what constitutes unacceptable risks that would require risk reduction actions. The Tolerable Risk Guidelines (TRG) were developed for this purpose, and to form a common basis for dam and levee safety evaluations and decisions. Protection of life is paramount, and there are four TRG related to (1) understanding the risks surrounding dams and levees, (2) building risk awareness, (3) fulfilling daily responsibilities, and (4) continually considering actions to reduce risks. The USACE policies have evolved over time, but the fundamental principles that underpin the TRG have been fairly consistent for the past 10 years. The evolution of the TRG have come as a result of the experiences using these principles to support more than 2,500 safety decisions. This paper describes the rationale behind the selection of the TRG.
The purpose of this paper is to document a limited review of the existing concrete chute spillways in the United States Army Corps of Engineers (USACE) portfolio of dams. This internal review was undertaken in response to the partial spillway failure of the Oroville Dam concrete chute spillway in February 2017, the partial spillway failure of the Guajataca Dam concrete chute spillway as a result of Hurricane Maria in September 2017, and to address the request by the United States Congress for USACE, United States Bureau of Reclamation (USBR), and the Federal Energy and Regulatory Commission (FERC) to review their respective portfolios for similar spillway vulnerabilities as Oroville Dam. The intent was to screen for existing concrete chute spillways within the USACE portfolio that may be susceptible to damage/failure during operation.
A new operating arrangement at Hume Dam is being developed to improve the transition from flood operations to the release of water set aside for delivering environmental flow demands. The arrangement also aims to help manage inherent downstream flood risks associated with this transition and with the requirement to fill the storage.
This paper describes particular flood risks and environmental impacts resulting from the current approach required to meet asset and water resource security priorities during airspace management operations at Hume Dam. It then considers how the new environmental demands have interacted with long-standing operating objectives and airspace management during high inflow periods in ways that have altered the dam operations required to meet operating priorities and manage flood risks.
Critically, requests by environmental managers to start releases can arise sooner and with greater uncertainty compared with releases for meeting irrigation demand following a period of flood operations or airspace management. This difference has led to a more rapid storage filling curve to maximise water resource during periods when inflow rates remain relatively high and catchments are still responsive to rainfall.
The paper details how the new operating arrangement provides greater volumes and more flexible flood mitigation airspace using a discretionary volume of ‘held’ environmental water without otherwise impacting on the flood operations decision-making process. A number of challenges in defining the potential level of benefit and risk, and in understanding trade-offs were faced in negotiating the arrangement. However, the successful development of the approach and agreement to trial it were ultimately achieved by framing the issue as an opportunity to adjust dam operations in a way that seeks mutual benefits for dam operators and environmental managers.
Full adoption of the arrangement would result in greater airspace flexibility during flood operations to better manage risks without affecting water resource. Simultaneously, it provides environmental benefit due to changes in the pattern of releases during the transition period from flood operations to the commencement of environmental water releases as well as during the pre-spill release period.
If a risk-based approach is used to assess the spillway adequacy for large dams, then an estimate of the annual exceedance probability (AEP) of extreme rainfalls is required up to and beyond the Probable Maximum Precipitation (PMP). This paper describes how two site-specific approaches described by Nathan et al. (2015; 2016) were used to estimate the AEP of extreme rainfalls for seven catchments, ranging from 1300 km2 to 114,000 km2, in the northern-coastal region of Australia. The results are then compared with the regionally-based Laurenson and Kuczera (1999) relationship for estimating the AEP of the PMP, which is recommended by the Australian Rainfall and Runoff 2019 guide to flood estimation (Nathan and Weinmann, 2019). This shows that the site-specific assessments have produced a rarer estimate of the AEP of the PMP compared with Laurenson and Kuczera (1999), particularly for the catchments >10,000 km2. For some of these locations, this has allowed the dam owners to plan risk-based upgrades with more confidence.