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.
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The Keepit Dam Safety Upgrade Project is being implemented to bring the 54m high concrete gravity dam in line with current guidelines for flood and earthquake loading. Stage 2A of the project involves the installation of two vertical 91 strand post-tensioned anchors on each monolith of the spillway section.
During coring of the anchor head blocks for the vertical anchors, deep cracks were observed across some monoliths, dipping diagonally in an upstream direction. In two of the monoliths the cracks were found to be continuous enough to possibly daylight at the upstream face and form freestanding blocks. If the freestanding blocks postulate is correct, the block stability could be currently relying on the friction of the cracked surface and on the engagement with shear keys of adjacent monoliths, which are provided in the vertical contraction joints.
This paper will explain the complex 3-D nonlinear Finite Element Analysis (FEA) conducted to replicate the conditions of the cracked spillway monoliths during the post-tensioned anchor installation. The nonlinearity captured the expected opening, closing and sliding of the crack, as well as its potential pressurisation, and the residual shear strength retention due to asperities of the crack surface. For the shear keys of the vertical contraction joints, the nonlinearity captured the force-deformation relationship of the plain concrete, up to a brittle failure condition if the shear strength threshold was reached.
The 3-D nonlinear FEA was also used to design the optimum number of Macalloy post-tensioned bars required to stitch the freestanding block to the monolith, so that the vertical anchors can be safely installed. In addition, the remedial design accounted for future extreme design flood and extreme earthquake loading conditions, the latter modelled with a time-history analysis.
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.
Trustpower’s Mahinerangi Dam in New Zealand’s South Island is a concrete arch and gravity abutment dam built in 1931, subsequently raised in 1946 and strengthened with tie-down anchors in 1961.
This paper discusses a 3D finite element analysis of the dam and the predicted performance of the arch section under Safety Evaluation Earthquake (SEE) loading against identified potential failure modes.
Current guidelines and recent seismic hazard assessments recommend earthquake loadings higher than what was originally accounted for in previous decades. A Comprehensive Safety Review identified stability under SEE loading as a potential deficiency, so a programme of works was commenced to evaluate and better understand the seismic risk by using modern day tools and technology to evaluate the dam against current performance standards.
The final model incorporated the results of extensive laboratory testing, high-resolution LiDAR survey data and dynamic calibration using ambient-vibration monitoring. Motion recordings across the face of the dam during the 2016 Kaikōura earthquake were also used to validate the model. The reservoir has been explicitly modelled together with the opening, closing and sliding of contraction joints and the foundation interface. This allowed the modelling of permanent displacements and the redistribution of loads within the dam under SEE loading, which had been shown to be an important behaviour from the previous stages of analysis.
New Zealand’s economy is heavily dependent on export revenues generated by primary industries such as dairy, meat, agriculture, horticulture and viticulture. For these sectors, securing water for irrigation has been a key factor for growth. New Zealand has a temperate climate with generally wet winters and dry summers. The availability of water in the dry summer period is very important for these sectors to maximise production. A considerable amount of investment has already been made in the construction and operation of reservoirs for irrigation purposes. However, because climate change effects (more frequent occurrences of extreme events such as droughts and flash floods) have been observed around the world and the need for restrictions imposed on the use of water resources by regulators for environmental reasons, the need for developing water storage reservoirs has become more essential than ever. Climate change effects are already being factored into current practice. Drawing on the author’s experience, this paper discusses the potential impacts of climate change, with an emphasis on the effects of drought, on the design, construction and operation of water storage facilities with changes necessary to improve the resilience of new dams in
response to climate change. The paper also aims at raising awareness among the farming community so they can appreciate the associated risks and issues with climate change and be more cautious about planning and budgeting for their future investments in dam and irrigation projects.
Estimation of the potential economic consequences of dam failure is becoming an issue of increasing importance in the Australian dams industry. As a result of the ongoing investment in dam safety upgrades, societal risk profiles for many dams are generally reducing. Additionally, there is evidence of the potential magnitude of economic and financial costs from recent overseas dam incidents. Whilst there is a well established framework for estimation of economic consequences, based on concepts of direct/indirect and tangible/intangible damages, there is a dearth of recent literature on the application of modern unit cost rates for various asset classes. This is particularly important in cases where direct, tangible damages are an important component of economic consequences.
Currently, unit cost rates used to estimate direct tangible economic consequences in Australia are typically taken from older sources such as the floodplain Rapid Assessment Method (RAM). The appropriate cost rates are then factored by CPI to represent ‘current day’ estimates of these costs. However, since the time when the RAM was first developed, there have been changes to the categorisations used to identify economic assets such as businesses in common databases such as the census. Additionally, there have been a number of large, recent flood events in Australia which provide very useful data to assist in deriving updated unit cost estimates.
This paper presents proposed unit rates for damaged and destroyed residential and commercial structures (including stage-damage curves) consistent with the Australian Bureau of Statistics categorisations used in the census data, agricultural land and assets such as roads. These rates have been derived based on a range of sources. The purpose of producing these unit rates is to promote ease-of-use and consistency, especially for large consequence assessment studies where numerous assets are impacted. A case study is presented showing the application of these unit cost rates and highlighting the variability in direct, tangible damages in different circumstances