The geographical location of New Zealand to the south west of the ‘Pacific Ring of Fire’ and in the ‘Roaring Forties’ of the Pacific Ocean exposes national infrastructure networks across the country to a range of natural hazards. Despite this, studies of built environment resilience to natural hazards in New Zealand, have historically focused on the robustness of individual physical assets, with less emphasis on the performance of infrastructure networks at a national level. This is particularly true for the stopbank (levee) network. Until recently, stopbanks have often been considered at regional scales and to varying degrees depending on what information has been catalogued, and the level of interest / requirements and local expertise available at the time.
We present the findings of a preliminary national level natural hazard exposure assessment of New Zealand’s stopbank network by adopting the newly developed New Zealand Inventory of Stopbanks (NZIS). Geospatial seismic hazard data from recent modelling is used as a case study to demonstrate how understanding the exposure of stopbanks in NZIS can inform multi-hazard risk and resilience assessments. Four seismic and co- seismic hazard metrics are considered in our stopbank network exposure assessment: surface rupture (through proximity to known active faults), the strength of ground shaking (i.e. probabilistic estimates of peak ground accelerations and velocities), and liquefaction and landslide susceptibility.
With over 20% of current catalogued NZIS stopbank length and a relatively high seismic hazard exposure (active fault proximity and liquefaction susceptibility) in Southland, the likelihood of stopbank failure or breaching due to seismic activity appears to be relatively high in this region of New Zealand. Large sections of the stopbank network in other regions including Manawatu-Wanganui, Wellington and Hawkes Bay are also particularly exposed to large seismic hazards in our preliminary assessment. However, further work is required to more appropriately understand stopbank attributes including design and safety considerations.
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The paper evaluates the stability of the reinforced rockfill at the downstream side of Waimea Dam, a new CFRD dam that is currently under construction in New Zealand. The reinforced rockfill is part of the overall diversion strategy for the dam during construction and has been designed to allow for safe overtopping to a depth of 2.9m, which corresponds to the 1 in 1,000 AEP flood.
Design of reinforced rockfill for overtopping allows for the safe passage of floods that exceed the capacity of the primary diversion works. This may be required for dam safety during construction, as is the requirement for Waimea Dam. It also serves to protect the works whist the dam is being built.
The focus of the paper is the stability assessment of the reinforced rockfill to prevent seepage induced instability during overtopping. As seepage forces have a considerable effect on the stability of the dam, a finite element seepage analysis was undertaken to estimate the seepage forces throughout the embankment, which was used in the design of the reinforcement system.
Details of the design process, including the seepage and stability analysis for a range of configurations are outlined, and recommendations for the design of similar future projects are provided.
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.
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.
Many numerical simulations have tried to model the failure-induced displacements of earth structures due to liquefaction. In this paper, the challenges in modelling such as the large displacement and non-immediate failure of earth structures due to liquefaction are discussed. An advanced bounding surface plasticity model is used to simulate the dynamic behaviour of saturated porous media. A series of benchmark welldocumented seismic events are analysed, and the results are compared to the reported laboratory and field observations. These analyses consist of one centrifuge test on liquefiable sand (Model #12 of the VELACS project) and one earthfill dam (Lower San Fernando Dam in California) subjected to seismic loading that leads to liquefaction. The capability of the model to capture the flow failure due to liquefaction is demonstrated and results are compared with other attempts in the literature to capture similar responses.