Papers
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2019 Papers
2019 – Breaking Through the Breach: Global perspectives on dam-failure flood estimation
Learn moreSamantha Watt, Alan Brown, Duane McClelland, Chriselyn Kavanagh, Peter Kinley
Estimating the likely extent, depth and velocity of flooding should a dam fail – and planning to both prevent and respond to such a failure – are important parts of managing risk from dams and ensuring community resilience. This paper compares and contrasts current standards and practices for dambreak analyses and flood routing in New Zealand, Australia, the US, and the UK. Comparisons highlight consistent and evolving practices and consider how dambreak modelling supports robust dam safety decision making. In addition, the paper offers opinions regarding selected areas for future research, and insights into the benefits and limitations of increasing complexity in breach modelling.
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2019 Papers
2019 – One Breach or More? Assessment of Potential Multiple Flood Overtopping Dam Breaches and Sequencing
Learn moreLindsay Millard, Michel Raymond
Many quantified risk assessments finish the failure mode event tree at the estimated occurrence of an embankment breach leading to dam failure outflows and downstream consequences. In some situations, for dams with multiple embankments with potentially different consequences downstream of each embankment, the possibility for further breaches may be pertinent if there may potentially be higher consequences for a multiple breach scenario. The location of an initial breach and sequence of subsequent breaches could also result in different contributions to total risk.
This paper discusses a method applied to investigate the conditional probability of flood overtopping breaches for multiple earth-fill embankments with grass covered downstream slopes.
For the subject dam, preliminary modelling identified that for a flood overtopping breach of an embankment the breach’s development may not be sufficient to reduce the lake level and sustained overtopping flow over the remaining embankment crests could lead to further embankment breaches.A Monte Carlo dam breach simulation modelling approach was used with a large number of flood events. The simulation modelling considered erosion initiation for a grass slope due to the combination of velocity and duration of flow, and erosion continuing to breach based on duration of flow after erosion initiation. Potential uncertainty of erosion initiation and erosion continuing to breach were represented with probability distributions in the Monte Carlo modelling.
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The results from the large number of dam breach simulations were then analysed with post processing to derive conditional probabilities for single or multiple breaches and breach sequence. -
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2019 Papers
2019 – Dam-Break Consequence Assessments: Framework for Estimation of Population at Risk
Learn moreBill Veale, Grant Webby, Matthew Henden
Identification of people impacted by a hypothetical dam-break flood is required to understand the potential hazard a dam poses to downstream communities. The New Zealand Dam Safety Guidelines and the Australian Consequence Categories for Dams define these people collectively as the “Population at Risk” (PAR) and recommend that evaluation of PAR should include both permanent and temporary populations. However, there is limited guidance on specific methods to determine these populations. This paper provides an outline of an evidence-based, repeatable method to determine the PAR (both permanent and temporary) within a dam-break flood inundation zone. The method is intended to provide guidance for people tasked with estimating PAR in accordance with the New Zealand Dam Safety Guidelines. The methodology provides a current practice framework for users to apply and estimate the PAR in a clear and defendable manner.
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2019 Papers
2019 – Evidence Based Procedure for Estimating Itinerant Loss of Life
Learn moreTim Rhodes
Loss of life estimates in dam breach circumstances are a key determining input in establishing the appropriate risk profile for these assets. They can also be useful in identifying the most effective emergency management responses. While there are a range of approaches described in the literature for assessing loss of life for concentrated population centres, there is little specific guidance on approaches to be taken when there is only a small number of properties or where itinerant loss of life has the potential to be the dominant risk element. Itinerants are most commonly considered to be road users, although, they can alternatively be any temporary users of the floodplain. The literature on flood fatalities indicates that the largest number of deaths occurs at vehicle crossings or otherwise when individuals voluntarily enter waterways. An approach has been developed for identifying the cases where itinerant loss of life has the potential to be the dominant vector for flood fatalities. In addition, the available flood fatality literature and associated databases have been reviewed to establish the precursors to fatalities.
A simple stepped procedure is presented which allows the user to identify cases where itinerant risk to life on roads should be considered with a separate procedure and a method presented by which itinerant life loss may be identified.
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2019 Papers
2019 – Systematic Condition and Structural Assessment of Extensive Flood Protection Levee Infrastructure to Support Resilient Communities in Otago
Learn moreScott Sutherland, Scott Forster and Tim Morris
Otago Regional Council (ORC) own and operate the Lower Taieri, Lower Clutha, and Alexandra Flood Protection Schemes. Collectively the schemes comprise over 220 km of earthfill levees, together with numerous appurtenant structures, such as major spillways, flood gates and pumping stations. The schemes provide flood protection to significant and varied communities and infrastructure adjacent to the Clutha and Taieri rivers, for example Dunedin Airport, and towns such as Balclutha and Outram. The works were constructed at various times since the 19th century to a range of standards, and assets are at various lifecycle stages.
Regular and systematic condition and structural integrity assessment is a key aspect of operating flood protection schemes for resilient communities. This can be challenging due to the large spatial extent of multiple schemes. Efficient and effective on-the-ground visual inspection of the entire network is key. A field assessment methodology was developed which combined on-the-ground visual assessment with innovative use of GIS technology, for field data capture, recording, analysis and presentation.
The structural assessment methodology used LiDAR-derived digital elevation models (DEMs) integrated with the field data to screen the levee networks based on geometry condition, to identify critical locations for analysis. Levee susceptibility to hazards such as overtopping scour, piping, seismic performance and slope instability was assessed utilising a semi-quantitative multi criteria analysis. Subsequent efforts were focused on critical locations enabling analysis which would not be efficient on a scheme-wide scale. An outcome included a GIS database to enable rapid future review of asset information and condition.
The assessment coincided with the July 2017 Taieri River flood – the largest event in almost forty years, and a timely reminder of the importance of flood protection infrastructure for community resilience. This event also highlighted the importance of making use of such events to field-truth assessment results and test assumptions about scheme performance and vulnerable locations.
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2019 Papers
2019 – Flood Detention Reservoirs (Retarding Basins): UK Experience in Designing for Resilience
Learn moreJ D Gosden, A J Brown, A P Courtnadge
There are currently around four new flood detention reservoirs (retarding basins) built each year in UK, which although only being modest structures with median height of 4m and reservoir capacity of 300,000m3 pose a significant risk to the community as they are located immediately upstream of the community they are protecting. These communities range from around five to several thousand households.
The cost and therefore viability of these structures can vary depending on the number of defensive features built into the design, which raises interesting conflicting issues of public safety contrasted to vulnerability to property inundation in operational (say, 1 in 100 chance) floods.
The authors have designed and supervised over 30 flood detention reservoirs in the UK in the last 20 years. This paper describes the engineering decisions which need to be made regarding defensive measures and the resilience of these structures to withstand flood loading on demand. Examples of measures to include resilience are described, with discussion of when selection of the options to increase resilience against a particular failure mode should be mandatory, and when it may be more appropriate to consider it on a case by case risk-based approach. The paper will also discuss more strategic issues of how to balance making flood detention reservoirs affordable, while at the same time maintaining high standards of public safety and compares Australian and UK approaches.
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2019 Papers
2019 – Auckland Council’s Dam Safety Management System
Learn moreJ.O. Grimston, M.S. Razak, D.M. Knappstein, P.D. McCallum
Auckland Council (Council) is developing a dam safety management system with an overall objective to protect people, property, infrastructure, and the environment, from the harmful effects of a dam failure.
Council has responsibilities as an owner and operator of approximately 600 stormwater ponds and wetlands, many associated with dams. Council also has wider responsibilities for safety in the Auckland region, which may be affected by dams owned by others and even by inadvertent dams, such as road or rail embankments across streams that have the unintended but potential function of diverting, storing or holding back water. Three categories of dams have been distinguished, associated with Council’s different types of responsibility. Each category of dam is managed differently in the dam safety management system.
Given the large number of structures, which are not always obviously dams, a key activity has been the initial identification of dams across the Auckland region. Prioritisation has also been a necessary tool to direct resources and programme. Once dams have been identified, the consequences and risk of dam failure have been assessed, and commensurate measures have been established to manage those risks. There is limited guidance for some of these activities, and new procedures and tools have been developed.
This paper describes the process and the challenges encountered, for consideration by other councils when developing their own systems, and for consideration by the wider dams’ community.
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2019 Papers
2019 – Natural Hazard Exposure Assessments of New Zealand’s Stopbank (Levee) Network: Integrating a New Stopbank Inventory and Recent Seismic Hazard Models
Learn moreDaniel M Blake, Liam Wotherspoon, Kaley Crawford-Flett, Eduardo Pascoal, Matthew Wilson
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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2019 Papers
2019 – Dam Safety Maturity Matrix to Evaluate Health of USACE Dam Safety Program
Learn moreJacob Davis, P.E. , David Blackmore, P.E.
The U.S. Army Corps of Engineers (USACE) has a robust Dam Safety Program (DSP) that utilizes risk- informed decision-making to prioritize its portfolio of dams in need of further study and modifications. USACE also utilizes a two-tiered governance structure in which one body makes portfolio recommendations around risk management while the other body oversees the execution of the agency’s routine DSP and makes policy recommendations. The routine program consists of the activities required for interim risk reduction measures, inspections, instrumentation, monitoring, assessments, operations and maintenance, emergency action planning, training, and other dam safety activities. An internal program management tool exists to monitor and track all these activities and generate metrics around execution of the routine DSP, however, it does not include metrics around other aspects of the DSP like governance, asset management, public safety and security, flow controls, or audits/reviews. USACE hopes to identify gaps in its DSP that can be used to correct shortcomings, continuously improve, and to increase the resilience of its DSP, which will enable each project to deliver benefits to the Nation. The Centre for Energy Advancement through Technological Innovation (CEATI), through its Dam Safety Interest Group (DSIG), collaboratively developed a spreadsheet tool known as the Dam Safety Maturity Matrix (DSMM). The DSMM is a facilitated exercise used to help evaluate how well-developed a program is across 12 elements considered to be typical and important of most dam safety programs. Each of the elements is then deaggregated into sub-elements, each of which can be evaluated by the team. The maturity ranges across 5 levels from Needing Improvement to Leading Edge. After all sub-elements are evaluated, an aggregate maturity level is computed that gives an estimation of the overall maturity level of the program. USACE will present the results of its pilot project using the DSMM and share lessons learned regarding its implementation. The short-term goal is to identify program strengths and areas for improvement, while the long-term goal of USACE using the DSMM is to participate in bench- marking across multiple agencies and international dam owners regarding their dam safety programs, for which has never been done to the knowledge of this author.
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2019 Papers
2019 – Using Maturity Matrices to Evaluate a Dam Safety Programme and Improve Practices
Learn moreDan Forster, Lizzie Smith
Dam owners manage many complex activities to maintain and operate their dams safely and resiliently. Identifying, and continually improving, the key elements of an effective dam safety program and associated practices can be challenging but are essential to support resilient dams and resilient communities; using the Dam Safety Maturity Matrices (DSMM) is an efficient and thorough way to do this. A maturity matrix is a tool to evaluate how well-developed and effective a process or program is. The matrices were developed within CEATI’s Dam Safety Interest Group (DSIG) for owners to assess the effectiveness of their dam safety program against industry practice, and to assist with identifying improvement initiatives.
This paper will present the matrices and demonstrate how they are used to evaluate the effectiveness (or maturity) of a dam safety program. It will also highlight the benefits associated with using the matrices as an assessment tool, including the identification of improvements that can be made to a dam safety program, and the prioritization of efforts across multiple facets of a dam safety program.
User case studies from dam owners in both New Zealand and overseas will be presented to elaborate on the tool and the process.
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2019 Papers
2019 – Rationale Behind the US Army Corps of Engineers Tolerable Risk Guidelines
Learn moreNathan J. Snorteland, M.S., P.E.
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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2019 Papers
2019 – Performance and Management of the Cromwell Gorge Landslides, Clyde Dam Reservoir
Learn moreDon Macfarlane and Peter Silvester
Prior to filling the Clyde Dam reservoir in 1992-93 large scale stabilisation works were undertaken on several pre-existing landslides along the reservoir margins. Monitoring and visual observations indicate that the landslides are behaving satisfactorily and have confirmed that the stability improvements undertaken have successfully offset the negative effects of the reservoir on the landslides.
This paper presents selected records detailing more than 25 years of landslide behaviour that demonstrate the effectiveness of the stabilisation works. Monitoring has been able to detect increasing water levels, drainage flow changes and, in some cases, deformation following periods of high rainfall.
However, the highly satisfactory performance of the landslides experienced to date does not allow complacency and although the surveillance monitoring has been progressively scaled back to a more focussed strategy, ongoing assessment and reviews will be required. The paper also briefly discusses the current challenges associated with changing personnel and aging instrumentation.
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