Andrew Balme, Dan Forster, Tim Logan
The MW7.8 Kaikōura earthquake on 14 November 2016, ruptured over 20 faults during the initial shaking,which lasted nearly two minutes. A complex series of fault ruptures propagated northeast for nearly 180 km from the initial rupture location. Instrumentation from dams across New Zealand shows that whilst most dams did not suffer physical damage, piezometric responses were measured in dams and their foundations. Earthquake related changes in seepage regimes are not unusual and depend on the characteristics of the ground motions,and site specific characteristics that influence how a dam and its foundation respond to ground motions. The ability to measure a piezometric response in a dam or foundation is heavily influenced by the instrumentation network and method of monitoring. Data collected during events such as the Kaikōura earthquake provides valuable information for both characterising performance of a dam during the event, and assisting future analysis such as failure mode assessments. Careful consideration must be given to the scope of installed instrumentation and the frequency of monitoring in order to provide these benefits,and the robustness of the system to ensure it adequately survives the event.
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Andrew Northfield, Simon Lang, Peter Hill
Melbourne Water currently manages more than230retarding basins (RBs). A large portion of these are less than 4 metres high, and traditionally structures of this size have not been subject to intermediate or detailed ANCOLD Consequence Assessments. However, the need to understand the failure consequences for smaller structures has increased over time, as risk based approaches to managing safety have expanded from large dams to other water retaining assets.
Undertaking detailed consequence assessments for all Melbourne Water’s RBs would not be practical, given the costs and time involved. Therefore, this paper describes a method for assessing the level of ANCOLD Consequence Assessment that is justified, based on the structure’s attributes. It also presents an equation that was used to estimate peak outflows from RB failure. The peak outflow estimates can be used to model approximate failure inundation extents downstream of small dams and RBs.
The paper draws on work that HARC have recently undertaken for Melbourne Water to assess the failure consequences for 88 RBs. The outcomes are relevant to other organisations that own or manage significant numbers of small water dams or RBs.
T. Allen, J. Griffin, M. Leonard, D. Clark and H. Ghasemi
Geoscience Australia (GA) has embarked on a project to update the seismic hazard model for Australia through the National Seismic Hazard Assessment (NSHA18) project.The draft NSHA18 update yields many important advances on its predecessors, including: 1) calculation in a full probabilistic framework using the Global Earthquake Model’s OpenQuake-engine; 2) consistent expression of earthquake magnitudes in terms of moment magnitude, MW; 3) inclusion of epistemic uncertainty through the use of alternative source models; 4) inclusion of a national fault-source model based on the Australian Neotectonic Features database; 5)the use of modern ground-motion models; and 6)inclusion of epistemic uncertainty on seismic source models, ground-motion models and fault occurrence and earthquake clusteringmodels.The draft NSHA18 seismic design ground motions are significantly lower than those in the current (1991-era) AS1170.4–2007 hazard map at the 1/500-year annual ground-motion exceedance probability (AEP) level. However, draft values at lower probabilities (i.e., 1/2475-year AEP) are entirely consistent,in terms of the percentage area of land mass exceeding different ground-motion thresholds,with other Stable Continental Regions(e.g.,central & eastern United States). The large reduction in seismic hazard at the 1/500-year AEP level has led to engineering design professionals questioning whether the new draft design values will provide enough structural resilience to potential seismic loads from rare large earthquakes. This process underscores the challenges in developing national-scale probabilistic seismic hazard analyses (PSHAs)in slowly-deforming regions, where a 1/500-year AEP design level is likely to be much lower than theANCOLD Maximum Credible Earthquake (MCE) ground motions. Consequently, a robust discussion among the Standards Australia code committee, hazard practitioners and end users is required to consider alternative hazard and/or risk objectives for future standards.Site-specific PSHAs undertaken for owners and operators of extreme and high consequence dams general-ly require hazard evaluations at lower probabilities than for typical structural designas recommended in AS1170.4.However, modern national assessments, such as the NSHA18, can provide a benchmark in terms of recommended seismicity models, fault-source models, ground-motion models, as well as hazard values, for low-probability site-specific analyses.With a new understanding of earthquake processes in Australia leading to lower ground-motion hazard values for higher probability events (e.g.,1/500-year AEP), we should also ask whether the currently recommended design probabilities provide an acceptable level of seismic resilience to critical facilities (such as dams)and regular structures.
James Stuart, Michael Hughes
Several recent rain events in Australia have resulted in impoundment flood levels where there was a surprising variability between the Annual Exceedance Probability (AEP) of the flood level and that of the rainfall. The issue was highlighted during the Queensland Flood Commission of Inquiry (QFCI, 2011) by the Queensland Dam Safety Regulator who suggested there may be a problem with design hydrology after a dam safety event that saw impoundment levels of around 1:9000 AEP with a 1:200 AEP catchment rainfall at North Pine Dam, north of Brisbane in 2011. Wide disparities have occurred at Wivenhoe Dam west of Brisbane, at Callide Dam, west of Gladstone and at other locations.
This paper examines the Generalised Short Duration Method (GSDM) (BoM, 2003) and the Revised Generalised Tropical Storm Method (GTSMR) (BoM, 2003) typically used for dam flood capacity assessments in an attempt to explain the variability outlined above and whether it is, in part, exacerbated by the methods themselves.
It finds that processes of generalising rainfall depth, intensity, temporal and spatial characteristics are working together with adopted hydrological methods to contribute to such variability, that in the worst case could lead to PMF levels in dams with much less rainfall than the associated PMP would infer.
Moreover, two key assumptions; that of AEP neutrality (AEP of rainfall is equal to that of the flood) and frequency of PMP based on catchment area, which are the foundations stones of our understanding of flood frequency for large structures, are found to be untested or simply interim advice. This leads to the conclusion that the likelihood of floods in the range 2000 year AEP to PMF may continue to show surprising variability, potentially of an order of magnitude or more, compared to the rainfall AEP.
There is a need for a review of these methods and potentially provision of interim guidance as these methods are currently being used in dam upgrade programs throughout Australia and are also the basis for emergency planning. The identification of these issues concerns current methods and are independent to any discussion on climate change.Prior to commencing, it is worth defining two terms that re-occur throughout the document:
Annual Exceedance Probability (AEP): The probability that a given rainfall total accumulated over a given duration will be exceeded in any one year. AEP Neutrality is the theory that assumes the probability of the rainfall can be transferred to the resulting flood.
Average Variability Method (AVM): Technique for estimating design temporal pattern of average variability to ensure AEP Neutrality in transition from PMP to PMP design flood
Petros Armenis, Malcolm Barker, Peter Christensen, Graham Harrington
The Canterbury Earthquake Sequence in September 2010 and February 2011 caused large areas of land to change by differing amounts throughout Christchurch, New Zealand. Land levels fell by more than 300 mm in some areas. This increased flood risk in the tidal reaches of the Avon River. Urgent repairs were completed with the objective to restore the tidal river defences to a crest level equivalent to a 1% AEP tide level. This work needed to be completed prior to impeding spring tides.
The levees will be required for up to 20 years and then probably be rebuilt on a new alignment. To better understand the risks associated with the ongoing reliance of the levees for flood protection in the interim, a risk assessment was undertaken using conventional Australian National Committee on Large Dams (ANCOLD) practices and levee design procedures. Careful consideration was made to the performance of the existing levees under seismic, flood and tidal loading from which the societal and individual risk profiles were derived. The work included the following:
This paper will present the levee design and the process applied for the analysis of the levee and the upgrade options selection
Barton Maher and Michael Peel
The Queensland Bulk Water Supply Authority (Seqwater) manages up to $12 billion of bulk water supply infrastructure and the natural catchments of the region’s water supply sources to ensure a reliable, quality water supply for more than 3million consumers across the region. Seqwater was formed on 1 January 2013 through a merger of three State-owned water businesses, the SEQ Water Grid Manager, LinkWater and the former Seqwater. Seqwater delivers a safe, secure and reliable water supply to South East Queensland, as well as providing essential flood mitigation services and managing catchment health. Seqwater also provides water for irrigation to about 1,200 farmers and offers community recreation facilities enjoyed by more than 2.5 million people each year.Seqwater owns and operates 26 referable dams which fall under the dam safety regulation in Queensland, 51 weirs, and two bore fields across the region. Twelve key dams across the region supply as much as 90% of South East Queensland’s drinking water.In 2011, Seqwater engaged a consultant team of URS (now AECOM) and SKM (now Jacobs) to undertake a portfolio risk assessment of the 26 referable dams and Mount Crosby Weir. At the completion of the project in December 2013 there were 12 dams with life safety risks assessed as being above the ANCOLD and DEWS Limit of Tolerability. A $6.2 million investigation was approved in 2014 to commence planning for the recommended dam safety upgrades and reduce uncertainties in the risk assessment.This program of work was completed in late 2016. The estimated costs of the identified dam safety upgrades exceed $900 million.Confronted with such a large capital program, Seqwater has instigated a number of key actions including:-benchmarking capital investment and rates of risk reduction achieved by other dam owners through a dam owners group-developing a dam safety investment policy to provide a clear guidance on the framework for prioritising and scheduling upgrades-undertaking targeted investigations to reduce uncertainty in the risk assessments including the use of detailed consequence assessment-preparing a prioritised schedule of planned upgrades to gain endorsement from Government and the Dam Safety Regulator. This paper presents the outcomes of the Portfolio Risk Assessment and key changes to the initial risk assessment following further studies. The basis for the dam safety investment policy is presented and the proposed prioritisation tools.The impacts of the risk assessment provisions in the most recent revision of Queensland Acceptable Flood Capacity Guidelines for Water Dams are also discussed. In particular,the application of the economic criteria for determining the minimum upgrade required by the Queensland Dam Safety Regulator and its relevance to other dam owners.