While structures such as a dam walls, pipelines, gas storage tanks, and nuclear facilities are vulnerable to the shaking from earthquakes, they are even more susceptible to differential movement on faults passing beneath their foundations.
In the past, the probability of surface rupture of a fault was calculated by making some simplistic assumptions about the distribution of earthquake magnitudes. Improved databases of earthquake ground faulting now allow the probability of surface rupture to be estimated in a more realistic fashion. Computing software that uses a Monte Carlo approach has been developed to allow the effect of various scenario choices on rupture probability to be investigated.
Using this software, it is found that the most significant influence on rupture probability is the long-term fault slip-rate. Other assumptions about the faulting style, maximum magnitude and conversion parameters have only a moderate influence on the results.
There have been several instances in recent history in Australia of surface faulting due to earthquakes, but there has been only limited damage to infrastructure due to the remoteness of these earthquakes. The software that has been developed will allow a considered assessment and comparison of the hazard and risk due to both ground shaking from earthquakes and from surface rupture.
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For intraplate regions such as Australia, identifying and quantifying activity on tectonic faults for inclusion in probabilistic seismic hazard assessments can be challenging due to the typically long return period for ground-rupturing earthquakes associated with these structures. Return periods of 10,000’s to 1,000,000’s of years mean that surface displacement evidence is prone to degradation through erosion and burial, and paleoseismological ‘trench’ excavations may not uncover geology old enough to reveal previous events. As a consequence, there is often little or no preserved evidence of past ground rupturing events on these structures. Rather than ignoring faults which show no evidence of neotectonic displacement, we present an alternative approach; in addition to considering active faults (movement in the last 35,000 years) and neotectonic faults (movement in the last 10 Myr) in seismic hazard assessments, we also consider faults which otherwise show no evidence of neotectonic activity but which are aligned favourably with the current stress regime and are therefore potential sources of earthquakes and accompanying strong ground motion.
There are many dams in Australia with appurtenant features such as spillway gates, large capacity outlet works, power stations and transfer tunnels. These features can play a significant role in how these dams are operated during flood events and allow for additional flexibility to implement flood mitigation activities such as pre-releases and surcharge depending on authorised operating procedures for the dam.
Typical practice in many dam flood hydrology studies has been to significantly simplify or even ignore the impacts of these features on the dam water level frequency curve. For example, it may have been assumed that spillway gates were either fully open or changed from fully closed to fully open in a uniform manner regardless of inflow rate. Whilst this approach significantly simplifies routing of floods through these storages, it may produce results which are inconsistent with the expected flood probability of the dam given its current operating procedures, especially for relatively frequent flood events. This is particularly critical for risk assessment where definition of the flood loading probabilities requires robust estimates of water level AEPs for all events.
In a number of recent studies, greater emphasis has been placed on detailed modelling of the effects of spillway gates and other outlet works on dam flood hydrology. This has required site-specific algorithms to be developed which incorporate the characteristics of the spillway gates or other features at each structure, as well as the flood operations procedures for the dam. This paper presents a number of case studies where explicit simulation of dam flood operations has had a significant impact on the resulting flood frequency curve and downstream flow rates and discusses the implications of that on dambreak modelling and risk assessment for those dams.
This paper provides an outline of the design and construction of the works undertaken to refurbish the 120 year old intake tower at Mundaring Weir. The project drivers included asset condition, hydraulic capacity, reduction in unusable storage, and reduction in evaporation from the reservoir. The one off sale of this water together with the present value of the reduction in evaporation pays for the project construction and is a significant response to climate change that is taking place in the region. The effects of Alkali Aggregate Reaction (AAR) compromised the efficacy of the Intake Tower operating as a dry-well, while the small diameter and significant corrosion of cast iron pipes and valves had severely diminished the service capacity of the structure. The solution implemented in this project included: lining the Intake Tower with a 37 m long by 2.7 m diameter 316 stainless steel liner; construction of a new inlet 15 m below the reservoir surface using a bespoke underwater coring rig; relining of existing pipes through the dam wall; and new outlet control pipework and valves downstream of the dam.
Lessons learned from recent major incidents and related enquiries in Victoria in concert with the adoption
of an all-emergencies all-communities philosophy have informed both the scope and reach of the current
emergency management and dam safety regulatory environment. Victorian dam owners now have a statutory
obligation to implement an all-emergencies all-communities approach to risk assessment at their assets and,
as part of that, to adopt this approach as part of their “business as usual” activities. A major outcome of
this requirement is that for major dams, risk management is now being driven from Board and senior
management level: the implementation of controls and actions is formalised. As a consequence, there is a
better understanding across the organisation of new and emerging risks that require new technologies,
thinking and expertise and an improved appreciation of asset interdependencies and the risk posed to reliant
stakeholders. With other reforms including oversight and audit arrangements in place, the move from “doing
enough” to striving for “good’ industry practice, aided by an improved regulatory regime and statutory
processes, is well established. A brief consideration of the lessons learned from the February 2017 Oroville
dam incident in this context concludes the paper.
Extreme flood analyses are routinely used as inputs to dam risk assessments, spillway adequacy assessments and spillway designs. Estimation methods applied in Australia using rainfall-runoff models in combination with a Probable Maximum Precipitation (PMP) estimate are consistent with the current best practice applied around the world. The estimation methods can, however, result in substantial variability in peak flow estimates depending upon the practitioner and the methods used to quantify model parameters. Around the world, validation procedures are commonly applied to combat this variability, but no such techniques are routinely applied in Australia. A method is proposed for application across Australia which may variously be applied to validate and constrain extreme flood estimates and also provide quick estimates.