Many mapped faults in the south-eastern highlands of New South Wales and Victoria are associated with apparently youthful topography, suggesting that faulting may have played a role in shaping the modern landscape. This has been demonstrated to be the case for the Lake George Fault, and may reasonably be inferred for the poorly characterised Murrumbidgee, Khancoban, Tantangara, Berridale Wrench and Tawonga faults. More than a dozen nearby major faults with similarly youthful topography are uncharacterised. In general, fault locations and extents are inconsistent across different scales of geologic mapping, and rupture lengths, slip rates and other fault behaviours remain largely unquantified. A more comprehensive understanding of these faults is required to support safety assessments for communities and large infrastructure in the region.
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The paper describes the development of UK guidance on reservoir drawdown capacity. The guidance provides for a consistent thought process to be used in determining the recommended capacity. A basic recommended standard is proposed for embankment dams which varies with the consequences of failure of a dam. The drawdown rate for the highest consequence dams is 5% dam height/day with an upper limit of 1m/day. Engineering judgement is used to vary this standard allowing for ‘other considerations’ including the vulnerability to rapid dam failure, surveillance and precedent practice. A different approach is proposed for concrete/masonry dam, which considers the prime purpose of drawdown being to lower the reservoir in a reasonable timeframe to permit repairs rather than rapid lowering to avert failure. The UK approach is compared with that used in Australia and suggestions made for where its use may be appropriate.
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.
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.
Recent advances in communication technologies have made available an array of new systems and functionalities that dam operators can use to improve automation and centralisation in the daily surveillance tasks of their portfolios. These functionalities include real-time monitoring, target-oriented video surveillance and the remote management of PLCs and data loggers.
The present paper aims to outline some integration possibilities using TCP/IP technologies for remote operations and video surveillance.
The case study features a comprehensive dam instrumentation upgrade, in which the acquisition systems were complemented with a series of IP cameras designed to be triggered by local and remote events.
A common concern for large spillways is erosion/abrasion of the receiving plunge pool and potential impacts on the stability of the dam. An example of this was presented at the 2017 ANCOLD Conference in a paper that discussed the detection and repair of spillway scour erosion at the base of Devils Gate Dam, an 84 m high, double curvature arch concrete dam. The focus of this paper is the partial repair of scour and abrasion within another concrete lined plunge pool, at the base of Repulse Dam in Southern Tasmania.
Repulse Dam consists of a 42 m high double curvature concrete arch with post-tensioned abutments and an adjoining earth embankment with a reinforced concrete upstream face. The stepped dam crest acts as a free-overflow spillway which discharges onto a concrete apron designed to protect the valley sides and floor immediately downstream of the dam. The permanent tailwater rises part-way up the dam during high flows which lessens the impact on the apron.
Previous underwater inspections had not identified a pressing need for maintenance. However, an upcoming twelve month Repulse Power Station outage would generate constant spill and therefore a more thorough assessment of the spillway apron was undertaken. Inspection was limited to underwater methods due to the inability to lower the tailwater; the downstream lake forming the tailwater is solely regulated by a hydro-power station and this station was being refurbished at the time. Sonar scanning enabled the spillway apron condition to be mapped and revealed areas of exposed reinforcing steel and deposits of river rock and gravel. The information provided by the scan justified temporary disruption to the lakes and power stations which form the Lower Derwent Power Development in order to dewater the area and work safely below the spillway. This was necessary to expose the apron for detailed inspection in dry conditions and thereby make a full assessment of the need for concrete repairs prior to the station refurbishment.
This paper presents a case study of the actual performance of a spillway apron below an arch dam and the inherent challenges in accessing and maintaining these types of structures when a permanent tailwater is present.