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.
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On February 7, 2017, the gated service spillway (also known as the Flood
Control Outlet or FCO Spillway) at Oroville Dam was being used to release water
to control the Lake Oroville level according to the prescribed operations plan.
During this operation, the service spillway’s concrete chute slab failed, resulting
in the loss of spillway chute slab sections and deep erosion of underlying
foundation materials. Subsequently, as the damaged service spillway was
operated in an attempt to manage multiple risks, the project’s free overflow
emergency spillway was overtopped for the first time since the project was
completed in 1968. Significant erosion and headcutting occurred downstream of
the emergency spillway’s crest structure, leading authorities to evacuate about
188,000 people from downstream communities.
The assessment of the geological foundations of arch dams is required as part of the asset owner’s safety obligations (ANCOLD 2003). The task is often made difficult due to steep topography where arch dams are commonly constructed. Between 2013 and 2017, GHD was engaged by South Australia Water (SA Water) to examine the geological and geotechnical conditions of the Sturt River Flood Attenuation Dam (South Australia) abutment foundations. The dam was constructed between 1964 and 1966 within the Proterozoic “Sturt Tillite”. The foundations of the dam are characterised by a folded and fractured rock mass which creates complex spatial relationships between discontinuities and outcrop expression, difficult to assess in two-dimensional space. In collaboration with Monash University’s School of Earth, Atmosphere and Environment, a high resolution ortho-photogrammetric survey of the downstream dam abutments was undertaken using an Unmanned Aerial Vehicle (UAV) in areas where traditional mapping could only be obtained by rope access methods. Monash also undertook digital geological mapping of inferred discontinuities based on the UAV imagery. The data was then used to construct a three-dimensional (3D) model of the shape and position of high-persistence discontinuities, potentially critical to abutment stability. In addition to digital data, a low cost, high value field investigation to “ground-truth” the digital data and reviewed existing geological information (including rope access scanline data, foundation mapping and rotary cored boreholes) to develop a holistic understanding of the persistent discontinuities in their geological context.
In 2018, DNRME released the latest revision of the Failure Impact Assessment (FIA) Guidelines and the first significant change since 2003. An FIA is the instrument for determining if a dam is referable and therefore regulated for dam safety purposes in Queensland.
The guidelines reflect upon changes in legislation and advances in methods and tools for assessing consequences of dam failure. The revised version tends to be less prescriptive and emphasises the responsibility of the engineer completing the assessment to develop appropriate and defensible methods.
The paper provides an overview of the FIA guidelines, key concepts, the steps to follow when preparing an FIA and a comparison to ANCOLD’s latest consequence assessment guideline.
Earthquakes are a well-known threat to the safety of dams. While this threat is subdued for Australian Dams, the potential for earthquake induced failure of a dam requires risk minimisation in the downstream community through monitoring and emergency response procedures. This paper details WaterNSW’s approach to their development of a Seismic Monitoring Strategy which was to align the business and ensure an appropriate post-seismic response.
The strategy also identifies that a proactive approach to seismic instrumentation can be taken to reduce business risk by aiding decision making should a dam be in a damaged post-seismic state.
The interim outcome of implementing the Seismic Monitoring Strategy resulted in a fast emergency
response time and less overreaction/distraction of dam safety resources in insignificant seismic events. There is opportunity for other Australian dam owners to implement similar systems to = WaterNSW and achieve similar results.
There are a number of software packages that have been developed to conduct Probabilistic Seismic Hazard Assessments (PSHA’s). Each one has advantages and disadvantages. Two such programs are compared; the licenced subscription-based EZ-FRISK software package developed by Fugro USA Land, Inc. and the open-sourced OpenQuake-engine (OQ) software package by the Global Earthquake Model (GEM) Foundation. Both of these packages use the classical PSHA methodology as described by Cornell (1968) and modified by McGuire (1976). Each of these packages offers different advantages; OQ is freely distributed, code based and provides easy access to a number of tools. EZ-FRISK doesn’t rely on command-line tools and instead provides an easy user interface with quick access to plots to check results. EZ-FRISK is computationally faster than the OQ program.
A simple rectangular source model with four sites was used to investigate the degree of agreement between these two software packages. Results indicate that hazard estimates from the two packages agree to within 4% for the two closest sites. At long return periods for the two furthest sites, the difference is larger.