Dan Forster, Murray Gillon
A robust and defensible dam surveillance process is considered to be the ‘front-line of defence’ in ensuring dams do not present an unacceptable risk to people, property and the environment. The concept of a ‘Quality Chain of Dam Surveillance’ describes the surveillance process as a multi-linked chain where each step in the process forms a critical link. Without rigorous attention given to quality assurance links in the chain can become tenuous or broken and thus compromise the integrity of the whole chain. Hydro Tasmania is currently re-engineering its existing surveillance process using the Quality Chain of Dam Surveillance as a basis.
This paper presents the concept of the quality chain and uses the Hydro Tasmania improvement initiative as an example application of the concept. The paper is intended to provide a fresh perspective on what is sometimes considered a stale topic and reinforces the need for a considered approach to dam surveillance.
2011 – The Quality Chain of Dam Surveillance
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R.J. Nathan, P.I. Hill, and P.E. Weinmann
The current definition of the Probable Maximum Flood (PMF) is open to subjective interpretation, and this lack of objectivity can lead to inconsistencies in the application of risk-based and standards-based criteria. This paper summarises the different approaches used to estimate the PMF, and highlights how these reflect differences in the availability of design information and local tradition and experience. A number of approaches are available that can aid the objective definition of the PMF. These approaches attempt to define the “reasonableness” of the manner in which the various flood producing factors are combined by reference to the relative shift in the annual exceedance probability of the event. The implications of the different approaches to deriving the PMF are summarised for a number of dams from across Australia. Guidance on deriving the PMF is provided in the paper with a view to seeking feedback from industry and consideration for inclusion in relevant guidelines.
Kirsty Carroll, Kelly Maslin, Richard Rodd
Melbourne Water manages over 210 retarding basins across Greater Melbourne ranging in size from 4ML to 4700 ML with embankment heights from 0.3m to 10m. Over the years the basins have been designed and constructed by a range of different owners and authorities. Varying design and construction standards with the majority of retarding basins generally being located in highly urbanised areas, has resulted in Melbourne Water having a large portfolio of assets that have potential to pose a significant risk to the downstream communities they are designed to protect.
High level hazard category assessments completed over the last10 years identified that approximately 90 structures were either High or Extreme hazard categories based on the ANCOLD Guidelines on Assessment of the Consequences of Dam Failure.
In an attempt to identify retarding basins requiring priority consideration for remedial works Melbourne Water embarked on a process of completing a dam safety risk assessment for five of the retarding basins in accordance with the ANCOLD Guidelines on Risk Assessment. The objective of the risk assessment was to develop an understanding of the key risk issues that might affect retarding basins as distinct from water supply storages, identify potential remedial works and develop a prioritised risk management strategy for the five basins considered. In completing the risk assessment there was also significant discussion about ways to streamline the process to allow assessment of the remaining basins.
This paper details the results obtained from the risk assessment, investigates the application of the base safety condition and implementation of a risk management strategy. It also looks at similarities between sites to enable common upgrades to be implemented across the range of retarding basins. This paper also discusses the need for guidelines specific to retarding basins to be developed.
How do you solve a problem like retarding basins? An asset owner’s perspective
John Grimston, Robin Dawson
The Ambuklao and Binga Hydro-Electric Power Projects are located in Luzon, Philippines and were privatised in early 2008 after public bidding. Ambuklao dam forms an impoundment on the Agno River. The nearest city, Baguio, is approximately 45km or 1.5hrs drive away. The key headworks feature is an embankment central core rockfill dam and reaches a maximum height of some 129 m above the bed of the Agno River. A gated spillway is located at the left abutment, with a steep chute and flip bucket. Binga dam forms an impoundment approximately 20 km downstream of the Ambuklao dam. The rockfill embankment with an inclined clay core reaches a height of about 107 m above the bed of the Agno River. The spillway is located at the left abutment.
Heavy tropical rains and typhoons can cause very high flows in the rivers leading into the Ambuklao and Binga reservoirs. PMF peak flow is 11,600 cumecs. Due to the steep slopes surrounding the reservoir and along the access roads to the Binga Dam, landslides can create a hazard in the reservoir or for emergency access to the dam. There are numerous active faults in the area, including the Abra, Digdig and Philippines Faults (the latter being one of the most active faults around the Philippines). The region around the dams is capable of and has experienced earthquakes with a magnitude of 7.8 on the Richter Scale. This was demonstrated by the 1990 earthquake (7.8 magnitude) and caused minor damage to the dam structures.
The Project owner commenced rehabilitation implementation planning immediately after purchasing the facilities aimed at reactivating the Ambuklao plant’s 75MW capacity (inoperable since 1999 due to reservoir siltation issues triggered by the 1990 earthquake) and increasing it to 105MW. Rehabilitation at the Binga plant will increase capacity from it’s current 100MW to 120MW. The overall rehabilitation works include plant, intakes, associated tunnels, etc. This paper will focus primarily on the dam and spillway related rehabilitation, studies and design including review of the PMF and spillway capacity for both dams, Ambuklao innovative upstream face rehabilitation, Ambuklao spillway studies and rehabilitation and Binga spillway works and reservoir sedimentation studies.
2011 – Refurbishment of Ambuklao and Binga Hydro Power Dams and Appurtenant Works
Awoonga Dam is the sole source of water for the City of Gladstone and the heavy industries in the region. The area’s distribution reservoirs hold little more than a day’s supply. Extended water supply disruption could have severe economic impacts.
The nine large valves in the inlet tower and river outlet of the dam cannot be inspected or maintained without shutting down the entire water abstraction system. Consequentially limited maintenance has been carried out in the 25 years since the valves were installed.
Recent Dam Safety inspections carried out for the dam owner, the Gladstone Area Water Board (GAWB,) noted some deterioration of the valves and recommended that the valves should be removed, inspected and refurbished as necessary
GAWB was thus presented with a daunting challenge to refurbish valves at Awoonga Dam, as it was generally believed that their removal for refurbishment would not be possible within the time limitations imposed by the system and customer requirements.
In 2008 GAWB commissioned GHD to develop a strategy to refurbish the valves within a 12 hour shutdown period. The strategy proposed and adopted required a rigorous risk management approach and close collaboration between GAWB’s operational staff, two contractors and the consulting engineers. The work was successfully completed during 2011.
This paper discussed the strategies and processes developed and how the project planning, supervision and execution was driven by the risk management based approach. It also highlights some of the experiences and lessons learnt during the project.
2011 – Refurbishing Outlet Valves utilising Shutdown Periods
Amanda Ament, Jon Williams, Malcolm Barker
Aplins Weir is located on the Ross River in Townsville, downstream from the Ross River Dam. Previous work had identified Aplins Weir as exhibiting factors of safety below 1.0 under normal operating conditions, with over 1000 persons at risk today in the event of failure. Originally constructed in the early 1920s, Aplins Weir has been upgraded and repaired following various failures on a number of occasions. The end result is a complex reinforced concrete and steel sheet pile composite structure reliant for stability on a number of unreliable components. This paper presents the historical data describing the current configuration of the weir, and the analyses required to evaluate the extisting structure, leading to the design of the proposed upgrade works. The final design involves a retrofit of large diameter cast-in-place lined piles and a heavily reinforced base overlay slab designed to completely bypass all existing vulnerable substructure elements.
2011 – Where is our Weir going – an Unusual Upgrade!