2012 – North Pine Dam – January 2011 Flood of Record Lessons Learned
Barton Maher, Michel Raymond, Mike Philips
The Queensland Bulk Water Supply Authority (trading as Seqwater) owns and operates North Pine Dam, situated on the North Pine River in the Northern Suburbs of Brisbane. North Pine Dam is an Extreme Hazard Dam consisting of a concrete gravity dam with earthfill embankments at both abutments and three earthfill saddle dams. The spillway consists of five radial gates which are manually operated. Flood operations at the dam are controlled in real time by the Seqwater Flood Operations Centre.
In January 2011, North Pine Dam experienced the flood of record at the dam site with a peak inflow of approximately 3,500 m3/s and a corresponding outflow of approximately 2,850 m3/s. This inflow was more than double the previously recorded flood of record. The inflow was generated by high intensity rainfall both at the dam and in the upper catchment resulting in a rapid rise of the storage. The system which caused this rainfall was also contributing to the major flooding occurring in the adjacent Wivenhoe – Somerset catchment, also being managed by the Seqwater Flood Operations Centre. The rapid rise and fall of the storage presented difficulties for both the Seqwater Flood Operations Centre and the operators at the dam site.
Following the flood event, an analysis of the rainfall and the resulting inflows indicated a significant difference between the Annual Exceedance Probability (AEP) of the rainfall in the catchment and the estimated AEP of the inflow and peak water levels from previous hydrology studies. A detailed review of the flood event was commissioned by Seqwater and undertaken by URS Australia Pty Ltd.
This paper presents details of the flood event, lessons learned for the operation of the dam, upgrade works undertaken to date, results of the hydrology review and the conclusions of the Acceptable Flood Capacity (AFC) study. A key implication for dam owners was the increase in the estimate of the Probable Maximum Flood (PMF) by over 30% due to changes in calibration of the hydrologic model for the catchment.
Keywords: Probable Maximum Flood, Flood Operations, North Pine Dam, Flood Estimation
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2012 Papers
2012 – Design and construction of seismic enhancement works for Stuart Macaskill Lakes, Upper Hutt, New Zealand
Learn moreM C N Taylor, Dr H E Cherrill, S F Croft, S F Eldridge
The Stuart Macaskill Lakes are two raw water storage lakes with a combined storage of approximately 3280 ML supplying Wellington City, New Zealand. The lakes are High Potential Impact Category (PIC) earth embankment dams constructed on terrace gravel deposits adjacent to the Hutt River and located within approximately 20 to 50 metres of the Wellington Fault Deformation Zone. Construction of the lakes began in 1982 and they were commissioned in 1985.
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In early 2008, the lake’s owner Greater Wellington Regional Council (GWRC), embarked on a programme to supplement Wellington City’s water supply storage. Whilst that study is ongoing, GWRC engaged Tonkin & Taylor (T&T) to investigate the feasibility of increasing the Stuart Macaskill Lakes capacity as an interim measure.
The feasibility study concluded in late 2009 that the lake dam embankments could be raised by up to 1.3 metres in height to gain an approximate additional 450 ML of water storage. An important finding of that feasibility study has been that the seismic requirements have increased significantly since the construction of the lakes. To address this issue GWRC is currently constructing Stage Two of a two stage construction programme to both raise the lakes and to incorporate seismic resistant features into the lakes.
The primary design features are downstream rock buttressing in the critical areas of the lakes and synthetic lining the inside of the lake embankments. The buttressing works were completed in early 2011 and the lining and crest raising works are due for completion in 2013.
This paper summarises the design, laboratory testing and construction to enhance the lakes performance during very strong seismic accelerations (Peak Ground Accelerations of up to 1.08g) expected during a maximum design earthquake originating from the Wellington Fault.
Keywords: Water Reservoir, Seismic Design, Geomembrane, Rock Buttressing, Seismic Risk Assessment, Wellington Fault -
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2012 Papers
2012 – Negotiating the Geological Structure of the Enlarged Cotter Dam Site During the Abutment Excavation Period
Learn moreAndrew Barclay, Greg Kotze
The Enlarged Cotter Dam (ECD) is under construction on the Cotter River, 18km west of Canberra. The new dam comprises an 85m high roller compacted concrete gravity dam, located 120m downstream of an existing 31m high concrete dam. This paper describes the geological structures that prevail at the site and their significance with respect to design and construction considerations.
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Geological mapping has confirmed that the abutment slopes are characterised by zones of prominent rock outcrop and thin mantles of colluvial soil that form overall slope angles of 45 degrees. The Cotter River valley in the ECD area has been eroded through a geological sequence of Early to Late Silurian age, comprised predominantly of porphyritic rhyolite and lapilli tuffs of the Walker Volcanics.
Geotechnical investigations for the ECD were extensive and comprehensive. The results obtained have enabled the compilation of a detailed geological model of the dam site. Particular attention was paid to defining, characterising and kinematically analysing prominent geological structures, including intersecting sheared or crushed seams and zones that traverse the dam footprint.
Prominent geological structures that were encountered during the abutment excavation had significant design and construction implications for:
Abutment stripping and foundation preparations;
Rock slope stabilisation;
The foundation of the intake tower that comprises a 66m high concrete structure; and
The foundations for 1 x 56m high and 2 x 78m high tower cranes that required positioning on the steep abutment slopes during construction.
This paper highlights the importance of understanding the geological origin, nature and distribution of rockmass defects within a complex rock foundation. Site specific construction requirements and engineering design solutions used to successfully negotiate adverse geological structures are described.
Keywords: Dam, Roller Compacted Concrete, Geological Structures, Abutment, Foundation. -
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2012 Papers
2012 – Estimating potential loss of life from dam failure in the digital age
Learn moreSimon Lang, Peter Hill, Wayne Graham
The empirical method developed by Graham (1999) is the most widely used in Australia to estimate potential loss of life from dam failure. It is likely to remain that way while spatially based dynamic simulation models are not publicly available (e.g. LIFESim, HEC-FIA and LSM). When the Graham (1999) approach was first developed the prevalence of spatial data and the speed of computers was much less. In addition, most people did not have mobile phones, social media was in its infancy, and automatic emergency alert telephone systems were 10 years from being used in Australia. Graham (1999) was intended to be applied to populations at risk (PAR) lumped into a discrete number of reaches. The selection of fatality rates for the PAR in each reach was based on average flood severity and dam failure warning times. Today, there is typically much more spatially distributed data available to those doing dam failure consequence assessments. Often a property database is available that identifies the location of each individual building where PAR may be, along with estimates of flood depths and velocities at those buildings. News of severe flooding is likely to be circulated by Facebook, Twitter and e-mail, in conjunction with official warnings provided by emergency agencies through radio and television and emergency alert telephone systems.
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This raises the question of how Graham (1999) is best applied in today’s digital age. This paper explores some of the issues, including the estimation of dam failure warning time, using Graham (1999) to estimate loss of life in individual buildings and the suitability of Graham (1999) for estimating loss of life for very large PAR.
Keywords: loss of life, dam safety, risk analysis. -
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2012 Papers
2012 – Estimating Individual Risk
Learn moreKelly Maslin, Mark Foster, Len McDonald
A key requirement of assessing the tolerability of dam safety risks is the assessment of individual risk. The ANCOLD Guidelines on Risk Assessment provides guidance on acceptable levels of individual risk and some general guidance on the calculation of individual risk.
Individual risk is a key measure in the consideration of the tolerability of risk, ALARP and development of risk mitigation works. It is essential that there is consistency in the approach to estimating individual risk used across the dams industry.
This paper reviews the approaches taken to estimating individual risk across the dams industry both locally and internationally as well as the experience of other industries.
The paper includes a review of the various methods for estimating the vulnerability of individuals subjected to flood inundation based on historical fatality rates as well as identification of the individual most at risk
The paper then describes a method that has been developed based on the principles used for assessing individual risk due to other hazards, such as landslides. The method includes consideration of a range of factors such as warning time, temporal variation and vulnerability of the individuals most at risk. The method developed provides a transparent, defensible and pragmatic approach to estimating individual risk. Practical guidance and examples are also provided on the application of the method.
Keywords: individual, risk, exposure, fatality
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2012 Papers
2012 – We need flood protection in WA too!
Learn moreGraeme Mann, Michael Smith, Louise Thomas
Regular flooding around the coastal town of Busselton, south of Perth, led to the construction of large rural drains in the 1920s to divert two of the major rivers around the town. Hydrologic studies after major floods in 1997 and 1999 showed that the existing drains were providing much less than the desired 1 in 100 AEP flood protection, particularly as subdivisions were being developed along both sides of the Vasse River Diversion Drain (VDD).
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Three compensating basins constructed in rural land south of Busselton provide a total storage capacity of 4.4 GL. The banks that form the three basins have a total length of 8.5 km, vary in height up to 6 m and are either zoned earthfill embankments, with a clay foundation cut-off through sandy soil horizons to a depth of 1 to 2 m below ground level or homogeneous earthfill embankments. The spillways are overflow sections on the embankments using concrete revetment mattresses. The outlet works are uncontrolled box culverts with a capacity of up to 14 m3/s. Peak outflows are typically about 30% of peak inflows to the basins.
The paper discusses the Busselton Flood Protection Project and associated diversion drains, including the design of long embankments for the compensating basins on very flat terrain that are required to survive their “first filling period” during each flood emergency.
Keywords: Earth embankments, flood mitigation, flood compensating basins, levee banks, diversion drains, Busselton, piping failures.