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
Steven Hare, Daniel Masters, Phil Farnik
State Water Corporation, New South Wales’ bulk water delivery business, is enhancing its maintenance planning and execution through a maintenance improvement project. This project incorporates a Reliability Centred Maintenance (RCM) analysis of water infrastructure assets including dams, weirs and pipelines and refining current maintenance procedures.
The project aims to address inconsistencies in maintenance frequencies and applications that have resulted from historically managing assets at a regional level. This project, coupled with State Water’s new “functional based” organisational structure, is expected to yield an effective and consistent preventive maintenance program across the organisation. The project is also expected to increase the reliability of critical dam infrastructure and aid in maintaining safe operation of the organisation’s assets.
This paper briefly describes the history of maintenance, principles of RCM, project implementation aspects and early outcomes. These outcomes include the reduction of maintenance frequencies on non safety critical assets with low failure rates, elimination of ineffective tasks and standardisation of maintenance frequencies on equipment common to all dam sites.
Keywords: Reliability centred maintenance, dams, weirs
Tim Griggs and Richard Herweynen
The river diversion is an important aspect to be considered in the design of a dam. It generally consists of an upstream cofferdam, river diversion conduit and downstream cofferdam and allows the dam to be constructed in a dry section of river.
This paper reviews the diversion design adopted at three recent Australian roller compacted concrete (RCC) dams and comments on the effectiveness of the design in providing risk mitigation during the construction of each of these dams. The dams considered are Paradise Dam (2005), Meander Dam (2007) and Wyaralong Dam (2011).
Rather than selecting an arbitrary design flood for the diversion, a risk-based assessment was used that generally resulted in a relatively low design capacity. Even though there were cases where the diversion capacity was exceeded, it is considered that the risk based design process provided an economical diversion design for these recent Australian dams.
Keywords: Diversion, roller compacted concrete dam, RCC.
David S. Bowles, Sanjay S. Chauhan, Loren R. Anderson, Ryan C. Grove
A risk assessment (RA) was conducted for 27 miles of Herbert Hoover Dike to better understand and estimate the Baseline failure risk. Unique aspects of this risk assessment included the following: high stillwater levels persisting for almost a year; highly dynamic and spatially variable wind loading; short-duration wind setup that reduces likelihood of piping; dike length that increases probability of failure; and multiple breaches with overlapping inundation areas that affect failure probability and consequences and the risk evaluations.
A wide range of stillwater and wind loading combinations were considered. Following a potential failure modes analysis (PFMA), failure modes included were: piping through foundation, embankment piping, piping along conduits, piping along structures through embankment, embankment and flood wall instability, and overwash and overtopping. System response probabilities (SRPs) were estimated using toolboxes, analyses and expert judgment. Life-loss consequences were estimated using LIFESim. RA calculations were performed using DAMRAE-HHD, which includes length effects. Estimated risks were evaluated against the US Army Corps of Engineers (USACE) tolerable risk guidelines (TRG). Uncertainties were explored using sensitivity analyses.
P C Styles, A L Garrard
The Victorian town of Nathalia was surrounded by flood water during the March 2012 floods in Northern Victoria.
Nathalia is protected by earthen levees of various sizes and age. Portable aluminium levees were installed during the March 2012 flood event, generally in areas where a permanent levee would restrict access to a park and views. The flood level came within 200mm of the crest of many of the levees and remained at a high level for nearly 2 weeks.
The paper describes the emergency management issues and procedures which relied on engineering advice to provide targeted and relevant remedial works on the levee system as potential problems arose. Engineers worked alongside the SES, CFA, Victoria Police, ADF and other volunteers to monitor, repair and reinforce the levee system on a 24 hour basis. The engineering support continued over a period of approximately 2 weeks, from the time the flood waters commenced rising until they had receded sufficiently for the orders for evacuation of the town to be rescinded.
Keywords: Nathalia, floods, levees, emergency management
Andrew 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.
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.