This paper highlights the importance of hydraulic diversion control structures during construction of large dams and the value of allocating sufficient resources during project planning and implementation.
The design of the diversion gate for construction of the Enlarged Cotter Dam presented various challenges, including operation for up to 38m head for discharge into a 3m diameter conduit and the need to serve as an upstream concrete form during eventual diversion closure.
The short duration of operation allowed acceptance of increased level of operational risk and a higher level of design uncertainty. The design used generally accepted gate design methods, but no hydraulic modelling. The hydrodynamic forces were estimated using published data. After installation, a 1 in 100 AEP flood event resulted in the gate being subjected to 90% of its design head while operating in conditions close to the maximum design down-pull force. Attempts to raise the gate succeeded only after increasing the hydraulic pressure above the design value.
Keywords: Guard gate design, outlet works, dam, construction.
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.
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
Richard R. Davidson, Nate Snorteland , Doug Boyer, John France
The US Army Corps of Engineers (USACE) has embarked upon a monumental journey in applying risk-informed decision making in the management of the safety of the 650 major dams for which it is responsible. This process has shifted safety criteria from fully deterministic to a probabilistic basis. There has also been a shift from de-centralized district-based decision-making to centralized management of resources through the new Risk Management Center (RMC) and the Senior Oversight Group (SOG), a group of senior engineers and managers from across the USACE organization. The risk process began about five years ago with a portfolio prioritisation using screening-level risk assessments of the entire dam inventory, culminating in Dam Safety Action Classifications (DSAC) for each of the dams. Based on this risk prioritisation, Issue Evaluation Studies (IES) were initiated for the highest risk DSAC I and II dams, with each study including detailed failure mode and risk analyses for each dam. Because the Corps was relatively new to dam safety risk analyses, and their dam design history was one of following codified manuals of practice, various risk tools were prepared to provide guidance when assessing the risk of potential static, seismic and flood failure modes, as well as life loss and economic consequences of dam failure. Although these tools provided useful guidance to a relative large population of inexperienced risk estimators, many of these early risk assessments were flawed; they provided unrealistically high estimates of failure probabilities and the tools did not help estimators understand or explain each failure mode. To assist the RMC in bringing more defensible risk estimates to the table and improve consistency of the evaluations, the Quality Control and Consistency (QCC) review process was initiated about two years ago. The QCC process provides high level review of IES activities, including detailed reviews of risk analyses, by a small group of experienced dam safety risk estimators. Not only has this brought risk estimates into a more reasonable range, it has provided valuable training for risk estimators, and important checks and balances on the risk-informed decision making process for moving dam safety upgrade projects forward. The justification for a number of very expensive projects has been challenged and, in some cases, re-prioritised, and other projects have risen to the prominence they deserve.
A.E. Bentley, P.I. Hill, S.M. Lang, M. Freund, A. Richardson
This paper describes the development of a detailed assessment approach using spatial data to estimate the consequences of dam failure across a portfolio of 18 dams in NSW. The assessment is made for potential loss of life; economic and financial losses and a qualitative assessment of environmental and social impacts. The approach is designed around the use and interrogation of spatial databases combined with outputs from hydraulic models. The assessment method is applicable to a wide range of dams in different valleys, each with different downstream characteristics. The paper provides discussion on the advantages of the approach and presents some insights into the effective application to a dam portfolio of significant size and scale.
Keywords: consequence assessment, spatial databases
John Grimston, David Leong, Robin Dawson
The Angat Multipurpose Project, originally constructed in the 1960’s, is located 60 km north-east of Manila, and provides power, irrigation and domestic water supply and flood mitigation. The major water-retaining structures of the scheme are a 131 m high main rockfill dam and a 55 m high rockfill saddle dam.
Previous seismology studies have identified the presence of a possible branch of the West Valley Fault crossing under the saddle dam. If the fault dislocated, the branch under the saddle dam could produce horizontal and vertical shear displacements. Further, earthquake shaking poses a risk outside the fault zone. If the main dam/saddle dam were to fail in such an event, there would be major consequences in respect to both the water supply (serves a population of approximately 10 million) and the large population living below the dams. The dams are thus in the highest hazard category under any internationally accepted standard.
A study to investigate the dam safety aspects and identify remediation works which would bring the seismic performance of the main dam/saddle dam system up to an acceptable level was undertaken and included:
The main conclusions were:
Keywords: Dam, Remedial, Seismic, Fault, Spillway.