James Toose, Lelio Mejia, Jorge Fernandez
The recently completed Panama Canal Expansion project required construction of a new, 6.7-km-long channel at the Pacific entrance to the Panama Canal, to provide navigation access from the new Post-Panamax locks to the existing Gaillard Cut section of the Canal. The new channel required construction of four new dams adjacent to the existing canal, referred to as Borinquen Dams 1E, 2E, 1W, and 2W. The dams retain Gatun Lake and the Canal waterway approximately 11 m above the level of Miraflores Lake and 27m above the Pacific Ocean.The largest of the dams, Dam 1E, is 2.4km long and up to 30 m high. The dam abuts against Fabiana Hill at the southern end, and against the original Pedro Miguel Locks at the northern end. This paper provides an overview of the key challenges in construction of Dam 1E including the foundation, seepage cut-offs and embankment.
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James Stuart, Michael Hughes
Several recent rain events in Australia have resulted in impoundment flood levels where there was a surprising variability between the Annual Exceedance Probability (AEP) of the flood level and that of the rainfall. The issue was highlighted during the Queensland Flood Commission of Inquiry (QFCI, 2011) by the Queensland Dam Safety Regulator who suggested there may be a problem with design hydrology after a dam safety event that saw impoundment levels of around 1:9000 AEP with a 1:200 AEP catchment rainfall at North Pine Dam, north of Brisbane in 2011. Wide disparities have occurred at Wivenhoe Dam west of Brisbane, at Callide Dam, west of Gladstone and at other locations.
This paper examines the Generalised Short Duration Method (GSDM) (BoM, 2003) and the Revised Generalised Tropical Storm Method (GTSMR) (BoM, 2003) typically used for dam flood capacity assessments in an attempt to explain the variability outlined above and whether it is, in part, exacerbated by the methods themselves.
It finds that processes of generalising rainfall depth, intensity, temporal and spatial characteristics are working together with adopted hydrological methods to contribute to such variability, that in the worst case could lead to PMF levels in dams with much less rainfall than the associated PMP would infer.
Moreover, two key assumptions; that of AEP neutrality (AEP of rainfall is equal to that of the flood) and frequency of PMP based on catchment area, which are the foundations stones of our understanding of flood frequency for large structures, are found to be untested or simply interim advice. This leads to the conclusion that the likelihood of floods in the range 2000 year AEP to PMF may continue to show surprising variability, potentially of an order of magnitude or more, compared to the rainfall AEP.
There is a need for a review of these methods and potentially provision of interim guidance as these methods are currently being used in dam upgrade programs throughout Australia and are also the basis for emergency planning. The identification of these issues concerns current methods and are independent to any discussion on climate change.Prior to commencing, it is worth defining two terms that re-occur throughout the document:
Annual Exceedance Probability (AEP): The probability that a given rainfall total accumulated over a given duration will be exceeded in any one year. AEP Neutrality is the theory that assumes the probability of the rainfall can be transferred to the resulting flood.
Average Variability Method (AVM): Technique for estimating design temporal pattern of average variability to ensure AEP Neutrality in transition from PMP to PMP design flood
John Harris, James Robinson, Ron Fleming
Haldon Dam Remediation: A Case Study of Earthquake Damage and RestorationJohn Harris, James Robinson, Ron FlemingAECOM New Zealand LimitedAECOM New Zealand Limited, Fleming Project Services Limited Haldon Dam is a 15m high zoned earth-fill embankment irrigation dam, located approximately 10 km south-west of Seddon, in the Awatere Valley, New Zealand. The crest and upstream shoulder of the embankment suffered serious damage during the 2013 Cook Strait earthquakes, and the Regulator enforced emergency lowering of the reservoir by 5.5m to reduce the risk of flooding to Seddon Township from a potential dam failure. AECOM was engaged by the owner to carry out a forensic analysis of the damaged dam and subsequently the design of the 2-Stage remedial works. The remedial works addressed the existing dam deficiencies and earthquake damage in order to restore the dam to full operational capacity and gain code compliance certification. Key features oft he approach included holding a design workshop with the owner prior to undertaking detailed design, careful rationalisation of the upstream shoulder to optimise the competing interests of strength and permeability, contractor and regulator involvement in the design and construction process, and balancing risk and constructability with the chimney filter retrofit. This paper presents a description of, and approach to, remedial works solution undertaken to remediate a substandard and earthquake-damaged dam to fully operational status in an area of high seismicity. Applying this approach, the objective of achieving a robust, safe, economical design that was acceptable to the regulators and the owner was achieved.
Alberto Scuero, Giovanna Lilliu, Marco Scarella, Gabriella Vaschetti
Hardfill dams present technical and cost advantages. Placement is like in embankment dams, thus construction is fast. The typical trapezoidal shape makes possible use of local aggregates and low cement content. Despite the low strength material, these dams can be built on weak foundation, and resist earthquake and overtopping. However, being the material semi-pervious, they require an impervious facing. Until 2014 this was typically made with conventional concrete slabs with waterstops, or grout enriched hardfill. Concrete facings require heavy and costly equipment, long construction time, are expensive, frequently require maintenance.Construction of the facing can have a big impact on the overall construction costs of the dam. Replacing the concrete facing with a geomembrane lining is a cost-effective solution. This paper describes two hardfill dams’ projects with an exposed geomembrane as upstream liner: Filiatrinos (Greece, 2015), 55.6 m high,and Ambarau(Democratic Republic of the Congo, 2017), 19.30 m high.
Andrew Balme, Dan Forster, Tim Logan
The MW7.8 Kaikōura earthquake on 14 November 2016, ruptured over 20 faults during the initial shaking,which lasted nearly two minutes. A complex series of fault ruptures propagated northeast for nearly 180 km from the initial rupture location. Instrumentation from dams across New Zealand shows that whilst most dams did not suffer physical damage, piezometric responses were measured in dams and their foundations. Earthquake related changes in seepage regimes are not unusual and depend on the characteristics of the ground motions,and site specific characteristics that influence how a dam and its foundation respond to ground motions. The ability to measure a piezometric response in a dam or foundation is heavily influenced by the instrumentation network and method of monitoring. Data collected during events such as the Kaikōura earthquake provides valuable information for both characterising performance of a dam during the event, and assisting future analysis such as failure mode assessments. Careful consideration must be given to the scope of installed instrumentation and the frequency of monitoring in order to provide these benefits,and the robustness of the system to ensure it adequately survives the event.
C.Jolly and J.Green
New rare design rainfalls were released for Australia in February 2017, for durations from one to seven days and probabilities from 1in 100Annual Exceedance Probability (AEP) up to 1 in 2000 AEP.The differences between the previous rare design rainfalls using estimated Cooperative Research Centre –FOcussed Rainfall Growth Estimation (CRC-FORGE) method and the new rare design rainfall estimates vary with location, duration and probability. In this paper, these differences are explored spatially through the use of national maps, comparing percentage change between the two datasets for selected durations and probabilities. Before this comparison with the new rare design rainfalls could be completed, the State-basedestimates had to be resampled and aggregated to form a national data set for Australia.For rare design rainfalls, it is often the catchment values that are required to determine the gross rainfall for design purposes. The impact of the revised areal reductions factors and rare design rainfalls is explored through case study catchments in Tasmania.