A common concern for large spillways is erosion of the receiving plunge pool and potential impacts on the stability of the dam.Devils Gate Dam is an 84m high, double curvature arch concrete dam, located in northern Tasmania and constructed between 1968 and 1970.The full 134m long crest is designed as a free-overflow spillway and spill flows impact the downstream valley sides and plunge pool below, where energy is dissipated to reduce riverbank erosion downstream.To protect foundation rock,the plunge pool and large portions of the valley sides were concrete lined with 450mm thick reinforced and anchored concrete. During spill events the area is inundated by up to 12m of tail-water.In 2016 damage to the plunge pool concrete was discovered by divers during a special inspection of the impact areas, but poor visibility limited the understanding of the extent and severity. Subsequent investigations, including detailed sonar scanning, improved the understanding but it was not until the plunge pool was fully dewatered that the full extent of the damage was quantified.The damage commenced around 35m downstream of the dam arch and consisted of approximately 330 square metres of moderately to severely eroded concrete and exposed, deformed, and in some areas completely removed reinforcing bars. The most significant feature was a penetration through the concrete up to 2.5m into the foundation rock.A number of stressed anchor heads were also damaged or destroyed.A full appreciation of the damage necessitated the decision for immediate repairs given the impending power station refurbishment (commencing January 2018) which will subject the plunge pool to nine months of constant spill.This paper outlines the diving and sonar investigations undertaken in 2016, discusses the challenging tasks of dewatering the plunge pool and gaining access through the narrow canyon, and presents the physical works to strengthen the damaged areas.It discusses the difficulty of identifying and treating such damage, and serves as a cautionary tale for other owners who have fully submerged plunge pools downstream of spillways.
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.
Mojtaba E. Kan, Hossein A. Taiebat and Mahdi Taiebat
In design of new embankment dams or evaluation of the performance of existing earthfill and rockfill dams, the Newmark-type Simplified Methods are widely used to estimate the earthquake-induced displacements. These methods are simple, inexpensive, and substantially less time consuming as compared to the complicated stress–deformation approaches. They are especially recommended by technical guidelines to be used as a screening tool, to identify embankments with marginal factor of safety. The methods would serve as a reliable screening tool had they always resulted in conservative estimates of settlements. However, a number of studies in the last 15 years show the contrary. This paper provides a critical review of the fundamental theory behind the simplified Newmark-type methods. Cases in which the results of the simplified methods are reportedly non conservative are further investigated and possible reasons are discussed, that may be taken into account in future design and investigations of Australian dams. The reliability of the simplified methods is examined based on the existing thresholds proposed in the literature and accounting for the embankment geometry and type, and for the seismic activity characterization. A recently proposed practical framework is further elaborated to demonstrate its effectiveness in the study of seismic behaviour of embankment dams. In particular, the case study of Zipingpu concrete faced rockfill dam in China is discussed where all widely used simplified procedures failed to predict the order of deformations experienced by the Dam under a recent strong earthquake event.
Dr Andy Hughes
Tailings dams continue to undergo failures at an unacceptable rate compared to water storage dams, including failures at operations owned by high profile mining companies.Tailings dams have often a different form and method of construction than water storage dams in that tailings dams continue to be raised over time as part of the mine operations and rise to considerable heights. These failures are often the result of a combination of design, construction and operations actions that are controlled by humans and must be better coordinated and managed in the future. The consequence of failure can be widespread flows of tailings and water over the landscape and water courses. This can have extreme consequences in terms of life loss, environmental damage, social license to operate, company value, and mining industry sustainability. Therefore,it is necessary that the mining industry strive for zero failures of tailings facilities. Any additional technology and information that enables an owner of a tailings dam to be more certain of its condition and thereby reduce the risk of failure is of tremendous value to reliable tailings and mine water management.The Willowstick method uses low voltage, low amperage, and alternating electrical current to directly energise the groundwater by way of electrodes placed in wells or in contact with seepage or leaks. This approach has been successfully used to identify water flow paths through, under and around tailings dam in plan and elevation.The Willowstick technology provides additional information to supplement the geological, geotechnical and hydrological, evaluations analyses and designs, and to further improve tailings dam safety by more robust designs if necessary. This paper, using several tailings dam case studies, illustrates the procedure, findings, and the benefits of the Willowstick methodology. The findings of many Willowstick surveys range from tailings dams where the methodology has confirmed the design evaluations, to tailings dams where new groundwater and leakage flow paths were identified. In the latter case, the dam designers were able to update the designs, based on the new information,to mitigate the identified risks and to improve the overall safety of the tailings dams in accordance with the goal of zero failure.
Lesa Delaere, Dr Natalie Clark, Dr Shayan Maleki
Waterway barriers, such as dams and weirs, have the potential to impact aquatic fauna species through the restriction of fauna movement and direct injury and mortality of individuals. Without suitably designed aquatic fauna passages and features to minimise injury and mortality, these barriers may adversely affect the viability of local and regional populations, through disruption to critical behaviours (e.g. breeding, dispersal).
The Lower Fitzroy River Infrastructure Project comprises of two weirs on the Fitzroy River in central Queensland. Two threatened turtle species, the Fitzroy River turtle and the white-throated snapping turtle, and a range of fish species needed consideration of species-specific requirements and development of targeted design solutions.
This paper discusses the ecological needs of these species as well as features incorporated into the design to reduce the impact of the weirs. The design incorporated modular fishlocks, gate, spillway and stilling basin features, an innovative turtle passage, special considerations for outlets and operational aspects. The design was further subject to complexity due to the variation in river flows, zero flow to approximately 9,000m3/sat bank full, and needed to account for a wide range of operational scenarios with respect to the species impacts.The paper also includes a discussion on computational fluid dynamics modelling (CFD) which was used to validate the design of fish passage structures.
D Stephens, S Lang, P Hill, M Scorah
Robust estimates of the duration of flood overtopping are a key input into the dam safety risk assessment process. For embankment dams, the likelihood of erosion of the dam crest, downstream face and eventual unravelling of the embankment are heavily dependent on the duration of water flowing over the crest. Similarly, the chance of erosion of the abutments of concrete dams is strongly linked to the duration of floodwaters overtopping the dam. Previously, it has been difficult to define the annual exceedance probability (AEP) of the flood required to cause overtopping of a certain depth for a certain duration, and coarse assessments have typically been made based on critical storm durations of the dam crest flood (DCF). This approach carries significant uncertainty, particularly for structures on smaller catchments where the critical storm duration on outflow may be relatively short. In these cases, it has been difficult to confirm with any reliability that the flood required to achieve a significant duration of overtopping has an AEP close to that of the DCF. This paper describes a new algorithm that has been incorporated into the RORB hydrological model which allows for a frequency curve of flood overtopping duration to be determined within a Monte Carlo framework. The results of this analysis are presented for a case study of a quantitative risk assessment, to demonstrate how the outcomes influenced numerous aspects of the risk analysis process.