Design floods for most dams and levees typically have an annual exceedance probability (AEP) of 1:100 (1E-2) or less frequent. In the U.S., high hazard dams are designed to pass the Probable Maximum Flood (PMF), which typically has an AEP of 1:10,000 (1E-4) or less frequent. In order to reduce epistemic uncertainties in the estimated AEP for extreme floods, such as the PMF, it is important to incorporate as much hydrologic information into the frequency analysis as reasonably possible. This paper presents a Bayesian analysis framework, originally profiled by Viglione et al. (2013), for combining at-site flood data with temporal information on historic and paleofloods, spatial information on precipitation-frequency, and causal information on the flood processes. This framework is used to evaluate the flood hazard for Lookout Point Dam, which is a high priority dam located in the Willamette River Basin, upstream of Portland, Oregon. Flood frequency results are compared with those from the Expected Moments Algorithm (EMA). Both analysis methods produce similar results for typical censored data, such as historical floods; however, unlike the Bayesian analysis framework, EMA is not capable of incorporating the causal rainfall-runoff information in a formal, probabilistic manner. Consequently, the Bayesian method considered herein provides higher confidence in the fitted flood frequency curves and resulting reservoir stage-frequency curves to be used in dam and levee safety risk assessments.
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The development of geological, engineering geological and geotechnical models is essential for all dams. These models provide the basis for understanding the engineering characteristics of foundation materials and geological structures that are critical to the safe design, construction and operation of the dam.
The use of digital three dimensional (3D) engineering geological modelling techniques is becoming more common for civil infrastructure projects. In addition to established design applications, 3D engineering geological models can be utilised by dam owners, operators and stakeholders for ongoing management of the dam.
The recent option studies at North Pine Dam in Brisbane, Australia, provides an example of collaboration between the owner (Seqwater) and the designer (GHD) to maximise the use of existing information and to enable future information to be efficiently integrated and utilised.
The initial North Pine Dam 3D engineering geological model was developed using historical records dating from the design and construction of the dam in the 1950’s and 1960’s. These records had been carefully stored, collated and digitised by the owner, so that they could be easily georeferenced and incorporated into the 3D engineering geological model.
The initial model was interrogated to identify data gaps and to plan targeted and cost-effective investigations that addressed critical geotechnical issues. The 3D engineering geological model was further refined using the newly acquired data, to develop a comprehensive “3D database” that can be used to visualise and interrogate all existing records as high- resolution georeferenced images and embedded data.
This provides an asset for the dam owner to maximise the use of existing information and reduce the cost of future safety reviews or design.
Dam owners manage many complex activities to maintain and operate their dams safely and resiliently. Identifying, and continually improving, the key elements of an effective dam safety program and associated practices can be challenging but are essential to support resilient dams and resilient communities; using the Dam Safety Maturity Matrices (DSMM) is an efficient and thorough way to do this. A maturity matrix is a tool to evaluate how well-developed and effective a process or program is. The matrices were developed within CEATI’s Dam Safety Interest Group (DSIG) for owners to assess the effectiveness of their dam safety program against industry practice, and to assist with identifying improvement initiatives.
This paper will present the matrices and demonstrate how they are used to evaluate the effectiveness (or maturity) of a dam safety program. It will also highlight the benefits associated with using the matrices as an assessment tool, including the identification of improvements that can be made to a dam safety program, and the prioritization of efforts across multiple facets of a dam safety program.
User case studies from dam owners in both New Zealand and overseas will be presented to elaborate on the tool and the process.
Trustpower’s Mahinerangi Dam in New Zealand’s South Island is a concrete arch and gravity abutment dam built in 1931, subsequently raised in 1946 and strengthened with tie-down anchors in 1961.
This paper discusses a 3D finite element analysis of the dam and the predicted performance of the arch section under Safety Evaluation Earthquake (SEE) loading against identified potential failure modes.
Current guidelines and recent seismic hazard assessments recommend earthquake loadings higher than what was originally accounted for in previous decades. A Comprehensive Safety Review identified stability under SEE loading as a potential deficiency, so a programme of works was commenced to evaluate and better understand the seismic risk by using modern day tools and technology to evaluate the dam against current performance standards.
The final model incorporated the results of extensive laboratory testing, high-resolution LiDAR survey data and dynamic calibration using ambient-vibration monitoring. Motion recordings across the face of the dam during the 2016 Kaikōura earthquake were also used to validate the model. The reservoir has been explicitly modelled together with the opening, closing and sliding of contraction joints and the foundation interface. This allowed the modelling of permanent displacements and the redistribution of loads within the dam under SEE loading, which had been shown to be an important behaviour from the previous stages of analysis.
Flood inundation consequence and emergency evacuation assessment using advanced numerical modelling tools such as HEC-LifeSim is progressively emerging as accepted best practice, due in part to the growing ease in obtaining the necessary datasets and hydraulic numerical modelling results and the increasing computational power readily available to perform analyses. In turn, these tools are being applied to assess dam failure consequence and the effectiveness of emergency response procedures.
An essential resource is an approved Emergency Action Plan (EAP, also known as a Dam Safety Emergency Plan), which describes how dam owners and disaster management groups notify and warn persons at risk of harm during an emergency event. There have been progressive improvements in the effectiveness of EAPs through a series of reviews and lessons learnt from emergency events, legislative and regulatory amendments and general improvements in communications, monitoring, alerts and public awareness. Effectiveness is measured through feedback from training exercises and expert reviews, however a more quantitative measure is not presently available. This limitation can challenge decision makers who need to balance costs associated with emergency preparedness with anticipated reductions in life safety risks.
The paper explores the feasibility of providing a quantitative assessment of the effectiveness of an EAP using advanced consequence modelling (HEC-LifeSim). Using consequence models for two dams in Queensland, EAP effectiveness is assessed for a range of emergency response measures. The accuracy and reliability of the model parameters applied to each simulation and their impact upon the reliability of predictions of potential loss of life (PLL) are analysed and discussed. The feasibility of the approach is discussed and recommendations to be considered for future applications made.
Leslie Harrison Dam is located on Tingalpa Creek in the Redlands region, approximately 18 km southeast of Brisbane. It is classified as an extreme hazard category dam with a large population at risk only a short distance downstream.
The dam comprises a 25 m high zoned earthfill embankment, with a dry well concrete intake tower and an outlet conduit located at the base of the dam near the old river channel. The spillway has a 43 m wide concrete gravity ogee crest, with a concrete lined chute terminating in an energy dissipator structure.
Seqwater is undertaking a staged upgrade of Leslie Harrison Dam to address deficiencies identified during the Portfolio Risk Assessment (URS 2013) and Geotechnical Investigations (GHD 2016).
While the dam has met the water supply needs of the community for the past 50 years, the upgrade ensures local residents will be well served into the future. Additionally, the structure will meet the most up to date requirements of dam safety management and national industry standards.
Construction of the Stage 1 upgrade commenced in June 2018 and involved the removal and replacement of liquefiable material in the foundation, modernisation and extension of the outlet works, addition of a new downstream filter buttress to the embankment, and lastly, the installation of both active and passive anchors within the spillway ogee and lower chute floor.
As with any major project, the works involved a number of challenges that had to be addressed. This paper provides an insight into the key challenges encountered and how these were overcome by the design and construction teams using practical engineered solutions. The intent is to provide the reader with an account of the “lessons learned” during the construction phase, along with recommendations for future dam upgrades.