Prior to filling the Clyde Dam reservoir in 1992-93 large scale stabilisation works were undertaken on several pre-existing landslides along the reservoir margins. Monitoring and visual observations indicate that the landslides are behaving satisfactorily and have confirmed that the stability improvements undertaken have successfully offset the negative effects of the reservoir on the landslides.
This paper presents selected records detailing more than 25 years of landslide behaviour that demonstrate the effectiveness of the stabilisation works. Monitoring has been able to detect increasing water levels, drainage flow changes and, in some cases, deformation following periods of high rainfall.
However, the highly satisfactory performance of the landslides experienced to date does not allow complacency and although the surveillance monitoring has been progressively scaled back to a more focussed strategy, ongoing assessment and reviews will be required. The paper also briefly discusses the current challenges associated with changing personnel and aging instrumentation.
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Fault displacement can occur due to primary faulting on a main fault intersecting a dam foundation or rim, as well as by secondary faulting. This secondary faulting may be triggered locally by the occurrence of primary faulting on a main fault; its occurrence is conditional on the occurrence of an earthquake on the main fault. A probabilistic approach is most viable for fault displacement hazard analysis. Unlike the case of probabilistic ground motion hazard, which is nonzero even for short return periods due to the occurrence of a broad range of earthquake magnitudes in a wide region around the site, probabilistic fault displacement hazard is zero for return periods less than the recurrence interval of surface faulting earthquakes on the fault. In Australia, these recurrence intervals typically lie in the range of 10,000 to 100,000 years.
Consequently, the fault displacement hazard due to primary faulting may be zero or negligible for return periods shorter than 10,000 or 100,000 years. For longer return periods, the hazard is best evaluated using a risk-based approach, as recommended by ANCOLD (2018); the alternative of using a deterministic approach, which disregards return period, could potentially yield a large fault displacement. The probability of triggered secondary faulting, conditional on the occurrence of a large earthquake on the main fault, is typically one or two orders of magnitude lower than that on the main fault, and so is even more likely to be zero or negligible for return periods shorter than 10,000 to 100,000 years
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.
In recent times two dimensional (2D) hydraulic modelling has become the most common type of modelling for undertaking dambreak assessments. Direct map outputs such as depth and depth-velocity product are very useful in assessing risk across a floodplain. The temporal output from 2D models also enables the tracking of flow across a floodplain, helping practitioners and dam owners alike make informed decisions on warning time and evacuation routes. These outputs form essential input to packages such as HEC-LifeSim an agent-based simulation model for estimating life loss by simulating population redistribution during an evacuation.
A number of investigations have shown the hydraulic model, TUFLOW, is able to simulate the hydraulic conditions expected in a dambreak flood wave, giving confidence in the model’s ability to correctly capture the flood wave propagation. Notwithstanding this ability, there remains uncertainty over the best methodology to adopt when assigning a breach hydrograph to the model and in turn the impact this choice has on assessing downstream populations at risk.
A commonplace method of assigning dam breach hydrographs is to model the reservoir and dam structure with a 1D model or spreadsheet, where the storage is represented with a stage storage relationship and outflow through a time-varying breach is calculated using level-pool routing. The resulting hydrograph is then applied directly to a 2D model immediately downstream of the dam to model the propagation of flow downstream.
An alternative approach consists of representing the entire reservoir, dam and downstream floodplain in the 2D model. This allows for the dynamic effects of bathymetric constrictions in the reservoir to be accounted for which could greatly impact on the timing and shape of the dam breach hydrograph. However, this comes at a cost, as representing the reservoir in 2D requires bathymetry data which can be expensive to capture and also may require a major extension of the model domain.
In this paper the ‘Fully 2D’ and ‘Stage storage relationship 1D/Spreadsheet’ approaches are compared for a number of case studies.
Dams and levees within the U.S. Army Corps of Engineers (USACE) inventory were constructed for a variety of purposes including flood control, navigation, hydropower, recreation, and fish and wildlife conservation. USACE transitioned to using life safety risk as a key input to all dam and levee safety decisions in 2006. This was implemented for many reasons, paramount among them is forming a consistent basis to evaluate the safety of dams and levees and prioritize the implementation of risk reduction measures in a consistent manner across the agency to best utilize available resources. This requires knowledge of what constitutes unacceptable risks that would require risk reduction actions. The Tolerable Risk Guidelines (TRG) were developed for this purpose, and to form a common basis for dam and levee safety evaluations and decisions. Protection of life is paramount, and there are four TRG related to (1) understanding the risks surrounding dams and levees, (2) building risk awareness, (3) fulfilling daily responsibilities, and (4) continually considering actions to reduce risks. The USACE policies have evolved over time, but the fundamental principles that underpin the TRG have been fairly consistent for the past 10 years. The evolution of the TRG have come as a result of the experiences using these principles to support more than 2,500 safety decisions. This paper describes the rationale behind the selection of the TRG.
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.