Deformation Survey is a simple and widely implemented technique to identify the early signs of dam failure and is regularly undertaken on many dams. Thanks to advances in equipment and more accurate survey records, there is now a better understanding of measurement and movement of embankments and previous records.
However, the “expected” range of transverse deformation and implications for failure modes of dams is not particularly well researched or understood.
This paper collates a case history of transverse deformation for a number of Tasmanian dams and examines the relative behaviour of the embankment dams. From this the “expected behaviour” of an embankment dam can be estimated and related to key influencing factors, such as observed settlements, height and age of the dams, and thereby providing guidance on when transverse deformation is considered unusual for similar dams.
Now showing 1-12 of 58 2982:
The U.S. Army Corps of Engineers (USACE) Risk Management Center (RMC) developed the Reservoir Frequency Analysis software (RMC-RFA) to facilitate, enhance, and expedite flood hazard assessments within the USACE Dam Safety Program. RMC-RFA is a stochastic flood modeling software that employs advanced statistical and computing techniques, allowing a user to perform a screening-level stage-frequency analysis on a desktop PC with runtimes on the order of seconds to a few minutes. RMC-RFA utilizes an inflow volume-based stochastic simulation framework that treats the seasonal occurrence of the flood event, the antecedent reservoir stage, inflow volume, and the inflow flood hydrograph shape as uncertain variables rather than fixed values. In order to construct uncertainty bounds for reservoir stage-frequency estimates, RMC-RFA employs a two looped, nested Monte Carlo methodology. The natural variability of the reservoir stage is simulated in the inner loop defined as a realization, which comprises many thousands of events, while the knowledge uncertainty in the inflow volume-frequency distribution is simulated in the outer loop, which comprises many realizations.
Stage-frequency curves derived with RMC-RFA are compared to those derived with more complex, precipitation-based simulation frameworks, such as the Monte Carlo Reservoir Analysis Model (MCRAM), the Stochastic Event Flood Model (SEFM), and the Watershed Analysis Tool (HEC-WAT). The inflow volume-based framework employed by RMC-RFA produces stage-frequency curves that strongly agree with the more complex, precipitation-based methods. Furthermore, the results from the alternative methods fall within the RMC-RFA uncertainty bounds, demonstrating its robustness. In this sense, the RMC-RFA simulation framework lends itself to a value of information approach to risk management, where knowledge uncertainty can be efficiently quantified at a screening-level assessment, and then the value of performing more complex and sophisticated studies to reduce uncertainty can be considered.
Kangaroo Creek Dam is a concrete face rockfill dam (CFRD) located on the Torrens River, approximately 22 km north east of Adelaide. The dam is currently undergoing a major upgrade to align it with updated safety guidelines set by the Australian National Committee on Large Dams (ANCOLD) to better withstand major flood events or earthquakes. As part of this upgrade, external omega-type waterstops have been installed on the vertical and perimetric joints to mitigate the impact of expected joint deformations due to seismic loading. Two profiles were selected for the external waterstops; one capable of extending 200 mm for the perimetric joint and the outer two vertical joints on each side, and one capable of extending 100 mm for the remaining vertical joints and the horizontal joint between the new face slab and the original face slab. Using the external omega-type waterstops as the second waterstop for the extended perimetric joint simplified construction, particularly with respect to reinforcement details adjacent to joints. It is understood that this is the first time in Australia that an omega-type waterstop is being fitted to a CFRD slab. This paper demonstrates the benefits of retrofitting waterstops to existing dam joints when required, provides general installation details, details for providing a continuous barrier with the existing waterstops by overlapping internal and external waterstops, and lessons learnt from the waterstop installation.
Installing a suite of appropriate instruments such as piezometers, settlement plates, extensometers, and inclinometers etc., in strategic locations to monitor the performance of an embankment built on soft soils is vital when there are major design uncertainties; the monitoring data can also be used to calibrate the design parameters. Questionable readings of pore water pressure (PWP) have been reported in various case studies involving the development of dams, embankment foundations and reclamation work in Australia and in South East Asia, especially in low-lying acid sulphate soil (ASS) floodplains. Despite having vertical drains (PVDs), excess pore water pressure readings from Vibrating Wire Piezometers (VWPs) do not always dissipate as fast as expected, especially after a certain period of time, typically a year. This paper describes the biological and geo-chemical factors affecting reliability of Vibrating Wire (VW) piezometers, filter-tip clogging, smearing of soil adjoining the filter, gas generation, chemical alteration or corrosion of the filter, as well as electro-osmotic effects and cavitation. To that end, several VW piezometers installed in ASS terrain were extracted after being in place for 1.5 years and the soil surrounding the tips was tested for iron related and sulphate reducing bacteria. It is found that sulphate reducing bacteria has medium to high aggressivity whereas iron related bacteria has very high aggressivity with the bacteria count exceeding 20,000. VWPs with ceramic/stainless steel filter tips installed in acidic ground with organic contents exceeding say 4-5% have shown impeded dissipation of excess pore water pressure after a year or so. Accordingly, it appears that this issue is likely in other types of piezometers fitted with such ceramic or stainless filters when installed in ASS soils. Further Scanning Electron Microscopy (SEM) analysis of the piezometer filter is also ongoing at the University of Wollongong (UOW) laboratory to determine how ionic precipitation causes a VW piezometer to clog. In addition, several samples were collected from Victorian Dams and are being tested in University of Wollongong (UOW) laboratory to quantify the clogging effect in Dam practice when installed in ASS terrain.
The paper describes the development of UK guidance on reservoir drawdown capacity. The guidance provides for a consistent thought process to be used in determining the recommended capacity. A basic recommended standard is proposed for embankment dams which varies with the consequences of failure of a dam. The drawdown rate for the highest consequence dams is 5% dam height/day with an upper limit of 1m/day. Engineering judgement is used to vary this standard allowing for ‘other considerations’ including the vulnerability to rapid dam failure, surveillance and precedent practice. A different approach is proposed for concrete/masonry dam, which considers the prime purpose of drawdown being to lower the reservoir in a reasonable timeframe to permit repairs rather than rapid lowering to avert failure. The UK approach is compared with that used in Australia and suggestions made for where its use may be appropriate.
Lake Buffalo located on the Buffalo River near Myrtleford in Victoria was constructed in the 1960s as a cofferdam for the then proposed Big Buffalo dam. Consequently, the dam was designed for a short life (<10 years) and design features and criteria for a permanent dam were not implemented.
Critical features include a primary spillway with three vertical lift gates, two outlet conduits located
through the spillway piers, a single upstream valve on each outlet conduit for regulation and isolation, and a multi-part bulkhead which is installed in front of the valves for inspection and maintenance.
With the continued operation of the dam beyond 60 years, upgrades appropriate to a permanent dam have been implemented, including addressing deficiencies with spillway gate hoists lifting equipment and redundancy of the outlet conduit vales. This proved challenging, as the operation of spillway structures does not readily align with industry or Australian Standards. This paper will outline the issues encountered, their resolution and the lessons learnt during this upgrade work.