This paper provides an outline of the design and construction of the works undertaken to refurbish the 120 year old intake tower at Mundaring Weir. The project drivers included asset condition, hydraulic capacity, reduction in unusable storage, and reduction in evaporation from the reservoir. The one off sale of this water together with the present value of the reduction in evaporation pays for the project construction and is a significant response to climate change that is taking place in the region. The effects of Alkali Aggregate Reaction (AAR) compromised the efficacy of the Intake Tower operating as a dry-well, while the small diameter and significant corrosion of cast iron pipes and valves had severely diminished the service capacity of the structure. The solution implemented in this project included: lining the Intake Tower with a 37 m long by 2.7 m diameter 316 stainless steel liner; construction of a new inlet 15 m below the reservoir surface using a bespoke underwater coring rig; relining of existing pipes through the dam wall; and new outlet control pipework and valves downstream of the dam.
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The U.S. Army Corps of Engineers (USACE) is responsible for flood risk management across the United States. USACE has more than 710 dams and is responsible for more than 24,000 kilometres of levees. Since 2008, USACE projects have prevented more than AU$1.2 Trillion (in 2017 dollars) in damages from flooding. Although some of this came as a result of dozens of smaller floods, much of that protection came during three events within the last five years. From 2010 through 2017, the U.S. has had three major inland floods and two coastal events where federal flood protection exists: in 2010 on the Cumberland River, in 2011 on the Missouri, Ohio, White, and Mississippi Rivers, in 2015 on several rivers in Texas and Oklahoma, and in 2017 along the Gulf Coast of the U.S. and its territories in the Caribbean. For many of these locations, these events produced record rainfall and the flood of record. USACE operated many large facilities on these systems and those systems overall performed as expected. However, USACE also experienced some operational issues, did a substantial amount of flood fighting, had several incidents, and several failures. This paper will describe the flooding experienced in those events, the operations of the flood protection systems, the performance overall, and some of the lessons learned.
The notion of probability and its various interpretations brings numerous opportunities for errors and misunderstandings. This is particularly true of contemporary risk analysis for dams that mostly consider geotechnical, hydraulic, and structural capacities subjected to extreme loads considered as independent evets. In these analyses subjective “degree of belief” probability has a major role, both in the modelling of the risk in the system by means of event trees based on inductive reasoning and in the assignment of probabilities to events in the event tree. There are numerous situations where physically possible conditions are eliminated from consideration in a risk analysis on the basis of probabilities that are judged to be too low to be of relevance. This is despite the fact that the assignment of a probability to a condition means that the occurrence of the event or condition is inevitable sometime, with the added complication that the time of occurrence is unknown and unknowable. Although there is no relationship between a remote probability and the possibility (or credibility) of the occurrence of the event in the event tree, it is quite common for physically feasible conditions to be either eliminated or their importance discounted on the basis of low probability in a risk assessment of a dam. Twenty five years ago, this elimination process might have been referred to as “judicious pruning of the event tree”. In more modern parlance, the elimination process is based on consideration of whether or not the condition or sequence of events is clearly so remote a possibility as to be non-credible or not reasonable to postulate. In contrast to the consideration of extreme loads vs. structural or geotechnical capacities, experience has shown that many dam failures and perhaps the majority of dam incidents do not result from extreme geophysical loads, but rather from operational factors. These incidents and failures occur because an unusual combination of reasonably common events occurs, and that unusual combination of events has a bad outcome. For example, a moderately high reservoir inflow occurs, but nowhere near extreme; the sensor and SCADA system fail to provide early warning for some unanticipated reason; one or more spillway gates are unavailable due to maintenance, or an operator makes an error, or there is no operator on site and it takes a long time for one to arrive; and the pool was uncommonly high at the time. This chain of reasonable events, none by itself particularly dangerous, can in combination lead to an incident or even a failure. This leads to the unnerving conclusions that; our estimates of risk made in terms of best available practice using the best available estimates will be underestimates of the actual risk, and the extent to which we underestimate the risk is unknowable. This paper examines why these improbable events occur and what can be done to prevent them. Some implications with respect to the endeavour of risk evaluation are also considered.
A common concern for large spillways is erosion/abrasion of the receiving plunge pool and potential impacts on the stability of the dam. An example of this was presented at the 2017 ANCOLD Conference in a paper that discussed the detection and repair of spillway scour erosion at the base of Devils Gate Dam, an 84 m high, double curvature arch concrete dam. The focus of this paper is the partial repair of scour and abrasion within another concrete lined plunge pool, at the base of Repulse Dam in Southern Tasmania.
Repulse Dam consists of a 42 m high double curvature concrete arch with post-tensioned abutments and an adjoining earth embankment with a reinforced concrete upstream face. The stepped dam crest acts as a free-overflow spillway which discharges onto a concrete apron designed to protect the valley sides and floor immediately downstream of the dam. The permanent tailwater rises part-way up the dam during high flows which lessens the impact on the apron.
Previous underwater inspections had not identified a pressing need for maintenance. However, an upcoming twelve month Repulse Power Station outage would generate constant spill and therefore a more thorough assessment of the spillway apron was undertaken. Inspection was limited to underwater methods due to the inability to lower the tailwater; the downstream lake forming the tailwater is solely regulated by a hydro-power station and this station was being refurbished at the time. Sonar scanning enabled the spillway apron condition to be mapped and revealed areas of exposed reinforcing steel and deposits of river rock and gravel. The information provided by the scan justified temporary disruption to the lakes and power stations which form the Lower Derwent Power Development in order to dewater the area and work safely below the spillway. This was necessary to expose the apron for detailed inspection in dry conditions and thereby make a full assessment of the need for concrete repairs prior to the station refurbishment.
This paper presents a case study of the actual performance of a spillway apron below an arch dam and the inherent challenges in accessing and maintaining these types of structures when a permanent tailwater is present.
Two tailings storage cells were raised by constructing new embankments upstream of the existing
embankment walls. The performance of the new embankments was mainly dictated by the underlying tailings that consisted of a thick layer of very soft to soft fine tailings. The fine tailings in one cell was capped by a layer of sand for more than 30 years hence the tailings had mostly consolidated under the load of the capping. The fine tailings in the other cell was under consolidated because the cell had only been capped for about 18 months before the construction of the new embankment. The capping material was sand extracted from the tailings.
Stratification of the tailings was determined by CPT. Undisturbed samples of fine tailings were obtained by a piston sampler for CIU and oedometer testing to obtain parameters required for advanced soil models SHANSEP and Soft Soil (SS) models. These models were incorporated in full 2-D FE models to analyse the stability and settlement of the new embankments at various locations.
The application of advanced soil models such as SHANSEP and Soft Soil by hand calculation and
conventional slope stability analysis is considered cumbersome and labour intensive. This paper
demonstrates that with the help of FE software (PLAXIS in this case), it is practical to implement such advanced soil models to simulate the behaviours of soft fine tailings with reasonable accuracy. A similar approach could be used to model other fine tailings and soft clays. One should be reminded that the reliability of any analysis method relies on validation of the analysis model and parameters adopted.
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.