Susantha Mediwaka, Nihal Vitharana, Badra Kamaladasa
Nalanda dam is the oldest concrete gravity dam on the Island built in the 1950s by the Ceylon Department of Irrigation. The dam was built in 9 monoliths having a dam crest length of approximately 125m and a maximum height of about 36m. The spillway consists of: (1) a low-level uncontrolled ogee-crested horse-shoe section with a crest length of 46m, and (b) a high-level broad crested weir with a crest length of 43m.
It was designed and constructed according to the then standard practices adopted throughout the world. Over the years, Nalanda dam has been showing signs of deterioration which is suspected to be Alkali-Aggregate Reaction (AAR). The dam was also shown to be deficient with respect to the stability levels required by modern standards. Under a program of dam safety improvement of the dams throughout Sri Lanka, it was decided to stabilise Nalanda dam as the first step in addressing a series of issues affecting the dam.
This paper presents the construction history, current issues, design assumptions and salient construction features in the upgrading of the dam to modern dam safety requirements.
Keywords: Concrete dams, dams Sri Lanka, concrete buttressing, upgrade, horse-shoe spillway
J.H. Green; C. Beesley; C. The and S. Podger
Rare design rainfalls for probabilities less frequent than 1% Annual Exceedance Probability (AEP) are an essential part of spillway adequacy assessment as they enable more accurate definition of the design rainfall and flood frequency curves between the 1% AEP and Probable Maximum events.
Estimates for rare design rainfalls were previously derived using the CRC-FORGE method which was developed in the 1990s. However, as the method was applied on a state-by-state basis, there are variations in the approach adopted for each region. Differences in the cut-off period for data, the amount of quality controlling of the data undertaken, the base used for the 2% AEP estimates, gridding settings and smoothing processes have created inconsistencies which are particularly apparent in overlapping state border areas.
The Bureau of Meteorology has derived new rare design rainfalls for the whole of Australia using the extensive, quality-controlled rainfall database established for the new Intensity-Frequency-Duration (IFD) design rainfalls. These data have been analysed using a regional LH-moments approach which is more consistent with the method used to derive the new IFDs and which overcomes the limitations of the spatial dependence model in the CRC-FORGE method. In particular, the selection and verification of homogenous regions and the identification of the most appropriate regional probability distribution to adopt relied heavily on the outcomes of the testing of methods undertaken for the new IFDs. However, to focus the analysis on the rarer rainfall events, only the largest events have been used to define the LH-moments.
Keywords: Rare design rainfalls; Intensity-Frequency-Duration (IFD); Annual Exceedance Probability
This paper discusses the common environmental issues and requirements project lenders have when financing hydropower dam projects in developing countries. The environmental specialist’s role, as part of the Lender’s Technical Advisor team, is discussed throughout the main phases of project finance (credit approval, financial close, lending/construction and loan repayment/operation). Further, how environmental issues are reviewed and monitored, thereby minimising reputational risks to the lenders are outlined.
Lenders typically consider hydropower dam financing, especially reservoir schemes, as high reputational risk loans. Finance is usually syndicated and although most international lenders are Equator Principles signatories or use the International Financing Corporations (IFC) Performance Standards, some lenders have additional environmental guidelines and requirements to enable financing. These differences are discussed.
Common environmental concerns include loss of habitat of endangered and/or threatened species, changes to river flows, erosion and sediment control during construction, and the minimisation and disposal of project wastes.
These issues are discussed drawing on the author’s experience in monitoring environmental issues of hydropower projects in Asia Pacific and Africa, including both smaller run-of-river schemes and larger storage reservoir projects.
Keywords: Environment, impacts, project financing, concerns, lenders, lenders technical advisor.
Mark Arnold, Chris Topham, Phil Cummins
A central tenet of the ANCOLD Guidelines on Dam Safety Management (2003) is that the higher the consequence of failure of a dam, the more stringent the surveillance scope, frequency, and safety criteria that should be applied to that dam. This concept has generally served the industry well to date in assisting regulators and dam owners to focus on the dams that could have the highest impacts if they failed. ANCOLD 2003 does also suggest that risk may be taken into consideration, however it is the experience of the authors that for dam surveillance and monitoring programmes, the majority of owners and consultants are reluctant to stray too far from the tables provided in the Guideline. However, two owners have recently embarked on a formal process to apply a risk based approach to the specification of surveillance and monitoring for their dams. This paper outlines how sub-optimal outcomes that can arise when the guideline tables are applied exclusively, presents the process undertaken by two owners of large portfolios of high consequence dams, and demonstrates the benefits achieved when a risk based approach is used. The paper concludes that any update or rewrite of the 2003 Dam Safety Management Guidelines should promote a risk based, rather than a consequence based approach to surveillance and monitoring.
Keywords: Risk, risk-based surveillance programme, instrumentation, monitoring.
Robert Kingsland, Michelle Black, Andrew Russell
Managing the vibration impacts associated with blasting is a challenge for mine planners and operators. In an open cut mining environment production blasting is often an integral part of operations. The management of surface water is a key operational requirement for open cut pits and mine water dams are often a part of the water management infrastructure. Consequently, mine water dams are often subject to blasting impacts.
For the mine operator the foremost questions are, “how close can mine blasting progress towards the dam?” and “what is the maximum vibration that the structure can be safely subjected to?” For the dam safety regulator the key concerns are around potential modes of failure, consequence of failure, the likelihood of failure and the management of risk.
With reference to case studies, this paper will discuss the acceptable blasting limits for earth dams, impacts on various dam elements and failure mode analysis. Failures modes discussed include embankment cracking, slope failure and deformation, foundation cracking and outlet structure cracking. Risk mitigation measures will be presented including design, operation and monitoring controls.
Keywords: blasting impacts, embankment dams, coal mine.
Jason Fowler, Robert Wark
Tropical Forestry Services (TFS) currently (2015) leases Arthur Creek Dam from the West Australian state government and utilises the water source to drip irrigate its Indian sandalwood (Santalum album) plantation. Arthur Creek Dam is located approximately 70 km south west of Kununurra in the East Kimberley region of Western Australia. TFS grows and processes the sandalwood to produce oil that is used extensively in the global fragrance perfume market. TFS took over the lease of the 26 m high zoned earth core and rock fill dam in 2007 and has systematically carried out remedial works to the structure to lower the f-N curve below the ANCOLD “Limit of Tolerability” and to well within the ALARP zone. This paper describes the proactive risk management approach TFS has undertaken to address dam safety issues. It also specifically describes the most recent management issue, being the outlet pipe refurbishment.
A number of dam safety issues were identified during the initial surveillance and subsequent annual surveillance inspections. Issues include insufficient spillway capacity, seepage from the right abutment and deterioration of the steel outlet pipe. The remedial works to the outlet pipe were completed in late 2014 and involved close collaboration between TFS, the contractor and the designer. The outlet pipe re-sleeving operation was complex as the dam had to remain in operation and the water level could not be artificially lowered. In addition, the original outlet pipe was asymmetrical along both the vertical and horizontal axes, close to the bulkhead gate structure. Contingency measures were employed to enable the dam to remain in operation with 3 DN 400 HDPE siphon pipes installed.
The completion of the refurbishment of the outlet pipe by sleeving the pipe reduced the risk posed by this structure by an order of magnitude. Planned future risk reduction measures include the treatment of seepage within the upper right abutment and rebuilding the crest. These actions will further reduce the risk of dam failure through piping and overtopping of the dam crest.
Keywords: risk, ALARP, outlet pipe, re-sleeving.