Keeping close tabs on your dams
Access to quality long-term records is crucial for dam safety. Read what our specialists have to say about keeping your records under control.
The ANCOLD (Australian National Committee on Large Dams) guideline on dam safety management recommends regular reviews on different timescales – including routine inspections undertaken by the operator (daily to monthly), intermediate inspections by a dams engineer (annually), comprehensive inspections undertaken every five years by a dams engineer and other specialists (e.g. mechanical engineers specialising in gates and valves), and dam safety reviews at least every 20 years. For these less frequent dam safety reviews, the ability to access quality long-term records is crucial.
Because dams are such large structures with a very long service life, they need to be reviewed regularly to ensure that any community risks are mitigated appropriately. Dam safety reviews assess the current condition and performance of the dam in depth, and reassess the design and construction of the dam against modern standards.
When the time comes to conduct a dam safety review, dam owners need good records at their fingertips. Poor recordkeeping can lead to significant extra costs and time to conduct the review, but can also compromise a dam owner’s duty of care.
To properly analyse the performance of a dam and assess it against modern design and construction criteria, the dam engineer needs a thorough understanding of the original design and construction, any modifications to the dam since its original build, and a complete set of surveillance and monitoring data.
For some dam owners, when the time comes to complete a dam safety review, all or some of the records are missing – they may have been lost or destroyed through corporate restructures or a failure to understand the importance of the historical records, or the records may never have been created at all.
Which records are important?
In our work as dam safety consultants, we’ve encountered numerous cases in which poor recordkeeping – across many areas and disciplines – has resulted in costs for our clients. The following are critical records to keep in good order and close to hand.
Records from stream flow and rainfall gauges in the dam catchment provide key hydrological information for dam safety reviews. The streamflow data can be used directly to statistically analyse the probability of the more frequent floods. It can also be used to calibrate rainfall runoff models used to estimate extreme floods for which dam spillways need to be designed. Without this data, flood studies are more difficult because data needs to be extrapolated from nearby catchments. This not only adds cost, but also reduces the reliability of the data, which may potentially lead to oversizing or undersizing the spillway.
Dam break and consequence assessment records
In Australia, dams are classified according to the consequences of failure – from very low (minimal impact on the community, environment and dam owner’s business) to extreme (catastrophic impact on the community, environment and dam owner’s business). The consequence categories are used to define the level of design (e.g. spillway capacity), the surveillance and monitoring requirements and, in some states of Australia, the regulatory requirements. Where these studies are not available, ‘rule of thumb’ methods may be used to estimate the inundation extent as an interim step, but much more confidence can be obtained by using a flood hydraulic modelling package with good survey data. These studies should be reviewed regularly for currency, especially when development is occurring in the downstream catchment.
Geological and geotechnical records
A detailed understanding of the geology of the dam foundations is essential for assessing the risk of excessive leakage (piping), the presence of any low-strength zones that could cause instability, and the potential for landslides around the reservoir that could cause a wave able to overtop the dam. Investigations undertaken before construction as well as mapping and recording of the foundation during construction provide the best possible information for assessing these potential failure modes.
When these records aren’t available, it is often necessary to undertake extensive and expensive geological investigations into the foundation of the dam, usually while the dam is operational. Such investigations need to be carefully planned to preserve the safety of the dam.
The starting point is review of existing geological records of the site, including geological maps, aerial photographs/satellite images and geological/geotechnical reports from nearby locations. This should be followed by geological mapping to confirm the information obtained from the desktop search, including the rock types and joint and defect orientation spacing. From this, a preliminary geological model of the site can be developed. To reduce the uncertainty of the model, intrusive test pitting and drilling investigations can be conducted. Careful consideration of the dam safety risks of undertaking these investigations is needed, including contingency plans to deal with unexpected conditions (e.g. high-pressure water intersected in boreholes or instability of ground around test pits). ANCOLD has produced a very good guideline on geotechnical investigations of dams, their foundations and appurtenant structures, which should be considered an essential guide for dam owners.
If records of original design information are unavailable, the dam safety reviewer won’t be able to fully understand the designer’s intent and assumptions. The information required includes the design drawings showing the overall arrangement and key dimensions, as well as the specifications and the design report. If drawings aren’t available or are illegible due to poor quality scans, detailed surveys of the structure may be required to determine the actual constructed geometry. To assess the stability of the dam, the material properties of the dam will be required; but if no data is available, sampling and laboratory testing may be needed.
To understand the types of defects that may be present in the dam, it’s important to know how the dam was constructed, whether it was actually constructed to the design, and what issues were encountered during construction. Key construction records include construction reports detailing progress, changes to the design and issues; results of quality testing; as-constructed drawings or mark-ups on the design drawings; and photographs of the construction process. If these records are not available, it may be necessary to confirm as-constructed details through survey, sampling and laboratory testing.
Surveillance and monitoring records
Time-series data – such as regular inspection reports, photographs and instrumentation readings – are invaluable in establishing if there are trends or changes over time that may indicate deterioration of the dam. Without these long term records, it can be difficult to assess whether observed features are longstanding (e.g. present since construction) and what recent developments may indicate about the condition of the dam. Without the full time-series of monitoring data, it can be difficult to observe trends and to understand the relationship between various performance parameters (e.g. leakage versus reservoir level).
Keeping your records under control
If adequate records can’t be located, a dams engineer will need to spend a great deal of time searching archives or undertaking investigations to build a historical picture of the design and construction of the dam in order to assess its safety. When records are well managed, dam owners can save time, money and frustration. To keep your records under control, make sure that they are:
- as complete as practical – which may require extensive archive searches and investigations to fill in the gaps
- securely stored (electronically as well as the original paper records) and retained for the long term (so investigations don’t have to be repeated)
- readily retrievable through efficient indexing and archiving systems.
A good dam may outlast generations of engineers, operators and owners. This makes recordkeeping a fundamental part of maintaining the safety and performance of the dam over its long service life and a key responsibility of every dam owner as part of your duty of care.
About the author
Paul Southcott has more than 34 years of experience in civil and dam engineering, as well as expertise in geotechnical, foundation, structural, hydraulic and hydropower engineering. Paul’s dam engineering experience spans geotechnical and hydrological investigation; feasibility and options studies; concept, preliminary and detailed design; engineering assessment, consequence assessment and risk assessment; safety reviews; monitoring and surveillance; and emergency planning. He has extensive experience in dam risk assessment and was a member of the ANCOLD committees that issued the Guideline on Consequence Categories for Dams in 2012 and drafted the Guideline on Geotechnical Investigations for Dams. Paul pioneered the development of a dam risk assessment methodology for concrete-faced rockfill dams (CFRD). He was the Engineers Australia (Tasmania Division) Engineer of the Year in 2021.
June 21, 2022