THOUGHT LEADERSHIP

Using risk assessment to guide dam safety upgrades

Whether your focus is on a single dam or a whole portfolio, a risk assessment process helps you decide how best to intervene, and when, to get the maximum improvement in safety from your investment.

Using a risk assessment process to assess a whole portfolio of dams helps asset owners prioritise their limited resources for safety upgrades. Richard Herweynen, Entura’s Principal Civil Engineering, introduced our portfolio risk assessment program in his article ‘Dam safety: protecting lives and driving efficiencies’. Now we explore how a similar process can be used for a single dam to guide investigations, prioritise the order of a staged upgrade, and manage safety risks during construction, using the example of Hydro Tasmania’s Rowallan Dam.

The challenges of an ageing dam

Rowallan Dam is a 43m-high earth and rockfill dam built in the late 1960s, with two embankments on either side of a spillway located on a knob of very hard quartzite. The reservoir plays a key role in Hydro Tasmania’s hydropower system: it is a large storage at the top of the catchment and operates over a large range of water levels, capturing the high winter flows and then releasing them over the summer months through a cascade of five power stations downstream.

A dam portfolio risk assessment (PRA) completed by Entura in 2006 had identified Rowallan Dam as one of the potentially higher risk dams in Hydro Tasmania’s portfolio, with a number of suspected but not confirmed deficiencies. The potential deficiencies included a relatively high probability of a piping failure (internal erosion of the earthfill) as the filters were suspected to not meet modern standards and the dam had suffered a piping incident adjacent to the spillway walls when it was first filled.

As well, the PRA identified other potential deficiencies including questions regarding the spillway capacity, liquefiable foundations, potential for landslide hazards at the reservoir rim, structurally deficient spillway walls, and a limited capacity for dewatering.

With multiple potential deficiencies such as these, a range of challenges arise: Which deficiencies do we tackle first? Do we need to address all the deficiencies? To what level do they need to be addressed? How quickly should we address the deficiencies? And how do we justify to the business a large capital expenditure that will not generate any additional returns?

Reducing uncertainty

The original scope of the PRA was to assess the dam portfolio based only on the existing available information with the exception of preliminary dam-break modelling and consequence assessment. To allow the PRA to be completed within a reasonable timeframe and budget, this inevitably meant that significant gaps in our knowledge of the dams were filled with assumptions and best estimates.

A very targeted investigation program was then required to fill the gaps in our knowledge of Rowallan Dam’s particular possible failure modes and to prepare a fully justifiable business case. Entura led the investigation and concept development phase, working closely with external parties. The necessary investigations included:

• geological mapping of the potential landslide features (to determine whether they were in fact glacial features and not of concern)
• investigation into the core, filters and rockfill of the embankment (to confirm their properties)
• more detailed dam-break modelling (to confirm whether the original estimates of consequences of failure were correct)
• drilling of the left embankment foundation (to demonstrate whether liquefaction was a significant concern)
• full modelling of the flood hydrology (to confirm the flood risk)
• structural analysis of the spillway walls supporting the embankment on either side of the spillway chute

The spillway wall failure mode had not been identified in the PRA and was included in the investigation program to demonstrate that it was not a concern; however, it turned out to be the failure mode with the highest probability, and became the highest priority of the upgrade program.

Developing the upgrade program

When the specific investigations for Rowallan Dam were complete, the risk assessment for the dam was revised. The revised risk assessment determined the potential failure modes (in order of probability) to be failure of the spillway walls; piping along the spillway walls; piping through the upper part of the embankments; piping through the lower part of the right embankment; and overtopping of the dam during an extremely rare flood (rarer than a 1 in 10 000 year flood).

Developing an integrated, effective and progressively implemented solution that would not increase the dam safety risks during construction was a major challenge. Many alternatives for the outlet works, spillway augmentation and spillway walls were developed by Entura for resolving each dam safety risk while considering the construction flood risks and overall project risks.  This allowed Entura to find the right package of solutions to minimise the risks as well as the capital outlay. The ultimate solution included:

• increasing the monitoring of the spillway walls: To provide early warning of a possible failure, inclinometers were telemetered and alarmed. Monitoring picked up a possible deterioration in one wall, which was temporarily propped until a permanent solution could be implemented.
• increasing the dewatering capability of the dam: The riparian valve was replaced with one of greater diameter, and the controls of the turbine relief valve were modified to allow emergency bypass. This increased dewatering capacity was critical to managing flood risks during the later stages of construction and improved the ability to control any potential piping failure.
• strengthening the spillway walls on the inside of the existing walls: This approach incurred less risk of overtopping during construction than if the walls were strengthened on the embankment side. The replacement walls were designed to accommodate higher future discharges.
• installing modern, compliant embankment core filters: Installing these filters would protect the embankment from piping, but required major localised excavation adjacent to the spillway walls, including from the embankment crest to foundation, and reconstruction of the top seven metres of the embankment.

Lower risks, greater compliance

The major upgrades at Rowallan Dam have now reduced the dam’s safety risks. By considering a wide range of upgrade options and demonstrating that the selected option would significantly reduce the risk of failure through very targeted capital works, the Rowallan Dam risk assessment played a key role in developing a sound business case to demonstrate that the capital investment was both necessary and efficient – for a safer dam, both now and long into the future.

To find out more about how Entura can work with you to assess and remedy the safety risks of your dams or other water infrastructure assets, please contact Paul Southcott on +61 3 6245 4145, Phillip Ellerton on +61 439 010 172 or Shekhar Prince on +61 412 402 110.

About the author

Paul Southcott is a specialist civil engineer at Entura. He has more than 28 years of professional expertise 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 particular expertise in dam risk assessment and project managed the Hydro Tasmania portfolio risk assessment of 55 large dams, Southern Water’s 18 dams and most recently SA Water’s 18 large dams.  He was the technical leader for the Rowallan Dam upgrade project overseeing all aspects of the project including the detailed design of the spillway wall upgrades.

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19 November, 2015