New technology and outputs from flood forecasting systems can raise issues for dam safety managers in how they use uncertain information to make critical dam safety decisions. In particular, making operational decisions around pre-releases based on forecast inflow presents challenges. In this case dam safety risk needs to be weighed up with other risks such as increasing downstream flooding, or being able to supply water into the future. The process of developing a flood forecasting system should be a close collaboration between the developers and the users. This ensures that outputs provide meaningful information that can be used to support operational decision-making in a flood or emergency response situation.
The volume-of-fluid (VOF) technique was employed to develop a Computational Fluid Dynamics (CFD) model for comparison to physical measurements available from the Eildon Dam model in Australia for validations purposes. The water surface in the downstream chute of the spillway was observed to be mostly comprised of fully developed aerated flow. The free surface is physically measured as located between the mixing and upper zones, thus investigator judgement is critical to achieve reliable measurements. The mixing zone is also characterized by surface waves to complicate matters even further. A challenge arose to develop a post processing methodology that replicates as closely as possible the measuring technique used by the physical modeller for direct comparison of results, using a novel method which utilises Poisson probability of exceedance applied to the free surface.
While structures such as a dam walls, pipelines, gas storage tanks, and nuclear facilities are vulnerable to the shaking from earthquakes, they are even more susceptible to differential movement on faults passing beneath their foundations.
In the past, the probability of surface rupture of a fault was calculated by making some simplistic assumptions about the distribution of earthquake magnitudes. Improved databases of earthquake ground faulting now allow the probability of surface rupture to be estimated in a more realistic fashion. Computing software that uses a Monte Carlo approach has been developed to allow the effect of various scenario choices on rupture probability to be investigated.
Using this software, it is found that the most significant influence on rupture probability is the long-term fault slip-rate. Other assumptions about the faulting style, maximum magnitude and conversion parameters have only a moderate influence on the results.
There have been several instances in recent history in Australia of surface faulting due to earthquakes, but there has been only limited damage to infrastructure due to the remoteness of these earthquakes. The software that has been developed will allow a considered assessment and comparison of the hazard and risk due to both ground shaking from earthquakes and from surface rupture.
The As Low As Reasonably Practicable (ALARP) principle was established in the Australian Dams
community in the ANCOLD Guidelines on Risk Assessment in 1994. Since that time, dam owners have been focused on reducing their societal risk to below the ANCOLD Limit of Tolerability (LoT) through dam safety upgrades and are now considering how to justify an ALARP position. This paper presents a framework that provides a systematic approach to assembling the inputs, applying a process and documenting the outcomes of an ALARP assessment. It is a pragmatic approach that aligns with the safety case, which is a legislated requirement for Major Hazard Facilities in Victoria.
The framework has been applied to two dams in Melbourne Water’s portfolio with differing societal risk, size, uses and criticality to the water supply system. It has highlighted the importance of dam safety governance, documentation of procedures, defensible technical analysis and an ongoing engagement with leading industry practice, in demonstrating risks are ALARP.
The revised magnitudes of the Geoscience Australia’s NSHA18 earthquake catalogue approximately halve the rate of occurrence of earthquakes of a given Mw magnitude in Australia. This yields probabilistic ground motion levels that are significantly lower than the present design levels at dam sites in Australia that are not near faults, and is expected to result in a general reduction in ground motion levels at dams not near faults estimated for all Risk Assessments, and for Deterministic Assessments for all consequence levels except Extreme Consequence. For the latter, the ANCOLD (2018) guidelines will tend to increase existing SEE ground motion estimates for both of the methods used to estimate the safety evaluation earthquake (SEE). By requiring the use of the Deterministic SEE if it is larger than the probabilistic SEE, and by requiring use of the 85th fractile of the Probabilistic SEE if it is larger than the Deterministic SEE, the ANCOLD (2018) guidelines for Deterministic Assessments are much more conservative than the ICOLD and NZSOLD guidelines for Extreme Consequence dams, especially at those located near faults.
Investigations into the core material of earth fill dams are undertaken reluctantly due to the potential to cause damage to the embankment. Where investigations are required, Cone Penetration Testing (CPT) is increasingly used to assist with the geotechnical assessment of dam embankments. The risk of hydraulic fracture within embankment core material is well known and procedures are typically adopted to minimise the risk of hydraulic fracture during remediation of the holes. Backfilling is typically done in stages allowing for an initial set of the cement/bentonite grout mixture prior to subsequent lifts.
While the risk of hydraulic fracture is well understood, the lesser known risk of pneumatic fracture is a possibility where certain conditions exist. This paper discusses CPT investigations at Fairbairn Dam, operated by Sunwater in Central Queensland, and the challenges faced in undertaking the remediation of the CPT holes. The potential for pneumatic fracture of the embankment core was highlighted during the investigations and details of alternative techniques adopted for reinstatement of the holes are presented. Recommendations are made to appropriately manage the risk of pneumatic fracture when undertaking CPT’s through embankment core.