Two-dimensional hydraulic modelling technology has advanced significantly in recent years, providing powerful and flexible tools that are now routinely used for a wide variety of flood risk assessments. Assessing the downstream impacts of catastrophic dam failure represents an extreme test for the accuracy and stability of hydraulic models. Catastrophic dam failure can present an extreme risk to downstream infrastructure and public safety. Hence, it is important to have confidence in the estimated magnitude of potential impacts to design suitable, costeffective mitigation measures. The highly visual output of two-dimensional models adds credibility to their results. However, validation data for extreme hydraulic conditions is rarely available, resulting in uncertainty in the accuracy of model predictions and in the risks associated with dam failure. By validating numerical model results against analytical solutions for cases of simple geometry and also against realworld data, an improved level of confidence can be obtained in the accuracy of the model representation of these extreme hydraulic conditions. In this paper, we assessed the capability of the TUFLOW hydraulic modelling software package to accurately simulate an idealised dam break scenario by comparing the model results to analytical solutions. We also compared the model results for coastal inundation by a tsunami to real-world data from the 2004 Banda Ache (Indonesia) tsunami. The results showed that the HPC solver version of TUFLOW correctly captures the dam break flood fronts and the flood wave propagation and TUFLOW HPC is well suited for dam break flood modelling.
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.
The rehabilitation of wet tailings storages is likely to become of increasing importance. In a setting of increasing environmental regulation and oversight, the environmental issues inherent in wet tailings storages will increase in visibility. This will translate through to increased regulatory attention, rehabilitation standards and costs. This scenario will necessitate increased engineering ingenuity and approaches to develop cost effective and robust/ defensible outcomes.
This case study of a coal fired power station ash dam rehabilitation compares a conventional (baseline) rehabilitation strategy and the development of a higher land use, with potentially beneficial outcomes for the owner, the community and the environment.
The baseline rehabilitation was a conventional fit-for-purpose rehabilitation approach consistent with the proposed final land use comprising the creation of a stable, open greenspace environment. The higher land use was an aspirational target style rehabilitation, with the assessed highest and best use for the site that was determined to be an industrial land development. While there will be limitations due to the low strength tailings foundation, this higher land use is considered an appropriate stretch target and is a feasible outcome for this site.
Deformation Survey is a simple and widely implemented technique to identify the early signs of dam failure and is regularly undertaken on many dams. Thanks to advances in equipment and more accurate survey records, there is now a better understanding of measurement and movement of embankments and previous records.
However, the “expected” range of transverse deformation and implications for failure modes of dams is not particularly well researched or understood.
This paper collates a case history of transverse deformation for a number of Tasmanian dams and examines the relative behaviour of the embankment dams. From this the “expected behaviour” of an embankment dam can be estimated and related to key influencing factors, such as observed settlements, height and age of the dams, and thereby providing guidance on when transverse deformation is considered unusual for similar dams.
There are many dams in Australia with appurtenant features such as spillway gates, large capacity outlet works, power stations and transfer tunnels. These features can play a significant role in how these dams are operated during flood events and allow for additional flexibility to implement flood mitigation activities such as pre-releases and surcharge depending on authorised operating procedures for the dam.
Typical practice in many dam flood hydrology studies has been to significantly simplify or even ignore the impacts of these features on the dam water level frequency curve. For example, it may have been assumed that spillway gates were either fully open or changed from fully closed to fully open in a uniform manner regardless of inflow rate. Whilst this approach significantly simplifies routing of floods through these storages, it may produce results which are inconsistent with the expected flood probability of the dam given its current operating procedures, especially for relatively frequent flood events. This is particularly critical for risk assessment where definition of the flood loading probabilities requires robust estimates of water level AEPs for all events.
In a number of recent studies, greater emphasis has been placed on detailed modelling of the effects of spillway gates and other outlet works on dam flood hydrology. This has required site-specific algorithms to be developed which incorporate the characteristics of the spillway gates or other features at each structure, as well as the flood operations procedures for the dam. This paper presents a number of case studies where explicit simulation of dam flood operations has had a significant impact on the resulting flood frequency curve and downstream flow rates and discusses the implications of that on dambreak modelling and risk assessment for those dams.
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.