By Joel Eadie, Experienced Engineer
When is a dam break required?
Any dam requires rigorous site investigation, engineering design, and construction quality assurance. Followed by ongoing surveillance and monitoring during operation, and post-closure when the structure is decommissioned and rehabilitated. If these aspects all aim to limit the risk of failure, why then would a dam break assessment be required?
The failure of water and tailings dams has the potential for loss to human life, and damage to the environment, economy, property, and infrastructure. According to ANCOLD’s Guidelines on Tailings Dams (2019), the rate of failure has not decreased over the years, with 1-2 major tailings dam failure occurring per year across the world. The cause of failure can be from issues in design, arise during construction or operation, or even years after the closure of the structure.
Acknowledging then that dam failure can and does occur, a dam break assessment does not consider the probability of failure, and rather aims to quantify the severity of loss of damage if a failure occurred. This is referred to as a Failure Impact Assessment (FIA). The FIA is a crucial component of risk analysis, decision-making and sometimes the permitting processes.
The Dam Break Modelling is therefore carried out to characterise such aspects as inundation extent, and the depth and velocity of flow as the contents of the dam are released. These conditions form the basis for selecting dam design criteria, as well as contributing to risk management and risk mitigation planning.
A dam break assessment considers a hypothetical, but most credible failure mechanism for the storage in question. The failure impact conditions are used to assess the severity of loss and damage within the inundation area focusing on:
- Harm to humans
- Environmental harm
- Economic loss or property damage
Practices for new and existing structures
Current practice is for a dam break assessment to be undertaken for all new structures of a particular size and setting. This requirement is triggered by recognised industry guidelines such as ANCOLD’s Guidelines on Tailings Dams (2019) but also prompted by licence conditions or the environmental permit attached to the structure.
A dam break assessment is equally essential for existing structures, although it is not always triggered by industry practice or regulation. The consideration for existing dams is that the risk profile may well change throughout its life cycle. Factors such as storage upgrades through embankment lifting, changes to processing throughputs, or adjacent developments can impact on potential risks. A change in the operating condition of the structure, such as the development of the embankment phreatic surface or porewater pressures within the tailings mass can also influence the failure mechanisms modelled. Each of these factors, either individually or combined, can impact the outcomes from a dam break assessment. It is, therefore, so important to review and update the dam break assessment throughout the structure’s life cycle.
A Consequence Category Assessment (CCA) is used to assign design and operating criteria for the dam based on the dam break assessment. These criteria can include:
- Geotechnical design parameters (e.g. earthquake loading parameters, minimum factors of safety for embankment stability)
- Hydraulic parameters (operating freeboard allowances, spillway design criteria
- Monitoring requirements
- Emergency action planning
What assessment methods are required?
With several catastrophic tailings dam failures occurring in recent years, the demand for a higher and more exhaustive level of analysis has intensified.
Various methods are available for undertaking a dam failure assessment, ranging from empirical analysis to 2 or 3-dimensional hydraulic models. Empirical methods can be appropriate where the judged or perceived severity of loss and damage is low; for example, small structures containing benign materials or water of low toxicity. For large structures, with exposure to areas of population or an environment of concern, a comprehensive assessment is used to develop an understanding of the potential extent of harm.
Dam break modelling is a speciality of ATC Williams, covering both water (Newtonian fluid) and tailings (non-Newtonian fluid), with embankment washout (sediment transport) and mudflow simulations. In particular:
- Our in-house laboratory and slurry transport group specialises in characterising rheological parameters of mine tailings and flow behaviours in the turbulent regime; and
- A combination of industry recognised techniques is used to model breach geometry, breach flow hydrographs, dam break wave simulation and release routing.
The simplest and most graphic form of model outputs is the mapping of inundation extents as a result of the dam break. Coupled with depths and velocities of the failure wave, which may or may not be combined with a concurrent flood event, the extent and severity of the potential inundation provide the basis for assessing Population at Risk (PAR) as well as the Potential Loss of Life (PLL). These outputs can be depicted in plan-view or in video form, to demonstrate the progression of the dam break flood wave.
As well as supporting the Consequence Category Assessment and dam design process, dam break modelling can be used to optimise the location of the dam, with a view to minimising exposure risk to areas of population or environment. This optimisation process may well achieve capital cost savings, but also will contribute to the effective siting of protection measures such as levees to reduce overall exposure.
Project stages of a dam break
A dam break is best completed in the earliest phase of the project to ensure risks are identified and managed through design. Throughout the lifecycle of the structure, updates to the dam break should be considered where changes to the storage, such as the storage capacity, or downstream environmental or population conditions change. The frequency of such updates is typically part of management and operational plans.
ATC Williams’ experience in dam designs, tailings rheology, hydrology and hydraulics and geotechnical investigations offers a unique perspective in developing dam failure models and evaluating the consequence of failure.