Tailings Dam Breach Analysis

Tailings dam breach analysis provides a basis for evaluating potential downstream impact from a tailings dam breach event and is needed for facility classification and to develop an appropriate Emergency Response Plan.

Recognizing the complexity of actual tailings dam breach process, the Canadian Dam Association (CDA) Technical Bulletin - Tailings Dam Breach Analysis (CDA 2021) presents two conceptual processes for tailings dam breach analysis purposes. Process I represents discharge of the supernatant pond that carries eroded tailings and dam fill materials; Process II represents discharge of flowable tailings due to tailings liquefaction or progressive slumping of unsupported tailings. Failure mechanisms for these two processes are different.

For Process I, the understanding of tailings hydraulic erosion characteristics and estimates of eroded tailings volume by pond water are essential. Chen (2022) provided an erosion-based methodology to simulate the hydraulic erosion process on deposited tailings using MADflow. The mixture of water and eroded tailings was simulated as non-Newtonian fluids with varying solids concentration from the hydraulic erosion, and the resulting outflow hydrograph of water and eroded tailings mixture contains time-dependent solids concentration during the erosion process. The study highlights that the eroded tailings volume is not directly connected to tailings storage volume. Geotechnical properties and integrity of the deposited tailings play a significant role in erodibility and influence the volume of eroded tailings.

In Process II, the tailings mobilization can be correlated with in-situ geotechnical properties of deposited tailings and plausible failure mechanisms, also using MADflow (Chen and Cunning 2021). It rationalizes tailings breach outflow volume and rate of outflow (i.e., breach outflow hydrograph), and contributes to a reasonable evaluation of tailings mobility during runout and the subsequent assessment of downstream impact. It is worth mentioning that rheology and its relevant parameters for tailings mobilization and runout analyses are not necessarily the same, and sometimes need to be different.

These studies underscore the critical role of tailored numerical simulations, like those facilitated by MADflow, in understanding and mitigating the potential impacts of tailings dam breaches through accurate prediction of flow behavior and erosion dynamics.

Tailings Dam Breach Simulation -- Process I & Process II

The video above is a synthetic example using MADflow, depicting the two processes in a hypothetical tailings dam breach event. The previously deposited tailings behind a cross-valley dam are covered with water in the facility. The saturated tailings are susceptible to static liquefaction. One of the critical failure scenarios is the flood-induced overtopping failure (Process I), following with discharge of the flowable liquefied tailings (Process II).

In Process I modelling, the tailings surface was updated/lowered with progression of the erosion process, depending on factors like erodibility and critical shear stress. With intentionally exaggerated erodibility, tailings near the breach opening are heavily eroded with deep scour by hydrodynamic force, whilst tailings located in far field from the breach opening are barely disturbed. The hydraulic erosion process unfolds gradually over hours, reflecting its nature.

Process II simulates liquefied tailings from mobilization to runout. This is a non-erosional failure mode; the modelling demonstrates that collapse and mobilization of liquefied tailings occur in a very short period of time, dictated by in-situ geotechnical properties of deposited tailings and failure mechanisms. The simulated failure behavior is consistent with typical field observations of the sudden, rapid, and catastrophic nature of this type of flow liquefaction failures.