Rheological properties of tailings greatly influence the non-Newtonian flow behaviour in a tailings dam breach analysis. To reliably model tailings behaviour, rheological measurements were performed on soft tailings with high solids content. The measured shear stress and shear rate relations consistently exhibit shear-thinning behaviour and are characterized with the Herschel-Bulkley rheology. Employing the Herschel-Bulkley rheology developed from the measurements, a dam breach study was conducted for an active oil sands tailings storage facility aligned with current industry guidelines and best practices. Numerically, the study applied a two-process approach to simulate a breach scenario classified as “Case 1A” relevant to their distinct discharge mechanisms. The breach outflow hydrograph was derived using physics-based algorithm that implements time-dependent tailings surface evolution and breach geometry formation. Varying tailings strength is coupled with spatial and temporal evolution of solids concentration and shear rate. Major rheological, hydrodynamic, geotechnical and erodibility considerations were incorporated. To produce flood hazard maps, equation of Equivalent Clear Water Hazard Map was applied to dynamically account for the non-Newtonian fluid properties. The study sheds light on understanding nonlinear constitutive relations of the soft tailings, establishes confidence in generating tailings dam breach outflow hydrograph, provides a more reasonable basis for estimating potential downstream impacts of tailings flows, and supports informative decision making.