Assessment of the risk associated with storage and handling of coal-combustion residues such as fly ash and bottom ash in slurry form requires a proper understanding of the flow behaviour of the materials in the event of a breach. Among all possible modes of failure, a breach of such containment facilities is known to cause maximum downstream damages due to its flowable nature and often the presence of a supernatant pond. To estimate the downstream runout and inundation behaviour, this paper presents a case study of an ash dyke failure caused by human error at Reliance Sasan, Singrauli, India on 10 April 2020, where the slurry travelled a distance of about 7 km and contaminated a nearby river and firm lands. Based on publicly available databases and information from failure investigation reports, the initial breach input parameters estimated through empirical approaches are used to determine the extent of downstream damages caused by the flow of slurry using HEC-RAS 2D sedimentation transportation module as a non-Newtonian Bingham fluid. Through back analysis, the modelled breach parameters such as runout distance, depth of deposition and runout travel time are estimated and compared with the observed values. The results obtained showed similar values, indicating the usefulness of the model to estimate the runout parameters for risk assessment and contingency planning. Keywords: Ash Ponds, Failure, Runout, Back Analysis