One of the largest known megafloods on earth resulted from a glacier dam-break,which occurred during the Late Quaternary in the Altai Mountains in Southern Siberia.Computational modeling is one of the viable approaches to enhancing the understanding of the flood events.The computational domain of this flood is over 9460 km2 and about 3.784 × 106 cells are involved as a 50 m × 50 m mesh is used,which necessitates a computationally efficient model.Here the Open MP(Open Multiprocessing) technique is adopted to parallelize the code of a coupled 2D hydrodynamic and sediment transport model.It is shown that the computational efficiency is enhanced by over 80% due to the parallelization.The floods over both fixed and mobile beds are well reproduced with specified discharge hydrographs at the dam site.Qualitatively,backwater effects during the flood are resolved at the bifurcation between the Chuja and Katun rivers.Quantitatively,the computed maximum stage and thalweg are physically consistent with the field data of the bars and deposits.The effects of sediment transport and morphological evolution on the flood are considerable.Sensitivity analyses indicate that the impact of the peak discharge is significant,whilst those of the Manningroughness,medium sediment size and shape of the inlet discharge hydrograph are marginal. One of the largest known megafloods on earth resulted from a glacier dam-break, which occurred during the Late Quaternary in the Altai Mountains in Southern Siberia. Computational modeling is one of the viable approaches to enhancing the understanding of the flood events. Computational domain of this flood is over 9460 km2 and about 3.784 × 106 cells are involved as a 50 m × 50 m mesh is used, which necessitates a computationally efficient model. Here the Open MP (Open Multiprocessing) technique is adopted to parallelize the code of a coupled 2D hydrodynamic and sediment transport model. It is shown that the computational efficiency is enhanced by over 80% due to the parallelization.The floods over both fixed and mobile beds are well reproduced with specified discharge hydrographs at the dam site. Qualitatively, backwater effects during the flood are resolved at the bifurcation between the Chuja and Katun rivers. Quantitatively, the computed maximum stage and thalweg are physically consiste nt with the field data of the bars and deposits. The effects of sediment transport and morphological evolution on the flood are considerable. Sensitivity analyzes: that the impact of the peak discharge is significant, whilst those of the Manningroughness, medium sediment size and shape of the inlet discharge hydrograph is marginal.