Quasi-3D two-phase model for dam-break flow over movable bed based on a non-hydrostatic depth-integrated model with a dynamic rough wall law

•A depth integrated two-phase model of fluid and sediment motions is proposed.•3D flows and flows near the bed is calculated with interphase momentum transfer.•The advantages are presented comparing with several single phase models.•The bed variation due to dam break flow with 3D structure is reproduced. The assumptions of equilibrium flow conditions neglecting the flow acceleration for vertical velocity distributions and of the equilibrium wall law applied to the bottom boundary condition, in which the flow acceleration is neglected in the vicinity of the bed, are questionable when used to calculate sediment transport for flows that vary rapidly over time and space. Examples of such flows include dam-break flows and flows around structures. This study proposes a two-phase depth-integrated model for large-scale geophysical flow applications involving sediment transport phenomena and bed morphology. The model for the fluid phase is based on the non-hydrostatic quasi-3D method, and uses a dynamic rough wall law that employs continuity and momentum equations for the bottom boundary conditions. It was confirmed that the proposed model reduces to the previous bedload formulae for uniform flow conditions under the weak sediment transport condition. The model was applied in an experiment involving a dam-break flow on a movable bed channel with a suddenly enlarged section. The comparison between the experimental results and the results calculated with the proposed model and previous models demonstrates the validity of the proposed model. The comparison also highlights the advantages of introducing a quasi-3D two-phase model to evaluate vertical velocity distributions and non-equilibrium sediment motions.