Modeling of sediment transport in estuaries and coastal areas requires a lot of compromises to keep the computational costs within acceptable limits. Due to that, existing sediment transport models do not account for particle-scale physics, e.g. particle-particle interaction and turbulence modulation by sediment, which play a significant role, especially in the non-dilute regime. In the current study, a newly developed physics-based sediment transport model for free surface flows and its numerical implementation within the OpenFOAM framework is introduced. The new model is based on the multiphase mixture theory to account for interactions between sediment and water while tracking the free surface at the same time. A modified VOF equation for sediment-laden free surface flow was derived and implemented. The interphase momentum transfer is considered by solving an additional closure for the slip velocity which includes the effects of drag force, turbulent dispersion, and shear-induced diffusion. Dense granular flow rheology is used to supply the required closures for particle stresses. Additionally, suitable closures for the mixture and turbulent viscosities are introduced. The model was validated using experimental data and analytical solutions of five test cases of variable complexity. This includes pure sedimentation, laminar bedload transport, turbulent sheet flow, local scour due to a submerged jet, and wave-induced scour under a submarine pipeline. (C) 2019 Elsevier Ltd. All rights reserved.