Water and sediment flow was modelled numerically in three dimensions in a tunnel-type sand trap. The numerical model solved the transient Navier-Stokes equations with thek-ε turbulence model. Simultaneously, the sediment flow was calculated by solving the transient convection-diffusion equation for sediment concentration. Multiple sediment sizes were used. Van Rijn's formula for sediment concentration was used as boundary condition for the bed. Sediment continuity for the cells close to the bed gave the bed changes with time. An adaptive grid was used, adjusting to the bed changes. The processes in the sand trap involved both sedimentation and erosion, as a large volume of sand was formed and moved through the geometry. A physical model test was carried out to verify the numerical model. Bed levels were measured at a longitudinal profile and a cross-section. The results from the physical and numerical model compared well.