In this paper we present a model for transport of substance with sedimentation governed by a water flow in a shallow basin. The model is developed by using a dimension reduction technique in an infinite layer. The concentration of substance is modeled by a 2D-approximation, corrected in the vertical direction. In applications 2D-models are often used in shallow basins as it was illustrated for instance in [Legović T., Limić N., Valković V., Estuar. Coast. Shelf Sci. 30, 619-654, (1990)]. Here we establish the acceptability criterion for the reduction, based on the mass conservation law. Thus we discuss the existence of solutions to the original 3D-model and the deduced 2D-model with L1 source terms. For the conclusion we prove that the relative error of sedimentation rates for the o-ri-gi-nal and deduced model tends to zero as the distance from the source of substance tends to infinity.