The turbulent flows through the channels with abrupt cross-sectional changes are common and important physical process in nature. For a better prediction of the mean flow and turbulent characteristics for this problem, a two-dimensional depth-averaged numerical model is developed. The model is robust and accurate in reproducing the recirculation flow behind a groyne and turbulent flows in channels with abrupt cross-sectional changes, when compared to the available experimental data of mean velocities and turbulence kinetic energy. Our results reveal that the abrupt cross-sectional change of a channel can affect the flow pattern significantly and introduces the complex turbulence characteristics. In particular, when the channel has an abrupt expansion, the mean flow pattern is mainly in longitudinal direction with rather small transverse component. Meanwhile, a recirculating region forms behind the expansion position and the turbulence has very strong intensity within this region. For the flow in the channel with an abrupt contraction, the longitudinal component of the flow is decreased by the obstruction on one side and accelerated on the other side, whereas the transverse velocity is small. The turbulence is extraordinarily strong in the regions adjacent to the contraction wall in the narrow channel. In both cases of abrupt cross-sectional changes, the TKE is generated dominantly by the shear of the longitudinal velocities.