The reverberation is an important issue in underwater acoustics, in particular in shallow water where it has serious consequences on the detection and classification performance of sonar systems. The prediction of the reverberation level is then required and several models have already been developed. In this paper, a shallow-water reverberation model REVPA is presented. On one hand, in order to deal with the forward propagation, a wide-angle parabolic equation model (based on the Fourth order Padé approximation) was developed. On the other hand, interface reverberation (backscattering) phenomena can be well described by scattering strength functions. The REVPA model is based on the coupling between a propagation model and a scattering law by means of a plane-wave decomposition. Whereas any scattering laws and/or bathymetry can be included in the REVPA model, we concentrated, in this paper, on Lambert's law to predict bottom reverberation in the range-invariant case. The simulations results obtained with the REVPA model were compared to the results given by two different models based on the normal-mode theory: for tens of simulations, peak errors ranged from 0.1 to 1 dB for constant sound-speed profiles and from 1 to 2 dB for variable ones.