In this paper, we analyze a nonlinear model of the viscous water-waves equation proposed in Dias et al. (2008). To this end, we first study the linear model in Dias et al. (2008). We then derive a new model which approximates the nonlinear model in Dias et al. (2008). We finally show the existence of a unique global-in-time solution and its decay rates to this new system with small initial data in energy spaces. (C) 2022 Elsevier Ltd. All rights reserved.