Tsunami loading can cause sediment instability, which can compromise the structural integrity of coastal buildings and infrastructure. To understand the process by which a tsunami can cause sediment instability, it is necessary to understand how the pore water pressure in the soil changes during tsunami loading. Based on the changes in pore water pressure due to overlying water and seepage of infiltrating water during a tsunami, a deformation model was developed and coupled with a seepage model. The proposed seepage-deformation model is able to model the broad range of drainage conditions of a soil bed, from the fully undrained condition to the fully drained condition. The coupled seepage-deformation model is formulated and implemented in MATLAB using the finite difference method for one-dimensional loading. The coupled seepage-deformation model is used to perform numerical experimentation after a convergence study is performed. The numerical experimentation focuses on saturated and unsaturated soil conditions, with linear and nonlinear soil constitutive models.