An improved partial standing-wave equation is proposed to express the variation of a water-surface in front of a vertical seawall that permits wave overtopping. The calculated surface displacements are compared with a set of test data taken near a vertical barrier model of an adjustable crest height in a two-dimensional wave flume. Although the horizontal asymmetry is seen in the measured surface displacements at several locations, the profiles calculated by the finite-amplitude equation show good agreement with the experimental results. To determine the volume of water that overtops the seawall, the numerical calculations are carried out by means of an energy approach using temporal surface displacements involved in both overtopping and non-overtopping conditions. In this study, the total amount of wave overtopping for irregular waves is estimated by applying the Rayleigh distribution for wave height variability. The theoretical results of the overtopping rate can be seen in the figures of this paper.