The modern coastal protection systems place higher demands on the crest elevation and permeable of breakwaters.A three-dimensional numerical wave flume based on the Reynolds-averaged Navier-Stokes equations is established to investigate the wave force of submerged perforated semi-circular breakwaters under the action of shallow water waves.The results indicate that there is a phase difference between the horizontal and vertical forces acting on the submerged semi-circular breakwater,with the critical sliding moment generally corresponding to the moment of maximum shoreward horizontal force.As the perforation rate increases from 0% to 25%,the dimensionless maximum shoreward and seaward horizontal forces on the submerged semi-circular breakwater decrease by 27.8% and 39.8%,respectively.With decreasing wave period and increasing submergence depth,the seaward sliding force on the submerged semi-circular breakwater increases.Empirical formulas provide conservative estimates for the total force on unperforated semi-circular breakwaters under long-period waves and extreme submergence conditions,and further overestimate the wave forces on submerged perforated semi-circular breakwaters.