The paper describes the development of the efficient interface sharpening procedure for viscous incompressible flows governed by the Navier–Stokes equations. The moving interface has been captured by a pseudo-concentration method. The solution domain has been discretised by the space-time finite elements, while numerical schemes have been stabilised by the Galerkin least squares method. The dam break problem including breaking waves has been solved in order to validate the performance of the numerical technique. The computed position of the leading edge of water column has been compared with the experimental measurements. The detailed investigation of numerical parameters governing the sharpness of the front and mass conservation has been presented.