A rapid and efficient algorithm for interactive geometric modelling of arch dams is presented. It combines the advantages of the traditional geometric-design with innovative computational capabilities offering simple procedures for otherwise complex process of laying out double-curvature arch dam-reservoir coupled systems. The key parameters taken into account are: terrain topography, shape and thickness of crown cantilever, reference cylinder, thickness and curvature of individual arches, excavation depth, concrete volume, vertical and peripheral construction joints and automatic generation of finite element and boundary element models. The proposed algorithm was implemented in and runs parallel to the ADAD-IZIIS FEM-BEM, a finite element-boundary element software for structural analyses of concrete arch dams. To demonstrate the performances of the proposed algorithm, an example of a 130m high double-curvature arch dam was considered in a narrow V-shape canyon. The number of graphical options available at the push of a button, such as vertical and horizontal cross sections and 3D perspectives, allows the user to rapidly conduct the dam design within the optimization process.