Computer-implemented method for civil engineering including obtaining a mesh representing a terrain and computing a watershed segmentation of the terrain based on the mesh, the computing of the watershed segmentation including identifying one or more saddle points on the mesh, for each identified saddle point, identifying paths each ascending from the saddle point according to a direction of a local maximal slope around the saddle point, and a path descending from the saddle point according to a direction of a steepest slope around the saddle point, the identified ascending paths dividing the mesh into connected components, and, for each identified saddle point, merging each connected component of which bottom point is the saddle point with a connected component including the identified path descending from the saddle point according to a direction of a steepest slope. The merging yields at least a part of a basin.