Effects of hydraulic structures (such as culverts, bridge openings, or gates) can be simulated in numerical surface-water-flow models. Generally the scale and dimension of the numerical model is insufficient to model flow through the structure directly. However, if for a particular hydraulic structure a unique relation exists among headwater elevation, tailwater elevation, and discharge through the structure, the effect of the structure can be introduced by enforcing the unique three-parameter relation among three dependent model variables: water-surface elevations immediately upstream and downstream of the structure and discharge through the structure. The three-parameter relation might be implemented through an equation or sets of equations specific to each particular structure. The technique described is more general, viewing the locus of points satisfying any unique three-parameter relation as a surface. The surface is described by a set of points with coordinates corresponding to the three parameters. Values and gradients required for implicit implementation of the unique three-parameter relation in a numerical flow model are interpolated from the surface. The technique described provides a general interface between numerical flow models and effects of any hydraulic structure characterized by a unique three-parameter relation.