In HEC-RAS versions 6.5 and earlier, pressurized flow through closed conduits is handled using the Preissman slot approximation. While this approximation has the advantage of allowing the same shallow water equation set and 1D finite-difference solution scheme as with open channel flow, it alters the geometry and physics of the problem, has problems with instabilities, and incorrectly predicts the propagation speed of surge flows through pipe networks (Fuamba, 2002).

The new implementation of pipe flow in HEC-RAS utilizes the same semi-implicit computational methods that drive the 2D and 1D finite volume shallow water solvers. The semi-implicit method has the advantages of being volume conservative, stable at relatively large computational time steps, able to handle wetting and drying, and utilizes subgrid bathymetry for computational efficiency. By treating the pressure term semi-implicitly, the Courant stability condition for pressure waves in the pipe network is bypassed and restrictively small time steps are not required. A consequence of this treatment, however, is that pressure waves and their consequences (e.g., the water hammer effect) are not explicitly modeled in HEC-RAS.

The numerical methods used by the HEC-RAS 1D finite volume solver were extended to handle pressurized flow using the methods described in Casulli and Stelling (2013). This included 1) decomposing the cell volume property tables into positive and negative components, and 2) modifying the solution scheme to include an additional iteration loop. When the computed water surface elevation increases above the ceiling of a closed conduit cell, the computed water surface elevation is taken to represent the sum of the cell's ceiling elevation and the pressure head, P/γ, where γ is the specific weight of water.