The Preissmann Slot is a well-established numerical approach for simulating pressurized conduit flow using the open-channel Saint-Venant equations. The free surface of the Saint-Venant equations represents the surcharge pressure as flow confined to an imaginary, infinitely-high, narrow slot at the crown of the conduit. The traditional Preissmann Slot requires the ad hoc specification of a slot width, which affects the resulting hydrostatic pressure celerity and overall model performance. Typically, the Preissmann Slot behaves quite well for conduits of uniform size that are continuously surcharged. However, numerical shocks may occur at pipe size transitions and where the true free-surface flow crosses into the slot surcharge. These numerical shocks often spawn undesirable oscillations and can result in model instability. Here, we developed a form of the Preissmann Slot which ensures a smooth solution across transitions. The new model behavior depends on the user's setting of a target pressure celerity, whose ideal value can be related to model time step and stability constraints.
