Surcharged flow (when the flow exceeds capacity of the conduit so that the conduit becomes pressurized) is common in storm and sewer systems. The Preissmann slot method is used for simulating pressure or surcharged flows for gravity conduits. An artificial slot is added to the conduit and the slot extends vertically from pipe crown to infinity and over the entire length the pipe, and the width of the slot is usually 0.1% of the characteristic conduit dimension (diameter for a circular pipe) but not larger than 0.01 ft. The significant advantages in using a hypothetical slot are apparent in simulating the moving transitional interface between open-channel flow and pressure flow, which can happen anywhere at any time in a sewer system, since the model uses same equations and numerical schemes and make no special switches between open-channel flows and pressure flows.

In a manhole or junction where flow has open access to the ground, the storage effects of the ground are accounted by including the surface area and elevation data in the continuity or the storage equation as described previously. When the water elevation in the junction rises above the ground, the ground starts to store the water, which significantly attenuates the unsteady wave. As the water elevation recedes, the stored water may drain back to the underground sewer system from the junction or may be lost to surface flow duo to overflow. The overflow occurs when water elevation at the manhole reaches above defined highest street elevation. The overflow from the manhole ground is similar flow over a weir so that a weir equation is used to model the overflow as

Where Q_over is the overflow discharge, L_w is the overflow length, C is the discharge coefficient and C=3.0 is used, H is the head over the overflow elevation.