A unique hydraulic condition in the storm sewer modeling is the overcharged-flow- resulted street surface flooding. This is the condition in which the drainage flow into the sewer pipe is much larger than the sewer capacity and the depth is built higher than the ground surface elevation. In addition, at the sewer junctions (manholes) where there may be open access to the ground, the flow starts to go upward through the manhole openings, overtop the manhole rims.

There are two scenarios after the street flooding occurs:

• If there is a surface gutter or channel connected to the manhole, the overflowing water will join the surface gutter or channel and will be accounted for and simulated as part of the flows in the gutter subsystem. These flows may drain back to the sewer subsystem somewhere downstream.
• If there is no surface gutter or channel connected to the overflowed manhole, the overtopped flows leave the sewer system and these flows are lost to the system; this will be reflected by a flow volume loss. In this condition, there may also be a storage area above the ground elevation and below the user-specified overtop elevation. The water stored in the storage area will drain back to the manhole when the water elevation recesses. Users can specify the storage areas and the street-flooding-overtop elevation. A default overtop elevation is the ground rim elevation, assuming there is no storage effects.

The implicit hydraulic engine treats the street overtopping overflow as weir flow and uses a weir equation to determine the overflow. The weir crest elevation is the user-specified street overtop elevation and the weir length is determined by an empirical equation: