The implicit model can model flows in a surface gutter. A surface gutter is an open channel from a ground of a manhole or junction so that its flow sources can be:

• Excessive catchment runoff inflow above the inlet interception capacity, an inlet can capture all or partial runoff, a partial runoff is determined by a specified maximum capacity or an inflow-capture curve and other portion of the flow will go to the surface gutter
• Manholes overflow, if the manhole is overflowed and there is a gutter on the ground then the overflow goes to the gutter and the gutter may flow the overflow into the system, the manhole overflow is lost from the system if there is no gutter.

Gutter flows are modeled in simple Muskingum-Cunge routing method for flow and Manning equation for depth. The gutter and its surface network are solved every time after the primary sewer network is solved.