The CivilStorm model simulates the gutter subsurface flows using diffusion routing algorithms. A nonlinear Muskingum-Cunge routing method is used to route the flows in gutters and the Manning’s equation is used to compute water depths in the gutters.

An inlet receives both runoff flow from the catch basin and flows from gutters. Since it is an open pathway to subsurface sewers, it is possible that the subsurface sewers can become pressurized and as the overloaded flow increases and sewer water elevation rises above the inlet elevation so that "street flood" or "overflow" occurs in which water flows from the subsurface sewer to the ground through the manhole and the inlet. Under this condition, the water also finds its way to gutters and flows downstream if there is a gutter connected to the catch basin. This reverse interaction between subsurface sewer and surface gutter is also properly modeled by CivilStorm model. Therefore a gutter can carry excessive flow from an inlet or overflow from a catch basin.

There is a difference between a gutter as a surface drainage network and an open channel as part of a sewer network in a CivilStorm model. A gutter (or channel) in a surface network is always associated with a catch basin inlet and the main source of its flow comes from the excess water of the inlet or the overflow from the overcharged sewer catch basin. On the other hand, an open channel can be a part of the subsurface sewer system and a channel can be directly linked to a conduit.