When performing a 1D calculation (SWMM, GVF, etc..) the gutters are explicitly defined on the surface and used to dictate how flow is transferred to and bypass from inlets on the surface. With 2D calculations, however, the gutters are part of the terrain.

All gutters elements which intersect the computation grid defining the 2D calculation area are ignored during the calculation.

All flow to and from inlets are transmitted via the shape of the terrain.

All gutters outside of the grid element are computed as standard 1D elements during the full 2D analysis.