Transects in SWMM5 and InfoSWMM
Transects refer to the geometric data that describe how bottom elevation varies with horizontal distance over the cross section of a natural channel or irregular-shaped conduit. The figure below displays an example of a transect for a natural channel.
Each transect must be given a unique name. Conduits refer to that name to represent their shape. The maximum depth of the transect is calculated automatically and displayed in the transect editor. A special Transect Editor is available for editing the station-elevation data of a transect. Transects may be extracted from a digital elevation surface as well. InfoSWMM H2OMap SWMM InfoSWMM SA internally converts these data into tables of area, top width, and hydraulic radius versus channel depth. In addition, as shown in the diagram above, each transect can have a left and right overbank section whose Manning's roughness can be different from that of the main channel. This feature can provide more realistic estimates of channel conveyance under high flow conditions.
Note: Since this data is internally converted to area, top width, and hydraulic radius versus channel depth, only the relative elevations are important. This feature allows a transection to be extracted at any representative section along the channel reach without concern for absolute elevation.
Excerpt from the EPA manual Storm Water Management Model Reference Manual Volume II – Hydraulics (PDF)