Transect data processing in SWMM


My model includes a canal and I am using a transect to represent the canal. One of the typical sections is shown in the  figure. I uses the actual elevation data for the transect cross section and hence some coordinate values are negative.

1. The profile view does not show the negative portion. Is there any way I can see the full depth profile view? will there be any problem in SWMM internal calculation if I use negative value ( as shown in the figure).

2. From the transect sections, does SWMM automatically calculate the slope ( using the lowest coordinates) or it uses the section only to calculate area and wetted perimeter. I am asking this because if it considers lowest coordinates than there will be problem from double counting if I input slope through offsets.

3. I will use stage data at outfall. what is the datum SWMM considers for stage data depths given?

Thank you in anticipation.



E-mail me when people leave their comments –

You need to be a member of SWMM 5 or SWMM or EPASWMM and SWMM5 in ICM_SWMM to add comments!



  • Thank you Mr. Dickinson for explaining the whole process. The example clarifies it all.

  • Subject:   How are Negative Transect Elevations Used in SWMM5? 


    You can have negative elevations in the Transects of SWMM 5 as the elevations are transformed internally to relative depths above the node inverts in the SWMM 5 engine (Figure 1).   The slope of the link is calculated from the link offset elevations (Figure 3) and the cross sectional information for the irregular link in SWMM 5 (Figure 2) is computed from the Transect data (Figure 4).   The Water Surface elevation of the link is based on the node inverts (Figure 5).


    Figure 1.  Transect Editor of SWMM 5


    Figure 2.  The Transect Data is Used in the Irregular of HEC-RAS Shape of SWMM 5


    Figure 3.  The slope of the link with the Transect is calculated from the link upstream and downstream offset elevations – not the Transect data which is relative.

    Figure 4.  Transect Data Transformed into Tables of Area, Hydraulic Radius and Width from the Transect Data internally in SWMM 5.



    Figure 5.  HGL of the Water Surface Elevation from the Node Invert and Link Offset Elevations.


  • Thank you Mr Dickinson. I gratefully appreciate your help.

  • SWMM 5 can  use negative elevations but the profile is show ABOVE  the inverts of the upstream and downstream nodes.

This reply was deleted.