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 Transectdata 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.