This note attempts to explain both how the head upstream and the head downstream of a weir in SWMM 5 is calculated compared to the weir crest elevation and also to explain how the weir is presented in the HGL plot of SWMm 5. There has been confusion in the past concering how the weir is shown compared to the actual weir calculations. The node head is calculated obviously at both ends of the weir but the head over the weir is always based on H1-Crest or H2-Crest (Figure 1) and hence the weir should look flat – to the weir the downstream head is important but NOT the downstream node invert so the weir really is flat and should look flat in the HGL Profile across the weir (Figure 2). The crest elevation is always relative to the upstream node invert elevation NOT the downstream node invert elevaation
Figure 1. How the Head across a Weir is calculated in SWMM 5
Figure 2. HGL Profile across a Weir in SWMM 5.0.022. The black line should be shown flat.
Note: Weir and Orifice Flow E
If you use a weir in SWMM 5 then two flow equations are used
1. The weir uses the weir flow equation when the head at the weir is between the invert elevation of the weir and the crown of the weir and
2. An orifice equation when the head is above the weir crown or the weir is submerged.