Modelling Steeply Sloping Pressurised Pipes
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Innovyze Howbery Park Wallingford Oxfordshire OX10 8BA United Kingdom +44 (01491 821400 |
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Technical Paper February 2013 |
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A previous technical note covered the modelling of pressurised pipes within InfoWorks ICM. This technical note provides a description of how to model pressurised pipes which slope (steeply) downhill.
In reality, it is possible that the top part of the pipe will be operating under free surface flow, whereas the lower portion will be heavily surcharged due to backing up from downstream, as shown in Figure 1.
Figure 1: Free surface and pressurised flow in the same pipe
There is no ideal way to model this configuration, but the best approximation is to model the entire pipe as ‘forcemain’ and to ensure that it is pressurised throughout the simulation. This is achieved by lowering the upstream end of the pipe, such that it is below the ‘static’ water level shown in Figure 1.
The issue is compounded because it is not possible to display the static water level and gravity flow water levels. InfoWorks simply joins the up and downstream water levels at each end of the pipe to form the HGL shown in Figure 2.
Figure 2: HGL in InfoWorks, compared with reality (dotted line)
Figure 3 shows the original forcemain (on the left.) This model may not initialise and could also generate flow during simulation. The revised forcemain (on the right) is the correct way to model this as the forcemain is now below the water level, throughout the simulation.
Figure 3: Original and corrected profiles