How best to model debris and blockages in sewer pipes, and when to do it, and when not to…
Like many things when it comes to hydraulic modelling, there are different ways to model blockages, and different modellers will prefer different solutions. Irrespective of exactly how you model any blockage, you should firstly be sure that the blockage should be modelled. Secondly, it’s a good idea to model blockages in a consistent manner and document clearly in the model that it’s a representation of a blockage.
Does it need modelled at all?
If it’s likely that the blockage will get flushed away after a medium storm, then perhaps modelling it isn’t going to add much confidence to your model. Consider whether the blockage is hard or soft or debris? Is it fixed? Is it likely to be flushed out the system, is it continual silt? Have a reason for modelling or not modelling blockages. How accurate does the model need to be?
So you’ve decided to model the blockage. Here are some (there are others) ways to model them
For a Fixed Blockage
- Insert 2 dummy nodes, link by an orifice corresponding to the clear part of the pipe. So if the pipe is 500mm dia. and you feel the blockage takes up half the pipe then you could model a orifice with a diameter half the original.
- Model a A single node with a higher user defined head loss or even a fixed head loss.
- Model a user-defined link. Use the unblocked results to obtain a head-discharge relationship for the reach you are interested in. Then add a user-defined link in this location and modify the head-discharge relationship to include representation of the blockage.
For Rubble or Silt
- Add Sediment in the pipe to represent the silt.
- Reduce the roughness factor of the section of pipe impacted (the bottom).
- Change the pipe geometry in a way which includes a representation of the blockage if the blockage is fairly consistent over a significant length.