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modeling Sewage Collection System

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waseem19

Civil/Environmental
Nov 23, 2002
82
All,

What are the main benefits of modeling a sewer system with a backwater analysis? I know that this way you can locate hydraulic jump locations and exact water depths in each pipe segment but are there any other advantages? I can see the benefit of locating a hydraulic jump but isn’t an approximate water depth enough ?

if you run your model in term of pipe capacity only, just to make sure that each pipe can handle the peak flow at the constructed slope without exceeding the velocity limits (0.75m/s - 2.5m/s ) and ensuring a maximum depth of 70% "the client is rich and he is happy with this number" would that be enough ? do I still have to do a backwater analysis ?

The discharge points are always either an existing manhole or a pumping station wet well so tail water is not really encountered or very localized.

The reason I’m asking is that I’ve just obtained Sewercad of Haested method and it uses the backwater analysis method. I’ve worked before on excel sheet that only checked the pipe capacity.

Please try to answer my questions one by one.

Thanks all for your inputs.
 
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I'm really sorry to hear that you just bought SewerCAD. If you try to model an extensive combined pressure gravity system you'll find it very frustrating. I built a model of 40+ lift stations and 1000+ pipes and it was an awful experience. Single lines with one lift station discharging into a gravity system are okay (although if the controling point is not at the end of the line you have to make the rest of your forcemaing a gravity line with bolted manholes).

Just looking at the capacity of the pipe leaves out two bits: the inlet capacity and the upstream effects of a surcharged pipe in your gravity system. If your system runs smoothly, you've nothing to worry about, but if you're trying to look for bypass points, you'll be able to pick up which manholes are overflowing, and in SewerCAD, you'll be able to color code them.

Also in SewerCAD you're able to do an extended period analysis to see where your bottleneck points are and if your lift stations will overflow if inflow exceeds maximum outflow for a certain period (for instance, during a 24-hour storm event).
 
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