Hydraulic Capacity of Back pitched pipes

[zahid106]

I am working on determining the carrying capacity of an old brick storm sewer system. Some of the segments in the system have sagged over the years resulting back pitched pipe runs. Manning formula does not work with negative slopes. Any ideas?

 

[RWF7437]

You could try modeling this in HEC-RAS. Remember that the Manning formula works for pipes under pressure but the slope is the slope of the hydraulic grade line, NOT the slope of the pipe. In HEC-RAS you may have to do the modeling by trial and error until you find the flow which just begins to surcharge the pipe, assuming that is what you mean by "capacity".

good luck

 

[ccor]

RWF has the right idea but I don't know that HEC-RAS would be very efficient. Basically a pipe with an adverse slope has no gravity flow capacity. This means that the pipe must flow under pressure, where capacity is determined by definition. Usually keeping the HGL below ground level to avoid surcharging is sufficient. So, the analysis consists of calculating the HGL lines for different flows until a surcharge condition is encountered, this becomes the max capacity. The EGL/HGL calcs can be programmed into a spreadsheet, but a program like Haestad/Bentley's StormCAD can make it even easier, particularly for large systems.

 

[bltseattle]

You need a backwater program. StormCAD works well and I recommend it althougth it is expensive. King County Washington has a free program that you may be able to download from this site:

http://dnr.metrokc.gov/wlr/dss/kcrts.htm#kcbw

 

[civilperson]

If the back slope change in elevation is less than the diameter, then the flow can be modeled by a flat bottomed culvert with a circular arch component.

 

[LeoSC]

The correct way of modelling this problem is how RWF7437 or bltseattle suggested.

Gravity flow is pushed by the difference in total energy, so what will happen here is that the flow will build up a head to push the flow through the pipe. This will happen even when the pipe is not flowing full. HEC-RAS can model this using a bridge or culvert routing in short increments (but it's tricky).

The other way of doing this is by hand, using the standard step method. This may be faster than a computer program for short segments, for one discharge.

Leo Sanchez

http://www.sanchezengineering.ca