modeling perf. pipe

[Cadi1]

I'm confussed as to how to model a buried perforated pipe that is capturing and discharging water. For the storage portion I have embedded the 12" pipe in stone and now am trying to set the discharge parameters for the pipe. Do I use the orifice routed to the 12" pipe for the perforations? If so,how do you set it up? Assume that the perf. diam. of 0.3 in with 6 holes every 2". The entire pipe length is approximately 200'.

Thanks for your help.

 

[psmart]

How you model this depends on how you expect the system to behave. A common approach is to model the pipe and stone as providing storage for a single underground "pond". This assumes that the perforations are sufficient to allow the water level to equalize between the pipe and the stone, and that the final discharge is determined by some other "outlet" control, such as exfiltration into the surrounding soil, or perhaps an overflow pipe.

For details on modeling this scenario see

http://www.hydrocad.net/chambers.htm

Here is the basic pond setup in brief:

Storage volume#1 = outer excavation with stone backfill and appropriate voids setting.

Storage volume#2 = horizontal cylinder (the perforated pipe) of appropriate length and/or quantity. Set the "Embedding" option to "Volume#1". This places the pipe inside the stone bed.

Now save the node and open the report window. Scroll down to the storage table, which will shown the detailed calculation for storage contributed by the stone and pipe.

If you want to perform a complete hydrograph analysis, you can add the appropriate outlet device, such as exfiltration and perhaps an overflow device. For a conservative design, you may want to include the overflow device only, since the actual exfiltration can often be very hard to predict. For details see

http://www.hydrocad.net/exfilt.htm

This discussion assumes that the pipe perforations are sufficient to create a "level pool", so that the actual perforations do not appear in the model. If the perforations are a significant restriction, such that the levels are different, the model becomes more complex. This may require the use of two coupled ponds: The first pond being the pipe volume with orifice outlets, and the second pond being the surrounding stone with exfiltration and/or overflow outlet. This will also require a tailwater-sensitive routing procedure, since the rising water level in the stone will constitute a "tailwater" for the orifice flow through the pipe perforations.

But remember, the two-pond approach is recommended ONLY if the perforations are small in relation to the inflow, and the "level pool" assumption does not hold. Otherwise, model it all as a single pond - MUCH easier.

 

[Cadi1]

Thanks for your advice. I wanted to make sure that I was not overlooking any restrictions that would effect the discharge of the buried pipe. Thanks again.

 

[psmart]

You should still assure yourself that the perforations are not an issue. But with 200' of pipe I suspect that the perforations can pass much more flow than the pond's exfiltration capacity.

I assume you've also checked the inlet capacity of the pipe. If you need to model this, treat it as a culvert outlet on another pond immediately upstream. (This may be a catch basin, pond, or other structure that is feeding the 12" pipe.)

Remember, HydroCAD deals primarily with OUTLET controls. To model a potential control point, it must be modeled as the outlet of an appropriate pond, and the pond storage will consist only of the volume that lies upstream of that point. Anything downstream of the control point would be modeled as part of a separate pond.