plug flow versus center of mass

[murdock]

In my state, the regulators typically look at the COM detention time to determine if their criteria are being met. In the model I'm working on now, the plug flow time is nearly double the COM time. Reviewing HydroCAD's website entry on the difference between the two, I believe that using COM results in an overly conservative design.

In my model, it says "66% of inflow" next to the plug flow time. I've read the help file, and understand that this is the volume included in the calculation. Why wasn't the whole volume included? What is the practical effect on the accuracy of the plug flow number? Does this make the calculation more conservative, or is it not counting part of my flow?

 

[psmart]

"66% of inflow" can be caused by several things:

1) Some of the inflow is permanently retained in the pond
2) The time span isn't long enough to include the entire discharge
3) You have a permanent pool which must be displaced before the plugs can exit.

Having a permanent pool will increase the plug-flow time, but has no effect on the COM method. Otherwise the two method will usually produce similar results.

For details see

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

If you have further questions please post your file or send it to HydroCAD support.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[murdock]

This is for a wet pond, so there will be some inflow held in the basin. I take it that means that 34% of my flow becomes the replaced volume of my permanent pool?

 

[psmart]

Correct. Since the plug flow calculation is based on a first-in first-out scenario, the existing volume must be displaced before any of the new inflow can exit the pond. This means that the existing volume will increase the plug flow detention time, but it will not alter the center-of-mass detention time which doesn't take the constant pool into account.


Peter Smart
HydroCAD Software

http://www.hydrocad.net