center of mass detention time 2

[amittilak]

hii guys
i really need your help here. i manually calculated the orifice size for releasing the CPv volume over the 24 hr duration and inputted the diameter into hydrocad. but inspite of that the center of mass detention time given by hydrocad is some 200 mins and my local agency is taking objection over that.
please help me in this matter
thanks,
at

 

[TerryScan]

What is the exact requirement of the regulating agency?

Here, one must have minimum 24 hours from when inflow begins and outflow ends.

On all of my models which meet this criteria, the Center-of-Mass and Plug-Flow Detention times are around the same value as your project. Yet, this has nothing to do with my local (state) requirements.

 

[psmart]

Detention requirements are written in many different ways. As Terry suggests, you need to check the regulations very carefully. The official requirements may (or may not) use the center-of-mass or plug-flow techniques as provided in HydroCAD.

If one of these techniques is appropriate, and your detention time values are low, you may need to adjust your outlet design. I assume that your initial orifice estimate was based on a specific head. But the head will actually vary over time, which is why a full hydrograph routing is required.

Also be SURE that your time span is long enough to watch the entire volume empty from the pond, otherwise the calculation will omit the end of the hydrograph, and may considerably reduce the calculated detention time.

Although the plug-flow value is hard to verify by hand, the center-of-mass result is the difference between the center-of-mass times of the outflow and inflow. These values are included in the summary report, and can be visually checked against the inflow/outflow hydrograph:

Center-of-Mass det.time= 98.7 min (1,020.0-921.3)

Note: If the regulations don't specifically cite the plug-flow or center-of-mass methods, then you may need to use the storage-vs-time values (as shown on the tabular hydrograph) to determine if you are meeting the specific detention requirements. For details see

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

 

[amittilak]

thanx guys for the insight. the guy from regulatory agency says that the center of mass detention time must be 24 hrs and that it must be seen in hydrocad report. I calculated the orifice size according to the MDE 2000 manual but still im not getting that value for center of mass time in hydrocad. i wonder if it is even possible to get value of 24 hrs center of mass detention time when the storm event itself is 24 hrs.
thanx,
amit

 

[psmart]

You probably need a smaller orifice. Although the rainfall event lasts for 24-hours, the pond will continue to discharge for some hours beyond that. Be SURE your span extends until the pond is empty, otherwise you'll be ignoring the later outflow from the detention time calculation.

If you have specific questions about the calculations, please send your HydroCAD project file to support@hydrocad.net