center of mass detention time
center of mass detention time
(OP)
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
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





RE: center of mass detention time
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.
RE: center of mass detention time
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 www.hydrocad.net/firstinch.htm
RE: center of mass detention time
thanx,
amit
RE: center of mass detention time
If you have specific questions about the calculations, please send your HydroCAD project file to support@hydrocad.net