What is the rationale for the Channel Protection Volume?

[murdock]

In Vermont, we are using the regulations developed with the help of the Center for Watershed Protection. One of the criteria, the channel protection volume (CPv), requires "12 or 24 hours of extended detention storage for the 1-year, 24-hour storm event." Most of the time I have used pre-approved sizing methodology (Harrington 1987) to satisfy this criterion.

Due to the inaccuracies in this method (for instance, one assumption is that the runoff from a watershed enters the pond instantaneously), I would like to use my calculated hydrographs/tables for a more realistic calculation. However, when reviewing the items necessary to meet the criterion, I come across the following item:

"The CPv shall be released at a roughly uniform rate...with the goal of achieving the requisite detention time between the inflow and outflow mass centroids."

I read "requisite detention time" to mean the 12 or 24 hour release rate mentioned above (the only place where detention times are given). If that is true, doesn't that mean that there should be either 12 or 24 hours between the peak inflow and peak outflow hydrographs? The Harrington method comes nowhere near that detention time (past projects have anywhere from 2 hours of detention time to 6 hours) and the pond size necessary for that would be (unacceptably) huge. My original ideas on this criterion is that there would be 12 (or 24) hours of time between the first drop of water entering the pond and the last drop leaving it. The Harrington method comes closer to meeting this rationale than the cited rationale.

Does anyone have any insight into this stormwater criterion?

--
Shane Mullen, Staff Engineer
Llewellyn-Howley Incorporated

http://www.lhinc.net

 

[TerryScan]

I have not used the Harrington method, but I glanced at it and see your point.

Here, our criteria for water quality is for 24 hours from beginning of inflow to the end of outflow.

Our CPv criteria is 2 year post peak kept at pre development rate.

I openned a small HydroCad project which has a nice flat outflow curve on the 2 year (1.81cfs in; 0.08cfs out- peaks). HydroCAd computes center of mass detention time. My initial modeling time was 0-36 hours. A center of mass dt. time reported was 602 min, but I still had small flow at the end time. I extended the modeling to 80 hours in order to get a center of mass det. time of 1535 min (exceeds 24hrs). Using the start-in end-out criteria, this was 60 hours of detention.

I understand part of what they are after. One complaint by reviewers here is that some outlet configurations produce a large peak...then a trickle out to 24 hours where water quality is not necessarily met (we don't have a time criteria for CPv) I can see where center-of-mass would help define the desired outflow shape, but 24 hours for that criteria seems extreme.

 

[psmart]

Murdock,

The text you quoted does seem to be specifying a center-of-mass detention time of 12 or 24 hours. However, the time lag between the inflow and outflow peaks will generally be much smaller. As TerryScan points out, it is the extended discharge for many hours after the peak that gives the larger detention time.

 

[sketchhwyman]

Here in New York, the DEC's Stormwater Manual says that you should detain the CPv for 24 hours. You use an average flow rate through your outflow oriface based on an average head. For example, if the full depth of your CPv extended detention pool is 4 feet, you assume an average flow rate through the oriface based on an average head of 2 feet.