Double criteria for a detention pond
Double criteria for a detention pond
(OP)
A 60 acres site will be developped with commercial + multi residential lots.
Review Angency #1 ask, for each of the commercial and multi-residential lots, that the 50-year peak flow rate doesn't exceed 4.2 cfs/acres after developpment.
Review agency (#2) ask that the overall peak flows = or < a 100-year storm associated with pre-developpement conditions.
Question: How to design a detention pond for both agency #2 ?
Or should we forget #1 and design it with pre/post conditions for a 2 to a 100-year storm.
Agency #1 is more concerned about conduits capacity (network) and Agency #2 by streams (Environment).
Review Angency #1 ask, for each of the commercial and multi-residential lots, that the 50-year peak flow rate doesn't exceed 4.2 cfs/acres after developpment.
Review agency (#2) ask that the overall peak flows = or < a 100-year storm associated with pre-developpement conditions.
Question: How to design a detention pond for both agency #2 ?
Or should we forget #1 and design it with pre/post conditions for a 2 to a 100-year storm.
Agency #1 is more concerned about conduits capacity (network) and Agency #2 by streams (Environment).





RE: Double criteria for a detention pond
Can you choke back 4.2 cfs/acre up to the 50, and then let the 100 rip up to the pre-development condition? (I'm not sure if you are saying 100 yr pre>post, or if it's 2 yr pre>100 yr post).
Either way, do you have na option as to which you cna design too? You need both approvals correct? You need to satisfy them both.
RE: Double criteria for a detention pond
RE: Double criteria for a detention pond
Thing is... how to manage a 100-year storm when you know each lots will be controlled for a 50-year storm. I didn't design every device or bassin on each lots. How will they let the flow pass when a 100-year storm hits.
It's peak flow only. Runoff volume will be > after developpment.
You're right, it's a Regulatory issue so I have to satisfy both.
RE: Double criteria for a detention pond
Mike McCann
MMC Engineering
RE: Double criteria for a detention pond
So you have a basin on each lot? So that 50-yr requirement is for each, and the 100 year requirement is for the entire 60 acres?
So I think you want to model all the upstream basins (use the as-built info or approved designs), design for the 4.2 cfs/acre for each, then some downstream basin that meets your 100 yr reduction for the entire watershed. The 100 year storm doesn't matter for the individual lots, correct, so they will route through some type of emergency spillway or overflow, then proceed on their way to the downstream basin.
RE: Double criteria for a detention pond
That's right each lot will have it's own "basin" controling the peak flow up to a 50-year storm. Then a single basin has to control the 100-year storm before the water's released to the stream.
My point is : 100-year storm will be somehow controled by each basin on each lot. That's pretty hard to model. How to design the single basin ?
RE: Double criteria for a detention pond
Your 100 year model for the upstream basins will be whatever it is based on the design. It is either controlled somehow, routes through an emergency spillway, or simply overtops.
Sounds like a very cool design project to be honest.
RE: Double criteria for a detention pond
RE: Double criteria for a detention pond
They're going to hire another engineer to design their basin.
Other way is to assume every lots are 4.2 cfs/acres up to 50-year, and retention for the roads only. Then when a 100-year storm hits, you need a space to store all the 100-year runoff volume (e.g. park).
RE: Double criteria for a detention pond
Do you have the ability to put the 100-year basin on hold while the upstream basins are being designed? Underground vs. above should have no bearing on your end. Your inflow will be what it is regardless. At least it is only one firm doing the upstream, that will make coordination better for you. Maybe you could push for all this work yourself?
RE: Double criteria for a detention pond
The 100-year basin has to be there before each basin on each lots. What we could do is ask for a 4.2 cfs up to 100-year. This would be easier to control.
RE: Double criteria for a detention pond
This way you can just go and do the pre-developed for the entire watershed, and know what you need to hit, if you can be assured there is no increase in 100
RE: Double criteria for a detention pond
But in terms of HOW to do it, I think you guys are over-thinking it.
If you're developing out all the infrastructure for the 60 acre mixed use parcel yourself, you shouldn't have to individually detain on each outparcel. You should be able to just size your conveyance systems to carry the undetained flow to your regional pond, and do all your detention there. Then check that pond on the 100 year for a pre/post, and on the 50 year for your fixed discharge limit.
When designing the regional pond, you make an assumption about watershed cover. I often assume 85% imperviousness for full build-out, but that can vary with land plan. Then when the client sells the outparcels to other developers, they have a condition in their sale that states they won't build over 85% impervious cover on their one site, and all that you need to comply with the regional system is a conformance letter, which can be filed with both regulatory agencies. This approach works in Georgia, NC, AL, MS, and most of Florida.
In SFWMD, they will instead require you to submit a watershed summary table that includes how much imperviousness was allowed by the permit, how much has currently been built, how much is left, etc, and they keep track of it that way. So there, you can borrow from PEter to pay Paul in a sense.
So yes, the pond will be bigger, but in the end the developer gets to sell parcels that don't have to provide their own stormwater management, which increases their value. You also get to stick the pond off in a corner somewhere that has less real estate value anyway. In my experience, they make a lot more money this way than selling parcels each of which have to have their own pond. But then again, a lot of that comes back to phasing.
I also agree that 4.2 cfs/ac seems high, but then again that can vary widely by land use, soils, and design storm.
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: Double criteria for a detention pond
I'm with you on the fact that there should only be one single basin to control both criteria and no detention on each lots.
It will be easier afterward to permit the construction on each lots and as you mention, it will probably cost less and increase the value of the lots.
Plus it's easier to design (2-100 year pre/post conditions will > the 4.2 cfs/ac criteria).
As the stormwater drainage system is designed with a 10-year storm and the starting diameter has to be 12'' (regulation), there won't be an incidence on the pipe diameter whenever using ther 4.2 cfs/acres criteria or not.
Assuming imperviousness % is a great idea.
Let's see how I can get through this Regulation issue.
RE: Double criteria for a detention pond
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com