Double criteria for a detention pond

[SMIAH]

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).

 

[Twinkie]

Well which is more restrictive?

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.

 

[Twinkie]

Do you need to manage the volume for either? Or is this just a rate thing?

 

[SMIAH]

4.2 cfs/acres is almost like a flow rate for undevelopped conditions here.
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.

 

[msquared48]

#2 is usually more restrictive overall (fisheries constraints), although, if you have more than one basin, #1 could come into play locally. 4.2 cfs/acre seems like an awful lot after development to me for 50. Sure it's not 0.42?

Mike McCann
MMC Engineering

 

[Twinkie]

I think I follow what you are getting at.

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.

 

[SMIAH]

50 L/s*ha = 4.2 cfs/acres?
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 ?

 

[Twinkie]

You need the individual design info for each upstream basin. There really is no other accurate way to model this. You realy should route each upstream guy to a reach which accurately models the travel from upstream basin to 100 year basin. Do you have access to the designs of all the individual basins? You can just rebuild all the basins based on the approved designs and drainage areas.

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.

 

[Twinkie]

Another way, if you have the designs, and they ran a 100 year hydrograph, you can just rebuild all those individual hydrographs in HydroCAD, route them to their respective reaches, then to your 100 year basin. Your 100-year basin design will just be like any other basin you do, you're just using an aweful lot of individual inflow hydrographs.

 

[SMIAH]

Yes you need to design every basin. Thing is... some might be a Wall Mart with undergroud retention and others with a open-air basin.
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).

 

[Twinkie]

I like that approach, but to assume absolutely no detention on the upstream basins for the 100 year, you may end up with something way oversized.

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?

 

[SMIAH]

Good idea.

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.

 

[Twinkie]

..or just ask for pre>post for each upstream 100 year basin.

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

 

[beej67]

Well first off, you have 2 criteria, not one, and you need to meet one of the criteria for the 50 year storm and the other criteria for the 100 year storm. Designing that is a snap, just check both storms.

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

 

[SMIAH]

Thanks beej67.

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.

 

[beej67]

Sometimes if I'm designing the pond for the 100 year storm, and the pipes for the 10 year or 25 year storm, I've had municipalities ask me to show a plan for how the surcharged water at the 100 year storm makes it into the pond, over land, jumping curbs, whatever. Basically a check against the surface water system accidentally passing water out into the street bypassing the pond. It's a smart idea to check that on your own whether your reviewer asks you for it or not.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com