Pond Drawdown Time 48 Hours after event

[jcfowler79]

Working on a project near a small municipal airport. FAA requires that dry bottom ponds be empty 48 hours after the storm event has concluded. City requires 2-year and 100-year restriction for the 24-hour event necessitating a dual stage restrictor. For the life of me I can not get the pond to drain down in 48 hours if the 2-year restriction meets the requirement. In this case 0.04 cfs/trib acre. The orifice is just too small to allow the water to drain out quickly enough. We can not have open water without bird - balls, netting etc so open water is out of the question. I need dry bottom that drains in 48 hours and meets a 2-year discharge rate, is it even possible? I have changed orifice shape from a circle to rectangle to maximize flow area at lower depths and still no progress. Any one have experience or documentation of a municipality waiving certain requirements near an airport?

 

[Ryb01]

In this instance the FAA may allow a 24hour draw down vs the 48hour draw down. This may work better for your outlet structure. We've been approved on a few airport projects of having drawdown times of approximately 18hrs due to timing of peak flows from external areas.

Hope this helps.

 

[jcfowler79]

I am having trouble getting the pond to draw down in 48 and certainly can not meet 24 hours. The current draw down is approximately 72 hours after the event. The issue is that the two requirements are working in the opposite direction, the FAA wants the water gone to keep the birds away and the city wants the detention time and lower release rate.

 

[gibfrog]

Did any nearby properties develop under the same stormwater criteria? How did they solve this problem. Did you ask your city stormwater engineer for advice? For public safety (ie FAA requirements), the city may be willing to modify their attenuation requirements or have a "design exception" process.
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If I assume that your allowable release rate analysis is correct and if I assume that you have no percolation to drawdown the pond, then the first answer that comes to me is underground storage - either a vault, pipe/rock bed, infiltrator/rock bed, etc., of the volume from 48 hours to the end of the drawdown. Not cheap!

Also, make your 2 year control, as broad as possible - maximize the rectangular orifice/wier width and minimize the hieght. Going under 4" high may require anti-clogging measures. Does not gain a lot, but usually is not a significant extra cost.


Clifford H Laubstein
FL PE 58662

 

[beej67]

Yeah, you're stuck, sir. FAA vs "The Locals" is always a fun one. (fun = challenging) Echo asking The Locals to bend their rules a bit. Although there might be some creative ways around it. Off the top of my head..

1) What's your real estate situation like? You might be able to divert the 2 year storm offline and detain it out of the flight path, and have a spillter box or diversion weir that only allows flows past the 2 year storm into your larger 100 year pond, which I presume stays in the flight path.

2) What sets the 2 year allowable discharge? Is it a pre/post regulation or a fixed rate per unit watershed area? If it's pre/post, can you reasonably make the case that your pre discharge is higher than you're currently assuming? For instance, if you're doing a redevelopment then your pre condition probably has runways already, which might set that allowable much higher and allow you to widen your 2 year control.

3) Can you detain the 2 year underground in pipes or an infiltration manifold and divert the 100 year to a surface pond?

4) Will "The Locals" give you storage credit for void space inside a stone / gravel matrix? You could conceivably over excavate your surface pond and backfill with gravel, assuming 40% voids, and hold whatever 2 year volume remains past 48 hours down in the gravel matrix. That would make the FAA happy (no surface ponding past the drawdown time) and still allow you to meet your allowable.

I've got a half dozen other ideas along this vein too, which are all out-of-the-boxers. Possibly pricier, but might meet both criteria. Shoot me an email if you want to talk more offline. I think if I were in your case, I'd start by asking The Locals to cut you some slack. If they don't, craft a design utilizing a combo of 3) and 4), price it, and then call a meeting with The Locals and The Owner that lays out the increased price of the new design and ask again, nicely. Then let The Owner talk to The Locals Boss if The Owner thinks the cost is worth the hassle.


my .02


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