Community- Ornamental Pond, mitigating storm spillage/overflow 4

[injunear457]

Hi guys...Firstly I am an EE so my knowledge of this kind of thing is limited.
I'll preface my question with this description of what we currently have in our community to manage storm overflow of a relatively large (approx 300'x150' by 8'?depth) ornamental fish pond.

When (severe) inclement weather occurs it appears, and I assume, that the pond's rising level is mitigated by a riser/overflow pipe connected to storm drain piping and subsequently to a smaller detention pond nearby.

The problem is that the smaller detention pond's also used for storm water (watershed) of the 'typical' surfaces, blacktop and green space and *I* believe that it is sized for the ordinary runoff rate. It always performs flawlessly until the ornamental pond overflows and then the detention pond is overwhelmed and floods private property at lower elevations by sourcing water(and fish) via the storm drain grates at street level, all of which are also connected to the aforementioned detention pond.
It appears to me, and again I'm an EE so I don't know much about this stuff, that this is 'flooding by design'. It appears to intentionally divert the larger pond overflow to lower elevations as a ‘circuit breaker’ without regard to flow rate.

My question(s) is this:

1) Is this the 'typical' hookup to mitigate overflow of an ornamental pond?

2) Am I correct in my assertion that all the methodologies used to size, volumetrically, the detention basin are 'thrown out the window' when making this extraneous riser/storm drain connection to the larger pond?

3) Any other ways that this might be implemented (I think pumps and controls) would be better to divert the overflow to a different detention pond a short distant away under controlled flow rate to share the burden.

I might of course follow up with a couple more questions later...

Oh, and thanks in advance!

 

[injunear457]

Well, I thank you guys for all the help but it's that time of the year to put this aside and take care of family obligations. My next step is to visit the authorities again (in Jan) and see if I can't get the district's engineer's to do me a favor in providing a little guidance and analysis concerning the fish pond and the dra...

For now I want to sincerly thank all and have a Merry Christmas. See you in January!

 

[LincolnPE]

Hmmm, every time I mention attorneys....

 

[beej67]

WOAH! You should have mentioned you were in SWFWMD up front. I've done several projects in SWFWMD, and I caution other engineers in this thread who have limited or no experience in Florida to do their homework before responding. Things can be quite different down there, and there's things being said above that are absolutely correct for most of the country yet flat wrong for you.

The design of these ponds would have been governed by the SWFWMD Basis of Review:

https://www.swfwmd.state.fl.us/files/database/site_file_sets/17/erp_basis_of_review.pdf

Chapter four covers water quantity discharge criteria. The peak discharge mitigation requirements are NOT the 100 year storm, they're the 25 year storm, since you're presumed to be evacuated for anything larger than that anyway, and watching your home on TV from the comfort of a Days Inn in Valdosta. The 100 year storm only kicks in if you're in a closed basin, meaning it doesn't eventually discharge to the ocean. (lots of places like that around Lakeland FL, hence the name) And even then, that criteria is to match runoff volumes, not rates. You're still held to the 25 year rate, or sometimes a unit area rate established by the district, with either a 100 year volume match for closed basins, or no volume match for open basins.

You do have to do a 100 year flood analysis, but that's not to set discharge rates, it's to set minimum FFEs. There are rules about not flooding out your downstream neighbors, but those rules are presumed to be met if you can show a pre/post match for the 25 year storm. I do not recall whether SWFWMD requires a "Glass Box" analysis, but I've done those quite a bit in SFWMD, and for those you drop the 100 year storm on your site with zero discharge, presuming the entire site acts as a pond with a glass wall at the property line, and you see how high the site stages up, setting FFEs above that. (and ensuring that's no higher than the FIRM elevation) The intent is to design for a case very similar to your photos. If I saw those photos out of a site in Naples or Miami (SFWMD) after a hurricane I'd say it was performing as designed!

The most likely source of this flooding in SWFWMD style engineering designs is the prior engineer did not properly account for variable tailwater on the system. You need to hire an engineer to review his model, which was probably in ICPR, to check his tailwater rating cuves / etc. Doubtful the District is willing to do this for you, considering they already approved the permit. Where the District may help you is if the HOA has done things to the pond post-permit that are impeding its performance. Certainly go to them, but don't expect them to overturn an ERP they've already issued without an engineer giving them good reason.



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

http://www.campbellcivil.com

 

[LincolnPE]

The OP: "As an aside, I did verify that the frequency is supposed to be 100 year interval."

Whoa! You slow down there big fella

You are getting all excited about Major design event volumes. We've already ruled (best you can in this forum without actually doing the work) he is not being flooded in the Major storm. Look at the pictures of Debby also.

He states his street, driveway and lot frontage are being flooded deeply frequently. That's why I advised him about how Minor storm criteria is the more restrictive criteria regarding street flooding performance for his perspective. I gave him Minor storm event street flooding design criteria for reference.

Also, remember to a layman, the Minor storm design events, such as the 5yr storm design event which has an exceedance probabiltiy of only 20% in any given year, is going to look like a small flood, with the street full and water surface out to the crown per that Miami-Dade criteria.

The flooding should not be at the depths as frequently as he is reporting. Something is up with design and/or maintenance that an engineer needs to review.

 

[beej67]

I'm just saying the ponds were assuredly not designed for 100 year events. Knowing everything we know now, I'd bet $100 that his problem is related to tailwater from the primary control staging through the system.

It's hard to tell where the primary control is from his diagram, but it appears that his inlet and the fish pond both drain to "DRA," and that "DRA" drains out of the project. "DRA" is highly likely to be a combination detention and retention pond, with a retention volume at the invert that either bleeds down through a bleeder orifice (not true retention) or infiltrates into the ground. Should say in the BOR linked above, but I think the recovery time is 72 hours for dry retention systems in SWFWMD, which would explain why it's still wet "48 hours after the storm" in the photo above. That's probably by design.

I'd bet that the primary control at DRA is either a slot or an orifice set at this retention volume, with an overflow weir above that, perhaps in a weir box. I'd further bet that the design screwup came in one of two very easy forms:

1) Civil Engineer forgot to check the elevation of the culdesac inlet vs the 25 year stage when sizing the pond, or
2) Contractor ran out of fill dirt when building the thing, and set his culdesac lower than it was intended to be in the design.

I have seen both happen before.

A third option, which is much simpler, is that the primary control is clogged. 457 should walk out to DRA and stand on the outlet control box, and site to his house - to see whether the top of the box coincides with the flooding elevation he's experiencing in the street. He should also see if some pesky critters haven't clogged up the primary control with sticks or what-have-you. Weir boxes in FL typically have a wooden baffle out in front of them to catch floatables, but they might not stop everything.

For what it's worth, Screwup Possibility (1) should be a pretty easy thing for even a EE to check (grin) if he's got the hydro report in hand. Find the 25 year design stage of the pond, and compare that to the rim elevation of your inlet in the culdesac. Post both numbers in the thread.

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

http://www.campbellcivil.com

 

[LincolnPE]

Don't have much time at the moment to post but the storm Debby brought us (locally) about 10"~12" in 24 hours.

Just FYI: From the isopluvials that would be in the 50 to 100 year range for SW Florida.


Anyway, to address his frequent flooding due to his reports about surcharged flow reversals coming from the pond and up out of that gutter drain grate in front of his house, it may be as simple as installing a flap gate on the end of that storm sewer run on the end that discharges into the pond.

That hydrograph from the short storm sewer run would more than likely beat the hydrograph peak of the detention pond system, making this a viable solution.

 

[beej67]

Depending on how flat it is and how much fall he's got in the system, that may work. But it's Florida, so there may only be four feet between the invert of "DRA" and the swale between he and his neighbor's house. There's also the possibility that it's not even pond related, that they're catching backwater from the regional floodplain.

As I say, the name of the game in Florida is tailwater and interconnected pond routing.

SFWMD FIRMs are hilarious, if you've never seen one. It's one big grey blob with a flood elevation number in the middle of it.



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

http://www.campbellcivil.com