Retention/Detention Ponds in Florida

[kmart30]

I am trying to help a friend of mine with a site and a drainage plan. I've done some very small single family retention designs in the past but this one is a bit larger. My ESH is fairly shallow which would give me a large pond area if I design it to where it accepts all the runoff and make sure it recovers in 72hrs. The other alternative I believe is to design a detention pond and outflow a portion of the runoff to the swale (In the R.O.W) in the front of the property but I am not sure if this is allowed. Even if I did design it to take all the volume and treat it I believe I would still need to have a way for the water to get to the front swale in case it overflows.

The site is still relatively small at about 0.5 acre but I am trying to come up with the best design possible for this. Any feedback on possible solutions would be greatly appreciated

 

[Ron]

I'm not sure if you read my post in your original query in the structural forum, so I'll repeat what I can remember of it!

You'll need to check with the municipality to determine their design criteria. You'll also have to submit to one of the water management districts for approval. There are water management districts that cover the entire state, with each having its own area of responsibility. They do not have the same rules or review criteria, so be aware that you'll need to contact them as well before you do your design.

Some municipalities require both retention and detention facilities, even for a small site. Usually they require automatic splitting between the ponds. For the detention facility, you can usually pop off to either the Florida DOT's master facility for the roadway area or to the municipality's system. You'll obviously have to get permission. Depending on the municipality's requirements you will likely be required to attenuate pre- and post development runoff and prove the stage/storage conditions. Not difficult, just another hoop to jump through.

Good luck.

 

[kmart30]

No i didn't get the chance to read your reply but thank you for re-posting. They took it down because it was in the wrong forum so I re-posted in the correct one.

Yes, I read through the water districts BPM and city requirements and this site falls under the exemption for permitting through the district. I still have to permit it through the city but they have less strict requirements and a little more flexibility with design. I am allowed to use a 5-year storm with a 20-min Tc. but if I design it as full retention they will require a DRI field test for infiltration. I can make it work either way but I am scratching my head on the best way to convey the runoff from the parking area in the front to the pond in the back of the property. I was thinking swales or small pipes running down the sides of the property to the pond in the back but I am not sure what is more practically accepted for construction and cost....

 

[Ron]

For a small site like this I would use swales rather than pipes. The construction cost will be significantly lower and the the maintenance costs will be also.

 

[Okiryu]

Sorry to jump in and ask something that is not directly related to the OP's question, but just curious: do you do infiltration basins in Florida? I am just asking because I know that there is a karst geology there with sinkholes potential...

 

[geomane]

Okiryun

Yes, the civil engineers try to construct dry retention ponds on all sites in the panhandle of Florida. Retention ponds directly infiltrate the stormwater into the shallow unconfined aquifer. When clayey soils are present at the surface, we will recommend a vertical sand chimney to key into deeper sand layers. Sand chimney construction observations are very important to verify the contractor is keying into the correct layer and to verify subsurface conditions are similar to the geotechnical borings.

 

[Okiryu]

It is interesting. I was expecting that infiltration methods are difficult to get approved in Florida due to it can create sinkholes problems. Appears that since you are discharging directly into the aquifer and no into the soils it may not create sinkholes problems since it will not be water flow thru the soils.

 

[Ron]

Okiryu...it does filter through the soils. Our aquifer depths vary in Florida. Usually it is below about 40 feet, but that varies a lot.

Most of our sinkhole activity is centered along an area known as the Florida Ridge. It is an area extending from just south of the Florida-Georgia border to south central Florida, north and west of Lake Okeechobee. This area has limestone (some of which is dolomitic) that extends to the surface in many areas.

 

[Okiryu]

Hi Ron, thanks for the information. Do local agencies allow for percolation drainage at the Florida Ridge area?

 

[Ron]

Okiryu....generally, yes

 

[geomane]

From what I hear, some firms in Gainesville are using sinkholes that have opened up for drainage basins. They are backfilling the sinkholes with granular materials, with the materials getting gradually smaller (bottom to top). So they are backfilling with boulders/cobbles followed by gravel sized particles and then sand. I guess this is serving as a filtering mechanism? The aquifer is shallow in this area and they are using the sinkholes as a means to recharge the aquifer. I have not been invloved with any of these projects; a coworker just mentioned it to me about a week ago.

 

[Okiryu]

Thanks! This is very useful information as I also have to deal with some karst geology in my area.