Determining Water Surface Elevation for a detention pond

[oengineer]

I am creating a detention pond for a site. In order to determine the Water Surface Elevation (WSEL) for the detention pond, I am using the current existing grade on site based on Topo drawings. The existing grade elevation is 36 ft. We will excavate the earth to create the proposed detention pond and size the pond that it should match the existing grade when it is full of water. Please let me know if this accurate? Any suggestions/comments are appreciated.

 

[msquared48]

Are you intending to have additional ponding above the catch basins in the parking lot?

Normally there is a 1 foot minimum height from the top of any pond berm to the maximum water surface elevation (100 year flood).

Mike McCann, PE, SE (WA)

 

[oengineer]

I am using the detention pond, parking lot, & some 24" HDPE pipe for detention storage volume.

 

[gbam]

msquared48 mentioned the 1-foot requirement. This is freeboard. Make sure you check with your governing agency on the freeboard requirements in regard to detention basins. There my be different requirements.

 

[oengineer]

My governing agency says:

Detention ponds less than one (1) acre in size shall have a minimum of 4
inches of freeboard. All ponds one (1) acre or larger in size shall have at
least one (1') foot of freeboard. Freeboard shall be measured from the
maximum 100-year water surface elevation and shall not be used in the
calculation of storage or mitigation.


My detention pond is 0.828 acres. Do I still need to satisfy the 4 inches requirement even though I am also using the parking and and pipe for my detention storage volume? I ask because is it okay that if the detention pond overflows that I am able to use the pipe and parking lot to hold the storm water?

 

[oengineer]

I just confirmed with my governing agency that they require 1 ft of freeboard on all ponds. Where is the best location to determine the freeboard? At the center of the pond or at the edges?

 

[msquared48]

Of course at the edges where the water would escape if overtopping occurred.

Mike McCann, PE, SE (WA)

 

[gbam]

Your maximum pond elevation, during your design storm routing, must be 1-foot lower than the lowest point on your "top" of basin. Note that, I have been directed by our local agencies to provide the 1-foot freeboard to my emergency spillway, this created a 2.5-foot difference to the top of basin.

 

[beej67]

Freeboard requirements vary.

In SFWMD, you can have zero freeboard at the 25 year storm, the general thinking being that anything past the 25 year storm the evacuate anyway. In many areas of Georgia, you need 1.5 ft of freeboard to the berm elevation. The freeboard in your storm structures then would either be required to be the same as the pond, or "under the ground," or sometimes "show your 100 year ponding limits on your plans," etc. In Durham I had to run a hypothetical analysis to determine how the 100 year storm tracked through the development if a secretive band of concrete ninjas snuck onto the site the day before a 100 year storm and somehow plugged every pipe with bags of readymix, causing them all to overflow. Had to show that on a surface drainage map.

So it all just depends.



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

http://www.campbellcivil.com