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# Determining the depth of the detention pond

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## Determining the depth of the detention pond

(OP)
Hello, I am working on a project were we are creating a detention pond for a car lot. I am trying to determine the maximum depth that I can use for the detention pond. We would like to go as deep as possible and use take of less area. I have been told by a co-worker that the outfall will govern how deep I can make my pond and not the water table. Any suggestions/ comments are appreciated.

### RE: Determining the depth of the detention pond

Your coworker is partly correct.

If the outlet elevation is above the maximum water table, then he is correct, but if the maximum water table at the out fall can rise above the out fall elevation, then the water table controls.

Mike McCann, PE, SE (WA)

### RE: Determining the depth of the detention pond

many cities set a limit to the depth of basin / depth of ponding. Here it is commonly limited to 3 feet deep or less

### RE: Determining the depth of the detention pond

But that would only include the active storage volume, not the additional depth of water for settling or dead storage.

Mike McCann, PE, SE (WA)

### RE: Determining the depth of the detention pond

the OP indicated:

"would like to go as deep as possible and use take of less area"

So the depth of flood storage above the permanent pool would be limited to 3 feet or whatever the requirement is for the AHJ

### RE: Determining the depth of the detention pond

If you are doing pure detention, and not retention, or wet detention, then your depths will either be determined by local criteria or by what your outlet elevation can be.

### RE: Determining the depth of the detention pond

Even if the water table doesn't control, it can significantly influence the pond depth. Excavation of soil below the water table does not yield a 1:1 volume for storage. Further, soil exfiltration is limited to the difference between your control elevation and the water table seasonal high level.

Another consideration is the side slopes of the pond. If greater than about 1:4, you should fence the pond for safety.

### RE: Determining the depth of the detention pond

While possible, it's not common to have an outfall elevation lower than the water table, because if you did then the water table would eventually stabilize at the outfall elevation.

Basically the question goes back to whether you're trying to retain water (and then infiltrate it into the ground) or whether you're just trying to store and detain it.

If your goal is detention, storage, and release through the outlet control structure, and your water table is below your lowest available discharge elevation, then the lowest available discharge elevation is how deep you can make your pond. If the water table is higher than the lowest available discharge elevation, then you might be able to dig your pond that deep and just let it drain the water table down over time. That will be subject to local and state control.

If your goal is to capture, retain, and infiltrate water into the ground as part of your design, the bottom of the pond will need to be X feet above the water table. Often I see 3 ft, or 1 ft, for this criteria, and your first discharge control then has to be higher still in your pond, dicated by the amount of volume you have to capture. There's also infiltration math you need to do.

If your goal is to have a permanent wet pond, then you can dig plenty down below the water table, and ideally you'd set your outlet control at the water table elevation, setting the water elevation of the pond such that it interfaces seamlessly with the groundwater. If you want your wet pond significantly higher than the groundwater, you'll need to think about either pond liners or water balance equations related to your watershed. But don't take credit for volume below the permanent pool surface in your routing, because it's not available for storage.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com

### RE: Determining the depth of the detention pond

(OP)
Thank you all for your input. I greatly appreciate it.

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