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Exfiltration from Underground Stormwater Vault

Exfiltration from Underground Stormwater Vault

Exfiltration from Underground Stormwater Vault

Looking for some assistance.

I have an underground stormwater vault (w/forebay (10'x18.67') & 12" outlet pipe) is located beneath a parking garage. See attached screen captures for details

On the bottom of the vault, we have located 4 - 5' dia. openings to allow stormwater to percolate into the ground. We are required to use a design rate of 0.8 in/hr for the surrounding soil which will be made up of import free draining engineered fill. The review engineer is adamant that my HydroCAD model is incorrect and pretty sure he is correct. Since this is an infiltration basing, the LGU (Local Government Authority) requirements of a 48-hour drawdown time have to be met.

The area of the circular openings in the bottom on the vault equal 62.832SF. Multiply area by infiltration rate - 0.001164 CFS. HydroCAD is using the entire surface area of the vault. Since we are using the structural footings to construct the walls of the vault, the openings are a way to keep the stormwater from affecting the structural integrity of the columns.

How do I model this correctly so that the stormwater will enter any of the 4 - 5' dia openings (elevation 898.33)and flow to the center of the earth all the while keeping the vault volume available for storage?


Sleepless in Minnesota

RE: Exfiltration from Underground Stormwater Vault

Without details on your HydroCAD model I don't know your current setup. Assuming you're using an exfiltration outlet, this will provide several options for the exfiltration calcs. Since you are restricting exfiltration to just a fraction of the footprint, you will need to select the option for "Constant Flow" and enter your hand-calculated CFS value.

Peter Smart
HydroCAD Software

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