DrainageCalculators

Underground Detention Calculator

Design underground detention systems for stormwater management. Calculates storage volume, outlet sizing, and drawdown time per ASCE MOP 77 and FHWA HEC-22 methodology.

What This Solves

Designs underground detention systems (pipe galleries, chambers, vaults) for stormwater management, calculating storage volume, outlet sizing, and drawdown time.

Best Used When

  • Space constraints prevent a surface detention pond and you need underground storage
  • You are sizing a pipe gallery, chamber system, or concrete vault for stormwater detention
  • You need to calculate the required number of pipes or chambers to provide the design storage volume

Do NOT Use When

Key Assumptions

  • Storage volume is calculated from the net void space in the underground system
  • Pipe gallery storage uses the pipe cross-sectional area times length minus any bedding volume
  • Outlet discharge is controlled by an orifice or weir at the downstream end
  • The system is watertight (detention, not infiltration) unless designed with perforations
  • Structural loading from soil overburden and traffic is within the system rated capacity

Input Quality Notes

Verify manufacturer specifications for chamber or pipe storage volumes — void ratios vary by product. Confirm structural ratings (AASHTO H-20, HS-25) match the expected loading conditions.

Input Parameters

Hydrology

Peak flows and design storm volume

acres

Contributing watershed area

cfs

Peak flow before development

cfs

Peak flow after development (uncontrolled)

cf

Total runoff volume to store

cfs

Maximum outflow rate permitted

Storage Structure

Underground storage dimensions

Type of underground storage

Cast-in-place or precast concrete vault

Efficiency: 95% typical (90% - 98%)

ft

Length of storage structure

ft

Width of storage structure

ft

Depth of storage structure

ft

Depth below lowest outlet

Outlet Structure

Outlet configuration and sizing

Type of outlet structure

in

Diameter of outlet orifice

Number of orifices

ft

Height of orifice centerline above floor

hours

Maximum time to drain storage

Infiltration (Optional)

For retention systems

Add infiltration through bottom

Professional Review: For projects requiring regulatory approval or where public safety is involved, have your calculations reviewed and stamped by a licensed Professional Engineer (PE).

Was this calculator helpful?

Last verified: February 2026

Disclaimer & Terms of Use

Educational Purpose: The calculations provided by DrainageCalculators.com are for informational and educational purposes only. While we strive for accuracy, these tools do not substitute for professional engineering judgment or designs stamped by a licensed Professional Engineer (PE) where required by law.

Professional Certification: Users are solely responsible for verifying all calculations, confirming compliance with local codes and regulations, and determining the appropriateness of any methodology for their specific application. Critical infrastructure and public safety projects require review and certification by a licensed Professional Engineer.

Limitation of Liability: DrainageCalculators.com, its owners, operators, and contributors assume no liability for damages or losses arising from the use of these calculators. By using this tool, you acknowledge that you have read, understood, and agreed to these terms.

PE Stamp Required: Many jurisdictions require drainage designs to be reviewed and stamped by a licensed Professional Engineer. Check your local requirements before using these calculations for construction or permit applications.