Design underground detention systems for stormwater management. Calculates storage volume, outlet sizing, and drawdown time per ASCE MOP 77 and FHWA HEC-22 methodology.
Peak flows and design storm volume
Contributing watershed area
Peak flow before development
Peak flow after development (uncontrolled)
Total runoff volume to store
Maximum outflow rate permitted
Underground storage dimensions
Type of underground storage
Cast-in-place or precast concrete vault
Efficiency: 95% typical (90% - 98%)
Length of storage structure
Width of storage structure
Depth of storage structure
Depth below lowest outlet
Outlet configuration and sizing
Type of outlet structure
Diameter of outlet orifice
Number of orifices
Height of orifice centerline above floor
Maximum time to drain storage
For retention systems
Add infiltration through bottom