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
- A surface detention pond is feasible and preferred — Use Pond Sizing Calculator
- You need a subsurface infiltration system rather than detention with a controlled outlet — Use Dry Well Calculator
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.
Size an underground detention or retention system — vaults, HDPE chambers, large pipe or plastic crates — by comparing the storage you provide against the design storm volume, then check the outlet (orifice and/or weir) and drawdown time against your allowable release rate.
How underground detention sizing works
Underground detention temporarily stores stormwater runoff and releases it at a controlled rate so the post-development peak discharge does not exceed the pre-development (or regulatory allowable) rate. The calculation proceeds in three parts: storage volume, outlet hydraulics and drawdown.
1. Storage volume
The gross (geometric) volume of a rectangular system is Vgross = L × W × D. Because walls, structural members and stone voids displace water, the usable volume is reduced by a storage efficiency η: Veff = Vgross × η. Any depth below the lowest outlet is “dead” storage and is subtracted to give the active storage compared against the required design storm volume. A storage ratio (active ÷ required) of 1.0 or more indicates adequate capacity.
2. Outlet hydraulics
Discharge at the peak stage is computed for each outlet type and summed.
- Orifice: Qorif = Cd · A · √(2gh), with Cd = 0.62, g = 32.2 ft/s² (9.81 m/s²) and h the head on the orifice centerline.
- Rectangular weir: Qweir = Cw · L · H1.5, with Cw = 3.33 (US customary) or 1.84 (SI) and H the head above the crest.
- Pump: constant rated capacity when the stage is above the pump-on level.
- Infiltration (retention systems): Qinf = f · Afloor, using the native soil rate f across the floor area.
The total outlet capacity at peak stage must be at or below the allowable release rate.
3. Drawdown time
The time to empty the active storage is estimated as tdrain = Vstored ÷ (Qavg × 3600) hours, using an averaged outlet flow because discharge decreases as the head falls. It is checked against your target drawdown window.
Methodology follows ASCE Manual of Practice 77 (Urban Stormwater Management) and FHWA HEC-22 (Urban Drainage Design Manual) outlet hydraulics. This is a screening tool; final design should use full stage–storage–discharge reservoir routing.
Storage efficiency by system type
The effective (usable) storage is the gross box volume multiplied by a system void efficiency η. Typical and range values used by this calculator are below; confirm the exact void ratio with your manufacturer’s data for the specific product and backfill.
| Storage system | Typical efficiency η | Range | Description |
|---|---|---|---|
| Concrete vault | 95% | 90–98% | Cast-in-place or precast concrete vault |
| HDPE chambers | 94% | 90–97% | Arch-shaped plastic chambers |
| Corrugated pipe | 90% | 85–95% | Large-diameter corrugated metal or plastic pipe |
| Concrete pipe | 92% | 85–95% | Reinforced concrete pipe storage |
| Plastic crate system | 95% | 90–97% | Modular plastic storage crates |
Values per manufacturer data and ASCE MOP 77. Effective storage = gross volume × η. A “custom” option lets you enter a measured void ratio directly.
Outlet equation coefficients
| Outlet | Equation | Coefficient | Notes |
|---|---|---|---|
| Orifice | Q = Cd A √(2gh) | Cd = 0.62 | Sharp-edged orifice; h measured to centerline |
| Rectangular weir (US) | Q = Cw L H1.5 | Cw = 3.33 | Suppressed weir, US customary units |
| Rectangular weir (SI) | Q = Cw L H1.5 | Cw = 1.84 | Suppressed weir, metric units |
Gravitational constant g = 32.2 ft/s² (US customary) or 9.81 m/s² (SI). Coefficients per FHWA HEC-22 outlet hydraulics.
Frequently asked questions
How is the required storage volume determined?
This calculator sizes the structure against the design storm runoff volume you supply (the total volume that must be held back so the controlled outflow stays at or below the allowable release rate). It compares the active storage you provide — gross volume × system efficiency, minus any dead storage below the lowest outlet — to that required volume and reports a storage ratio that should be at least 1.0. The design storm volume itself typically comes from a separate hydrologic method such as the Modified Rational or SCS/NRCS curve-number method.
What is storage efficiency and why is the effective volume smaller than the box?
Underground systems rarely store water through their full geometric volume. Chambers, pipes and crates contain the walls, end caps, stone backfill voids and structural members that displace water, so only a fraction of the bounding box actually holds water. That fraction is the storage (void) efficiency η. The calculator multiplies gross volume (L × W × D) by η to get effective storage. Concrete vaults are the most efficient (about 95%) while large-diameter pipe arrangements store roughly 85–95% because of the gaps between barrels.
How is the outlet discharge calculated?
Orifice outlets use the standard orifice equation Q = Cd·A·√(2gh) with a discharge coefficient Cd of 0.62. Rectangular (suppressed) weirs use Q = Cw·L·H^1.5 with a weir coefficient Cw of 3.33 in US customary units (1.84 in SI). Both are evaluated at the peak water stage, and the combined orifice + weir + pump + infiltration flow is checked against your allowable release rate. These follow FHWA HEC-22 outlet hydraulics.
Why does drawdown time matter for a detention system?
Drawdown time is how long the storage takes to empty after a storm. Many jurisdictions require detention storage to recover within a set window (commonly 24–72 hours) so that capacity is available for the next storm and, for systems with infiltration, so standing water does not persist. If your calculated drawdown exceeds the target, the outlet is undersized for the volume stored. The tool estimates drawdown from the stored volume and an averaged outlet flow, so treat it as a screening value and confirm with full reservoir routing for final design.
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Last verified: February 2026