Stormwater DetentionSystems & Vaults
Underground detention stores storm runoff beneath parking and buildings, then releases it at the rate your ordinance allows. This guide compares modular plastic systems like ACO StormBrixx and NDS StormChamber against concrete vaults and pipe galleries, with honest 2026 installed costs, sizing methodology, and the regulations that decide how big your system must be.
How Detention Works
Storm inflow
Catch basins & storm pipe deliver runoff
Underground storage
90%+ void chambers, crates, or a concrete vault
Controlled release
Orifice-metered outlet drains down in 24–72 hrs
Post-development discharge never exceeds the pre-development rate
Detention at a Glance
What Is Underground Stormwater Detention?
A buried storage bank that absorbs the runoff surge development creates, then meters it out at the rate the receiving system can handle.
An underground stormwater detention system is buried storage — plastic chambers or crates, large-diameter pipe, or a concrete vault — that catches runoff from roofs and pavement faster than the site is allowed to discharge it. Development replaces absorbent ground with impervious surface: a one-acre parking lot sheds roughly 27,000 gallons in a one-inch storm. Detention holds that surge so the storm sewer downstream never sees the spike.
The mechanics are simple. Catch basins and storm pipe deliver runoff to the storage bed, water fills the void space inside and between the units, and an outlet control structure — typically an orifice plate or weir in a downstream manhole — restricts release to the rate the reviewing agency set. Most systems draw down within 24 to 72 hours, restoring full capacity ahead of the next storm. A pretreatment row or sump upstream traps sediment before it reaches the storage bed.
The case against the traditional detention pond is arithmetic. A pond doing the same job consumes buildable land at the low point of the site — often the corner you would rather lease, sell, or park on — and brings fencing, mowing, mosquitoes, and open-water liability with it. Underground storage costs more per cubic foot, but on commercial land it usually wins the moment you price the parking stalls it gives back.
When a Project Typically Needs Detention
Detention vs. Retention: Which Does Your Site Need?
The words get used interchangeably on plan sets, but they solve different regulatory problems — and the distinction decides your outlet design.
Detention
Temporary storage, controlled release
Runoff fills the system during the storm and drains out through a restricted outlet over hours to a few days. Detention controls the rateof discharge — peak flow — so downstream pipes and channels see no more water per minute than they did before development.
Choose it when discharge to a storm sewer or surface water is permitted, when tight clay soils or high groundwater rule out infiltration, or when the ordinance is written around peak-flow matching.
Retention
Permanent capture, no piped discharge
Water is held until it infiltrates into native soil or is reused — it never leaves through a piped outlet. Retention controls the volume of runoff, recharges groundwater, and gains water-quality treatment as the soil filters what passes through.
Choose it when soils percolate (roughly 0.5 in/hr or better) with adequate separation to groundwater, when the jurisdiction mandates volume reduction first, or when there is simply no downstream capacity to accept a discharge.
The same hardware does either job: wrap a geocellular tank in geotextile over permeable soil and it retains; wrap it in an impermeable membrane and pipe the outlet and it detains. Many projects run both in series — infiltrate the water-quality volume, detain the big storms.
System Specifications
Overview of detention system options, materials, and capabilities. Specific specifications vary by system type and manufacturer.
Detention System Options
Different system types serve different site conditions and requirements. Select based on storage needs, site constraints, and budget.
Arch Chamber Systems (NDS StormChamber)
Open-bottom, arch-shaped HDPE infiltration chambers installed in rows on a stone bed. NDS StormChamber comes in 18" and 34" heights and can stack in layers to add storage while preserving usable surface above. The open bottom provides high void space and works in permeable or non-permeable soils for detention, retention, or conveyance.
Commercial sites, parking lots, residential, infiltration and stacked-storage projects
Geocellular Modular Tanks (ACO StormBrixx)
Crate-style polypropylene cells assembled into a bonded 3D structure. ACO StormBrixx uses patented brick-bonding and cross-bonding for high load capacity (US variants: 300 = H-20/HS-25, 600 HD = H-25/HS-25, 900 SD = H-20/HS-20) and offers 3-dimensional internal access for CCTV inspection and jet cleaning. Wrap in geotextile for infiltration or impermeable membrane for detention/retention.
High-load sites under traffic, projects needing inspectable/cleanable storage, LID/SUDS
Large-Diameter Pipe Storage
Corrugated HDPE or concrete pipe arranged in manifold configurations. Offers 30-40% void space with robust structural capacity. Can be watertight for detention or perforated for infiltration. Familiar technology for contractors.
Linear sites, high groundwater, projects requiring watertight storage
Precast Concrete Detention Vaults
Buried watertight boxes assembled from precast sections (or cast in place) for maximum structural capacity. Vaults take the heaviest wheel loads, work under very shallow cover, and allow walk-in inspection — at a higher installed cost and with crane placement during construction.
Heavy industrial, airports, loading docks, shallow-cover urban sites
Infiltration Galleries
Chamber or pipe systems designed specifically for groundwater recharge. Open bottoms rest on engineered stone beds over permeable native soils. No surface discharge—all water infiltrates. Provides water quality benefits.
Sites with permeable soils, groundwater recharge mandates, retention requirements
Water Quality Units
Specialized chambers or vaults that provide enhanced pollutant removal through sedimentation, filtration, or other treatment processes. Often installed upstream of standard detention to pretreat runoff.
Water quality compliance, sensitive discharge areas, enhanced treatment needs
Rainwater Harvesting Tanks
Lined or solid-wall chambers designed to store rainwater for reuse. Includes pumping, treatment, and overflow provisions. Dual-purpose systems provide stormwater management and supplemental water supply.
Irrigation supply, gray water systems, LEED projects, water conservation
Deep-dive the two modular systems we cover in detail: ACO StormBrixx geocellular tanks and NDS StormChamber arch chambers.
Modular Plastic vs. Concrete Vaults vs. Pipe Storage
The three families solve the same storage problem with different structural strategies. Here is how they trade off on the factors that actually move project budgets.
| Factor | Modular PlasticGeocellular & arch chambers | Concrete Detention VaultPrecast or cast-in-place | Pipe GalleriesHDPE / RCP manifolds |
|---|---|---|---|
| Storage per excavated volume | 90–97% void — the smallest dig for a given volume | High inside the box, but thick walls and a base slab grow the hole | Pipe interior is open, but stone bedding drops net yield to roughly 30–40% |
| Typical installed cost (2026) | $8.50–$13.00/cu ft geocellular; $12.50–$17.00/cu ft arch chambers | Often $15.00+/cu ft once crane time and freight are counted | Between the two; climbs with pipe diameter and burial depth |
| Traffic loading | AASHTO H-20/HS-20 to HS-25 with required cover | Highest — loading docks, fire apparatus, even aircraft aprons | High with adequate cover depth |
| Cover depth | Shallow — commonly 12–24 in. minimum | Designed for very shallow cover when needed | Large diameters bury deeper, which lowers the outlet too |
| Inspection & cleaning | Internal CCTV and jet-vac access; StormBrixx offers full 3-D internal access | Walk-in entry — confined-space procedures apply | Standard pipe CCTV and jetting |
| Installation | Hand-placed units, no crane, fast assembly | Crane picks, heavy hauls, larger crews | Machine-set — the most familiar method for pipe crews |
| Best fit | Parking fields, tight urban sites, infiltration designs | Extreme loads, shallow cover, agencies requiring rigid watertight structures | Linear parcels, high groundwater, watertight detention |
Installed cost figures are 2026 U.S. commercial budgetary ranges — site conditions and region shift them substantially. Get a project-specific quote before committing a budget line.
How Detention Systems Are Sized
Four steps take a site from ordinance language to a chamber layout. The storage volume is never a guess — it falls out of the hydrology.
Set the allowable release rate
Pull it from the local stormwater ordinance. The usual rule: post-development peak discharge may not exceed pre-development rates for the 2-, 10-, and 100-year storms. Some utilities instead cap release at a fixed cfs per acre.
Estimate site runoff
For drainage areas up to about 20 acres, the Rational Method (Q = CiA) gives peak flow from area, runoff coefficient, and rainfall intensity. Larger or staged sites use NRCS curve-number hydrographs (TR-55).
Compute required storage
Storage is the gap between the inflow hydrograph and your capped outflow. As a first screen, commercial sites commonly land between one and three inches of runoff depth over the impervious area — routing analysis refines the number.
Configure and route
Lay out chambers or vault cells to reach the volume within the available footprint and depth, size the outlet orifice and overflow weir, then verify with pond-routing software. A licensed civil engineer seals the final design.
Run the numbers first
Our free Rational Method calculator turns drainage area, runoff coefficient, and rainfall intensity into peak flow — the starting point for every detention sizing exercise.
What Underground Detention Costs
Fully installed U.S. commercial systems run $8.50 to $17.00 per cubic foot of storage in 2026. Where your project lands in that range depends more on the hole than the hardware.
Modular geocellular tanks
Crate-style cells such as ACO StormBrixx. Hand-placed units and 95%+ void ratio keep the excavation small.
Arch chamber beds
Open-bottom chambers such as NDS StormChamber set on stone. The stone backfill adds material and haul cost.
Precast concrete vaults
Highest structural capacity. Crane placement and heavy freight push installed cost above the plastic options.
What Moves the Number
- Excavation and haul-off — regional rates span roughly $18/cu yd in rural markets to $55/cu yd in the Northeast corridor
- Depth to invert and groundwater — dewatering and shoring add cost quickly below the water table
- Traffic loading — HS-25 fire lanes demand more cover and engineered backfill than landscaped areas
- Liner choice — geotextile wrap for infiltration vs. welded impermeable membrane for watertight detention
- Pretreatment and outlet structures — isolator rows, sump catch basins, and the outlet control manhole
- Engineering and permitting — drainage report, routing analysis, and agency review cycles
A worked example: a 20,000 cu ft geocellular system budgets roughly $170,000–$260,000 installed — before you count the land a surface pond of the same capacity would have taken out of production.
The Regulations Behind Detention Requirements
Detention volume is not a design preference — it is a compliance output. These are the layers of regulation that produce the number on your plans.
Federal: Clean Water Act & NPDES
Construction disturbing one acre or more falls under NPDES construction general permits, and Phase II MS4 permits require municipalities to enforce post-construction stormwater controls on new development — which is where local detention rules come from.
Local peak-flow ordinances
The near-universal performance standard: post-development peak discharge may not exceed pre-development rates for named design storms — the 2-, 10-, and 100-year events are the most common trio. Your detention volume is whatever it takes to meet that.
Water quality & channel protection
Many ordinances add a water quality volume — often the first inch of runoff or the 90th-percentile storm — plus channel protection: extended detention of the 1-year storm drawn down over roughly 24 hours to prevent downstream erosion.
The approval process
Expect to submit a PE-sealed drainage report with routing calculations, record an operations & maintenance agreement, and schedule post-construction inspections. Some reviewers keep approved-product lists for chambers — check it before you specify.
Requirements vary block by block — see our stormwater regulations guide for how federal, state, and municipal layers fit together.
Key Benefits
Common Applications
Detention System Manufacturers
We partner with leading manufacturers to offer detention solutions for every project scale.
NDS
StormChamber open-bottom arch infiltration chambers (18" & 34" heights)
ACO
StormBrixx geocellular modular tanks (300 / 600 HD / 900 SD, H-20–HS-25)
Detention is one piece of a Low Impact Development strategy. Reduce the runoff volume reaching storage with permeable pavers, capture surface flow upstream with catch basins, and control discharge from your system with the right outlet valves.
Detention Systems by Application
Underground storage frees up buildable land across these project types.
Detention System FAQs
Costs, vault vs. modular decisions, and when detention is required
On U.S. commercial sites, fully installed underground detention typically runs $8.50–$17.00 per cubic foot of storage (2026 industry cost guides). Modular geocellular tanks tend toward the low end at roughly $8.50–$13.00/cu ft installed, arch chamber beds run about $12.50–$17.00/cu ft, and precast concrete vaults often exceed $15.00/cu ft once crane placement is included. A 20,000 cu ft system therefore budgets in the $170,000–$340,000 range. Excavation depth, dewatering, traffic loading, and regional labor rates are the biggest swing factors.
A detention vault is a buried watertight structure—usually precast concrete box sections, occasionally cast-in-place—that temporarily stores runoff and releases it through a controlled outlet. Vaults offer the highest structural capacity (fire lanes, loading docks, airports), work under shallow cover, and provide walk-in access for inspection. The trade-offs are higher cost per cubic foot than modular plastic, heavy crane picks during installation, and confined-space entry procedures for maintenance.
Modular plastic—geocellular tanks like ACO StormBrixx or open-bottom arch chambers like NDS StormChamber—wins on installed cost, 90%+ void ratios, and crews placing units by hand, and current systems carry AASHTO H-20/HS-20 to HS-25 ratings suitable for parking lots and access roads. Specify a precast concrete vault instead when the design calls for extreme wheel loads, very shallow cover, walk-in maintenance access, or a reviewing agency that requires a rigid watertight structure.
Detention stores runoff temporarily and releases it downstream at a controlled rate—water leaves the site within hours to days, which controls peak flow. Retention holds water permanently or infiltrates it into the ground with no direct surface discharge, which controls total volume. The same chamber or tank often does either job: pipe the outlet to storm sewer for detention, or leave an open bottom (or geotextile wrap) over permeable soil for retention and infiltration.
Most U.S. municipalities require detention when a project adds impervious surface beyond a local threshold. Federal NPDES construction permits apply at one acre of land disturbance, and many cities and counties regulate far smaller additions through their own stormwater ordinances. The near-universal performance rule: post-development peak discharge cannot exceed pre-development rates for specified design storms, commonly the 2-, 10-, and 100-year events. Confirm your local ordinance before schematic design—it drives the storage volume.
Yes—keeping the surface usable is the main reason to go underground. Chamber and geocellular systems carry AASHTO H-20/HS-20 to HS-25 ratings when installed with the manufacturer’s minimum cover (typically 12–24 inches) and specified backfill compaction, so they sit under parking stalls, drive aisles, and fire lanes. Precast concrete vaults handle even heavier loading. The storage footprint keeps earning as parking while the site meets its discharge limits.
Resources & Tools
Design guides, sizing calculators, and regulatory information
StormChamber Design Guide
NDS StormChamber specifications and sizing
StormBrixx Design Guide
ACO StormBrixx geocellular tank specs and load classes
Runoff Calculator (Q=CiA)
Estimate peak flow for detention sizing
Installation Guidelines
Best practices for chamber installation
Regulatory Compliance
Meeting stormwater regulations
Shop Drainage Products
Browse drainage products online
Need Help Sizing Your Detention System?
Our specialists can help you select the right chamber system and configuration for your site conditions and stormwater requirements.