1 Scope
NOTE This standard covers the design, supply, and installation of post-construction stormwater management systems that detain, retain, or infiltrate site runoff. (1.1)
NOTE Such systems are required where the local Municipal Separate Storm Sewer System (MS4) permit, the municipal stormwater ordinance, or the land-development code conditions site-plan approval on on-site volume control, peak-rate attenuation, or groundwater recharge. In practice this applies to nearly every commercial or institutional site disturbing more than one acre in the United States. (1.2)
NOTE Included systems and components are as follows. (1.3)
- Modular plastic arch chambers (polypropylene and HDPE corrugated wall).
- Precast concrete modular vaults and box sections.
- Cast-in-place reinforced concrete basins and cisterns.
- Surface dry detention basins and wet retention ponds.
- Outlet control structures (orifice plates, weirs, riser-and-barrel assemblies).
- Forebays and water-quality treatment inserts and vaults.
- Geotextile separation and filtration fabrics, impermeable pond liners.
- Aggregate bedding beds associated with chamber systems.
NOTE "Detention" and "retention/infiltration" are distinct engineering functions with different permit pathways. (1.4)
NOTE A detention system temporarily stores runoff and releases it at a controlled rate; a retention or infiltration system stores runoff and exfiltrates it to the ground. The specifier shall identify which function governs before a system type is selected — a closed-bottom detention chamber and an open-bottom infiltration chamber look alike but behave and permit very differently. (1.5)
1.6The system function shall be identified and recorded before the system type is selected.
NOTE The federal floor for new-development volume control is 40 CFR 122.26, but the controlling instrument is the local MS4 permit and the governing state stormwater design manual, which frequently exceed the federal baseline. (1.7)
NOTE The federal rule establishes a 1.0 inch retention floor for new development; the enforceable criteria — design storms, release rates, and treatment targets — come from the state design manual and the municipal ordinance. Design to the more stringent of the applicable instruments. (1.8)
2 Referenced Standards
2.1Equipment, materials, and installation shall comply with the latest adopted edition of each of the following unless a specific edition is cited.
2.2Where referenced standards conflict, the more stringent requirement shall govern unless the Engineer of Record directs otherwise in writing.
2.3Where a local stormwater design manual or MS4 permit imposes a criterion more stringent than a referenced consensus standard, the local instrument shall govern.
| Standard |
Title |
| ASTM F2418 |
Polypropylene (PP) Corrugated Wall Stormwater Collection Chambers |
| ASTM F2787 |
Structural Design of Thermoplastic Corrugated Wall Stormwater Collection Chambers |
| ASTM C857 |
Minimum Structural Design Loading for Underground Precast Concrete Utility Structures |
| ASTM C858 |
Underground Precast Concrete Utility Structures |
| ASTM C1433 |
Precast Reinforced Concrete Monolithic Box Sections for Culverts, Storm Drains, and Sewers |
| ASTM C478 |
Precast Reinforced Concrete Manhole Sections |
| ASTM D2321 |
Underground Installation of Thermoplastic Pipe for Sewers and Other Gravity-Flow Applications |
| ASTM D3385 |
Infiltration Rate of Soils in Field Using Double-Ring Infiltrometer |
| ASTM D4751 |
Determining Apparent Opening Size of a Geotextile |
| ASTM D4632 |
Grab Breaking Load and Elongation of Geotextiles |
| ASTM D7001 |
Geocomposites for Pavement Edge Drains and Other High-Flow Applications |
| AASHTO LRFD |
LRFD Bridge Design Specifications (HS-20 / HS-25 live load) |
| ACI 350 |
Code Requirements for Environmental Engineering Concrete Structures |
| 40 CFR 122.26 |
NPDES Stormwater Discharges — Post-Construction Requirements |
3 Submittals
NOTE Action submittals are required before fabrication or installation. (3.1)
NOTE Action submittals establish that the proposed system meets the approved hydraulic design, the structural loading, and the geotechnical basis. Because system selection drives invert elevations shared with the storm-drainage network, action submittals shall be reviewed against the approved grading and drainage plans before any product is released. (3.2)
3.2.1The Contractor shall submit the following action items for review and approval before fabrication or installation:
- Product data for chambers, vaults, outlet structures, geotextiles, and liners.
- Shop drawings showing plan layout, profiles, invert elevations, bedding sections, and manhole/cleanout locations.
- Structural calculations for the selected loading class, sealed by a licensed engineer.
- Storage-volume and stage-storage routing calculations demonstrating compliance with each required design storm.
- Geotechnical infiltration test report (for open-bottom or infiltration designs) with measured saturated hydraulic conductivity and seasonal high groundwater elevation.
- Manufacturer cut sheets confirming minimum and maximum cover for the proposed traffic class.
☑ Product data (chambers, vaults, OCS, geotextile, liner)
☑ Shop drawings (layout, profiles, inverts, bedding, access)
☑ Sealed structural calculations
☑ Stage-storage routing calculations
☐ Geotechnical infiltration / SHGWT report
☐ Cover-depth confirmation for traffic class
NOTE Informational submittals document materials compliance and the geotechnical basis of design. (3.3)
NOTE These items are submitted for record and do not require approval before installation, but they substantiate the certificates of compliance that the reviewing authority and the Engineer of Record rely on at closeout. (3.4)
3.4.1The Contractor shall submit the following informational items:
- Aggregate gradation test report confirming gradation and fines content.
- Geotextile property certification (AOS, grab tensile, permittivity).
- Concrete mix design and compressive-strength reports for precast or cast-in-place elements.
- Manufacturer certificate of compliance with ASTM F2418, C858, or C1433 as applicable.
☑ Aggregate gradation test report
☑ Geotextile property certification
☐ Concrete mix design and strength reports
☑ Manufacturer certificate of compliance
NOTE Closeout submittals establish the long-term maintenance obligation. (3.5)
NOTE Most MS4 permits make a recorded operation-and-maintenance (O&M) easement and maintenance schedule a condition of the certificate of occupancy. Omitting them from the contract documents is a common and costly oversight. (3.6)
3.6.1The Contractor shall submit the following closeout items:
- Record drawings showing as-built inverts, dimensions, and access locations.
- Manufacturer's operation and maintenance manual with inspection and cleaning intervals.
- Recorded O&M easement and maintenance agreement, where required by the permit.
- Post-installation video or inspection report of chamber rows and outlet structure.
☑ Record (as-built) drawings
☑ O&M manual with inspection intervals
☑ Recorded O&M easement / maintenance agreement
☐ Post-installation inspection video or report
4 Quality Assurance
4.1The stormwater management design shall be performed by, or under the direct supervision of, a professional engineer licensed in the jurisdiction of the work.
4.2Hydraulic storage and routing calculations shall be sealed by the engineer of record for the site civil design.
4.3Structural calculations for underground systems under trafficked surfaces shall be sealed by a professional engineer and shall conform to ASTM F2787 for thermoplastic chambers or ASTM C857 for precast concrete structures.
4.4Chamber and precast manufacturers shall be regularly engaged in the production of the specified product and shall furnish a certificate of compliance with the governing material standard.
4.5A geotechnical infiltration evaluation shall be completed before any open-bottom or infiltration system is committed to design.
NOTE Specifying open-bottom infiltration chambers on soils that were never tested is a leading cause of permit rejection and post-construction flooding. The infiltration rate must be measured in the field, not assumed from a soil-survey texture class. (4.6)
4.6.1Field infiltration testing shall be performed by double-ring infiltrometer per ASTM D3385 or an equivalent method accepted by the reviewing authority.
4.6.2The design infiltration rate shall not exceed 50 percent of the field-measured saturated hydraulic conductivity.
4.6.3An infiltration system shall not receive infiltration credit where the measured saturated hydraulic conductivity is below the minimum rate accepted by the governing design manual.
● Field infiltration test required (open-bottom / infiltration system)
○ Soil boring and SHGWT only (closed-bottom detention)
○ No subsurface investigation (surface basin, lined)
5 Environmental and Service Conditions
NOTE The system shall be designed for the hydrologic, geotechnical, and loading conditions of the specific site. (5.1)
NOTE Storage volume, release rate, separation from groundwater, and structural loading are all site-specific and interdependent. The datasheet selections in this section define the design envelope; the routing and structural calculations demonstrate compliance with it. (5.2)
5.3 Design storms and release rate
5.3.1The system shall provide the storage volume required to meet each design-storm criterion imposed by the governing permit, evaluated by an accepted routing method.
NOTE Where the governing instrument requires matching multiple recurrence intervals, the system shall satisfy the most restrictive controlling event without violating any other. (5.3.2)
NOTE A frequent design error is sizing for a single return period — for example the 10-year event — when the permit requires the 2-year, 10-year, and 100-year peaks to be matched simultaneously. Routing must demonstrate compliance across the full required set, because the controlling event differs by orifice and stage. (5.3.3)
5.3.4The controlled release rate shall not exceed the pre-development peak discharge for each required recurrence interval unless the permit specifies a different allowable rate.
☐ 2-year
☑ 10-year
☐ 25-year
☑ 100-year
● SCS / NRCS curve number (TR-55 / TR-20)
○ Rational method
○ Continuous simulation
● Match pre-development peak (per interval)
○ Fixed allowable rate per local ordinance
○ Downstream-capacity limited
5.4 Groundwater and infiltration
5.4.1A minimum vertical separation of 2 ft shall be maintained between the bottom of an infiltration system and the seasonal high groundwater table, unless the governing design manual requires more.
NOTE Inadequate separation eliminates infiltration capacity, and in cohesive soils it allows the empty system to float. The seasonal high groundwater elevation, not the elevation measured on the day of investigation, governs this dimension. (5.4.2)
● Detention only (closed bottom, controlled release)
○ Infiltration / recharge (open bottom)
○ Combined detention and infiltration
○ Water quality treatment with detention
5.5 Structural loading
5.5.1Any system installed beneath a drivable surface shall be designed for AASHTO HS-20 live load (32 kip axle) as a minimum.
5.5.2Systems beneath major roadways, loading docks, or heavy-truck zones shall be designed for AASHTO HS-25 live load (40 kip axle) or the heavier site-specific load.
NOTE The finished cover over the system shall fall within the manufacturer's rated minimum and maximum cover for the selected loading class. (5.5.3)
NOTE Cover below the rated minimum invites live-load crushing; cover above the rated maximum overloads the chamber arch with dead load. Coordinating finished grade with the manufacturer's cover table is a structural requirement, not a finishing detail. (5.5.4)
○ Non-traffic (H-10 equivalent, landscaped)
● Standard highway HS-20 (32 kip axle)
○ Heavy HS-25 (40 kip axle, dock/roadway)
6 System Type and Configuration
NOTE The system type shall be selected to match the identified hydraulic function, the available footprint, the loading class, and the geotechnical conditions. (6.1)
NOTE The four broad families — surface basins, modular plastic chambers, precast concrete vaults, and cast-in-place concrete structures — trade footprint, depth capacity, traffic capacity, and cost differently. Modular plastic chambers dominate commercial sites under parking lots; precast and cast-in-place vaults suit deep, high-load, or tight-footprint installations; surface basins remain the economical choice on suburban and rural land where area is available. (6.2)
● Modular plastic arch chamber (open or closed bottom)
○ Precast concrete modular vault / box
○ Cast-in-place reinforced concrete basin / cistern
○ Surface dry detention basin
○ Surface wet retention pond
6.3 Modular plastic arch chambers
6.3.1Polypropylene corrugated wall chambers shall comply with ASTM F2418 for materials, dimensions, wall stiffness, marking, and qualification.
6.3.2HDPE and polypropylene corrugated wall chambers shall be structurally designed in accordance with ASTM F2787 for the specified live load and cover range.
NOTE Chambers shall be installed as an open-bottom infiltration bed or a closed (lined) detention bed consistent with the selected hydraulic function. (6.3.3)
NOTE The same chamber module serves either function; the distinction is the base treatment — an open stone bed over native soil for infiltration, or an impermeable liner beneath the stone for closed detention. (6.3.4)
● Polypropylene corrugated arch (ASTM F2418)
○ HDPE corrugated arch (ASTM F2787)
● Open bottom over stone bed (infiltration)
○ Impermeable liner beneath stone (closed detention)
6.4 Precast and cast-in-place concrete systems
6.4.1Precast concrete modular vaults shall comply with ASTM C858, and minimum design loading shall conform to ASTM C857.
6.4.2Precast box sections used as detention cells shall comply with ASTM C1433.
6.4.3Cast-in-place reinforced concrete basins and cisterns intended to retain water shall be designed in accordance with ACI 350.
NOTE Underground concrete structures shall be designed to resist hydrostatic uplift when the system is full and the surrounding soil is saturated. (6.4.4)
NOTE A water-filled vault in saturated ground is subject to buoyant uplift and surcharge that an empty-structure analysis misses. The engineer shall confirm anti-flotation provisions — ballast slab, tie-down anchors, or dead-man anchors — for any below-grade concrete structure. (6.4.5)
● Precast modular vault (ASTM C858)
○ Precast box section (ASTM C1433)
○ Cast-in-place reinforced concrete (ACI 350)
● Ballast / anti-flotation slab
○ Tie-down anchors to base slab
○ Dead-man anchors
○ Not required (above groundwater)
6.5 Surface basins and ponds
6.5.1A surface dry detention basin shall drain completely between storms and shall be graded to prevent standing water and the resulting nuisance and vector conditions.
NOTE A surface wet retention pond shall maintain a permanent pool and a vegetated littoral shelf for water-quality treatment. (6.5.2)
NOTE Wet ponds are dual-purpose: the permanent pool settles and biologically treats runoff, while the temporary storage above it attenuates peaks. Many southeastern jurisdictions require a wet pond specifically for its water-quality performance. (6.5.3)
6.5.4Basin and pond side slopes shall be no steeper than the maximum slope permitted by the governing design manual for safety and maintainability.
● Dry detention basin
○ Wet retention pond
○ Extended detention (dry with micropool)
7 Aggregate Bedding and Geotextile
NOTE The aggregate bed surrounding a chamber system provides both storage void and structural support, and its gradation governs how long that void survives. (7.1)
NOTE Stone with excess fines lets fines migrate into the void space, permanently reducing storage volume, while too-fine a gradation chokes infiltration. The bedding specification is as important to long-term performance as the chamber itself. (7.2)
7.3Bedding aggregate shall be clean, angular, crushed stone meeting ASTM D2321 Class IA or IB, with a fines content not exceeding 2 percent.
7.4The Contractor shall furnish a gradation certificate confirming compliance before placement of any aggregate.
○ ¾ in clean angular crushed stone (ASTM D2321 Class IA)
● ¾ in to 1.5 in clean angular crushed stone (Class IB)
NOTE Geotextile shall fully encapsulate the aggregate bed — base, sides, and top — to exclude fines on all faces. (7.5)
NOTE Wrapping only the base and sides leaves the top open to windblown silt that intrudes at the surface and clogs the bed. Full encapsulation is the correct detail; a partial wrap is a defect. (7.6)
7.7Geotextile in filtration applications shall have an apparent opening size determined per ASTM D4751 no coarser than the No. 40 sieve (0.425 mm).
7.8Non-woven geotextile shall have a grab tensile strength of at least 200 lb per ASTM D4632.
○ Filtration (around aggregate bed)
○ Separation (subgrade interface)
● Full encapsulation wrap
0.300 (No. 50)
0.425 (No. 40)
0.600 (No. 30)
NOTE An impermeable liner used for closed detention shall be continuous, seam-welded or overlapped per the manufacturer, and protected from puncture by the bedding stone. (7.9)
NOTE The liner is what converts an otherwise-infiltrating stone bed into a sealed detention cell. A breached liner silently defeats the design where infiltration is not permitted, for example over contaminated soils or a shallow water table. (7.10)
● None (open-bottom infiltration)
○ HDPE geomembrane liner
○ Reinforced polypropylene liner
○ PVC liner
8 Water Quality Pretreatment
NOTE Sediment-laden runoff shall pass through a pretreatment device before it enters the storage bed. (8.1)
NOTE Without pretreatment, sediment bypasses the geotextile and clogs the aggregate bed within five to ten years, destroying infiltration capacity and storage volume. A forebay, hydrodynamic separator, or filter vault upstream of the storage is the standard defense and is typically required by the permit. (8.2)
8.3A pretreatment forebay, hydrodynamic separator, filter insert, or media-filter vault shall be provided upstream of the primary storage in accordance with the governing water-quality criterion.
8.4The pretreatment device shall be sized and configured to meet the total suspended solids (TSS) removal target of the governing permit.
● Forebay (settling cell)
○ Hydrodynamic separator
○ Catch-basin filter insert
○ Media-filter vault
9 Outlet Control Structure
NOTE The outlet control structure (OCS) regulates the release rate that the entire detention design depends on, and its details are the most clog-prone part of the system. (9.1)
NOTE The OCS converts stored volume into a controlled discharge through orifices and weirs. A multi-stage riser couples a small low-flow orifice to higher weirs so that different recurrence intervals are released at different rates. Smart, sensor-actuated outlets can modulate release in real time where the permit and budget allow. (9.2)
9.3The OCS shall be sized by routing calculation to deliver the required release rate for each controlled recurrence interval, with the smallest orifice typically governing the most frequent event.
9.4A multi-stage riser shall provide a separate control feature for each recurrence interval the permit requires to be matched.
NOTE The outlet orifice and trash rack shall be cleanable or self-cleaning, with a maintenance protocol stated in the O&M manual. (9.5)
NOTE An undersized or non-cleanable trash rack clogs with leaf litter and debris, causing premature ponding and bypass. Specifying a self-cleaning or readily cleanable arrangement prevents the most common chronic maintenance failure. (9.6)
9.7Trash rack clear bar spacing shall not exceed 2 in to limit debris passage while remaining cleanable.
9.8An anti-vortex baffle shall be provided on barrel outlets 24 in in diameter and larger.
○ Single-stage orifice plate
● Multi-stage riser (orifice + weir)
○ Inline flow restrictor
○ Smart / adaptive automated gate
● Precast concrete (ASTM C478 manhole sections)
○ HDPE manhole / riser
○ Cast-in-place concrete
10 Maintenance Access
NOTE The system shall be configured so that every storage row and the outlet structure can be inspected and cleaned with conventional jet-vacuum equipment. (10.1)
NOTE Chamber rows longer than the practical jetting reach without an access point cannot be maintained, and contractors routinely raise an RFI when a long row has no cleanout. Designing access in from the start avoids both the RFI and the future maintenance failure. (10.2)
10.3Inspection and cleanout manholes shall be spaced no more than 50 ft on center along chamber rows.
10.4Access manholes shall provide a clear opening of at least 24 in and shall comply with ASTM C478 where precast.
10.5An isolator or inspection row with an accessible cleanout manifold shall be provided to concentrate sediment for periodic removal where the manufacturer's system offers one.
● Provided (concentrated sediment capture)
○ Not provided
11 Coordination
NOTE The system inlet and outlet inverts shall match the connecting storm-drain network inverts shown on the approved drawings. (11.1)
NOTE Mismatched inverts between the detention system and the conveyance network are the most common civil RFI generator on stormwater projects. The detention designer and the
Storm Drainage conveyance designer shall reconcile inlet and outlet inverts before fabrication.
(11.2) NOTE Setbacks from building foundations, property lines, and drinking-water wells shall comply with the local code. Infiltration near a foundation can saturate bearing soils, and infiltration near a well can compromise water quality; the required separations are set by the local ordinance and the health authority. (11.3)
11.4The system shall be located to satisfy code-required setbacks from building foundations, property lines, and drinking-water wells, with locations as shown detention system plan. NOTE Coordinate underdrain and subdrainage interfaces with
Subdrainage where a closed detention system discharges to a perforated underdrain. Where soils do not permit infiltration but a slow drawdown is still desired, an underdrain beneath a lined bed provides a controlled outlet — a hybrid detail that must be coordinated with the subdrainage design.
(11.5) NOTE Coordinate construction-phase protection of the storage bed with the erosion and sediment control plan. (11.6)
NOTE A finished infiltration bed exposed to construction-phase sediment is clogged before it is ever placed in service. The bed shall be protected from sediment until the contributing area is stabilized, per
Erosion And Sediment Control.
(11.7) 11.8The storage bed shall not be placed in service to receive runoff until the contributing drainage area is stabilized.
12 Installation
NOTE Installation shall follow the manufacturer's published instructions for the specific chamber or vault system, which take precedence on bedding, backfill lift, and compaction details. (12.1)
NOTE Thermoplastic chamber performance depends on the embedment doing the structural work; deviating from the manufacturer's embedment and backfill sequence voids the structural basis of the ASTM F2787 design. (12.2)
12.3The subgrade shall be prepared, the bedding placed and compacted, and the system set to line and grade before backfill begins.
12.4Backfill shall be placed in balanced lifts on opposite sides of chambers and vaults to prevent differential loading and lateral displacement.
12.5Construction equipment shall not cross the installed system until the minimum cover for the equipment load is in place.
12.6Open-bottom infiltration beds shall not be compacted at the native soil interface, to preserve infiltration capacity.
● Per manufacturer embedment instructions (thermoplastic chambers)
○ Per ASTM D2321 (Class IA/IB embedment)
○ Per ASTM C857 / C858 (precast bedding)
13 Testing
NOTE The installed system shall be inspected and tested to confirm it performs as designed before acceptance. (13.1)
NOTE Underground systems are unforgiving of latent defects because they are inaccessible after backfill. A post-installation inspection of the rows and outlet, plus verification of the controlling drawdown behavior, catches problems while they are still correctable. (13.2)
13.3Chamber rows and the outlet structure shall be visually inspected, by video where rows are not directly accessible, after installation and before final acceptance.
13.4For an infiltration system, a drawdown or saturated-conductivity verification shall confirm that the design infiltration rate is achieved.
13.5For a closed detention system, the outlet control structure shall be verified to release at the designed rate at each control stage.
☑ Visual / video inspection of rows and OCS
☐ Infiltration drawdown verification
☑ Outlet release-rate verification
☐ Liner integrity verification (closed systems)
14 Delivery, Storage, and Handling
NOTE Chambers, vaults, and outlet structures shall be handled with equipment and procedures that prevent cracking, deformation, and ultraviolet degradation. (14.1)
NOTE Thermoplastic chambers are light but vulnerable to point loads and prolonged sun exposure; precast units are heavy and crack if lifted improperly. Protecting both classes of product before installation preserves the structural and durability basis of the design. (14.2)
14.3Thermoplastic chambers shall be stored out of prolonged direct sunlight and protected from ultraviolet exposure exceeding the manufacturer's allowance.
14.4Precast units shall be lifted only at the designated lifting points and shall not be rolled or dropped.
14.5Geotextile and liner rolls shall be stored off the ground, covered, and protected from ultraviolet exposure and physical damage until installed.
15 Warranty
15.1The manufacturer shall warrant chambers, vaults, and outlet structures against defects in materials and manufacture for the period stated below from the date of substantial completion.
15.2The installing contractor shall warrant the installation, including bedding, backfill, and connections, against settlement and defective workmanship for the same period.
○ 1 year
● 2 years
○ 5 years
16 Spare Parts
NOTE The Contractor shall furnish spare components needed for routine maintenance so that the system can be serviced without procurement delay. (16.1)
NOTE The serviceable, consumable parts of a stormwater system are the outlet trash rack, the orifice inserts, and the filter media; keeping spares on hand keeps a clogged or damaged outlet from becoming a compliance failure. (16.2)
16.2.1The Contractor shall furnish the following spare parts and maintenance items:
- One spare trash rack matching the installed outlet.
- One set of replacement orifice inserts for each adjustable orifice.
- One replacement filter cartridge or media charge for each filter device, where used.
☑ Spare trash rack
☑ Replacement orifice inserts
☐ Replacement filter media / cartridge