Corpus sync: neutrality remakes, note hygiene, datasheet relocation, transformer-split cross-refs
---
title: Stormwater Management and Detention Systems
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## "Detention" and "retention/infiltration" are distinct engineering functions with different permit pathways. {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. {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.
## The system function shall be identified and recorded before the system type is selected.
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## Action submittals are required before fabrication or installation. {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. {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.
### The Contractor shall submit the following action items for review and approval before fabrication or installation:
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# Environmental and Service Conditions {toc}
−## The system shall be designed for the hydrologic, geotechnical, and loading conditions of the specific site. {note}
+## The system shall be designed for the hydrologic, geotechnical, and loading conditions of the specific site.
## 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. {note}
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### The system shall provide the storage volume required to meet each design-storm criterion imposed by the governing permit, evaluated by an accepted routing method.
−### Where the governing instrument requires matching multiple recurrence intervals, the system shall satisfy the most restrictive controlling event without violating any other. {note}
+### Where the governing instrument requires matching multiple recurrence intervals, the system shall satisfy the most restrictive controlling event without violating any other.
### 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. {note}
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### A 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.
+```datasheet
+label: Minimum separation to seasonal high groundwater
+type: range
+unit: ft
+min: 2.0
+max: 6.0
+step: 0.5
+default: 2.0
+```
+
### 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. {note}
+### The hydraulic function of the stormwater management system shall be specified as detention, infiltration, combined detention and infiltration, or water quality treatment with detention.
+
```datasheet
label: System hydraulic function
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```
−```datasheet
−label: Minimum separation to seasonal high groundwater
−type: range
−unit: ft
−min: 2.0
−max: 6.0
−step: 0.5
−default: 2.0
−```
−
## Structural loading {toc}
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### Systems 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.
−### The finished cover over the system shall fall within the manufacturer's rated minimum and maximum cover for the selected loading class. {note}
+### The finished cover over the system shall fall within the manufacturer's rated minimum and maximum cover for the selected loading class.
−### 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. {note}
−
```datasheet
−label: Traffic loading class
−type: radio
−options:
− - Non-traffic (H-10 equivalent, landscaped)
− - Standard highway HS-20 (32 kip axle)
− - Heavy HS-25 (40 kip axle, dock/roadway)
−default: Standard highway HS-20 (32 kip axle)
−```
−
−```datasheet
label: Minimum cover over chambers
type: range
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```
−# System Type and Configuration {toc}
+### 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. {note}
−## The system type shall be selected to match the identified hydraulic function, the available footprint, the loading class, and the geotechnical conditions. {note}
+### The traffic loading class for the stormwater management system shall be specified based on the surface loading occurring above the system.
−## 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. {note}
+```datasheet
+label: Traffic loading class
+type: radio
+options:
+ - Non-traffic (H-10 equivalent, landscaped)
+ - Standard highway HS-20 (32 kip axle)
+ - Heavy HS-25 (40 kip axle, dock/roadway)
+default: Standard highway HS-20 (32 kip axle)
+```
+# System Type and Configuration {toc}
+
+## The system type shall be selected to match the identified hydraulic function, the available footprint, the loading class, and the geotechnical conditions.
+
```datasheet
label: Primary system type
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```
+## 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. {note}
+
## Modular plastic arch chambers {toc}
### Polypropylene corrugated wall chambers shall comply with ASTM F2418 for materials, dimensions, wall stiffness, marking, and qualification.
−### HDPE and polypropylene corrugated wall chambers shall be structurally designed in accordance with ASTM F2787 for the specified live load and cover range.
−
−### Chambers shall be installed as an open-bottom infiltration bed or a closed (lined) detention bed consistent with the selected hydraulic function. {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. {note}
−
```datasheet
label: Chamber material and wall type
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```
+### HDPE and polypropylene corrugated wall chambers shall be structurally designed in accordance with ASTM F2787 for the specified live load and cover range.
+
+### Chambers shall be installed as an open-bottom infiltration bed or a closed (lined) detention bed consistent with the selected hydraulic function.
+
```datasheet
label: Chamber base treatment
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```
+### 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. {note}
+
## Precast and cast-in-place concrete systems {toc}
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### Cast-in-place reinforced concrete basins and cisterns intended to retain water shall be designed in accordance with ACI 350.
−### Underground concrete structures shall be designed to resist hydrostatic uplift when the system is full and the surrounding soil is saturated. {note}
+### Underground concrete structures shall be designed to resist hydrostatic uplift when the system is full and the surrounding soil is saturated.
−### 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. {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.
```datasheet
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### A surface dry detention basin shall drain completely between storms and shall be graded to prevent standing water and the resulting nuisance and vector conditions.
−### A surface wet retention pond shall maintain a permanent pool and a vegetated littoral shelf for water-quality treatment. {note}
+### A surface wet retention pond shall maintain a permanent pool and a vegetated littoral shelf for water-quality treatment.
### 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. {note}
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```
−## Geotextile shall fully encapsulate the aggregate bed — base, sides, and top — to exclude fines on all faces. {note}
+## Geotextile shall fully encapsulate the aggregate bed — base, sides, and top — to exclude fines on all faces.
## 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. {note}
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```
−## 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. {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.
−## 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. {note}
−
```datasheet
label: Impermeable liner
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```
+## 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. {note}
+
# Water Quality Pretreatment {toc}
−## Sediment-laden runoff shall pass through a pretreatment device before it enters the storage bed. {note}
+## Sediment-laden runoff shall pass through a pretreatment device before it enters the storage bed.
## 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. {note}
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## A multi-stage riser shall provide a separate control feature for each recurrence interval the permit requires to be matched.
−## The outlet orifice and trash rack shall be cleanable or self-cleaning, with a maintenance protocol stated in the O&M manual. {note}
+## The outlet orifice and trash rack shall be cleanable or self-cleaning, with a maintenance protocol stated in the O&M manual.
## 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. {note}
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# Maintenance Access {toc}
−## The system shall be configured so that every storage row and the outlet structure can be inspected and cleaned with conventional jet-vacuum equipment. {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.
## 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. {note}
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# Coordination {toc}
−## The system inlet and outlet inverts shall match the connecting storm-drain network inverts shown on the approved drawings. {note}
+## The system inlet and outlet inverts shall match the connecting storm-drain network inverts shown on the approved drawings.
−## 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 [[sync/storm-drainage]] conveyance designer shall reconcile inlet and outlet inverts before fabrication. {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 [[sync/storm-drainage]] conveyance designer shall reconcile inlet and outlet inverts before fabrication.
## 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. {note}
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## Coordinate construction-phase protection of the storage bed with the erosion and sediment control plan. {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 [[sync/erosion-and-sediment-control]]. {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 [[sync/erosion-and-sediment-control]].
## The storage bed shall not be placed in service to receive runoff until the contributing drainage area is stabilized.
# Installation {toc}
−## 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. {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.
## 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. {note}
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# Testing {toc}
−## The installed system shall be inspected and tested to confirm it performs as designed before acceptance. {note}
+## The installed system shall be inspected and tested to confirm it performs as designed before acceptance.
## 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. {note}
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# Spare Parts {toc}
−## The Contractor shall furnish spare components needed for routine maintenance so that the system can be serviced without procurement delay. {note}
+## The Contractor shall furnish spare components needed for routine maintenance so that the system can be serviced without procurement delay.
## 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. {note}
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- Replacement orifice inserts
```