Small-Scale Bioretention Systems¶
Small-scale bioretention systems are vegetated stormwater BMPs that collect runoff in a shallow soil-bed system for filtration, uptake, detention, and, where the design allows, infiltration into the subsoil. In the live source corpus, the governing source chapter is BMP Manual Chapter 9.7.
Structured citation evidence for this page lives in the sibling claim manifest.
Small-scale bioretention is the Chapter 9 green infrastructure form of bioretention. It is used near runoff sources such as parking areas, landscaped islands, streetscapes, and similar constrained locations where a vegetated treatment feature can be integrated into the site.
The BMP can support more than one stormwater objective:
- water quality treatment through filtration and soil/vegetation processes
- runoff-volume reduction where infiltration is allowed
- groundwater recharge when the system is designed to infiltrate into the subsoil
- runoff-quantity control when the design includes appropriate storage and routing
| Parameter | Source-backed requirement |
|---|---|
| Maximum contributory drainage area | 2.5 acres |
| Maximum runoff depth for the WQDS | 12 inches |
| Maximum design-storm drain time | 72 hours |
| Minimum subsoil design permeability rate | 0.5 inches/hour, tested under Chapter 12 |
| Soil-bed mix | 85 to 95% sand; no more than 15% silt and clay with 2 to 5% clay content; amended with 3 to 7% organics by weight |
The source set also distinguishes between infiltrating and underdrained configurations. Only small-scale bioretention systems designed to infiltrate into the subsoil may be used to meet groundwater recharge requirements. If the system is designed with an underdrain, it cannot be used to satisfy that recharge standard.
The BMP is flexible, but the siting analysis still has to clear the normal infiltration and hydraulic-impact checks:
- verify the soil evidence chain under Chapter 12 before relying on infiltration
- evaluate seasonal high water table separation and hydraulic impacts using the governing NJAC and Chapter 13 framework
- screen for high pollutant loading, contamination conflicts, and other recharge prohibitions
- confirm the drainage area stays within the 2.5-acre small-scale threshold
Where those conditions are not met, the design may need an underdrain or a different BMP.
Routine maintenance still centers on the same performance indicators found in the source corpus:
- confirm the system drains after storm events within the design drain-time window
- keep pretreatment areas, inlets, and overflow structures free of sediment and debris
- protect vegetation cover and avoid compaction of the soil bed
- inspect underdrains and outlet structures where the design includes them
The most important failure patterns in the live source set are not exotic. They are usually design or construction misses:
- drainage area exceeds the Chapter 9.7 small-scale limit
- soil-bed media does not match the specified sand / fines / organics range
- infiltration is credited without a defensible Chapter 12 testing record
- the system fails the drain-time requirement after construction
- an underdrained system is treated as if it still provides recharge credit
Governing source chapters
- BMP Manual Chapter 9.7 for small-scale bioretention design criteria
- BMP Manual Chapter 12 for soil testing criteria
- BMP Manual Chapter 13 for groundwater mounding analysis
Governing NJAC sections
- N.J.A.C. 7:8-5.3 for green infrastructure applicability and drainage-area limits
- N.J.A.C. 7:8-5.4 for groundwater recharge use
- N.J.A.C. 7:8-5.5 and 5.6 where the BMP is used in water quality or quantity demonstrations
Related Pages
Source Mapping¶
The live citations layer resolves this route directly to the following canonical source bundles.
- Route type:
canonical - 2023 source bundle: Chapter 9.7 (
2023_BMP_9_7) - Small-Scale Bioretention Systems - 2026 source bundle: Chapter 9.7 (
2026_BMP_9_7) - Small-Scale Bioretention Systems
Key verified claims already tied to this page manifest: - Small-scale bioretention is capped at 2.5 acres in both eras. - Chapter 9.7 design summary controls the key area, depth, drain-time, and permeability limits. - Recharge credit depends on infiltration design rather than underdrained design. - The soil-bed mix is the 85-95 percent sand and 3-7 percent organics formula, not a 60/20/20 blend.
Crosswalk Snapshot¶
The reconciled design criteria below come from the live crosswalk and page-level claim set, not from the older narrative snapshot.
| Parameter | 2023 source value | 2026 source value | Change |
|---|---|---|---|
| Contributing drainage area | <= 2.5 acres | <= 2.5 acres | unchanged |
| Maximum ponding depth | 12 in | 12 in | unchanged |
| Typical ponding depth | 6 in | 6 in | unchanged |
| Engineered media depth | 18 in minimum (80% TSS removal) to 24 in minimum (90% TSS removal) | 18 in minimum (80% TSS removal) to 24 in minimum (90% TSS removal) | unchanged |
| Filter media composition | 85-95% sand (<=25% fine/very fine); <=15% silt and clay (2-5% clay content); 3-7% organics by weight | 85-95% sand (<=25% fine/very fine); <=15% silt and clay (2-5% clay content); 3-7% organics by weight | unchanged |
| Drawdown time | <= 72 hours | <= 72 hours (24 hrs if pedestrian-traffic area) | unchanged |
| Underdrain required when | Infiltrating design requires subsoil design permeability >= 0.5 in/hr (tested >= 1.0 in/hr); underdrain used when infiltration impractical, recharge credit not required, or subsoil permeability below threshold | Infiltrating design requires subsoil design permeability >= 0.5 in/hr (tested >= 1.0 in/hr); underdrain used when infiltration impractical, recharge credit not required, or subsoil permeability below threshold | unchanged |
| SHWT separation | 2 ft below bottom of media - measured from static SHWT | 2 ft below bottom of media - measured from static SHWT | unchanged |
| K_design calculation | K_design = K_field / 2.0 (factor of safety = 2.0) | K_design = K_field / 2.0 (factor of safety = 2.0) | unchanged |
| Pretreatment required | Required | Required | unchanged |
Source-backed criteria that still control this route: - Small-scale bioretention is capped at 2.5 acres in both eras. - Chapter 9.7 design summary controls the key area, depth, drain-time, and permeability limits. - Recharge credit depends on infiltration design rather than underdrained design. - The soil-bed mix is the 85-95 percent sand and 3-7 percent organics formula, not a 60/20/20 blend. - Bioretention must drain within 72 hours, with a 24-hour target in pedestrian areas.
Source Bundles¶
Use these source bundles when checking the live extracted text or paired OCR evidence:
- 2023 bundle: Chapter 9.7 (
2023_BMP_9_7) - Small-Scale Bioretention Systems - 2026 bundle: Chapter 9.7 (
2026_BMP_9_7) - Small-Scale Bioretention Systems