Overview of Small-Scale Green Infrastructure
Small-scale GI practices are the workhorse of site-level stormwater compliance in New Jersey. They are the primary tools through which individual development and redevelopment projects meet the GI Requirement under N.J.A.C. 7:8 — demonstrating that a volume of runoff equal to the Water Quality Volume (WQV) is reduced through infiltration, evapotranspiration, or beneficial reuse rather than merely treated and discharged.
In the 2026 edition, Chapter 9 is reorganized to make the GI/Non-GI classification explicit for every listed BMP type. Certain practices present in the 2023 list — particularly Manufactured Treatment Devices and lined sand filters — are moved to Chapter 11 (Non-GI), reflecting the regulatory distinction codified in the 2026 N.J.A.C. 7:8 GI Requirement framework.
| BMP Type | Chapter | Primary Mechanism | GI Status (2026) |
|---|---|---|---|
| Cisterns | 9.1 | Beneficial reuse | GI — with reuse demand |
| Dry Wells | 9.2 | Native soil infiltration | GI |
| Grass Swales | 9.3 | Infiltration + conveyance | GI — Ch. 12 required |
| Green Roofs | 9.4 | Evapotranspiration | GI — ET credit method |
| Pervious Paving | 9.6 | Infiltration through surface | GI — Ch. 12 required |
| Small-Scale Bioretention | 9.7 | Infiltration + ET | GI — no impermeable liner |
| Small-Scale Infiltration Basins | 9.8 | Infiltration | GI |
| Vegetative Filter Strips | 9.10 | Sheet flow + limited infiltration | GI — limited VRC |
| Small-Scale Sand Filters (lined) | 9.9 | Filtration only | Non-GI if lined |
| Manufactured Treatment Devices | → Ch. 11 | Hydrodynamic / media | Non-GI (moved Ch. 11) |
Design Characteristics of Small-Scale GI BMPs
Each Chapter 9 GI BMP has distinct design requirements that govern how it generates Volumetric Reduction Credit. The following cards summarize key design parameters, siting requirements, and GI classification criteria for each BMP type.
Small-Scale Bioretention Systems
Shallow ponding area over an engineered filter media bed, planted with native vegetation. Stormwater ponds temporarily and infiltrates into native soil below or drains through an underdrain.
Pervious Paving Systems
Pervious concrete, porous asphalt, or permeable interlocking concrete pavers allow stormwater to pass through into a stone reservoir base, from which it infiltrates into native soil.
Green Roofs
Waterproofing membrane, drainage layer, lightweight growing medium (3–6 in), and vegetation on a rooftop. VRC is credited through evapotranspiration, formalized in 2026 Chapter 14.
Dry Wells
Subsurface stone-filled trenches or preformed chambers that receive concentrated inflow from roof drains and disperse it below grade into surrounding native soil.
Cisterns
Above- or below-ground tanks collecting roof runoff for beneficial reuse in irrigation, toilet flushing, or industrial processes. GI status requires demonstrated regular drawdown.
Grass Swales
Vegetated open channels that convey stormwater while providing water quality treatment through filtering, settling, and partial infiltration into underlying soil.
Vegetative Filter Strips
Uniformly graded, densely vegetated areas receiving sheet flow from adjacent impervious surfaces. Primary role is water quality treatment (TSS removal); limited volumetric reduction credit.
Small-Scale Infiltration Basins
Dry retention basins without permanent pools or outlets, designed to capture the WQV and infiltrate entirely into native soils within a 72-hour drawdown period.
GI Performance and Stormwater Treatment
VRC performance determines what fraction of the WQV is credited toward meeting the GI Requirement. Systems achieving higher VRC reduce the volume of runoff that must be managed through supplemental Non-GI treatment.
| BMP Type | Typical VRC | Primary Credit Mechanism | TSS Removal |
|---|---|---|---|
| Small-Scale Bioretention (no liner) | 80–100% | Infiltration into native soil | ≥80% |
| Pervious Pavement (GI-qualifying) | 75–100% | Infiltration through stone reservoir | ≥80% |
| Dry Well | 80–100% | Infiltration into native soil | ≥80% |
| Small-Scale Infiltration Basin | 80–100% | Infiltration into native soil | ≥80% |
| Green Roof (extensive) | 40–60% | Evapotranspiration from growing medium | N/A (detention) |
| Cistern (documented reuse) | 50–90% | Rainwater harvesting + demand drawdown | N/A (storage) |
| Grass Swale | 20–50% | Partial infiltration along channel bottom | 50–70% |
| Vegetative Filter Strip | 10–25% | Limited sheet flow infiltration | Up to 60% |
Key Updates: 2023 → 2026 BMP Manual
- Manufactured Treatment Devices (MTDs) removed from Ch. 9 and moved to Ch. 11 (Non-GI)
- Lined sand filters removed from Ch. 9 and reclassified as Non-GI in Ch. 11
- Ch. 9 is now exclusively GI-classified practices in the 2026 edition
- Bioretention split into small-scale (Ch. 9.7, ≤1 ac.) and large-scale (Ch. 10.1, >1 ac.)
| BMP / Topic | 2023 | 2026 | Change Type |
|---|---|---|---|
| Sand filters (lined) in Ch. 9 | Listed in Ch. 9 | Removed to Ch. 11 (Non-GI) | Reclassification |
| MTDs in Ch. 9 | Listed in Ch. 9.5 | Moved to Ch. 11 (Non-GI) | Reclassification |
| Bioretention size split | Single chapter all sizes | Ch. 9.7 (small) vs. Ch. 10.1 (large) | New Split |
| Pervious pavement GI criteria | General Ksat reference | Explicit Ch. 12 investigation + Ch. 13 trigger | Specification Added |
| Cistern reuse documentation | Not required | Required — demand calculations in SWM Report | New Requirement |
| Green roof ET credit | General performance language | Default ET credit table per roof type (Ch. 14) | Formalized Method |
Practical Implications for Designers and Reviewers
The 2026 reorganization makes GI/Non-GI classification a prerequisite step before sizing. Before applying any sizing table or design formula, the engineer must confirm whether the proposed BMP type is GI-classified in Chapter 9 or 10, whether site conditions support the required soil infiltration performance (Chapter 12 investigation), and whether the BMP footprint or tributary area triggers groundwater mounding analysis (Chapter 13).
2026-Compliant SWM Submission Checklist — Chapter 9 GI BMPs
- Chapter 12 soil investigation report (LPSS-certified SHWT determinations for all infiltrating systems)
- Chapter 13 groundwater mounding analysis if footprint > 3,000 ft² or conditions trigger analysis
- Chapter 14 VRC compliance table — GI/Non-GI classification documented for each BMP
- Cistern reuse demand calculations (if cisterns used for GI credit)
- Green roof ET credit calculation per 2026 Chapter 14 default table or site-specific method
- Vegetation species list per 2026 Chapter 7 zone-specific planting requirements
- Two-year establishment monitoring plan per 2026 Chapter 7
- Long-term O&M agreement referencing 2026 Chapter 8 maintenance standards
- Design drawings explicitly showing absence of impermeable liner in any GI-classified bioretention
- Bioretention tributary area ratio >25:1 (ponding capacity exceeded in the WQV event)
- Pervious pavement installed over HSG C/D soils without Ch. 12 Ksat confirmation
- Green roof VRC credited without ET calculation supporting the volume
- Cisterns credited for VRC without demand-side documentation showing regular drawdown
- Bioretention labeled as GI without explicit note on drawings prohibiting impermeable liner