Phase 2B — Small-Scale Green Infrastructure (GI) BMPs¶
Source Materials: NJ Stormwater BMP Manual, Chapter 9 (all sub-sections) — 2023 and 2026 Editions; GI Requirement Fact Sheets Date: March 5, 2026 | OPAL Stormwater Engineering Knowledge System
Overview¶
Small-scale Green Infrastructure BMPs 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. Chapter 9 of the BMP Manual covers ten BMP types ranging from passive infiltration systems to proprietary collection systems.
In the 2026 edition, the Chapter 9 BMP list is reorganized to more clearly align individual BMP types with their GI or Non-GI classification. Certain BMP types present in the 2023 Chapter 9 — particularly Manufactured Treatment Devices and sand filters with impermeable liners — are moved to Chapter 11 (Non-GI) in the 2026 edition, reflecting the regulatory distinction codified in the 2026 N.J.A.C. 7:8 GI Requirement framework.
Section 1: Overview of Small-Scale Green Infrastructure¶
1.1 Purpose of GI in Stormwater Management¶
Small-scale GI practices serve two interrelated functions. First, they reduce the volume of stormwater runoff that leaves a site by capturing rainfall and routing it into soil, vegetation, or storage systems that remove it from the conventional conveyance network. Second, they improve stormwater quality by providing physical filtration, biological uptake, and chemical sorption of common pollutants — notably total suspended solids (TSS), total nitrogen, total phosphorus, and metals — before the runoff enters receiving waters.
Both functions depend on the practice being correctly sized, correctly sited, and maintained to preserve its functional capacity. Small-scale GI is distinguished from large-scale practices (Chapter 10) primarily by its distributed placement — integrated into the site layout at the parcel or drainage area level — and by the smaller impervious drainage area each individual practice serves.
1.2 BMP Types in Chapter 9¶
The 2026 Chapter 9 includes the following GI BMP types, each addressed in a dedicated sub-section:
| BMP Type | Primary Volume Reduction Mechanism | GI Status (2026) |
|---|---|---|
| Cisterns (9.1) | Beneficial reuse (rainwater harvesting) | GI — with demonstrated reuse demand |
| Dry Wells (9.2) | Infiltration into native soil | GI |
| Grass Swales (9.3) | Infiltration + conveyance | GI — when meeting Chapter 12 criteria |
| Green Roofs (9.4) | Evapotranspiration + detention | GI — ET credit method (Ch. 14) |
| Pervious Paving Systems (9.6) | Infiltration through pavement surface | GI — when meeting Chapter 12 criteria |
| Small-Scale Bioretention (9.7) | Infiltration + ET | GI — without impermeable liner |
| Small-Scale Infiltration Basins (9.8) | Infiltration | GI |
| Vegetative Filter Strips (9.10) | Sheet flow filtration + infiltration | GI — limited WQV credit |
| Small-Scale Sand Filters (9.9) | Filtration (no native infiltration) | Non-GI in 2026 if lined |
| Manufactured Treatment Devices (9.5) | Hydrodynamic separation / media filtration | Non-GI (moved to Ch. 11 in 2026) |
Section 2: Design Characteristics of Small-Scale GI BMPs¶
2.1 Small-Scale Bioretention Systems (Chapter 9.7)¶
Small-scale bioretention is the most widely used single-parcel GI BMP in New Jersey. It consists of a shallow ponding area over an engineered filter media bed — typically 18 to 30 inches deep — planted with native vegetation. Stormwater ponds temporarily in the ponding area and infiltrates through the media into native soil below, or drains through a perforated underdrain to the storm sewer system.
Design features: - Minimum ponding depth: 6 inches above filter media surface - Maximum ponding depth: 12 inches (to protect plant root zones from extended saturation) - Filter media: typically 60% sand / 20% topsoil / 20% compost by volume (NJDEP-specified gradation) - Surface area: sized so that inflow from the 1.25-inch WQV storm ponds to no more than 12 inches depth; typical ratio of tributary impervious area to bioretention surface area is 10:1 to 20:1 - Ponding drawdown: 72 hours or less (24–48 hours preferred) - Pretreatment: grass buffer strip or stone diaphragm at inflow to settle coarse sediment
GI classification (2026): Bioretention without an impermeable liner — native soil infiltration required. Systems with impermeable liners and underdrains but no native infiltration are classified as Non-GI filtration systems in the 2026 edition and do not generate Volumetric Reduction Credit (VRC) toward the GI Requirement.
2.2 Pervious Paving Systems (Chapter 9.6)¶
Pervious pavement (pervious concrete, porous asphalt, permeable interlocking concrete pavers) allows stormwater to pass through the pavement surface into a stone reservoir base, from which it infiltrates into native soil or drains through an underdrain. The stone reservoir provides temporary storage and distributes infiltrated volume over the base area.
Design features: - Stone reservoir: clean NJDEP-specified open-graded stone, depth sized to store WQV from contributing tributary area plus pavement surface area - Native soil Ksat ≥ 0.52 in/hr required for GI credit without underdrain; Chapter 12 investigation required - Minimum 2-foot SHWT separation (Chapter 12) or 3-foot separation for areas serving vehicle traffic (to provide pollutant attenuation before groundwater contact) - Surface maintenance: vacuum sweeping ≥2× per year to prevent surface clogging
GI classification (2026): GI when native soil infiltration capacity is demonstrated per Chapter 12. Non-GI if underlain by an impermeable liner and underdrain only.
2.3 Green Roofs (Chapter 9.4)¶
Green roofs consist of a waterproofing membrane, drainage layer, filter fabric, lightweight growing medium (typically 3–6 inches deep), and vegetation installed on a roof surface. They provide volumetric reduction through evapotranspiration and temporary storage in the growing medium substrate.
Design features: - Extensive type (2–6 in growing medium): lower ET demand and load; primarily succulents (Sedum spp.) - Intensive type (6+ in growing medium): higher water retention, greater plant diversity, structural load requirements - GI VRC credit in 2026: based on ET credit factor applied to roof area served (see Chapter 14 ET method) - Minimum slope: typically flat to 2:12; steeper roofs require drainage layer design to prevent saturated media migration
Site placement: Applicable to commercial, residential, and institutional rooftops over occupied structures with adequate structural capacity. Not suitable as a standalone WQV compliance strategy for large impervious areas due to low surface area ratio.
2.4 Dry Wells (Chapter 9.2)¶
Dry wells are subsurface infiltration structures — typically stone-filled trenches or preformed plastic chambers — that receive concentrated stormwater inflow from roof drains or small impervious areas and disperse it below grade into the surrounding native soil.
Design features: - Receiving area: rooftop drainage from individual residential or small commercial downspouts - Minimum 10-foot setback from foundation walls and septic systems - Chapter 12 Ksat and SHWT investigation required at the proposed bottom-of-dry-well elevation - Not suitable for areas with Ksat < 0.52 in/hr or SHWT within 2 feet of dry well bottom
2.5 Cisterns (Chapter 9.1)¶
Cisterns are above-ground or below-ground storage tanks that collect and store roof runoff for beneficial reuse — irrigation, toilet flushing, or industrial process water. GI classification requires that stored volume is beneficially reused on a schedule that empties the tank before subsequent storm events.
Design features: - Storage volume: sized so that average demand cycle empties the cistern between storms generating ≥0.5 inches - Overflow routed to vegetated or infiltrating surface — not directly to storm sewer - Pump system or gravity-feed to end use; first-flush diverter recommended to exclude initial roof runoff quality of lesser quality
GI classification: 2026 Chapter 14 requires documented demand-side calculations demonstrating the cistern will be regularly emptied. Systems that remain near-full year-round do not receive full VRC credit.
2.6 Grass Swales (Chapter 9.3)¶
Grass swales are vegetated open channels that convey stormwater while simultaneously providing water quality treatment through filtering, settling, and infiltration. Unlike pipes, they expose runoff to vegetated surfaces that reduce velocity, remove coarse sediment, and allow partial infiltration into underlying soils.
Design features: - Maximum flow velocity: 1–2 ft/s during the WQV storm to prevent erosion and maintain infiltration function - Bottom width: 2–8 feet; parabolic or trapezoidal cross-section - Check dams at grade changes to increase contact time and infiltration opportunity - Grass height maintained at 3–6 inches; mow no shorter than 3 inches - GI credit: partial WQV infiltration volume quantified based on bottom infiltration area, measured Ksat, and hydraulic residence time
2.7 Vegetative Filter Strips (Chapter 9.10)¶
Filter strips are uniformly graded, densely vegetated areas that receive sheet flow runoff from adjacent impervious surfaces. They provide filtration and limited infiltration — primarily credited for TSS removal (up to 60%) at the site level rather than volumetric reduction.
2026 GI credit: Filter strips can generate limited VRC based on demonstrated native soil infiltration capacity beneath the strip, but their primary compliance role is water quality treatment, not volumetric reduction. They are commonly used as pretreatment for adjacent bioretention or infiltration systems.
Section 3: GI Performance and Stormwater Treatment Role¶
3.1 Volumetric Reduction Performance¶
The primary performance metric for GI BMPs under the 2026 standard is Volumetric Reduction Credit (VRC) — the fraction of the WQV demonstrated to infiltrate, evapotranspire, or be beneficially reused. Performance benchmarks:
| BMP Type | Typical VRC as % of Tributary WQV | Primary Credit Mechanism |
|---|---|---|
| Small-scale bioretention (native infiltration) | 80–100% | Infiltration into native soil |
| Pervious pavement (GI-qualifying) | 75–100% | Infiltration through stone reservoir |
| Dry well | 80–100% | Infiltration into native soil |
| Green roof (extensive) | 40–60% | ET from growing medium |
| Cistern (with documented reuse) | 50–90% (demand-dependent) | Rainwater harvesting |
| Grass swale | 20–50% | Partial infiltration along channel bottom |
| Vegetative filter strip | 10–25% | Limited infiltration under sheet flow |
3.2 Pollutant Removal Performance¶
All Chapter 9 GI BMPs provide some degree of pollutant removal as a secondary benefit of volumetric reduction. The 2026 manual aligns TSS removal performance claims to systems that achieve at least partial WQV capture:
- Small-scale bioretention: ≥80% TSS removal by mass when sized for WQV; 40–60% total phosphorus (dependent on media composition); 30–50% total nitrogen
- Pervious pavement: ≥80% TSS removal; pollutant contact time in stone reservoir governs water quality performance
- Green roofs: limited pollutant removal function; some phosphorus leaching possible from growing media — both editions note this and require appropriate receiving conditions
- Grass swales: 50–70% TSS removal at design flow; dependent on vegetation density and lack of short-circuiting
Section 4: Key Updates Between the 2023 and 2026 Manuals¶
4.1 Chapter Reorganization: GI vs. Non-GI Separation¶
The most structurally significant Chapter 9 change in the 2026 edition is the explicit removal of Non-GI practices from the chapter. In the 2023 manual, Chapter 9 included both GI and practices that operate without native soil infiltration (sand filters with liners, certain MTD applications). The 2026 edition removes these systems to Chapter 11 (Non-GI BMPs), making Chapter 9 exclusively GI-classified systems.
This change requires designers to determine GI vs. Non-GI classification at the outset of BMP selection — before sizing — and to explicitly document the classification rationale in the SWM Report.
4.2 Small-Scale Bioretention Split from Large-Scale¶
In the 2023 manual, bioretention was addressed in a single Chapter 9.7 entry covering all size ranges. The 2026 manual separates bioretention into: - Small-Scale Bioretention (Ch. 9.7): drainage areas ≤1 acre impervious; typical parcel-scale rain gardens and parking lot islands - Large-Scale Bioretention (Ch. 10.1): drainage areas >1 acre; larger facilities with higher engineering analysis requirements (Chapter 12 and 13 analysis mandatory)
This requires designers to identify applicable sub-chapter before proceeding with design, as sizing tables and analysis requirements differ between the two.
4.3 Pervious Pavement: SHWT and Soil Investigation Now Explicit¶
The 2023 Chapter 9.6 described pervious pavement soil requirements generally. The 2026 edition explicitly requires: - Chapter 12 soil investigation and Ksat testing at the proposed stone reservoir base elevation for all GI-qualifying pervious pavement installations - Chapter 13 groundwater mounding analysis when the stone reservoir footprint exceeds 3,000 ft² or conditions trigger the analysis requirement
4.4 Cistern Reuse Demand Documentation (New 2026 Requirement)¶
The 2026 manual adds an explicit requirement for cistern GI compliance: the SWM Report must include demand-side calculations demonstrating that the cistern will be reliably emptied before the next WQV storm event. This prevents the common design approach of sizing a cistern for storage volume without verifying that actual water consumption will maintain available capacity.
4.5 Green Roof ET Credit Method Formalized¶
The 2026 manual formalizes the use of ET credit for green roofs under the Chapter 14 VRC framework, providing default ET credit percentages by roof type (extensive, semi-intensive, intensive) to replace the ambiguous performance language of the 2023 edition. Designers may use site-specific ET calculations with meteorological data in lieu of default values.
4.6 Summary of Changes¶
| BMP / Topic | 2023 | 2026 | Change Type |
|---|---|---|---|
| Sand filters (lined) in Ch. 9 | Present — listed in Ch. 9 | Removed to Ch. 11 (Non-GI) | Reclassification |
| MTDs in Ch. 9 | Present — 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 |
Section 5: Practical Implications for Designers and Reviewers¶
5.1 GI BMP Selection Must Precede Sizing¶
The 2026 reorganization makes GI/Non-GI classification a prerequisite step. Before applying any sizing table or design formula, the engineer must confirm: 1. Whether the proposed BMP type is GI-classified in 2026 (Chapter 9 or 10) or Non-GI (Chapter 11) 2. Whether site conditions support the required soil infiltration performance (Chapter 12 investigation for all infiltrating systems) 3. Whether the BMP footprint or tributary area triggers groundwater mounding analysis (Chapter 13)
5.2 Documentation Expectations in SWM Submissions¶
A complete 2026-compliant SWM submission for a project using Chapter 9 GI BMPs must include: - Chapter 12 soil investigation report (with LPSS certification for all SHWT determinations) - Chapter 13 mounding analysis if triggered - Chapter 14 VRC compliance table with GI/Non-GI classification for each BMP - Cistern reuse demand calculations (if applicable) - Vegetation species list aligned to Chapter 7 (2026) zone-specific planting requirements - Two-year establishment monitoring plan per Chapter 7 (2026)
5.3 Reviewer Checklist Items (Common Deficiencies)¶
Both the 2023 and 2026 editions highlight the following common design deficiencies that reviewing authorities evaluate: - Bioretention sized with tributary drainage area ratio below 5:1 (undersized pretreatment) or above 25:1 (exceeds ponding capacity for WQV event) - Pervious pavement installed over HSG C/D soils without Chapter 12 testing demonstrating GI-qualifying Ksat - Green roof VRC credit claimed without ET calculation supporting the credited volume - Cisterns credited for VRC without demand-side documentation demonstrating regular drawdown - Bioretention designated as GI without explicitly calling out the absence of impermeable liner on design drawings