Large-Scale GI Systems — Chapter 10 | OPAL NJ BMP Manual 2026

Section 1

Overview of Large-Scale Green Infrastructure

Large-scale Green Infrastructure systems represent the top tier of stormwater BMP design complexity in New Jersey. While Chapter 9 small-scale practices operate at the parcel or sub-parcel level, Chapter 10 systems are engineered for larger drainage areas — typically exceeding one acre of impervious surface per facility — and carry correspondingly greater geotechnical, hydraulic, and regulatory analysis requirements.

The 2026 BMP Manual reorganizes Chapter 10 to align with the GI/Non-GI classification framework and introduces the large-scale bioretention category as a formally separate section, explicitly defined as a distinct system from its Chapter 9.7 small-scale counterpart.

BMP Type	Chapter	Primary Mechanism	GI Status (2026)
Large-Scale Bioretention	10.1	Infiltration + ET	GI — no impermeable liner
Infiltration Basins	10.2	Native soil infiltration	GI
Sand Filters (GI config.)	10.3	Filtration + partial infiltration	GI — specific configuration
Standard Constructed Wetlands	10.4	Extended detention + biological	Conditional — Ch. 12 required
Wet Ponds	10.5	Extended detention + settling	Non-GI in standard configs

GI Classification Context for Chapter 10 Systems

The GI status of constructed wetlands and wet ponds depends on whether the system infiltrates water into native soil or retains it over an impermeable base. A constructed wetland over undisturbed native soil with confirmed infiltration capacity may qualify for partial GI credit; the same system over a clay liner is Non-GI. Wet ponds, in most configurations, rely on a low-permeability base and are Non-GI for VRC purposes. The 2026 manual clarifies both cases explicitly — eliminating the gray area that existed in 2023.

Section 2

Design Characteristics

Large-Scale Bioretention Systems

Chapter 10.1 · Drainage Area > 1 Acre Impervious

GI — No Impermeable Liner

Identical design concept to small-scale bioretention — vegetated filter media bed with ponding area — but the scale demands additional engineering rigor. All large-scale bioretention requires a mandatory forebay, greater filter media depth, and Chapter 12 + Chapter 13 analysis.

ForebayRequired — volume ≥ 10% of total WQV; designed for sediment trapping and maintenance access

Media Depth24–36 inches (increased from 18–24 in for small-scale)

Side SlopesMaximum 3:1 (H:V); 4:1 preferred for vegetation stability

Drawdown72-hour maximum; underdrain converts to Non-GI unless partial infiltration documented

Ch. 12Mandatory — LPSS-certified SHWT and Ksat at BMP bottom elevation

Ch. 13Required — mounding analysis for footprints > 3,000 ft² or SHWT < 4 ft below proposed BMP bottom

Infiltration Basins

Chapter 10.2 · Dry Retention — No Permanent Pool

GI

Dry retention basins designed to capture the WQV and infiltrate it entirely into native soils within a 72-hour drawdown period. No permanent pool and no outlet structure below overflow elevation.

Basin BottomLevel or ≤2% slope; all WQV exits via native soil infiltration during design storm

Drawdown72 hours; sized using safety-factored measured Ksat

PretreatmentRequired — forebay ≥ 10% WQV or equivalent upstream pretreatment device

Exclusion Zones≥ 25 ft from potable wells; ≥ 50 ft from septic drainfields; ≥ 10 ft from structures with basements

2026 AdditionExplicit minimum setback from NJDEP Protected Water Supply Sources (new — not in 2023)

Water BalanceRequired (2026) when native Ksat is at or near minimum qualifying threshold

Standard Constructed Wetlands

Chapter 10.4 · Biological Treatment System

GI Conditional — Ch. 12 Required

Shallow basin systems with permanent shallow water and extensive emergent wetland vegetation. Runoff is treated biologically as it slows across the shallow wetland plain. GI status conditional on native soil infiltration under the wetland footprint.

Permanent Pool≥ WQV — ensures capture before displacement of existing pool

Forebay≥ 10% WQV; deep zone ≥ 3 ft for sediment trapping

Shallow Marsh≥ 70% of pool surface at ≤ 18 in depth for emergent vegetation

L:W RatioMinimum 1.5:1 to prevent short-circuiting; riser inlet + skimmer outlet preferred

GI EligibilityGI only if built over confirmed infiltrating native soils with no impermeable liner

Ch. 13Mounding analysis required due to large water surface area

Most constructed wetland installations include a liner or use naturally low-permeability soils — these configurations are Non-GI but still satisfy the Water Quality Standard.

Wet Ponds

Chapter 10.5 · Permanent Pool Retention System

Non-GI — Standard Configuration

Stormwater ponds maintaining a permanent pool of water between storm events. Among the most widely used stormwater BMPs in New Jersey for large sites and regional management facilities. Explicitly classified as Non-GI in the 2026 edition for standard configurations.

Permanent PoolTypically 2.5× WQV — mean HRT > 21 days for fine TSS settling

Active StorageWQV above permanent pool; released over ≥ 24 hours via outlet riser or skimmer

Safety Bench≥ 10 ft wide at normal pool elevation

Aquatic Bench≤ 18 in deep at pool perimeter — supports emergent vegetation, discourages geese

GI StatusNon-GI — typical liner or clay base prevents infiltration; satisfies TSS and Quantity standards only

Section 3

Hydrologic & Treatment Performance

All Chapter 10 systems are designed to achieve the dual regulatory objectives of peak rate attenuation (Quantity Standard) and ≥80% TSS removal by mass (Water Quality Standard). The mechanisms and relative performance vary substantially by BMP type.

BMP	Primary TSS Mechanism	TSS Removal	TP Removal	TN Removal
Large-Scale Bioretention	Filtration through engineered media	≥80%	50–70%	30–50%
Infiltration Basin	Filtration through native soil	≥80%	High	Moderate
Constructed Wetland	Settling + vegetative treatment	≥80% at design HRT	Moderate	High (nitrification zone)
Wet Pond	Settling in permanent pool	≥80% at design HRT	25–40%	20–30%

Large-scale GI systems provide peak flow attenuation as a core design objective alongside water quality treatment. Attenuation performance depends on the ratio of active storage volume to the inflow hydrograph peak — for the regulatory 2-year storm, infiltration basins and large-scale bioretention typically achieve full attenuation of the WQV storm and partial attenuation of larger events.

NJ BMP Manual, Chapter 10 — Performance Context

Section 4

Key Updates: 2023 → 2026

Most Significant Change: Bioretention Chapter Split

2023: All bioretention guidance in a single section (Ch. 9.7)
2026: Small-scale (Ch. 9.7, ≤1 ac.) vs. Large-scale (Ch. 10.1, >1 ac.) — separate chapters, separate requirements
Impact: Designers must identify applicable sub-chapter before proceeding — sizing tables and geotechnical requirements differ by scale

Topic	2023	2026	Change Type
Bioretention categorization	Single section all scales	Ch. 9.7 (small) / Ch. 10.1 (large)	New Split
Wet pond GI status	Not explicitly classified	Explicitly Non-GI in standard configs	Reclassification
Constructed wetland GI eligibility	Described generally	Requires Ch. 12 + Ch. 13 documentation	Documentation Added
Infiltration basin setbacks	General guidance	Explicit distances from protected water supplies	New Requirement
Infiltration basin drawdown (marginal Ksat)	Not required	Water balance required when Ksat near threshold	Analysis Added

Section 5

Practical Implications for Stormwater Design

Scale Determines Chapter and Analysis Requirements

The 2026 manual structure requires the designer to establish drainage area and BMP footprint dimensions early in the design process because these determine which chapter applies, what geotechnical investigations are required, and whether groundwater mounding analysis is triggered. For bioretention particularly, the one-acre impervious drainage area threshold is a key decision point — a system just above the threshold must comply with the full Chapter 10.1 large-scale requirements, including mandatory LPSS investigation and forebay design.

Wet Ponds and the GI Requirement Gap

Because wet ponds are classified as Non-GI in the 2026 edition, a development project that relies primarily on a wet pond for stormwater management must supplement it with GI BMPs sufficient to meet the full WQV volumetric reduction requirement. The wet pond provides water quality and quantity compliance, but does not satisfy the volumetric reduction standard. The design must identify additional Chapter 9 or Chapter 10 GI capacity — this is a significant design implication for constrained sites.

Documentation for Large-Scale GI Submissions

2026-Compliant SWM Submission Checklist — Chapter 10 GI BMPs

LPSS-certified Chapter 12 investigation report for all infiltrating GI BMPs with drainage areas > 1 acre
Chapter 13 mounding analysis for all bioretention, infiltration basins, and constructed wetlands meeting analysis triggers
Chapter 14 VRC compliance table — GI BMP-by-BMP credit documented
Post-construction as-built survey establishing baseline sediment depth in all forebays
Long-term O&M agreement referencing 2026 Chapter 8 maintenance standards
Vegetation establishment plan per 2026 Chapter 7 for all vegetated systems
For constructed wetlands claiming GI status: no-liner confirmation on as-built drawings + Ch. 12 investigation under wetland footprint