# Phase 2C — Large-Scale Green Infrastructure (GI) Systems **Source Materials:** NJ Stormwater BMP Manual, Chapter 10 (sub-sections 1–5) — 2023 and 2026 Editions **Date:** March 5, 2026 | OPAL Stormwater Engineering Knowledge System --- ## Overview Large-scale Green Infrastructure systems represent the top tier of stormwater BMP design complexity in New Jersey. Where 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 regulatory, geotechnical, and design scrutiny 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 distinct sub-section, formally separating it from the small-scale guidance of Chapter 9. --- ## Section 1: Overview of Large-Scale Green Infrastructure ### 1.1 Purpose and Role in Stormwater Management Large-scale GI systems serve stormwater management objectives at the site, multi-parcel, or subwatershed scale. Their functional role extends beyond the volumetric reduction mandate of the GI Requirement to include: - **Peak rate attenuation:** Reducing the rate of stormwater discharge during design storm events (2-year and 100-year frequency storms per N.J.A.C. 7:8) by providing significant storage volume - **Water quality treatment:** Providing TSS removal (≥80% by mass), phosphorus and nitrogen reduction through settling, filtration, and biological treatment - **Aquifer recharge:** Where site conditions support native infiltration, large infiltrating systems contribute meaningfully to groundwater recharge and baseflow restoration in receiving streams - **Runoff temperature attenuation:** Wet ponds and constructed wetlands that detain runoff for extended periods allow thermal equilibration, reducing thermal pollution of cold-water fisheries ### 1.2 BMP Types in Chapter 10 (2026) | BMP Type | Chapter | Primary Mechanism | GI Status (2026) | |---|---|---|---| | Large-Scale Bioretention | 10.1 | Infiltration + ET | GI — without impermeable liner | | Infiltration Basins | 10.2 | Infiltration | GI | | Sand Filters (GI) | 10.3 | Filtration + partial infiltration | GI — specific configuration | | Standard Constructed Wetlands | 10.4 | Extended detention + biological treatment | Context-dependent — see §1.3 | | Wet Ponds | 10.5 | Extended detention + settling | Non-GI in most configurations | ### 1.3 GI Classification Context for Chapter 10 Systems The 2026 edition brings important classification nuance to the Chapter 10 BMP types. 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 constructed over a clay liner or compacted sub-base is classified as Non-GI. Wet ponds, in most configurations, rely on permanent pool retention over a compacted or low-permeability base and are classified as Non-GI in the 2026 framework for purposes of volumetric reduction credit. The 2026 manual clarifies both cases explicitly — eliminating the gray area that existed in 2023. --- ## Section 2: Design Characteristics of Large-Scale GI Systems ### 2.1 Large-Scale Bioretention Systems (Chapter 10.1) Large-scale bioretention serves drainage areas exceeding one acre of impervious surface. The fundamental design concept is identical to small-scale bioretention (Chapter 9.7) — vegetated filter media bed with ponding area — but the scale requires additional design rigor: **Key design features:** - **Forebay:** Required for all large-scale bioretention. Volume ≥10% of the total WQV; designed as a settling chamber to intercept coarse sediment before it reaches the filter media zone. Forebay cleanout access must be designed for maintenance equipment entry. - **Filter media depth:** 24–36 inches (increased from 18–24 inches in small-scale systems) to provide greater pollutant contact time and storage volume - **Ponding area and side slopes:** Maximum side slope 3:1 (H:V); 4:1 preferred for vegetation establishment stability - **Overflow/bypass:** Primary overflow via an internal water level riser or outlet structure with emergency overflow spillway independent of the primary outlet system - **Underdrain:** On sites where native Ksat does not support 72-hour drawdown without underdrain, a perforated underdrain may be used — but converts the system from GI to Non-GI unless a verified partial infiltration amendment is documented in the design **Chapter 12 and 13 requirements:** Mandatory for all large-scale bioretention per the 2026 cross-reference in Chapter 6. LPSS certification of SHWT required. Mounding analysis required for footprints exceeding 3,000 ft² or when SHWT < 4 feet below proposed bottom-of-BMP. ### 2.2 Infiltration Basins (Chapter 10.2) Infiltration basins are dry retention basins with no permanent pool, designed to capture a specified volume of runoff and infiltrate it entirely into native soils within a 72-hour drawdown period. They are classified as GI in both editions when native infiltration is demonstrated per Chapter 12. **Key design features:** - **Basin bottom:** Level or gently sloped (<2%); no outlet structure below the overflow elevation — all outflow is via native soil infiltration during the design storm - **Emergency overflow:** Spillway or bypass to convey flows exceeding the basin design capacity safely - **Pretreatment:** Required — equivalent to one forebay volume of 10% WQV minimum, or equivalent pretreatment device upstream - **Sizing:** Basin bottom area sized so that WQV divided by measured (safety-factored) Ksat × drawdown time ≤ available basin ponding depth - **Exclusion zones:** Setback ≥25 feet from potable water wells, ≥50 feet from septic drainfields, ≥10 feet from any structure with a basement **2026 refinement:** The 2026 edition provides explicit minimum exclusion distances from NJDEP Protected Water Supply Sources, which were not included in the 2023 edition. This change reflects NJDEP Sole Source Aquifer regulations and adds a site-screening step to the early feasibility assessment. ### 2.3 Standard Constructed Wetlands (Chapter 10.4) Constructed stormwater wetlands are shallow basin systems with permanent shallow water and extensive emergent wetland vegetation. They function as biological treatment systems — runoff enters, slows as velocity decreases across the shallow wetland plain, and suspended solids settle while nutrients are taken up by wetland plants and cycled through the wetland food web. **Key design features:** - Permanent pool volume ≥ WQV (ensures the WQV storm is captured before displacement of the existing pool) - Forebay: ≥10% WQV; deep zone (≥3 feet) for sediment trapping - Shallow marsh zone: ≥70% of permanent pool surface area at ≤18 inches depth — target depth for emergent vegetation establishment - Inflow/outflow separation: minimum L:W ratio of 1.5:1 to prevent short-circuiting; riser inlet and skimmer outlet to force flow through the emergent zone - Emergency spillway: designed for the 100-year design storm - Anti-seep collar or clay liner required where the underlying soils are permeable — prevents short-circuiting through subsurface flow **GI status:** Classified as GI only when the wetland is constructed over demonstrated infiltrating native soils (confirmed by Chapter 12) and no impermeable liner is used. 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. ### 2.4 Wet Ponds (Chapter 10.5) Wet ponds maintain a permanent pool of water between storm events. They are among the most widely used stormwater BMPs in New Jersey for large sites and regional stormwater management facilities. The permanent pool extends mean hydraulic residence time, allowing settling of fine particles that would not settle during a short-duration runoff event alone. **Key design features:** - Permanent pool volume: sized to provide extended retention — typically equal to 2.5× the WQV, providing mean hydraulic residence time exceeding 21 days for the median inter-storm period - Active storage volume: WQV stored above the permanent pool elevation and released over ≥24 hours through an outlet riser or skimmer - Forebay with cleanout access - Emergency overflow spillway - Safety bench: ≥10 feet wide at the normal pool elevation to prevent direct access to water from adjacent slopes - Aquatic bench at permanent pool perimeter: ≤18 inches deep to support emergent vegetation and discourage geese **GI status (2026):** Wet ponds are classified as Non-GI in the 2026 edition in typical configurations because the permanent pool is underlain by a clay liner, compacted soil, or naturally very low-permeability material that prevents infiltration. They satisfy the Water Quality Standard (≥80% TSS removal) and the Quantity Standard (peak rate attenuation) but do not generate VRC toward the GI Requirement. --- ## Section 3: Hydrologic and Treatment Performance ### 3.1 Peak Rate Attenuation Large-scale Chapter 10 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 bioretention: typically achieve full attenuation of the WQV storm and partial attenuation of larger events, depending on the total storage volume above the overflow - Wet ponds: designed to attenuate 2-year and 100-year design storms through large active storage volume and controlled outlet sizing - Constructed wetlands: primarily designed for water quality; peak attenuation secondary ### 3.2 TSS Removal All Chapter 10 systems are designed to achieve ≥80% TSS removal by mass from the WQV storm event, per N.J.A.C. 7:8 requirements. Performance mechanisms differ: | BMP | Primary TSS Mechanism | Typical TSS Removal | |---|---|---| | Large-Scale Bioretention | Filtration through engineered media | ≥80% | | Infiltration Basin | Filtration through native soil | ≥80% | | Constructed Wetland | Settling + vegetative filtration | ≥80% at design HRT | | Wet Pond | Settling in permanent pool + active storage | ≥80% at design HRT | ### 3.3 Nutrient and Metal Removal Large-scale bioretention and constructed wetlands provide the highest nutrient removal performance among Chapter 10 systems: - Total phosphorus: 50–70% removal (bioretention with iron-amended media approaches 70–85%) - Total nitrogen: 30–50% removal (constructed wetlands with nitrification/denitrification zones achieve higher rates) - Metals (Pb, Zn, Cu): ≥80% removal in bioretention through sorption to organic matter in filter media - Wet ponds: moderate nutrient removal (25–40% TP; 20–30% TN) — dependent on permanent pool residence time and internal nutrient cycling --- ## Section 4: Key Updates Between the 2023 and 2026 Manuals ### 4.1 Bioretention Chapter Split The 2026 manual separates bioretention across two chapters: - **Small-Scale (Ch. 9.7):** Drainage areas ≤1 acre impervious; less detailed geotechnical analysis requirements - **Large-Scale (Ch. 10.1):** Drainage areas >1 acre; mandatory Ch. 12 LPSS investigation, Ch. 13 mounding analysis, mandatory forebay, dedicated maintenance access requirements In the 2023 manual, all bioretention guidance was consolidated in a single section. The split eliminates ambiguity about which analysis and design requirements apply based on BMP scale. ### 4.2 Wet Ponds: Explicit Non-GI Classification The 2026 edition adds explicit non-GI classification language to Chapter 10.5 (Wet Ponds). The 2023 edition described wet pond performance without explicitly categorizing the BMP as Non-GI. The 2026 language directly states that wet ponds, in standard lined or low-permeability base configurations, do not qualify for VRC credit and cannot be used to meet the GI Requirement volumetric reduction standard. ### 4.3 Constructed Wetland GI Eligibility Clarified Both editions allow constructed wetlands over infiltrating native soils to count toward GI requirements. The 2026 edition adds a specific documentation requirement: when GI credit is claimed for a constructed wetland, the SWM Report must include a Chapter 12 soil investigation demonstrating native infiltration capacity under the wetland footprint, confirmation that no impermeable liner is installed, and a Chapter 13 mounding analysis (required given the large water surface area involved). ### 4.4 Infiltration Basin Exclusion Distances (New 2026) The 2026 Chapter 10.2 adds explicit minimum setback distances from NJDEP-designated protected water supply sources — a requirement absent from the 2023 edition. Additionally, the 2026 edition requires a water balance analysis for infiltration basin sizing on sites where the native Ksat is at or near the minimum qualifying threshold, confirming 72-hour drawdown under wet antecedent conditions. ### 4.5 Summary of Changes | Topic | 2023 | 2026 | Impact | |---|---|---|---| | Bioretention categorization | Single section all scales | Ch. 9.7 (small) / Ch. 10.1 (large) | Clearer analysis requirements by scale | | Wet pond GI status | Not explicitly classified | Explicitly Non-GI in standard configs | VRC not available for typical wet ponds | | Constructed wetland GI eligibility | Described generally | Requires Ch. 12 + Ch. 13 documentation | Documentation obligation added | | Infiltration basin setbacks | General guidance | Explicit distances from protected water supplies | New exclusion screening step | | Infiltration basin drawdown verification | Not required under marginal Ksat | Water balance required when Ksat near threshold | Added analysis requirement | --- ## Section 5: Practical Implications for Stormwater Design ### 5.1 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 1-acre impervious drainage area threshold is a design decision point — a system just above the threshold must comply with the full Chapter 10.1 large-scale requirements, including mandatory LPSS investigation. ### 5.2 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. This is a significant design implication for projects where site constraints limit infiltration opportunities — 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. ### 5.3 Documentation for Large-Scale GI Submissions A complete 2026-compliant SWM Report for a project using Chapter 10 GI BMPs must include: - LPSS-certified Chapter 12 investigation report for all infiltrating GI BMPs with drainage areas > 1 acre - Chapter 13 mounding analysis for all bioretention cells, infiltration basins, and constructed wetlands meeting analysis triggers - Chapter 14 VRC compliance table documenting GI BMP-by-BMP credit - Post-construction as-built survey establishing baseline sediment depth in forebays - Long-term operation and maintenance (O&M) agreement referencing the 2026 Chapter 8 maintenance standards (see Phase 2A) - Vegetation establishment plan per 2026 Chapter 7 (for all vegetated systems)