# Phase 1B Planning Report
## Development Impacts & Regional Stormwater Management Plans
### NJ Stormwater BMP Manual – 2023 vs. 2026 Comparison

**Prepared for:** OPAL Stormwater Engineering Knowledge System  
**Phase:** Phase 1B – Planning Content  
**Date:** March 4, 2026  
**Source Documents:** NJ Stormwater BMP Manual, Chapters 1–3 (2023 and 2026 editions)

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## Section 1: Impacts of Development on Runoff

### 1.1 Overview of the Hydrologic Cycle and Development Effects

Undeveloped land in its natural state supports a hydrologic cycle characterized by high rates of infiltration, evapotranspiration, groundwater recharge, and relatively low and slow surface runoff. As land is converted to developed uses — commercial, residential, industrial, or transportation — the natural balance of this cycle is disrupted. The primary mechanism of disruption is the introduction of impervious surfaces, which prevent infiltration and redirect precipitation directly to surface drainage systems.

Both the 2023 and 2026 BMP Manual editions (Chapter 1) describe the following chain of consequences from impervious cover:

- **Increased runoff volume:** Because infiltration is eliminated, more of each precipitation event becomes surface runoff. For a given storm, a highly impervious site may generate three to five times more runoff volume than the same site in a forested condition.
- **Increased peak flow rates:** Concentration time (the time for runoff to travel from the most distant point in a watershed to the outlet) decreases dramatically on paved and graded surfaces compared to vegetated land. This produces sharper, higher-magnitude hydrograph peaks for any given storm event.
- **Reduced baseflow:** With less infiltration, groundwater recharge is diminished, reducing the subsurface contribution to streamflow between storm events. This leads to lower baseflows and, in some cases, intermittent streams that were perennial in pre-development conditions.
- **Increased pollutant loads:** Impervious surfaces accumulate pollutants — metals, petroleum hydrocarbons, nutrients, sediments, and pathogens — between storm events. These are mobilized and transported during runoff, creating a "first flush" effect that degrades water quality in receiving waters.
- **Channel erosion and instability:** Higher peak flows and increased runoff frequency subject receiving streams to erosive conditions more often, leading to channel widening, bank erosion, and loss of riparian habitat.
- **Thermal impacts:** Impervious surfaces heat during solar radiation and transfer that heat to stormwater. Runoff temperatures may be significantly elevated above natural conditions, stressing coldwater fish communities and aquatic ecosystems.

### 1.2 Impervious Cover as a Threshold Indicator

Both the 2023 and 2026 manual editions recognize the relationship between impervious cover percentage and watershed degradation, drawing on the established body of research that identifies impervious cover thresholds correlated with measurable ecological decline:

- **Below 10% impervious cover:** Streams generally retain good to excellent ecological health with functioning riffle-pool sequences, diverse macroinvertebrate assemblages, and stable banks.
- **10–25% impervious cover:** Stream quality begins to degrade. Channel instability, reduced macroinvertebrate diversity, and increased nutrient concentrations are commonly observed.
- **Above 25% impervious cover:** Streams typically exhibit significant degradation. Stable channel geomorphology is difficult to maintain, and biological indices indicate impaired conditions.

The practical implication for stormwater management is that minimizing net impervious cover on a per-site and per-watershed basis is as important as managing runoff from what impervious cover is created. This underpins the hierarchy of stormwater management that prioritizes source reduction — minimizing impervious surface creation — before relying on end-of-pipe treatment.

### 1.3 Cumulative and Watershed-Scale Impacts

A critical planning principle reinforced in both editions of the manual is that individual-site compliance does not guarantee downstream water quality outcomes. Even when each project in a watershed complies with stormwater standards, the cumulative effect of multiple developments may degrade the hydraulic and ecological function of receiving waters. This "cumulative impact" concept is the foundation of regional and watershed-scale stormwater planning described in Chapter 3 of the BMP Manual.

### 1.4 Updates Between the 2023 and 2026 Manuals (Chapter 1)

The 2026 edition of Chapter 1 expands the discussion of **thermal impacts** to include a more explicit connection to NJDEP's temperature standards for Category 1 (C1) waters. The 2026 edition notes that stormwater runoff temperature is now formally considered in the context of anti-degradation review for discharges to C1 streams, requiring that new impervious surfaces near thermally sensitive waters be evaluated for thermal loading. This guidance was implicit in the 2023 edition but is more directly stated in 2026.

The 2026 edition also includes updated **runoff coefficient tables** reflecting revised CN (Curve Number) values calibrated to New Jersey soil and land cover data developed through NJDEP's ongoing watershed monitoring program. These updated CNs affect both the volume calculations for GI sizing and the peak flow calculations for flood control design.

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## Section 2: Low Impact Development (LID) Techniques

### 2.1 Definition and Philosophy

Low Impact Development (LID) is a land planning and engineering design approach that manages stormwater at the source by mimicking pre-development hydrology through infiltration, evapotranspiration, and storage. LID differs from conventional "collect-and-convey" stormwater management in that it distributes management across the site rather than concentrating it in a single end-of-pipe facility.

Chapter 2 of the NJ BMP Manual establishes LID as the preferred approach to stormwater management in New Jersey, consistent with the regulatory hierarchy established in N.J.A.C. 7:8. The foundational principle is that the best stormwater management is that which reduces the volume of runoff generated rather than simply treating runoff after it is generated.

### 2.2 Site Planning and Layout

The first and most impactful LID strategy is **site planning and layout optimization**. Both the 2023 and 2026 Chapter 2 editions emphasize that stormwater outcomes are largely determined by design decisions made early in the site planning process, before individual BMP sizing begins. Key site planning LID strategies include:

- **Minimizing impervious footprint:** Evaluating building configuration, parking geometry, and road layout to reduce total impervious area. Shared parking, compact development patterns, and reduced setback requirements all contribute to impervious area reduction.
- **Preserving natural drainage features:** Retaining existing stream corridors, wetlands, depressions, and forest cover reduces the disruption to pre-development hydrology. Existing permeable soils and natural storage areas (e.g., closed depressions) should be identified early and protected from disturbance.
- **Directing runoff to permeable areas:** Grading the site so that rooftop, parking, and roadway runoff drains to pervious landscape areas (bioretention cells, grass swales, rain gardens) rather than directly to storm sewers.
- **Disconnecting impervious surfaces:** Routing runoff from rooftops, driveways, and small parking lots through pervious areas before it reaches the storm drain system reduces both volume and pollutant load.

### 2.3 LID Practice Types

Chapter 2 of both editions catalogs LID techniques and describes their hydrologic functions:

**Bioretention (Rain Gardens):** Engineered shallow depression filled with amended soil media, a mulch layer, and adapted vegetation. Runoff is treated through filtration, biological uptake, and infiltration. Provides volumetric reduction, groundwater recharge, and TSS/nutrient removal. One of the most versatile LID practices for New Jersey conditions.

**Pervious Paving Systems:** Porous asphalt, pervious concrete, or permeable interlocking pavers that allow precipitation to infiltrate through the pavement surface into a storage reservoir. Particularly effective for parking lots and low-traffic surfaces. Requires regular maintenance to prevent clogging of surface voids.

**Green Roofs:** A vegetated roof system with engineered growing media, drainage layer, and waterproofing membrane. Reduces runoff volume through evapotranspiration and temporary storage. Most effective for flat or low-slope commercial roofs. As clarified in the 2026 rules, green roofs qualify as GI for volume reduction credit only when paired with cisterns or reuse systems; evapotranspiration alone does not satisfy recharge requirements.

**Cisterns and Rain Barrels:** Containers that capture rooftop runoff for reuse in irrigation or other non-potable uses. Provide volumetric reduction through runoff harvesting. Effectiveness depends on demand patterns and tank sizing relative to contributing drainage area.

**Grass Swales:** Vegetated open channels that convey runoff while providing treatment through filtration, sedimentation, and biological uptake. May be designed with check dams or level spreaders to enhance infiltration. Effective as a conveyance replacement for curb-and-gutter systems in low-density development contexts.

**Dry Wells:** Underground infiltration chambers that receive rooftop or other clean runoff and allow it to percolate into the surrounding soil. Effective for residential applications with suitable soils. Require setbacks from foundations, wells, and septic systems.

**Small-Scale Infiltration Basins:** Shallow graded depressions designed to temporarily pond runoff and allow it to infiltrate into the soil. Differ from large-scale infiltration basins in drainage area served (typically less than 1 acre per the 2026 split-chapter structure).

### 2.4 LID Site Design Process

Both editions of Chapter 2 describe a sequential site design process for integrating LID:

1. **Delineate and characterize natural resources:** Map soils (HSG classification), wetlands, floodplains, existing drainage features, and vegetated areas that should be protected.
2. **Set site layout to minimize disturbance:** Locate buildings, roads, and parking to avoid sensitive areas; reduce total impervious footprint.
3. **Route runoff to natural or engineered pervious areas:** Use site grading to direct sheet flow from impervious surfaces toward LID practices.
4. **Size LID practices for the Water Quality Design Storm:** Ensure adequate volume reduction and treatment for the 1.25-inch/2-hour design storm.
5. **Verify recharge, water quality, and flood control compliance:** Confirm that the integrated design satisfies all three standards of N.J.A.C. 7:8.

### 2.5 Updates Between the 2023 and 2026 Manuals (Chapter 2)

The 2026 edition of Chapter 2 introduces a more explicit discussion of **volumetric reduction accounting** at the LID design stage. Where the 2023 edition described LID as contributing to volume reduction qualitatively, the 2026 edition connects each LID practice category to the quantitative volumetric reduction methodology in the new Chapter 14 of the BMP Manual. Engineers are instructed to calculate volumetric reduction credit for each proposed LID practice using the methods in Chapter 14 as part of the design documentation process.

The 2026 edition also provides updated guidance on **disconnected impervious areas (DIAs)**, clarifying the criteria under which impervious surfaces that drain over vegetated buffers may be credited as partially managed for the purposes of the GI and volumetric reduction standards. The 2023 edition referenced DIA concepts but did not provide consistent crediting guidance; the 2026 edition standardizes this calculation.

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## Section 3: Regional and Municipal Stormwater Management Planning

### 3.1 Purpose and Framework

Chapter 3 of the NJ BMP Manual addresses stormwater management at scales larger than individual development projects — the municipal and regional watershed scale. This chapter describes the legal and planning framework for Municipal Stormwater Management Plans (MSWMPs) and Regional Stormwater Management Plans (RSWMPs) in New Jersey, explains the process for developing and approving these plans, and describes how approved plans affect the stormwater requirements applied to individual development projects within their boundaries.

The statutory basis for MSWMPs and RSWMPs is found in the Stormwater Management Act (N.J.S.A. 40:55D-93 et seq.) and is implemented through N.J.A.C. 7:8. The fundamental premise is that stormwater management is most efficiently and effectively achieved when planned at the watershed or municipal level, rather than only project-by-project.

### 3.2 Municipal Stormwater Management Plans (MSWMPs)

A Municipal Stormwater Management Plan is prepared by a municipality (sometimes in cooperation with a county or watershed-level planning body) and establishes the framework for how stormwater will be managed within the municipality's jurisdiction. Key elements of an MSWMP include:

- **Watershed characterization:** Description of existing land use, impervious cover, receiving water conditions, and identified stormwater problem areas (flooding, erosion, water quality impairments).
- **Goals and objectives:** Measurable targets for runoff volume reduction, pollutant load reduction, and receiving water quality improvement.
- **BMP selection and siting:** Identification of preferred BMP types for different portions of the municipality based on soil conditions, topography, land use, and proximity to sensitive receiving waters.
- **Alternative compliance mechanisms:** Definition of off-site GI alternatives, payment-in-lieu (PIL) programs, or regional BMP options available to developers when on-site compliance is infeasible.
- **Implementation schedule and responsible parties:** Identification of capital projects, ordinance amendments, and programmatic actions required to achieve plan goals, with assignment of responsible departments or agencies and funding mechanisms.
- **Monitoring and adaptive management:** Description of how plan performance will be tracked and how the plan will be updated as monitoring data become available.

When an MSWMP has been approved by NJDEP, development projects within the municipality may follow plan-specific requirements rather than the default state standards, in cases where the MSWMP provides equal or greater environmental protection.

### 3.3 Regional Stormwater Management Plans (RSWMPs)

RSWMPs are developed for multi-jurisdictional areas, typically coinciding with watershed boundaries that cross municipal lines. They provide a framework for coordinating stormwater management across multiple municipalities, enabling:

- **Regional Green Infrastructure facilities:** Shared bioretention areas, infiltration basins, or wetland restoration projects that serve multiple development projects or municipalities, providing economies of scale.
- **Cumulative impact analysis:** Evaluating how the combined effect of anticipated development within a watershed will affect receiving water quality and hydraulics, allowing proactive mitigation planning.
- **Equity in stormwater burden allocation:** Ensuring that upstream municipalities are not disproportionately exporting stormwater impacts to downstream communities.

NJDEP provides approval and oversight for RSWMPs, with technical review focused on consistency with state water quality standards, the adequacy of the GI and recharge components, and the feasibility of the implementation plan.

### 3.4 Relationship Between Plans and Project-Level Compliance

Both the 2023 and 2026 editions of Chapter 3 describe the hierarchy of compliance pathways:

1. **On-site full compliance** with N.J.A.C. 7:8 requirements (default pathway when no MSWMP/RSWMP alternative is available).
2. **On-site compliance with MSWMP alternative standards** where an approved MSWMP establishes different or additional criteria.
3. **Off-site compliance through a regional or municipal BMP** where an MSWMP/RSWMP authorizes contribution to a regional facility in lieu of on-site management.
4. **Payment-in-lieu contributions** to a municipal or regional stormwater management fund, used to finance public GI retrofits, where authorized by an approved MSWMP and NJDEP.

### 3.5 Stormwater Management Plan Committee (SMPC)

Plans are developed and managed through a Stormwater Management Plan Committee (SMPC) composed of planning board representatives, the municipal engineer, and other relevant stakeholders. Chapter 3 of both editions describes the SMPC's role in plan preparation, public participation requirements, and the NJDEP approval process, including the required public comment period and response procedures.

### 3.6 Updates Between the 2023 and 2026 Manuals (Chapter 3)

The 2026 edition of Chapter 3 contains the most significant updates in the planning chapters. Key changes include:

**MSWMP Update Obligation:** Consistent with the regulatory requirement in the 2026 N.J.A.C. 7:8 amendments, the 2026 Chapter 3 describes the 24-month timeline for existing MSWMPs to be updated to reflect the new volumetric reduction standards and GI/Non-GI BMP classifications. Plans not updated within 24 months revert to direct application of state standards. The 2023 edition did not contain this update obligation.

**Integration of Volumetric Reduction into Plan Goals:** The 2026 edition requires that newly prepared or updated MSWMPs quantify volumetric reduction targets for the municipality or watershed area, expressed as either a percentage reduction in total impervious runoff volume or a specific volume target derived from watershed loading models. This metric was absent from 2023 MSWMP guidance.

**Payment-in-Lieu (PIL) Program Criteria:** The 2026 edition expands guidance on PIL programs, establishing minimum criteria for acceptable PIL programs including: (a) project-specific volumetric reduction calculation demonstrating what cannot be achieved on-site; (b) identification of specific off-site GI projects funded by PIL contributions; (c) a fund administration mechanism ensuring PIL payments are spent within the same watershed assessment unit as the generating project; and (d) annual reporting to NJDEP on fund disbursement and GI project completion status.

**Green Infrastructure Priority Areas:** The 2026 Chapter 3 introduces the concept of **GI Priority Areas** within an MSWMP — designated zones where NJDEP and the municipality identify high-value opportunities for GI retrofits based on soil characterization, proximity to water quality-impaired receiving waters, and social equity considerations. Developers within GI Priority Areas may face enhanced on-site GI requirements or may access streamlined PIL approval processes.

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## Section 4: Key Updates Between the 2023 and 2026 Manuals

### 4.1 Structural Additions

| Chapter | 2023 Content | 2026 Change |
|---|---|---|
| Chapter 1 – Runoff Impacts | Thermal impacts discussed qualitatively | C1 water anti-degradation context added; updated CN tables |
| Chapter 2 – LID | LID techniques described; volumetric credit qualitative | Volumetric reduction calculation linked to Chapter 14 methods; DIA crediting standardized |
| Chapter 3 – Planning | MSWMP/RSWMP framework; off-site compliance options | 24-month update obligation; PIL program criteria codified; GI Priority Areas concept introduced; volumetric targets required in plans |

### 4.2 Alignment with Regulatory Changes

All three chapters in the 2026 edition are explicitly updated to align with the January 2026 N.J.A.C. 7:8 amendments. Cross-references throughout Chapters 1–3 now direct the reader to Chapter 14 (Volumetric Reduction Standards) for design calculations and to the regulatory definitions of "GI BMP" and "Non-GI BMP" for practice classification. The 2023 editions used internal cross-references to the now-superseded GI standard definitions.

### 4.3 Design Storm References

Both editions reference the 1.25-inch/2-hour Water Quality Design Storm as the controlling event for GI sizing and TSS removal calculations. The 2026 editions update the CN values used to calculate runoff volumes from this storm event, based on revised NJ-specific soil data. This will produce modestly different WQV calculations in some HSG B and C soil classifications.

### 4.4 Vocabulary and Terminology

The 2026 editions introduce and consistently use the terms "volumetric reduction," "GI BMP," "Non-GI BMP," "treatment train," and "Internal Water Storage Zone (IWZ)" as defined in the 2026 N.J.A.C. 7:8 amendments. The 2023 editions lacked these standardized terms, leading to inconsistency in how practitioners described and calculated stormwater management performance.

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## Section 5: Planning Implications for Stormwater Design

### 5.1 Early-Phase Planning is Critical

The planning principles articulated in Chapters 1–3 of the BMP Manual underscore that stormwater outcomes are largely determined before detailed engineering design begins. Site planning and layout decisions — impervious footprint, grading philosophy, preservation of natural features — establish the baseline conditions that determine how much GI is needed and what sites are suitable for infiltration-based practices. Designers who defer site layout optimization to the permit review stage face more constrained and costly compliance pathways.

### 5.2 GI Feasibility Assessment as a Planning Step

Under the 2026 framework, the GI Feasibility Analysis (required prior to proposing Non-GI alternatives) is essentially a planning-phase deliverable. It requires soil investigation, SHWT mapping, evaluation of site constraints, and analysis of available permeable areas — tasks that are most efficiently performed during conceptual and preliminary design, not during final design or permit submission. Engineering firms should integrate GI feasibility evaluation into their phase 1 or concept-level scope of services.

### 5.3 Municipal Planning Context Affects Project Strategy

Whether a municipality has an NJDEP-approved and 2026-compliant MSWMP directly affects the compliance pathway available to a project:

- In municipalities **with** a 2026-compliant MSWMP: PIL programs, regional BMPs, and GI Priority Area designations may be available, expanding the designer's toolkit and potentially reducing on-site compliance costs for constrained sites.
- In municipalities **without** a 2026-compliant MSWMP (whether because no MSWMP exists or because the update has not yet been completed): State standards apply directly, and on-site GI compliance is required to the maximum extent feasible without PIL or regional BMP alternatives.

Understanding the status of the applicable MSWMP is therefore a prerequisite to scoping the stormwater compliance strategy for any New Jersey major development project.

### 5.4 Watershed-Scale Thinking in Site-Scale Design

Even where no RSWMP governs a project, the cumulative impact principles in Chapter 1 and the regional planning framework in Chapter 3 encourage designers to think beyond the property boundary. Documenting how a proposed project contributes to or detracts from watershed-scale runoff and water quality goals — through voluntary assessment or as required by MSWMP conditions — positions projects favorably in regulatory review and builds a stronger technical record for the design decisions made.

### 5.5 Implications for OPAL Knowledge System

For the OPAL system, the planning chapters establish the conceptual framework within which individual BMP design decisions are made. OPAL's BMP selection guidance, feasibility screening logic, and compliance pathway recommendations must be sensitive to:

- Site-specific soil and hydrologic conditions (Chapter 1 context)
- LID site layout options and their volumetric reduction potential (Chapter 2 context)
- Whether the project is in a municipality with an approved MSWMP offering alternative compliance mechanisms (Chapter 3 context)

These three planning-layer inputs should be captured in the OPAL intake workflow before BMP-specific design guidance is initiated.

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*Report generated for OPAL Phase 1B Planning Content.*  
*Reference documents: NJ Stormwater BMP Manual Chapters 1–3, 2023 and 2026 editions.*