Phase 2B - Small-Scale Green Infrastructure (GI) BMPs¶
Source Documents: - 2023: BMP 9.1, BMP 9.2, BMP 9.3, BMP 9.4, BMP 9.5, BMP 9.6, BMP 9.7, BMP 9.8, BMP 9.9, BMP 9.10, GI Requirement Fact Sheet - 2026: BMP 9.1, BMP 9.2, BMP 9.3, BMP 9.4, BMP 9.5, BMP 9.6, BMP 9.7, BMP 9.8, BMP 9.9, BMP 9.10, GI Requirement Fact Sheet
Overview¶
Small-scale green infrastructure remains the core site-level stormwater toolkit in both the 2023 and 2026 source sets. The GI fact sheets in both editions frame compliance around managing runoff close to its source, distributing BMPs throughout the site, and staying within the drainage-area limits attached to the qualifying BMPs. Chapter 9 is the main family of small-scale practices used for that work, covering cisterns, dry wells, grass swales, green roofs, GI-qualified manufactured treatment devices, pervious paving systems, small-scale bioretention systems, small-scale infiltration basins, small-scale sand filters, and vegetative filter strips. (2023 GI Fact Sheet; 2026 GI Fact Sheet; 2023 BMP 9.1-9.10; 2026 BMP 9.1-9.10)
The more stable reading of the manuals is continuity rather than wholesale reclassification. The 2026 manual still carries Chapter 9.5 as Manufactured Treatment Devices (GI) and Chapter 9.9 as Small-Scale Sand Filters; the design question is not whether those subjects disappeared from Chapter 9, but which configurations qualify as green infrastructure and which are addressed elsewhere. (2023 BMP 9.5; 2026 BMP 9.5; 2023 BMP 9.9; 2026 BMP 9.9)
Section 1: Overview of Small-Scale Green Infrastructure¶
1.1 Purpose of GI in Stormwater Management¶
Both editions describe green infrastructure as stormwater management close to the point where runoff is generated, through infiltration into subsoil, filtration by vegetation or soil, or storage for reuse. In practice, that means Chapter 9 BMPs are selected not simply as treatment devices, but as distributed site elements that can satisfy water quality, groundwater recharge, quantity control, or a combination of those functions depending on the BMP and configuration. The GI fact sheets make the same point from the rule side: a compliant design must manage runoff close to its source, distribute qualifying BMPs throughout the site, and meet the applicable drainage-area limitations. (2023 GI Fact Sheet; 2026 GI Fact Sheet; 2023 BMP 9.1-9.10; 2026 BMP 9.1-9.10)
1.2 Typical BMP Types Used for GI Compliance¶
The Chapter 9 family is broad rather than uniform. Some practices are reuse-based, such as cisterns; some are infiltration-led, such as dry wells and small-scale infiltration basins; some can be configured either with infiltration or underdrains, such as pervious paving and small-scale bioretention; and some rely on specialized filtration or plant-soil treatment, such as GI MTDs and small-scale sand filters. The GI fact sheets also make clear that not every Chapter 9 BMP uses the same drainage-area limit. Dry wells are capped at 1 acre, GI MTDs at 2.5 acres, pervious paving is limited by a maximum 3:1 ratio of additional inflow area to pervious surface area, and small-scale bioretention, small-scale infiltration basins, and small-scale sand filters are capped at 2.5 acres. Green roofs are treated differently in their chapter, which states that they have no maximum contributory drainage area limitation but should only receive precipitation falling directly on the vegetated surface and adjacent walkways, not runoff from other structures. (2023 GI Fact Sheet; 2026 GI Fact Sheet; 2023 BMP 9.2; 2026 BMP 9.2; 2023 BMP 9.4; 2026 BMP 9.4; 2023 BMP 9.5; 2026 BMP 9.5; 2023 BMP 9.6; 2026 BMP 9.6; 2023 BMP 9.7; 2026 BMP 9.7; 2023 BMP 9.8; 2026 BMP 9.8; 2023 BMP 9.9; 2026 BMP 9.9)
Section 2: Design Characteristics of Small-Scale GI BMPs¶
2.1 Small-Scale Bioretention Systems¶
Small-scale bioretention remains one of the most versatile Chapter 9 BMPs in both editions. The manuals describe it as a vegetated soil-bed system that can either infiltrate into subsoil or discharge through an underdrain. Just as importantly, both editions explicitly treat many named urban forms as members of the same family: rain gardens, stormwater planters, stormwater islands, downspout planter boxes, street trenches, bioswales, and enhanced or continuous tree pits are all presented as variations on small-scale bioretention rather than as separate BMP families. The manuals keep the contributory drainage-area limit at 2.5 acres, require even distribution of inflow across the surface, cap the water-quality design storm ponding depth at 12 inches in a flat-bottom system, and require drain time within 72 hours. Only systems designed to infiltrate into subsoil may be used for groundwater recharge; underdrained systems remain part of the Chapter 9 family, but they do not meet recharge by naming alone. (2023 BMP 9.7; 2026 BMP 9.7)
2.2 Pervious Paving Systems¶
Pervious paving is also presented consistently across the two editions as a system with a permeable surface course, transition layer, and open-graded storage bed, with either an underdrained or infiltrating configuration. The practical design distinctions are configuration-specific rather than era-specific. Both editions keep the 3:1 limit on additional inflow area, require the Water Quality Design Storm to be treated without overflow for the adopted 80% TSS removal rate, and require drain-down within 72 hours. Only systems designed to infiltrate into subsoil may be used for groundwater recharge. Underdrained systems remain viable Chapter 9 practices, but they are treated as filtered-storage systems rather than recharge BMPs. The 2026 chapter also states the seasonal high water table or bedrock must be at least 1 foot below the bottom of the storage bed for underdrained systems and 2 feet below for infiltrating systems. (2023 BMP 9.6; 2026 BMP 9.6)
2.3 Green Roofs¶
Green roofs retain their role as a distinct small-scale GI practice based on roof-area storage and evapotranspiration rather than subsoil infiltration. In both editions, the chapter says a green roof has no maximum contributory drainage area limitation, but it is limited to the precipitation that falls directly on the vegetated roof area and adjacent walkways; runoff from other surfaces should not be directed onto it. The manuals also continue to describe green roofs as BMPs that can be incorporated with other practices, including dry wells, cisterns, rain gardens, and pervious paving, when a design seeks to retain more of the roof runoff volume. The practical implication is that green roofs are not a generic substitute for every other small-scale BMP, but neither are they isolated from the rest of the Chapter 9 family. (2023 BMP 9.4; 2026 BMP 9.4)
2.4 Dry Wells and Cisterns¶
Dry wells and cisterns illustrate the manuals' emphasis on clean-runoff source control. In both editions, dry wells are limited to 1 acre of contributory drainage area and are described as facilities for clean roof runoff rather than general paved-area collection. They require suitable subsurface conditions, a 72-hour drain time, and evaluation of potential hydraulic impacts on the groundwater table, with Chapter 13 identified as the source for groundwater mounding assessment. Cisterns are likewise tied to roof runoff and beneficial reuse. Both editions require the system to empty within 72 hours, size reuse demand around the lowest three consecutive days of need during the year, and treat overflow that mixes with regulated runoff in a downstream facility. These chapters read less like a new 2026 regime than a continuing source-control logic that remained in place across both manuals. (2023 BMP 9.1; 2026 BMP 9.1; 2023 BMP 9.2; 2026 BMP 9.2)
2.5 GI MTDs, Small-Scale Sand Filters, and Other Distributed Practices¶
The source set also shows why Chapter 9 cannot be reduced to a single hydrologic mechanism. Both editions title Section 9.5 as Manufactured Treatment Devices (GI) and explain that most MTDs are not approved as green infrastructure, but a subset of specialized devices with engineered media and plant-soil treatment qualifies as GI. Those GI MTDs are capped at 2.5 acres and carry an 80% TSS removal rate when sized in accordance with Department certification. Small-scale sand filters follow a similar pattern. In both editions, the chapter says there are two types of small-scale sand filters, but the underdrained type does not meet the GI definition and is not included in Chapter 9.9; the chapter therefore addresses the infiltrating small-scale sand filter, keeps the 2.5-acre drainage limit, and requires pretreatment. Open vegetated practices such as grass swales and vegetative filter strips remain part of the Chapter 9 family as well, but their compliance role depends on the actual configuration and performance criteria in their chapters rather than a universal small-scale GI shorthand. (2023 BMP 9.3; 2026 BMP 9.3; 2023 BMP 9.5; 2026 BMP 9.5; 2023 BMP 9.9; 2026 BMP 9.9; 2023 BMP 9.10; 2026 BMP 9.10)
Section 3: GI Performance and Stormwater Treatment Role¶
3.1 Runoff Management Functions¶
The Chapter 9 family contributes to runoff management through several different pathways, and the manuals are more useful when read that way than when compressed into a single generic credit table. Dry wells and small-scale infiltration basins are explicitly infiltration-led practices. Cisterns reduce runoff through storage for reuse. Green roofs rely on roof-area retention and evapotranspiration. Small-scale bioretention and pervious paving can be configured either as infiltrating systems or as underdrained treatment systems, which changes their role in recharge calculations. GI MTDs and small-scale sand filters emphasize water-quality treatment within small drainage areas, even though they do not function the same way hydraulically. The common theme is that the BMP must be matched to the compliance function actually claimed in the report and calculations. (2023 BMP 9.1-9.9; 2026 BMP 9.1-9.9)
3.2 Stormwater Quality Treatment¶
The manuals do provide some performance anchors, but they are BMP-specific rather than universal. GI MTDs are assigned an 80% TSS removal rate when sized under their certification documents. Pervious paving systems receive the adopted 80% TSS removal rate when designed to treat the entire Water Quality Design Storm without overflow. Small-scale sand filters are likewise presented with an 80% TSS removal rate in both editions. Small-scale bioretention is somewhat more nuanced: both editions describe pollutant removal through settling, vegetation uptake, and filtration, and the chapters tie TSS removal performance to soil-bed depth and vegetation, noting an 80-90% range. The broader report takeaway is that Chapter 9 supports strong treatment expectations, but it does so practice by practice, not through a single performance percentage that can be carried across the whole family. (2023 BMP 9.5; 2026 BMP 9.5; 2023 BMP 9.6; 2026 BMP 9.6; 2023 BMP 9.7; 2026 BMP 9.7; 2023 BMP 9.9; 2026 BMP 9.9)
Section 4: Key Updates Between the 2023 and 2026 Manuals¶
4.1 Continuity in the Chapter 9 Family¶
The most important comparison point is how much continuity remains between the two editions. The small-scale Chapter 9 family is materially stable across 2023 and 2026, and several of the sharper reclassification narratives that circulated in earlier drafts are not borne out by the chapter texts themselves. GI MTDs are not a new 2026 invention; both editions already distinguish GI-qualified MTDs from the broader non-GI MTD universe. Small-scale sand filters were not simply removed from Chapter 9 in 2026; both editions already state that underdrained sand filters do not meet the GI definition and are not included in Chapter 9.9. (2023 BMP 9.5; 2026 BMP 9.5; 2023 BMP 9.9; 2026 BMP 9.9)
4.2 Clearer Separation Between Small-Scale and Large-Scale Bioretention¶
Where the 2026 source set does sharpen the framework is in its separation of small-scale and large-scale bioretention. Chapter 9.7 continues to define the small-scale family and keeps the 2.5-acre drainage-area limit, while 2026 Chapter 10.1 separately addresses large-scale bioretention and states that the bioretention systems in that chapter have no contributory drainage-area maximum. Chapter 10.1 also ties large-scale GI BMPs to the rule concept of systems that exceed the Chapter 5.3(b) drainage-area limits and may therefore be used only for stormwater runoff quantity standards. That makes the small-scale versus large-scale distinction easier to read in the 2026 manual, but it does not change the basic identity of small-scale bioretention itself. (2026 BMP 9.7; 2026 BMP 10.1)
4.3 GI Compliance Framing¶
The newer rule environment gives the 2026 report context a more explicit GI-compliance frame, but the design chapters themselves remain remarkably consistent. The 2026 GI fact sheet, like the 2023 fact sheet, still anchors compliance in three practical questions: is runoff managed close to its source, are BMPs distributed throughout the site, and are the drainage-area limits met. That is a clearer basis for comparing the two editions than treating 2026 as a wholly new Chapter 9 program. (2023 GI Fact Sheet; 2026 GI Fact Sheet)
4.4 Summary of Changes¶
| Topic | 2023 | 2026 | Practical Reading |
|---|---|---|---|
| Chapter 9 small-scale GI family | Full Chapter 9 family in place | Full Chapter 9 family still in place | Read as continuity, not a purge |
| GI MTDs | Section 9.5 already limited to GI-qualified MTDs | Section 9.5 still limited to GI-qualified MTDs | Classification issue is configuration and certification, not chapter removal |
| Small-scale sand filters | Underdrained type excluded from Chapter 9.9 | Underdrained type still excluded from Chapter 9.9 | The chapter continues to address the GI-qualifying infiltrating configuration |
| Small-scale bioretention | 2.5-acre Chapter 9 practice | 2.5-acre Chapter 9 practice | Small-scale family remains stable |
| Large-scale bioretention | Addressed without the same 2026 chapter split | Separate 2026 Chapter 10.1 with no drainage-area maximum | 2026 makes the small-scale/large-scale distinction easier to read |
Section 5: Practical Implications for Designers and Reviewers¶
5.1 Match the BMP to the Claimed Compliance Function¶
The practical lesson for design reports is that Chapter 9 BMP selection should come before any shorthand about GI credit. A reviewer should be able to see not only which Chapter 9 BMP is proposed, but which configuration is being relied upon. That matters because the same BMP family may support different compliance functions depending on whether it infiltrates, filters through an underdrain, or stores runoff for reuse. Small-scale bioretention, pervious paving, sand filters, and GI MTDs all illustrate that point in different ways. (2023 BMP 9.5-9.9; 2026 BMP 9.5-9.9)
5.2 Keep the Drainage-Area and Source-Runoff Logic Visible¶
The manuals and fact sheets repeatedly return to drainage-area limits and runoff source. Dry wells are for clean roof runoff and are limited to 1 acre. Cisterns are likewise tied to roof runoff and demand-based reuse. Small-scale bioretention, infiltration basins, sand filters, and GI MTDs operate within 2.5-acre limits, while pervious paving relies on the 3:1 additional-inflow ratio. A good report should make those thresholds visible in the narrative and calculations instead of assuming that every Chapter 9 BMP works under the same sizing logic. (2023 GI Fact Sheet; 2026 GI Fact Sheet; 2023 BMP 9.1; 2026 BMP 9.1; 2023 BMP 9.2; 2026 BMP 9.2; 2023 BMP 9.5-9.9; 2026 BMP 9.5-9.9)
5.3 Document the Conditions That Matter¶
The source set also supports a clear reviewer checklist. If infiltration is claimed, the report should address subsoil suitability, groundwater separation, and hydraulic impacts on the water table. If reuse is claimed, the report should show how the cistern empties within the required time window. If a Chapter 9 BMP is being counted for water-quality treatment, the design should reflect the chapter conditions tied to that adopted performance. And if a designer is relying on a distributed small-scale GI strategy, the GI fact sheets support asking whether runoff is actually being managed close to its source throughout the site rather than concentrated in one corner. Those are the report-level questions that survive close comparison of the 2023 and 2026 sources. (2023 GI Fact Sheet; 2026 GI Fact Sheet; 2023 BMP 9.1; 2026 BMP 9.1; 2023 BMP 9.2; 2026 BMP 9.2; 2023 BMP 9.6-9.7; 2026 BMP 9.6-9.7)