Overview of Small-Scale Green Infrastructure
Small-scale GI remains the main site-level stormwater toolkit in both the 2023 and 2026 sources. The GI fact sheets in both editions frame compliance around three linked ideas: manage runoff close to its source, distribute BMPs throughout the site, and stay within the drainage-area limits attached to the qualifying BMPs.
The Chapter 9 family stays materially stable across both manuals. 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 all remain part of the small-scale conversation. The sharper claim that 2026 removed Chapter 9.5 and 9.9 from Chapter 9 is not supported by the source chapters themselves.
| BMP Type | Primary Role | Key Limit or Condition | Source-Backed Note |
|---|---|---|---|
| Cisterns | Storage for reuse | Must empty within 72 hours | Reuse demand is based on the lowest three consecutive days of need. |
| Dry Wells | Roof-runoff infiltration | 1-acre maximum contributory drainage area | Chapter applies to clean roof runoff, not general paved-area inflow. |
| Pervious Paving | Filtered storage with infiltration or underdrain | Additional inflow area limited to 3:1 | Only infiltrating systems meet groundwater recharge. |
| Small-Scale Bioretention | Vegetated treatment with infiltration or underdrain | 2.5-acre maximum contributory drainage area | Rain gardens, planters, islands, trenches, and tree pits are part of the same family. |
| GI MTDs | Certified small-footprint GI treatment | 2.5-acre maximum contributory drainage area | Both editions limit Chapter 9.5 to the GI-qualified subset of MTDs. |
| Small-Scale Sand Filters | Infiltrating small-scale sand filtration | 2.5-acre maximum contributory drainage area | The underdrained type does not meet the GI definition and is excluded from Chapter 9.9 in both editions. |
| Green Roofs | Roof-area storage and evapotranspiration | No maximum contributory drainage area limitation | Runoff from other structures should not be directed onto the green roof. |
Design Characteristics of Small-Scale GI BMPs
The chapter family is broad, but the design logic is not generic. The source-backed comparison is BMP-specific and configuration-specific, especially for practices that can either infiltrate or discharge through underdrains.
Small-Scale Bioretention Systems
Both editions describe small-scale bioretention as a vegetated soil-bed system that may infiltrate into subsoil or discharge through an underdrain. The manuals also treat rain gardens, stormwater planters, stormwater islands, downspout planter boxes, street trenches, bioswales, and enhanced or continuous tree pits as forms of the same family.
Pervious Paving Systems
Pervious paving remains a Chapter 9 system with either an infiltrating or underdrained storage bed. The manuals keep the design focus on surface permeability, storage bed sizing, and the relationship between additional inflow area and pervious surface area.
Cisterns and Dry Wells
Both chapters keep these BMPs tied to source-control logic. Dry wells are described as roof-runoff infiltration structures, while cisterns are beneficial-reuse systems that depend on actual reuse demand and timely drawdown.
GI MTDs and Small-Scale Sand Filters
These chapters are where the unsupported reclassification story breaks down. Both editions keep Section 9.5 for GI-qualified MTDs, and both editions keep Section 9.9 for the GI-qualifying small-scale sand filter treatment while excluding the underdrained type from that chapter.
Green Roofs
Green roofs remain a distinct Chapter 9 practice built around roof-area storage and evapotranspiration. Their design logic is different from subsoil infiltration BMPs, but the manuals continue to show them working alongside other Chapter 9 practices.
Grass Swales and Vegetative Filter Strips
These open vegetated practices remain part of the small-scale Chapter 9 family, but their role depends on their actual chapter criteria and site configuration. They fit the distributed, close-to-source GI logic of the fact sheets without collapsing into the same hydraulic story as dry wells or bioretention.
GI Performance and Stormwater Treatment Role
The performance story in Chapter 9 is practice-specific rather than a single universal VRC table. Some systems reduce runoff through infiltration, others through reuse or evapotranspiration, and others through tightly bounded small-footprint treatment within the GI drainage-area limits.
| BMP | Runoff Management Pathway | Source-Backed Performance Anchor | Important Qualification |
|---|---|---|---|
| GI MTDs | Certified small-footprint GI treatment | 80% TSS | Applies to the GI-certified subset described in Chapter 9.5, not to all MTDs in the broader market. |
| Pervious Paving | Filtered storage with infiltration or underdrain | 80% TSS | Quality rating depends on treating the full Water Quality Design Storm without overflow. |
| Small-Scale Sand Filters | Infiltrating sand-bed filtration | 80% TSS | Chapter 9.9 excludes the underdrained type from the GI chapter treatment. |
| Small-Scale Bioretention | Vegetation, soil filtration, and optional infiltration | 80-90% TSS | Performance depends on soil-bed depth and vegetation selection. |
| Cisterns | Storage for reuse | No chapter-wide TSS percentage | Performance depends on actual reuse demand and 72-hour drawdown. |
| Green Roofs | Roof-area storage and evapotranspiration | No chapter-wide TSS percentage | The chapter focuses on roof-area retention and integration with other BMPs. |
Key Updates Between the 2023 and 2026 Manuals
- The strongest source-backed reading is continuity in the Chapter 9 family, not a dramatic removal of GI topics from Chapter 9.
- Both editions already distinguish GI-qualified MTDs from the broader non-GI MTD universe.
- Both editions already exclude the underdrained small-scale sand filter from Chapter 9.9.
- The 2026 source set makes the small-scale versus large-scale bioretention distinction easier to read by pairing Chapter 9.7 with a separate Chapter 10.1.
| Topic | 2023 | 2026 | Practical Reading |
|---|---|---|---|
| Chapter 9 family | Full small-scale family present | Full small-scale family still present | Continuity is the stronger comparison than a purge or simple move-to-Chapter-11 story. |
| GI MTDs | Section 9.5 already limited to GI-qualified MTDs | Section 9.5 still limited to GI-qualified MTDs | The real distinction is certification and GI qualification, not disappearance from Chapter 9. |
| Small-scale sand filters | Underdrained type excluded from Chapter 9.9 | Underdrained type still excluded from Chapter 9.9 | Chapter 9.9 continues to cover the GI-qualifying infiltrating configuration. |
| Small-scale bioretention | Chapter 9.7 keeps a 2.5-acre small-scale limit | Chapter 9.7 keeps the same 2.5-acre limit | The small-scale family remains stable across both editions. |
| Large-scale bioretention framing | No parallel 2026 chapter split | Chapter 10.1 states no contributory drainage-area maximum for large-scale bioretention | 2026 makes the small-scale/large-scale boundary easier to read. |
Practical Implications for Designers and Reviewers
The practical design lesson is to match each Chapter 9 BMP to the compliance function actually being claimed. The same named BMP may support different outcomes depending on whether it infiltrates, discharges through an underdrain, or stores runoff for reuse.
Reviewer Focus
- Confirm that the selected BMP and configuration match the claimed compliance role, especially for bioretention, pervious paving, sand filters, and GI MTDs.
- Keep drainage-area limits visible in the report narrative and calculations instead of assuming every Chapter 9 BMP follows the same sizing rule.
- Where infiltration is claimed, document subsurface suitability, groundwater separation, and the hydraulic impact assessment pathway referenced by Chapter 13.
- Where reuse is claimed, show how the cistern will empty within the required 72-hour window.
- Use the GI fact sheet test directly: runoff managed close to its source, BMPs distributed throughout the site, and drainage-area limitations met.