Grass Swale¶
Source: NJ Stormwater BMP Manual, Chapter 9, Section 9.3 (2026)
Grass swales are vegetated open channels that convey stormwater while simultaneously providing water quality treatment through filtering, settling, and infiltration. Unlike pipes, swales expose runoff to vegetated surfaces that reduce flow velocity, remove coarse sediment, allow partial infiltration along the channel bottom, and provide biological uptake of dissolved pollutants through the root zone and soil interface.
Grass swales are linear BMPs typically used for site drainage along roadways, parking lot perimeters, and between developed lots. They replace conventional curb-and-gutter conveyance with a treatment-active channel and serve as a distributed stormwater management measure integrated into the site circulation infrastructure.
GI Classification (2026): Grass swales qualify as Green Infrastructure when native soil infiltration capacity is demonstrated per Chapter 12. VRC credit is limited to the partial volume infiltrating through the swale bottom during the design storm hydraulic residence time. Swales with impermeable channel liners are Non-GI.
Primary stormwater functions:
- Water quality treatment — 50–70% TSS removal at design flow through velocity reduction
- Partial groundwater recharge — limited infiltration through channel bottom into native soil
- Conveyance — replaces conventional curb and gutter while providing treatment function
- Pre-treatment for downstream BMPs — removes coarse sediment before reaching engineered media
When engineers choose this BMP:
Grass swales are selected when linear drainage corridors exist between impervious surfaces; conventional curb-and-gutter is being replaced as part of a GI-focused site design; a distributed, low-cost pretreatment measure is needed between BMP cells; or the project is a roadway, subdivision street, or parking lot perimeter with adequate right-of-way for a vegetated channel.
Source: NJ Stormwater BMP Manual, Ch. 9, Section 9.3 (2026)
| Parameter | 2026 Requirement | 2023 Requirement | Notes |
|---|---|---|---|
| Maximum design flow velocity | 1–2 ft/s during WQV storm | Same | Prevents erosion and maintains infiltration function |
| Channel shape | Parabolic or trapezoidal cross-section | Same | Parabolic preferred for self-maintaining grass cover |
| Bottom width | 2–8 ft | 2–8 ft | Wider bottom improves infiltration contact area |
| Side slopes | Maximum 3:1 (H:V) | Same | Flatter slopes allow better vegetation establishment |
| Minimum grass height | 3–6 inches; mow no shorter than 3 inches | Same | Short mowing reduces filtration and infiltration function |
| Check dams | Required at grade changes (max 6-inch drop per dam) | Same | Increase residence time and infiltration opportunity |
| Longitudinal slope | Maximum 4% without check dams | Same | Steeper slopes require check dams at all grade breaks |
| GI infiltration credit | Based on bottom area × field Ksat × hydraulic residence time | Same | VRC is partial; typically 20–50% of WQV |
| Ksat field investigation | Required per Chapter 12 for GI credit | General Ksat reference | 2026: explicit requirement added |
Source: NJ Stormwater BMP Manual, Ch. 9, Section 9.3; Ch. 12; Ch. 14 (2026)
Minimum Slope
- Swales require a minimum longitudinal slope of approximately 0.5% to prevent standing water between storm events; standing water supports mosquito breeding
- Slopes < 0.5% are possible with flat-bottomed channels and check dams but require drainage verification in final grading design
Maximum Slope Without Check Dams
- Slopes > 4% without check dams produce erosive velocities; check dams required at all grade transitions on steeper slopes
Soil Permeability for GI Credit
- GI VRC requires confirmed native soil infiltration in the channel bottom; Chapter 12 field investigation required
- HSG D soils or impermeable liners eliminate GI credit; swale functions as water quality conveyance only
- See Soil Permeability Testing
Drainage Area Limitation
- Appropriate for drainage areas typically ≤ 5 acres; larger areas generate flows exceeding vegetation erosion resistance limits
- See Drainage Area Limits
Right-of-Way and Spatial Requirements
- Minimum vegetated corridor width needed; typically not feasible on sites with curbed road sections or tight property line constraints
- Coordinate with roadway designer to accommodate swale grade and section within right-of-way
Source: NJ Stormwater BMP Manual, Ch. 9, Section 9.3 (2026)
Routine Mowing — Minimum Every 4–6 Weeks During Growing Season
- Mow to maintain grass height between 3 and 6 inches; do not mow below 3 inches
- Remove clippings from the channel bottom after mowing to prevent organic accumulation
Sediment and Debris Inspection — After Major Storms and Twice Annually
- Inspect swale bottom for sand or silt deposits from storm events; remove accumulations that reduce channel depth or channel-bottom permeability
- Remove trash, leaves, and woody debris from channel section
- Inspect check dams (if present) for erosion, undermining, or damage; restore damaged check dam structures to design height
Erosion Repair
- Inspect inlet and outlet transitions for scour erosion; repair with rock, sod, or erosion control blanket as appropriate
- Re-seed or re-sod any bare soil areas on channel floor or side slopes promptly
Source: NJ Stormwater BMP Manual, Ch. 8; Ch. 9, Section 9.3 (2026)
Design Errors
- Velocity exceeds 2 ft/s at WQV design flow — grass cover eroded from bottom; swale converts to bare-earth channel with no TSS removal or infiltration function; consequence of undersized channel cross-section
- Check dams not provided on slopes > 4% — high-velocity flow scours channel bottom and destabilizes side slopes; downstream erosion and sediment export
- GI credit claimed without Chapter 12 investigation — VRC credited without confirming native soil infiltration; regulatory deficiency requiring correction
Construction Issues
- Grade not maintained as designed — low points accumulate standing water; mosquito breeding; wet soil kills grass cover; channel bottom compacts
- Inlet protection not installed during construction — construction sediment loads scour freshly seeded swale before vegetation establishes
Long-Term Performance Risks
- Vegetation wear on high-use sections — foot traffic, vehicle overrun, or concentrated inflow eliminates grass cover locally; bare scour spots expand with each storm event
- Invasive species colonization — Phragmites, reed canary grass establish in wet sections; reduces hydraulic efficiency; may require herbicide treatment
- Sediment accumulation reducing channel capacity — upstream disturbed areas deliver high sediment loads; check dam impoundment fills; channel conveyance capacity reduced
Source: NJ Stormwater BMP Manual, Ch. 9, Section 9.3 (2026)
Governing Regulations
| Rule Section | Topic | Engineering Relevance |
|---|---|---|
| N.J.A.C. 7:8-5.3 | Green Infrastructure Requirement | Grass swale qualifies as GI (partial VRC) when infiltration confirmed |
| N.J.A.C. 7:8-5.3(d) | Water Quality Treatment | TSS removal credit available (50–70% at design flow) |
| N.J.A.C. 7:8-5.4(a) | Groundwater Recharge | Limited recharge credit based on infiltration rate and HRT |
BMP Manual Sources
- NJ Stormwater BMP Manual, Chapter 9, Section 9.3 (2026) — Grass Swales
- NJ Stormwater BMP Manual, Chapter 12 (2026) — Soil Testing and Ksat Investigation
- NJ Stormwater BMP Manual, Chapter 14 (2026) — Volumetric Reduction Standards
- NJ Stormwater BMP Manual, Chapter 8 (2026) — Operation and Maintenance
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