Extended Detention Basin¶
Source: NJ Stormwater BMP Manual, Chapter 10, Section 10.3 (2026)
Extended detention basins are earthen impoundments with an outlet structure configured to slowly release captured stormwater volume over an extended period (24–72 hours) after the storm event ends. Unlike dry detention basins (Chapter 11), which release runoff as fast as possible without a quantity standard violation, extended detention releases are deliberately slowed to allow gravitational settling of suspended solids over the extended drawdown period.
The Chapter 10 extended detention basin is the GI-qualifying version: in the 2026 classification, extended detention basins that are sited over demonstrably infiltrating native soils (confirmed per Chapter 12), are constructed without an impermeable liner, and include native soil infiltration as part of their operation qualify as Green Infrastructure and generate partial VRC. Basins without native soil infiltration are classified as Non-GI (see Chapter 11 — Extended Detention).
TSS Removal (2026): Extended drawdown promotes settling of medium particles: approximately 60–70% TSS removal at 24-hour drawdown time for properly designed basin geometry. This approaches — but may not consistently achieve — the 80% TSS standard on its own; a forebay and careful outlet design are required to reach compliance.
Primary stormwater functions:
- Water quality treatment — 60–70% TSS removal through extended settling (24-hr drawdown)
- Peak flow attenuation — above-pool active storage attenuates post-development peak flows
- Partial groundwater recharge — limited infiltration from basin bottom (GI config only)
- Partial VRC credit — generated from portion of stored volume that infiltrates into native soil
Source: NJ Stormwater BMP Manual, Ch. 10, Section 10.3 (2026)
| Parameter | 2026 Requirement | 2023 Requirement | Notes |
|---|---|---|---|
| Minimum drawdown time | 24 hours for WQV (water quality volume) | 24 hours | 24-hr drawdown provides ≥ 60% TSS removal |
| Maximum drawdown time | 72 hours maximum ponding | 72 hours | Prevents extended standing water; vector control |
| Outlet structure | Extended-period outlet: small orifice at WQV invert; riser and principal spillway above | Same | Multi-stage: WQV drawdown orifice + flood control riser + emergency spillway |
| Forebay | ≥ 10% WQV; ≥ 3 ft deep; accessible for sediment cleanout | Same | Required for GI qualifying configuration to protect native soil infiltration surface |
| Side slopes | 3:1 maximum (H:V); 4:1 preferred | Same | Stable grass cover and mowing access |
| GI configuration (2026) | No liner; Chapter 12 Ksat investigation; Chapter 13 mounding analysis if applicable | Chapter 12 reference | 2026 explicit documentation requirements for GI credit |
| Emergency spillway | Designed for 100-year storm with primary outlet blocked | Same | Independent of primary outlet structure |
| Dam Safety | N.J.A.C. 7:20 jurisdiction if embankment ≥ 6 ft or impoundment ≥ 0.5M cf | Same | Triggers separate dam safety permit |
| TSS performance | 60–70% TSS removal; combine with bioretention or sand filter to reach 80% if needed | Same | Check basin geometry and L:W ratio to achieve design HRT |
2026 GI Qualification Requirements: When GI credit is claimed for Ch. 10 extended detention, the SWM Report must include a Chapter 12 soil investigation confirming Ksat at the basin bottom, confirm no impermeable liner is installed, and include a Chapter 13 mounding analysis for larger basin footprints.
Source: NJ Stormwater BMP Manual, Ch. 10, Section 10.3; Ch. 12; Ch. 13 (2026)
Native Soil Permeability (for GI Qualification)
- GI status requires confirmed native Ksat at basin bottom elevation per Chapter 12
- In HSG C or D soils, basin must be constructed with liner; GI credit not available; system functions as Non-GI extended detention (see Ch. 11 version)
- See Soil Permeability Testing
Seasonal High Water Table
- Confirm SHWT does not seasonally rise into the basin bottom during wet seasons; SHWT at or above basin bottom eliminates infiltration capacity
- See Seasonal High Water Table
Groundwater Mounding
- Chapter 13 analysis required when basin footprint is large or SHWT is shallow relative to basin bottom
- See Groundwater Mounding
Dam Safety
- Basins with embankment ≥ 6 ft or impoundment ≥ 0.5 million cubic feet trigger N.J.A.C. 7:20
- See Dam Safety
Source: NJ Stormwater BMP Manual, Ch. 10, Section 10.3 (2026)
Annual Inspection
- Inspect embankment for erosion, animal burrowing, seepage, and settlement; document condition
- Inspect extended drawdown orifice for debris, biological growth, or clogging; confirm orifice is clear immediately after each significant storm event
- Confirm basin drains completely within 72 hours; standing water beyond 72 hours indicates orifice blockage or native soil infiltration rate below design
- Inspect emergency spillway for vegetative cover and debris obstruction
Forebay Maintenance
- Remove sediment from forebay every 3–5 years or when depth is 50% of design; forebay equipment access must be maintained free of obstruction
- Forebay sediment removal extends the service life of the native soil infiltration zone
Embankment Vegetation
- Mow embankment twice per year; no trees on embankment; remove woody vegetation within 15 ft of embankment toe
Source: NJ Stormwater BMP Manual, Ch. 8; Ch. 10, Section 10.3 (2026)
Design Errors
- GI credit claimed without Chapter 12 investigation — VRC based on assumed soil Ksat; actual in-situ permeability below design; regulatory deficiency in SWM Report
- Orifice oversized — WQV draws down in < 24 hours; TSS removal below design; water quality compliance not achieved at 60–70% TSS design target
- No forebay provided — coarse sediment reaches native soil surface in basin bottom; progressive clogging of infiltration zone; GI performance degrades over first 3–5 years
Construction Issues
- Principal spillway barrel invert not at design elevation — WQV drawdown orifice activated prematurely; active storage volume reduced; peak attenuation function impaired
- Embankment not compacted per specification — seepage paths; differential settlement
Long-Term Performance Risks
- Orifice clogging — low-flow orifice is small by design; leaf litter, debris, or biological growth plugs orifice; basin doesn't drain; TSS removal eliminated; vector breeding
- Native soil infiltration decline — sediment loading without maintained forebay progressively reduces infiltration capacity at basin bottom; GI performance declines
Source: NJ Stormwater BMP Manual, Ch. 10, Section 10.3 (2026)
Governing Regulations
| Rule Section | Topic | Engineering Relevance |
|---|---|---|
| N.J.A.C. 7:8-5.3 | Green Infrastructure Requirement | GI config qualifies with Ch. 12 documentation; no liner; VRC credit for infiltrated volume |
| N.J.A.C. 7:8-5.3(d) | Water Quality Treatment | 60–70% TSS removal at 24-hr drawdown; may need treatment train for 80% standard |
| N.J.A.C. 7:8-5.4(b) | Stormwater Quantity Control | Peak attenuation from active storage above WQV stage |
| N.J.A.C. 7:20 | Dam Safety | Jurisdiction threshold: embankment ≥ 6 ft OR impoundment ≥ 0.5M cf |
BMP Manual Sources
- NJ Stormwater BMP Manual, Chapter 10, Section 10.3 (2026) — Extended Detention (GI)
- NJ Stormwater BMP Manual, Chapter 11, Section 11.2 (2026) — Extended Detention (Non-GI)
- NJ Stormwater BMP Manual, Chapter 12 (2026) — Soil Investigation for GI
- NJ Stormwater BMP Manual, Chapter 13 (2026) — Groundwater Mounding
- NJ Stormwater BMP Manual, Chapter 8 (2026) — Operation and Maintenance
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