Dry Detention Basin¶
Source: NJ Stormwater BMP Manual, Chapter 11, Section 11.1 (2026)
Dry detention basins are earthen impoundments that store stormwater runoff temporarily after a storm event and then release it at a controlled rate through an outlet structure (typically a riser and principal spillway). The basin is normally dry between storms; all stored volume drains completely between storm events. Dry detention provides peak flow attenuation by capturing and storing the excess runoff that exceeds the downstream conveyance or channel capacity and releasing it slowly over several hours.
Dry detention is a quantity-control BMP, not a water quality BMP. The 2026 NJ BMP Manual explicitly classifies dry detention as Non-GI. It does not generate VRC credit toward the GI Requirement; it does not provide groundwater recharge through a liner-free native soil contact; and its TSS removal efficiency (40–60%) is below the 80% TSS standard required for primary water quality compliance. It must be used in combination with dedicated water quality BMPs when both quantity and quality standards apply.
Primary stormwater functions:
- Peak flow attenuation — primary function; controls post-development peak discharge to pre-development rates for 2-, 10-, and 100-year design storms
- Runoff volume management — temporary storage during peak event; no volume reduction
- Limited water quality — 40–60% TSS removal from settling; does not meet 80% TSS standard
- Non-GI: does not generate VRC toward GI Requirement
When engineers choose this BMP:
Dry detention basins are selected when: downstream channel or storm sewer is undersized for post-development peak discharge; flood control is a primary regulatory driver; site area exists for an earthen basin that would be dry between storms; water quality compliance will be met through an upstream or parallel water quality BMP.
Source: NJ Stormwater BMP Manual, Ch. 11, Section 11.1 (2026)
| Parameter | 2026 Requirement | 2023 Requirement | Notes |
|---|---|---|---|
| Design storms | 2-year, 10-year, 100-year peak attenuation per N.J.A.C. 7:8-5.4 | Same | Match pre-development peak discharge rates |
| Outlet structure | Low-flow orifice at basin bottom + riser + principal spillway | Same | Multi-stage outlet controls multiple design storms |
| Emergency spillway | Designed for 100-year storm with primary outlet blocked | Same | Separate from principal spillway |
| Maximum ponding depth | At 100-year stage: ≤ 1 ft below top of embankment | Same | 1-ft freeboard minimum |
| Drawdown time | Fully drain within 72 hours of storm event end | Same | Prevent extended standing water; vector control |
| Side slopes | 3:1 (H:V) maximum; 4:1 preferred for mowing access | Same | Minimum slope for stable grass cover and maintenance |
| Forebay | Recommended for sediment management; 10% of total WQV if included | Same | Equipment access for periodic cleanout |
| Dam Safety | N.J.A.C. 7:20 jurisdiction if embankment ≥ 6 ft height or impoundment ≥ 0.5 million cf | Same | Requires separate dam safety permit and inspection program |
2026 Note: The 2026 manual reinforces that dry detention alone does NOT satisfy Water Quality Volume (WQV) treatment standards. A separate water quality measure (bioretention, sand filter, wetland, etc.) is required to achieve ≥80% TSS removal for WQV compliance.
Source: NJ Stormwater BMP Manual, Ch. 11, Section 11.1; N.J.A.C. 7:8-5.4; N.J.A.C. 7:20 (2026)
Soil Conditions
- Dry detention basins are appropriate in any soil type including HSG D soils
- In HSG A and B soils, ensure basin bottom does not inadvertently infiltrate at a rate that undermines peak attenuation storage volume calculations
- Verify foundation soil suitability for embankment stability — geotechnical borings may be required for larger embankments
Dam Safety Jurisdiction
- Basins meeting N.J.A.C. 7:20 thresholds (embankment ≥ 6 ft or impoundment ≥ 0.5M cf) require a separate Dam Safety permit; coordination with NJDEP Dam Safety Section required during project design, not at final design submission
- See Dam Safety
Drainage Area
- Appropriate for large contributing drainage areas; typically ≥ 10 acres for practical earthen basin sizing
- Small drainage areas can be controlled with smaller structures; dry detention basins are not the most efficient solution for very small sites
- See Drainage Area Limits
Water Quality BMP Integration
- Site design must include a separate water quality BMP to meet WQV TSS standard; dry detention cannot be the sole stormwater management measure on regulated projects
- Coordinate basin outlet to capture or route WQV through a dedicated water quality BMP before peak attenuation stage is required
Source: NJ Stormwater BMP Manual, Ch. 11, Section 11.1 (2026)
Annual Inspection
- Inspect embankment for erosion, animal burrowing (groundhog, muskrat), seepage, and settlement; document embankment condition per O&M Agreement requirement
- Inspect principal outlet riser for debris, blockage, or structural damage; confirm low-flow orifice is clear and sized orifice is clean
- Inspect emergency spillway for vegetative cover, erosion, and debris blockage
- Confirm basin floor is draining completely between storms (no standing water > 72 hours)
Vegetation
- Mow embankment slopes at least twice per year; maintain grass at ≤ 6-inch height for visual inspection of burrowing and seepage
- Inspect and maintain forebay vegetation (if present) after major storm events
- Trees and shrubs should not be allowed to establish on or within 15 ft of the embankment toe; root systems compromise embankment integrity
Sediment Management
- Inspect forebay for sediment accumulation after major storms; remove when 50% of design volume is silted in
- Inspect basin floor for sediment layer buildup; periodic removal required every 10–20 years depending on drainage area land use
Source: NJ Stormwater BMP Manual, Ch. 8; Ch. 11, Section 11.1 (2026)
Design Errors
- Dry detention sole WQV measure — project relies on dry detention for water quality compliance; 40–60% TSS removal is far below 80% standard; regulatory deficiency
- Outlet sizing error — low-flow orifice oversized; basin drains before peak of design storm; attenuation function not achieved; downstream flooding occurs
- Dam Safety jurisdiction not identified — dam safety permit application not initiated early enough; dam safety submittal timeline delays entire project
Construction Issues
- Embankment not compacted per specification — differential settlement and seepage paths develop through embankment; potential piping failure risk
- Principal spillway barrel not connected to riser watertight — water shortcuts around outlet structure; full basin storage not utilized
Long-Term Performance Risks
- Animal burrowing — groundhogs create burrow channels through embankment fill; seepage and piping failure follow; high consequence failure if basin has significant downstream hazard potential
- Outlet clogging — low-flow orifice blocked by debris; basin does not drain between storms; becomes de facto wet basin; vector breeding; embankment seepage risk rises
Source: NJ Stormwater BMP Manual, Ch. 11, Section 11.1 (2026)
Governing Regulations
| Rule Section | Topic | Engineering Relevance |
|---|---|---|
| N.J.A.C. 7:8-5.4(b) | Stormwater Quantity Control | Primary regulatory function; match pre-development 2-, 10-, 100-year peaks |
| N.J.A.C. 7:8-5.3 | Green Infrastructure Requirement | Non-GI — does not generate VRC; GI must be provided separately |
| N.J.A.C. 7:20 | Dam Safety | Jurisdiction trigger: embankment ≥ 6 ft or impoundment ≥ 0.5M cf |
BMP Manual Sources
- NJ Stormwater BMP Manual, Chapter 11, Section 11.1 (2026) — Dry Detention Basins
- NJ Stormwater BMP Manual, Chapter 8 (2026) — Operation and Maintenance
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