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Extended Detention Basins

Extended detention basins (EDB) are stormwater retention facilities that capture runoff and release it slowly over an extended period — typically 24 to 72 hours — through a small outlet orifice. The extended drawdown time provides water quality treatment through settling of suspended solids and associated pollutants.

Source: NJ Stormwater BMP Manual, Chapter 11, Section 11.2 (2026)

Extended detention basins provide stormwater management by impounding runoff behind a berm or embankment and metering its release through a small outlet. The extended residence time allows gravity settling of suspended solids, sediment-bound nutrients, and metals. They do not rely on infiltration and can be used in all soil types, including HSG C and D.

Extended detention basins are non-GI (Chapter 11) BMPs. They do not qualify as Green Infrastructure under N.J.A.C. 7:8-5.3 and cannot be counted toward the Green Infrastructure requirement. They are typically used where site conditions prohibit GI BMPs or as downstream controls to manage large drainage areas.

Primary stormwater functions:

  • Water quality treatment — TSS removal through gravity settling during extended drawdown; typically 40–60% TSS removal for standard EDB; higher with modifications
  • Stormwater quantity control — peak flow attenuation through detention storage
  • Limited or no recharge credit — EDBs do not infiltrate by design

When engineers choose this BMP:

Extended detention basins are selected when:

  • The site has inadequate soil permeability for GI infiltration BMPs
  • The contributing drainage area is large (multiple acres to hundreds of acres)
  • Quantity control is a primary requirement alongside minimum water quality treatment
  • Land is available for a basin footprint with berms and an outlet structure

They are often used in combination with GI measures that treat the first inch of runoff from individual developed areas, with the EDB capturing combined downstream flows.

Source: NJ Stormwater BMP Manual, Chapter 11, Section 11.2 (2026)

Confirm all values against the current NJ Stormwater BMP Manual, Section 11.2, before use in design. Run Agent 1 with the Chapter 11.2 PDFs to generate a verified 2023/2026 comparison table.

Parameter 2026 Requirement 2023 Requirement Notes
Extended detention volume (WQv) ≥ WQV stored above any permanent low-flow pool Same All WQV must enter and be released slowly
Minimum drawdown time 24 hours for WQV release 24 hours for WQV release Provides settling time for TSS removal
Maximum drawdown time 72 hours 72 hours Prevents anaerobic conditions
Outlet orifice sizing Sized to release WQV over 24–72 hours; low-flow orifice Same Orifice sizing controls drawdown rate
Forebay required Yes — ≥10–15% of WQV; minimum 3 ft deep Yes Coarse sediment trapping before main basin
Side slopes (interior) Maximum 3:1 (H:V); 4:1 preferred 3:1 max For safety and mowing access
Emergency spillway Required; sized for 100-year design storm Same Independent of primary outlet
Embankment design Subject to NJDEP Dam Safety Act (N.J.A.C. 7:20) if height/storage thresholds triggered Same Check thresholds early in design

2026 Update: No fundamental changes to EDB design criteria in the 2026 edition. The key 2026 change is the explicit Non-GI classification statement added to Chapter 11.2: extended detention basins do not qualify for VRC credit and cannot satisfy the GI Requirement. Supplemental GI BMPs are required to address the volumetric reduction obligation.

Source: NJ Stormwater BMP Manual, Ch. 11, Section 11.2 (2026)

Extended detention basins require adequate land area for the basin footprint, berm, and outlet structure. The following constraints govern siting:

Drainage Area

  • EDBs are typically designed for larger contributing areas (multiple acres to hundreds of acres); small-scale applications exist but are less common
  • See Drainage Area Limits

Soil Type

  • EDBs can be constructed on all soil types including HSG C and D because they do not rely on infiltration
  • Basin must be lined or have low-permeability soils if permanent pool (wet EDB) is intended; verify seepage analysis requirements

Topography and Land Area

  • Sufficient land must be available for basin footprint plus berm, emergency spillway, and access for maintenance
  • Generally not feasible in dense urban infill settings without large footprint availability

Dam Safety

  • Embankment height ≥ 6 feet from lowest point of natural ground at the downstream toe to the top of the embankment, or impounded storage ≥ 0.5 million cubic feet, triggers NJDEP Dam Safety Act (N.J.A.C. 7:20) permitting requirements
  • Verify applicable Hazard Classification (Low, Significant, High) against N.J.A.C. 7:20 criteria at the earliest design stage — it determines embankment design standard and emergency spillway sizing requirements

Setbacks

  • Minimum setback from property lines: check municipal and county stormwater ordinances; no statewide minimum in N.J.A.C. 7:8 but county/municipal requirements may apply
  • Setbacks from sanitary sewer and potable water lines per local utility requirements
  • Locate access roads to outlet structure and forebay for maintenance equipment access; access routes must be designed to support tracked excavator loads for sediment removal

Source: NJ Stormwater BMP Manual, Ch. 11, Section 11.2 (2026)

Semi-Annual Inspection

  • Inspect inlet, inlet protection, and forebay for sediment accumulation and erosion
  • Inspect outlet structure (low-flow orifice, riser, barrel pipe) for blockage, structural damage, or evidence of uplift/flotation
  • Inspect embankment for erosion, seepage zones, cracking, slope instability, or animal burrows
  • Inspect emergency spillway for obstruction, erosion, or vegetative encroachment
  • Confirm embankment vegetation is maintained (grass cover intact; no woody vegetation)
  • Inspect high-water mark to confirm basin is draining between storm events

Sediment Removal

  • Forebay: remove accumulated sediment when depth reaches approximately 50% of design capacity (annual inspection; cleanout every 3–10 years depending on sediment loading)
  • Main basin: cleanout when storage capacity is measurably reduced by sediment accumulation; typically every 10–25 years; sediment depth survey recommended every 5 years
  • Removed sediment must be characterized per applicable solid waste regulations before disposal

Embankment and Outlet Maintenance

  • Mow embankment slopes per O&M schedule (minimum twice per year); remove clippings
  • Remove all woody vegetation (trees and shrubs) from the embankment; root systems create seepage pathways and compromise embankment stability
  • Inspect and clean outlet orifice annually; small orifices are prone to debris blockage; orifice screens require cleaning after each significant storm event
  • Test outlet drawdown rate annually; document in O&M log; increasing drawdown time indicates orifice partial blockage or forebay/basin sedimentation

Source: NJ Stormwater BMP Manual, Ch. 8; Ch. 11, Section 11.2

Design Errors

  • Outlet orifice oversized — basin drains too quickly; TSS settling time insufficient; water quality treatment objective not achieved; most common EDB design error when quantity control takes priority over water quality orifice sizing
  • Forebay undersized or omitted — coarse sediment bypasses pretreatment and accumulates in main basin; storage capacity lost more rapidly; costly excavation required sooner
  • Emergency spillway inadequately sized — embankment overtopping during extreme events; embankment erosion risk; most dam failure incidents originate at undersized spillways
  • Dam Safety thresholds not identified at design stage — embankment built to standard detention basin specifications that do not meet N.J.A.C. 7:20 structural requirements; costly regulatory correction required
  • Basin does not drain between storms — inlet invert or outlet control set at wrong elevation; permanent pool forms unintentionally; converts to wet pond without water quality design for permanent pool management

Construction Issues

  • Outlet orifice blocked during construction — debris not cleared from riser pipe before basin becomes operational; ponding occurs at undesigned elevation
  • Embankment compaction deficient — improper fill placement or insufficient lift compaction increases seepage and long-term settlement risk
  • Inlet pipe invert elevation incorrect — creates unintended permanent pool or prevents basin from draining to dry-bottom condition

Long-Term Performance Risks

  • Outlet orifice clogging — small orifice diameters are highly susceptible to debris and biological fouling; orifice screens accumulate trash and leaves; loss of extended detention function without regular cleaning
  • Loss of forebay capacity — forebay fills with sediment and is not maintained; fine sediment load reaches main basin; TSS removal efficiency decreases progressively
  • Embankment vegetation succession — woody shrubs and trees establish on embankment; root intrusion creates preferential seepage paths; structural integrity compromised
  • Reduced TSS removal over time — cumulative sediment accumulation raises effective basin floor elevation; outlet draws from shallower water depth; settling efficiency proportionally reduced

Governing Regulations

Rule Section Topic Engineering Relevance
N.J.A.C. 7:8-5.3(d) Water Quality Treatment (non-GI) EDB must achieve required TSS and/or pollutant load reduction
N.J.A.C. 7:8-5.4(b) Stormwater Quantity Control EDB commonly used to meet peak flow attenuation requirements
N.J.A.C. 7:8-5.3 Green Infrastructure Requirement EDB does NOT qualify as GI; GI requirement must be met separately
N.J.A.C. 7:20 Dam Safety Act Embankment height/storage volume thresholds may apply

BMP Manual Source

  • NJ Stormwater BMP Manual, Chapter 11, Section 11.2 (2026)
  • NJ Stormwater BMP Manual, Chapter 8 — Operation and Maintenance
  • NJ Stormwater BMP Manual, Chapter 11 (general) — Non-GI BMPs overview

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