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Stormwater Quantity Control

Regulatory Summary

N.J.A.C. 7:8-5.6 establishes flood control standards requiring that post-development peak discharge rates do not exceed pre-development values for specified design storms. This standard protects downstream properties, infrastructure, and stream channels from increased flooding and erosion caused by new impervious surfaces.

The flood control standard applies to all major developments and is evaluated independently of the water quality and groundwater recharge standards.

Design Storm Requirements

Design Storm Purpose Requirement
2-year, 24-hour Channel protection and minor flooding Post-development peak flow must not exceed pre-development peak flow at point of discharge
10-year, 24-hour Moderate flood control (referenced in some MSWMPs) Where required by municipal plan; peak attenuation to pre-development rate
100-year, 24-hour Major flood control, dam safety, culvert design Post-development peak flow must not exceed pre-development peak flow; also used for spillway and emergency overflow sizing

NJ-Specific Rainfall Depths (Representative)

Rainfall depths are derived from NOAA Atlas 14 NJ-specific Intensity-Duration-Frequency (IDF) data. Values vary by geographic location within the state:

Storm Event Approximate Depth Range (NJ)
2-year, 24-hour 3.2 - 3.6 inches
100-year, 24-hour 9 - 13 inches

Engineers must use the applicable depths for the specific project location from the current approved Atlas 14 tables. The BMP Manual does not provide a single statewide depth — site-specific lookup is required.

2023 vs. 2026: Key Changes

The flood control standard remained substantively unchanged between the July 2023 and January 2026 rules. The 2-year and 100-year peak flow requirements are the same. However, the 2026 amendments provide updated guidance on several points:

Topic July 2023 January 2026
Core standard Pre/post peak flow match for 2-yr and 100-yr Unchanged
GI flood credit Not explicitly addressed GI practices may receive flood volume credit toward 2-year storm attenuation where the practice provides detention storage in addition to infiltration
Extended detention basins General flood control eligibility Non-GI Extended Detention Basins designed solely for WQ treatment do not satisfy flood control unless specifically sized for 2-year peak attenuation
Channel protection (Cpv) Referenced general MSWMP guidance Explicit that channel protection requirements flow from MSWMP; direct application when no MSWMP exists
Atlas 14 citation Referenced Explicit citation of current Atlas 14 version added
Future precipitation factor Not addressed Under discussion in 2026 guidance documents; climate-adjusted rainfall depths may be incorporated in future amendments

Channel Protection Standard

Some municipalities or MSWMP-governed projects require a channel protection volume (Cpv) standard, typically applied to the 1-year storm or a storm-specific channel erosion threshold. This requirement is geographically variable and depends on whether the municipality has an approved MSWMP that includes channel protection provisions.

Where no MSWMP exists, the state-level 2-year and 100-year standards apply directly. Engineers should confirm the applicable channel protection requirements with the local review authority before sizing detention facilities.

Engineering Interpretation

Hydrologic Modeling Approach

Peak flow calculations use the NRCS TR-55 methodology (or equivalent accepted hydrologic model) with the following inputs:

  • Curve Number (CN) — Area-weighted composite CN for pre-development and post-development conditions. The 2026 BMP Manual updates CN values for several land use categories (residential lawn on HSG B reduced from 69 to 66; commercial/industrial unchanged).
  • Time of Concentration (Tc) — Calculated for the longest flow path through the drainage area, accounting for sheet flow, shallow concentrated flow, and channel flow segments.
  • Design storm distribution — SCS Type III rainfall distribution, applicable for most of New Jersey.
  • Rainfall depth — Atlas 14 NJ-specific values for the project location.

Sizing Detention Facilities

Detention basins (or equivalent storage) are typically sized using a stage-storage-discharge routing analysis:

  1. Develop pre-development hydrograph using TR-55 or equivalent for 2-year and 100-year storms
  2. Develop post-development hydrograph with proposed site conditions
  3. Route post-development hydrograph through proposed detention facility
  4. Verify that routed peak outflow does not exceed pre-development peak for each design storm
  5. Size outlet structure — Typically a multi-stage outlet with low-flow orifice (2-year control), weir or riser (10-year if applicable), and emergency spillway (100-year)

GI Practices and Flood Control Credit (2026)

Under the 2026 guidance, GI practices that provide both infiltration and detention storage may receive flood volume credit toward the 2-year storm attenuation requirement. This applies to practices such as:

  • Bioretention systems with surface ponding depth designed for temporary storage
  • Pervious pavement systems with sub-base stone reservoir providing detention
  • Infiltration basins sized with freeboard above the infiltration volume

The key distinction: the GI practice must demonstrably attenuate peak flow, not merely reduce total runoff volume. An infiltration basin that fills and overflows before the peak of the 2-year storm hydrograph does not provide peak flow attenuation, even though it reduces total volume.

CN Table Updates (2026) — Impact on Flood Control

Updated CN values in the 2026 BMP Manual affect both WQV and flood control calculations:

Land Use HSG B (2023) HSG B (2026) Impact
Residential lawn CN 69 CN 66 Lower post-development peak; modest reduction in required detention
Commercial/industrial paved CN 98 CN 98 No change
Forest CN 55 CN 55 No change to pre-development baseline

Projects designed under 2023 standards that are being updated should recalculate hydrographs with 2026 CN values, as the changes may affect detention basin sizing.

BMP Implications

Flood control is typically achieved through detention — temporarily storing excess runoff and releasing it at a controlled rate. The following practices serve flood control functions:

Practice Flood Control Role Notes
Detention Basin Primary Standard approach; multi-stage outlet for 2-yr and 100-yr control
Extended Detention Basin Dual (WQ + flood) Must be specifically sized for flood control; WQ-only sizing is insufficient under 2026 rules
Bioretention (with detention storage) Supplemental May receive 2-year flood credit under 2026 guidance if detention storage is demonstrated
Pervious Pavement (with reservoir) Supplemental Sub-base reservoir provides temporary storage and peak attenuation
Infiltration Basin Supplemental Reduces total volume; peak attenuation depends on sizing relative to storm hydrograph
Wet Pond Primary Non-GI; provides permanent pool plus temporary flood storage
Underground Detention Primary Used on constrained sites; outlet controls sized for peak attenuation

Key BMP Pages

Cross References