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Water Quality Design Storm

Regulatory Summary

N.J.A.C. 7:8-5.5 establishes the Water Quality Design Storm as the controlling event for sizing stormwater quality treatment practices. The design storm is defined as:

1.25 inches of rainfall over a 2-hour duration

This event represents the target storm for which all water quality BMPs must be designed. The resulting runoff volume — the Water Quality Volume (WQV) — is the fundamental sizing parameter for both GI and Non-GI stormwater treatment practices in New Jersey.

The Water Quality Design Storm remained unchanged between the July 2023 and January 2026 amendments. However, the 2026 rules fundamentally changed how compliance with the WQV standard is demonstrated.

Water Quality Volume Calculation

The WQV is computed using:

WQV = P x Rv x A

Where:

Variable Definition Value/Source
P Design storm rainfall depth 1.25 inches
Rv Volumetric runoff coefficient (post-development) Derived from Curve Number methodology or composite impervious fraction
A Contributing drainage area to BMP Acres or square feet (consistent units with P)

The Rv coefficient captures the relationship between rainfall and runoff for the post-development land cover mix. Higher impervious percentages produce higher Rv values and larger WQV requirements.

TSS Removal Standard

The primary water quality performance metric is Total Suspended Solids (TSS) removal:

Condition TSS Removal Required
New impervious surfaces 80% removal from WQV runoff
Existing impervious surfaces being modified 50% removal from WQV runoff

TSS removal performance values are assigned to individual BMP types in the BMP Manual performance tables (Table 4-1), which list removal ranges based on published research.

2023 vs. 2026: Compliance Pathway Changes

Topic July 2023 January 2026
WQV design storm 1.25 in / 2 hr Unchanged
TSS standard (new impervious) 80% removal 80% removal (unchanged)
TSS standard (existing impervious) 50% removal 50% removal (unchanged)
GI BMP compliance TSS removal calculated for all BMPs using Table 4-1 GI BMPs achieving full volumetric reduction (VRv >= WQV) are deemed compliant — no separate TSS calculation required
Partial GI compliance Not explicitly codified Residual volume (WQV - VRv) must be treated by Non-GI BMP at 80% TSS removal
Volumetric reduction as pathway Performance goal within GI standard Standalone compliance pathway with Chapter 14 calculation methods
Treatment train calculation Mentioned; method specified (joint probability) Expanded guidance; sequential removal efficiency for GI + Non-GI combinations
Nutrient removal Secondary; TMDL-specific Strengthened for impaired waters; updated TP/TN removal tables; IWZ credit for bioretention
CN values for Rv 2023 tables Updated for several land use/HSG combinations (residential lawn HSG B: 69 to 66)

Volumetric Reduction as Compliance Pathway (2026)

The single most significant change in the 2026 rules is the formalization of volumetric reduction as a distinct compliance pathway for water quality. Under this framework:

  • A GI BMP that captures and retains the entire WQV with no surface discharge from the design storm is deemed to meet the 80% TSS standard automatically. The logic: if no runoff leaves the site, no pollutants are discharged.
  • This eliminates the need for a separate TSS removal efficiency calculation for practices achieving full volumetric capture.
  • For partial volumetric reduction (VRv < WQV), only the residual volume requires TSS treatment. This creates a quantifiable credit for every gallon infiltrated or evapotranspired, even when full capture is not achieved.

The volumetric reduction methodology is documented in BMP Manual Chapter 14 — Volumetric Reduction Standards, introduced in the 2026 edition.

Nutrient Removal Updates (2026)

The 2026 amendments strengthen nutrient removal requirements:

  • Updated total phosphorus (TP) and total nitrogen (TN) removal rate tables for both GI and Non-GI BMPs
  • Modified removal rates for bioretention systems that include an Internal Water Storage Zone (IWZ) — the saturated zone promotes denitrification and phosphorus uptake, significantly improving nutrient removal performance
  • Non-GI Wet Ponds and Non-GI Extended Detention Basins no longer receive default nutrient removal credits without specific design documentation supporting those rates
  • For sites within a designated nutrient management area, a treatment train incorporating bioretention or constructed wetland with confirmed nutrient uptake capability is the preferred compliance pathway

Engineering Interpretation

Design Workflow for WQV Compliance

Under the 2026 framework, the water quality compliance workflow is a multi-step process:

  1. Calculate WQV — Use 1.25-inch design storm with post-development Rv and contributing area
  2. Propose GI BMP(s) — Select practice(s) based on site conditions and GI feasibility analysis
  3. Calculate VRv — Determine volumetric reduction volume per Chapter 14 for each GI practice
  4. Evaluate compliance:
    • VRv >= WQV — Full compliance; document volumetric reduction calculation; no TSS calculation needed
    • VRv < WQV — Calculate residual (WQV - VRv); select Non-GI BMP for residual treatment at 80% TSS
  5. Document treatment train — If multiple practices treat the same runoff in series, use joint probability method for combined TSS removal (not simple addition of individual removal percentages)
  6. Verify nutrient compliance — If discharging to C1 waters or TMDL-regulated waters, confirm TP/TN removal meets applicable targets

WQV Sensitivity to CN Updates (2026)

The 2026 CN table updates affect WQV calculations:

  • Lower CN for residential lawn on HSG B soils (69 to 66) reduces computed Rv, producing a modestly smaller WQV for residential sites on B soils
  • No change for commercial/industrial or forested land uses
  • Engineers reworking existing designs under 2026 standards should recalculate WQV if the applicable CN has changed, as BMP sizing may be affected

Treatment Train Calculations

Where two or more BMPs treat the same runoff in series, combined TSS removal uses the joint probability method:

Combined Removal = 1 - [(1 - R1) x (1 - R2) x ... x (1 - Rn)]

Where R1, R2, ... Rn are the individual practice removal efficiencies (as decimals). Simple addition of percentages is not acceptable — a bioretention at 80% followed by a sand filter at 85% does not equal 165% combined removal.

Internal Water Storage Zone (IWZ) Impact

The 2026 rules introduce the IWZ as a design feature that significantly affects bioretention performance ratings:

  • With IWZ — Enhanced denitrification and phosphorus uptake; higher TP and TN removal credits in the updated performance tables
  • Without IWZ — Standard removal rates apply
  • The IWZ is a saturated zone maintained by an elevated underdrain or other control mechanism within the bioretention cell

BMP Implications

The WQV standard drives sizing for all water quality practices. The choice between GI and Non-GI determines the compliance pathway:

Practice WQ Compliance Pathway TSS Calculation Required?
Bioretention (full capture) Volumetric reduction (VRv >= WQV) No — deemed compliant
Bioretention (partial capture) VRv credit + residual TSS treatment Yes — for residual volume only
Pervious Pavement (full capture) Volumetric reduction No
Dry Well (full capture) Volumetric reduction No
Sand Filter w/ Underdrain Non-GI TSS treatment Yes — Table 4-1 rate applied to full WQV
MTD Non-GI TSS treatment Yes — verified removal rate from NJDEP MTD program
Wet Pond Non-GI TSS treatment Yes — Table 4-1 rate; no default nutrient credit without documentation (2026)
Extended Detention Basin Non-GI TSS treatment Yes — Table 4-1 rate; no default nutrient credit without documentation (2026)

Key BMP Pages

Cross References