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Water Quality Design Storm

The Water Quality Design Storm is the core storm event used in New Jersey's stormwater runoff quality standard. In the live source corpus, the governing rule text is N.J.A.C. 7:8-5.5, with related 2026 volumetric-reduction provisions in N.J.A.C. 7:8-5.6(d).

Structured citation evidence for this page lives in the sibling claim manifest.

Regulatory Summary

N.J.A.C. 7:8-5.5(d) defines the Water Quality Design Storm as:

  • 1.25 inches of rainfall
  • over 2 hours

That definition remains stable in the live 2023 and 2026 source sets.

Water quality calculations also have to use the rainfall distribution in Table 5-4. In other words, the WQDS is not just a single depth value; the source rule requires the time-distribution table to be part of the calculation framework.

TSS standard

The general runoff-quality standard is still the familiar 80 percent total suspended solids removal requirement. The current citations layer also resolves an important edge case: the 50 percent language in N.J.A.C. 7:8-5.5(b)2 is not a general rule for modified existing impervious cover. It is a narrow public-transportation-entity exception tied to right-of-way constraints.

2026 Change That Matters

The most important 2026 shift is not a new Water Quality Design Storm value. It is the added volumetric-reduction subsection at N.J.A.C. 7:8-5.6(d).

That means:

  • the WQDS itself still anchors runoff-quality design
  • the 2026 rule set adds a retention-focused compliance pathway for that storm
  • the volumetric-reduction standard should not be confused with a separate redesign of the WQDS definition

Engineering Interpretation

For source-backed authored guidance, the cleanest workflow is:

  1. Use the WQDS as the governing runoff-quality storm event.
  2. Apply the general 80 percent TSS framework unless the narrow transportation exception actually applies.
  3. In 2026 work, check whether the project is also using the volumetric-reduction path in N.J.A.C. 7:8-5.6(d).

The live evidence layer does not support the older shorthand that treated 50 percent TSS as a general existing-impervious standard.

If more than one BMP in series is needed to achieve the required 80 percent TSS reduction, N.J.A.C. 7:8-5.5(e) requires the rule formula for combined removal rather than simple addition of BMP percentages.

The same section also keeps nutrient reduction in play. N.J.A.C. 7:8-5.5(f) requires stormwater management measures to reduce post-construction nutrient load from WQDS runoff to the maximum extent feasible and to include GI BMPs that optimize nutrient removal while still meeting the recharge and quantity standards.

BMP Implications

The WQDS remains the common runoff-quality sizing anchor across BMP types, but the compliance logic differs by BMP configuration:

BMP type Typical WQDS role in the live source set
Infiltrating GI BMPs May retain or reduce the WQDS volume
Underdrained or non-infiltrating treatment BMPs Treat runoff generated by the WQDS
2026 volumetric-reduction designs Demonstrate retention or equivalent compliance under N.J.A.C. 7:8-5.6(d)

Cross References