Skip to content

Stormwater Quantity Control

N.J.A.C. 7:8-5.6 governs stormwater runoff quantity control for major development. The rule requires quantity review against the current and projected 2-year, 10-year, and 100-year storm events, and it works alongside - not instead of - the groundwater recharge and water quality standards.

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

Regulatory Summary

The runoff-quantity standard can be satisfied through one of three compliance paths:

  1. Show that the post-construction hydrograph does not exceed the pre-construction hydrograph at any point in time for the current and projected 2-year, 10-year, and 100-year storms.
  2. Show that there is no increase in peak runoff rates for those storms and that any volume or timing changes do not increase downstream flood damage.
  3. Design stormwater measures so the post-construction peak runoff rates are limited to 50 percent, 75 percent, and 80 percent of the pre-construction peak runoff rates for the 2-year, 10-year, and 100-year storms, respectively.

This means quantity review is not limited to a single detention-basin formula. It is a rule-based compliance framework tied to hydrologic and hydraulic analysis.

What Changed in 2026

The base runoff-quantity storm set did not change between the July 2023 and January 2026 rule versions: both eras still use the current and projected 2-year, 10-year, and 100-year storms for quantity review.

The significant 2026 addition is the volumetric reduction subsection at N.J.A.C. 7:8-5.6(d). That subsection creates a separate retention-focused pathway tied to the water quality design storm and the green infrastructure BMP tables. It should be read as an added 2026 compliance layer, not as a replacement for the quantity-control options in N.J.A.C. 7:8-5.6(b).

Rainfall Inputs

Storm-event depths for quantity calculations are not fixed statewide handbook values. The rules direct the design engineer to:

  1. Start with NOAA Atlas 14 precipitation data for the site.
  2. Apply the county-specific adjustment factors in Table 5-5 for current storms.
  3. Apply the county-specific change factors in Table 5-6 for projected storms.

That rainfall-input framework matters because the quantity standard is tied to both current and projected storms. The adjustment tables are part of the governing rule, not optional background guidance.

Where the site or its drainage area lies in more than one county, the rule allows the rainfall values to be adjusted according to the percentage of drainage area in each county or developed separately for each county.

Application Point

N.J.A.C. 7:8-5.6(c) applies the runoff-quantity standards at the site's boundary to each abutting lot, roadway, watercourse, or receiving storm sewer system. That boundary-level application matters when a project has more than one discharge path.

Engineering Interpretation

Quantity compliance usually relies on some combination of:

  • hydrograph generation for pre-construction and post-construction conditions
  • storage and routing through detention or retention BMPs
  • outlet control design
  • downstream impact review when the selected compliance path requires it

The BMP Manual's stormwater management computations chapter explains how BMP routing, hydrograph comparison, and rainfall-depth selection fit into these demonstrations. In practice, detention basins, wet ponds, extended detention systems, underground detention, and some green infrastructure BMPs can all participate in quantity compliance when they are designed and routed to meet the applicable option under N.J.A.C. 7:8-5.6.

In tidal flood hazard areas, the quantity analysis still applies unless the design engineer shows through hydrologic and hydraulic analysis that the increased volume, changed timing, or increased runoff rate will not cause additional flood damage below the point of discharge. No quantity analysis is required where the stormwater discharges directly to the ocean, bay, inlet, or the qualifying downstream tidal reach identified in the rule.

BMP Implications

Quantity control is not limited to traditional detention infrastructure. Depending on the selected compliance path and the project layout, the following practice types may contribute:

Practice type Typical quantity role
Detention basins and underground detention Peak-rate attenuation and hydrograph routing
Wet ponds and extended detention basins Storage plus controlled release
Green infrastructure BMPs from Tables 5-1 and 5-2 Retention, detention, or both, depending on design
Volumetric reduction BMPs in 2026 Retention of the water quality design storm under N.J.A.C. 7:8-5.6(d)

The key legal point is that the selected BMP or BMP train must be shown to satisfy the rule's quantity standard for the required storms and runoff conditions.

Cross References