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Drainage Area Limits

In the live source corpus, drainage-area limits are not one universal number. They are BMP-specific constraints that appear in the NJAC tables and the BMP Manual chapters.

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

Source-Backed Limits That Matter

The current citations layer supports several high-value limits clearly:

  • small-scale bioretention remains capped at 2.5 acres of maximum contributory drainage area
  • pervious paving uses a maximum 3:1 ratio of additional inflow contributory drainage area to pervious system surface area

Those are better source-backed anchors than the older page's generic range table.

Why the limits are BMP-specific

The live source set treats drainage area as a chapter-level design limit, not just a planning convenience. A drainage-area limit can change:

  • whether a Chapter 9 small-scale practice is still the correct source chapter
  • whether the proposed BMP keeps the hydraulic behavior assumed in its design chapter
  • whether the project needs to move from a single practice to a different configuration or treatment train

Engineering Interpretation

The authored takeaway is straightforward:

  1. read the drainage-area limit from the governing BMP source, not from a generic matrix
  2. if a proposed contributing area exceeds the small-scale chapter limit, move to a different configuration or a different BMP chapter
  3. avoid treating centralized routing as a substitute for the actual contributory-drainage limits in the source text

For bioretention specifically, the live citations layer has already corrected the old < = 1 acre impervious shorthand. The stable source-backed threshold is 2.5 acres total contributing drainage area for small-scale bioretention.

For pervious paving, the live source set points to the 3:1 additional-inflow ratio instead of a generic acreage cap. That distinction matters because it ties the limit to the paving surface area and drainage relationship, not just to a single watershed-size number.

Cross References