9.5 Post-Construction Stormwater, SCMs & the NCDEQ Stormwater Design Manual

9.5 Post-Construction Stormwater, SCMs & the NCDEQ Stormwater Design Manual

The Board's reference list names the NCDEQ Stormwater BMP Manual — now published as the NCDEQ Stormwater Design Manual — as a state-specific exam resource. Post-construction stormwater is a different regulatory program from erosion control: erosion control governs the construction period and is administered by the Sedimentation Control Commission, while post-construction stormwater governs the completed development in perpetuity and is administered by the Environmental Management Commission through the Division of Energy, Mineral, and Land Resources.

The rules were comprehensively readopted effective January 1, 2017, replacing the old "BMP" vocabulary with Stormwater Control Measure (SCM) and consolidating the design requirements into Minimum Design Criteria (MDC).

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1. Where the Rules Live

Rule .1050 opens with the sizing principle that governs everything downstream: "The design volume of SCMs shall take into account the runoff at build out from all surfaces draining to the system. Drainage from off-site areas may be bypassed. The combined design volume of all SCMs on the project shall be sufficient to handle the required storm."

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2. The Low Density / High Density Fork

Every regulated project takes one of two paths.

+-----------------------------------------------------------------------------+
|                        15A NCAC 02H .1003 - THE FORK                        |
|                                                                             |
|   LOW DENSITY (.1003(2))                    HIGH DENSITY (.1003(3))         |
|   ------------------------                  ---------------------------     |
|   Built-upon area at or below the            Any project that does not       |
|   applicable density threshold               conform to the low density      |
|                                              criteria                        |
|   Stormwater conveyed primarily by                                          |
|   VEGETATED CONVEYANCES:                     Must achieve either             |
|     - trapezoidal, min bottom width 2 ft       RUNOFF TREATMENT  or          |
|     - side slopes no steeper than 3:1          RUNOFF VOLUME MATCH           |
|     - minimum length 100 ft                  through engineered SCMs         |
|     - or treatment swales per .1061                                         |
|                                              SCMs must meet the MDC in       |
|   No engineered SCM required                 .1050 and the type-specific     |
|                                              rules in .1051 - .1062          |
+-----------------------------------------------------------------------------+

The density threshold is not a single statewide number — it varies by program, watershed, and local ordinance. Greenville, for example, applies a 24 percent built-upon-area threshold. The design decision on a subdivision is therefore always a two-part question: what program applies here, and what threshold does it set?

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3. The Design Storm

The Simple Method (Schueler) is the conventional volume computation in the NCDEQ manual:

$V = 3630 \cdot R_D \cdot R_v \cdot A$

where V is the design volume in cubic feet, R_D is the design storm rainfall depth in inches (typically 1.0 or 1.5), R_v is the dimensionless runoff coefficient derived from impervious fraction, and A is the drainage area in acres. The constant 3,630 converts acre-inches to cubic feet. If the discrete SCS Curve Number method yields a greater volume, that greater volume may be used.

Drawdown. Volume-based SCMs are conventionally designed to release the treatment volume over two to five days — long enough to provide treatment, short enough to restore storage before the next storm.

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4. Vegetated Setbacks

Rule .1003 requires vegetated setbacks from perennial waterbodies, perennial streams, and intermittent streams of at least 30 feet in width, for both low and high density development.

This 30-foot setback is a stormwater requirement and is distinct from three other buffers a North Carolina surveyor encounters, all of which can apply to the same project simultaneously:

Mapping all applicable buffers is the surveyor's job on a subdivision plat regardless of who designs the SCM.

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5. Common SCM Types

The MDC rules address, among others, infiltration systems, bioretention cells, wet ponds, stormwater wetlands, permeable pavement, sand filters, rainwater harvesting, green roofs, level spreaders and filter strips, disconnected impervious surface, treatment swales, and dry ponds.

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6. The Scope Line — Read This Before Designing an SCM

Post-construction SCM design is where a surveyor is most likely to drift out of the incidental exception without noticing. Run the Section 9.1 checklist:

[!CAUTION] A great many post-construction SCMs fail at least one of those tests. A wet detention pond with a riser-and-barrel outlet in an earthen embankment implicates exclusions 4 and 5 immediately. The realistic incidental scope is closer to vegetated conveyances, treatment swales, and simple grading-based measures — the low-density path — plus the survey, topography, drainage-area delineation, and plat work that any SCM project needs and that only a surveyor can provide.

That is not a small role. Delineating drainage areas, computing built-upon area, mapping buffers and setbacks, establishing SCM as-built geometry and elevations, and platting the drainage easements and access corridors that maintenance requires are all squarely within the practice of land surveying, and they are what most stormwater projects actually need from a PLS.