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Trainers emphasize NRCS method, separate curve‑number handling after reviewers find frequent calculation errors
Summary
In Module 2 of the stormwater design review course, instructors urged reviewers to use the updated NRCS/NEH methodology (Part 630) and to treat pervious and impervious curve numbers separately, saying averaging consistently underestimates runoff and can undersize BMPs.
Peter, the course presenter, reviewed why New Jersey requires the NRCS methodology (NEH Part 630, formerly TR‑55) for computing runoff, hydrographs and peak flows and why the older TR‑55 equations are now superseded.
He said reviewers must not average curve numbers across a site’s pervious and impervious areas. Using a classroom example, Peter showed that computing runoff with a single averaged CN produced only 1,089 cubic feet, while treating impervious and pervious areas separately produced 3,811 cubic feet — a 71% difference. “Averaging CNs can grossly understate runoff and lead to undersized BMPs,” he said.
The presentation covered time of concentration (Tc) methods and segmenting flow paths into sheet flow, shallow concentrated flow and channel flow. Peter walked through a worked example for a wooded drainage area in Mercer County, calculating sheet‑flow length, segment travel times and a total Tc of roughly 74.6 minutes. He stressed there is no longer an assumed minimum Tc; each drainage area’s Tc must be calculated from the flow path.
Hydrographs and dimensionless unit hydrographs (DUHs) were explained next. Peter described the SCS DUH and the Delmarva DUH, noting Delmarva is appropriate for flat coastal‑plain regions with slopes under 5% but should not be used to size manufactured treatment devices (MTDs) because it can underestimate peak flows. He also described the required workflow to obtain NOAH precipitation values and apply current and projected adjustment factors when computing current vs projected storm events.
Why it matters: the instructors said their audit work shows recurring mistakes — outdated methods, averaged CNs, incorrect Tc assumptions and improper DUH choice — that directly affect BMP sizing, TSS removal credit and downstream flood analyses. The course materials and examples direct reviewers to verify model inputs, use NEH Part 630 equations, check shape/peak factors in hydrology printouts and ensure different rainfall adjustments are applied to current and projected storms.
Looking ahead: the course will continue with module examples showing reviewer checks for software outputs, CN segregation, Tc computations and DUH selection.
