Stormwater Methodology

When It Applies

Rational Method is a simple peak-flow calculation appropriate for small, site-scale drainage analysis. In New Jersey it is explicitly limited in scope:

• NJAC 7:8 (state major-development rule) does not permit Rational Method. Major development requires NRCS methodology.
• Newark §41:17-4-6 permits Rational Method for minor development — sites below the major-development thresholds.

On a typical Newark minor-development project (a 2–6 unit residential infill, a small addition, a parking lot expansion), Rational Method is the correct tool and the tool the ordinance expects. On anything that triggers major-development status under §7:8-1.2, the tool switches to NRCS automatically.

Rational Method is a peak-flow-only calculation. It produces a single maximum discharge value in cubic feet per second. It does not produce a hydrograph, a runoff volume, or a storage size. Detention sizing from Rational Method uses a separate triangular-hydrograph approximation (covered in Phase A of the calculator).

The Formula

Where:

• Q = peak discharge (cfs)
• C = runoff coefficient (dimensionless, 0–1)
• I = rainfall intensity at the site’s time of concentration (Tc) duration (inches per hour)
• A = drainage area (acres)

The coincidence that cfs = in/hr × acres works out to within 1% (the exact conversion is 1.008). This is why Rational Method uses the units it does — they balance out naturally.

Each variable answers one question. C asks: how much of the rain becomes runoff? I asks: how hard is it raining? A asks: how big is the site? Multiply the three and you get the peak flow.

Where the Numbers Come From

C (runoff coefficient) is a single number per surface type. Building roofs are 0.95–0.99. Pavement is 0.95. Lawn is 0.20. The tool uses Newark’s published values from §41:17 Tables 17-4 and 17-5. For sites with multiple surfaces, a weighted-average C is computed the same way as composite curve number (CN) — area-weighted across the contributing surfaces.

C is simpler than CN. It does not depend on soil type, slope, or storm size. A lawn is 0.20 whether it sits on sand or clay, whether the storm is a 2-year or 100-year event. This simplicity is why Rational Method is restricted to small sites — the simplifications break down at larger scales.

I (rainfall intensity) is specific to the site’s time of concentration (Tc). Unlike NRCS methodology, which uses a 24-hour storm depth, Rational Method uses the rainfall intensity at the duration equal to Tc. A site with Tc of 15 minutes uses the 15-minute rainfall intensity; a site with Tc of 30 minutes uses the 30-minute intensity.

Intensity values come from NOAA Atlas 14, which publishes depth-duration-frequency tables for every location. For Newark, the 10-year 15-minute intensity is about 5.00 in/hr; the 10-year 30-minute intensity is about 3.04 in/hr. Shorter durations produce higher intensities — the rain falls at peak rate for a shorter time, but that rate is higher.

A (drainage area) is the site contributing to the outlet, measured in acres.

The 10-Minute Tc Floor

NJAC 7:8 (and Newark §41:17 by reference) impose a minimum time of concentration (Tc) of 10 minutes for Rational Method, regardless of the computed value.

On most Newark infill sites, the three-segment Tc calculation lands below 10 minutes — small lots have short flow paths and moderate slopes. The 10-minute floor kicks in, and Rational Method uses the 10-minute rainfall intensity for I. Getting the Tc geometry exactly right on a 25’ × 106’ lot doesn’t change the answer, because the computed Tc is floored regardless.

This is a regulatory safeguard, not a modeling choice. The 10-minute floor exists because Rational Method’s assumptions break down at very short Tc values — the method assumes the whole drainage area contributes to peak flow simultaneously, and at sub-10-minute Tc that assumption becomes unreliable. Flooring at 10 minutes keeps the method in its validity range.

What the Equation Does

The NRCS Runoff Equation takes a rainfall depth and returns the portion of that rainfall that becomes surface runoff. Not all rain runs off. Some is absorbed into the ground, some is held in surface depressions, some evaporates, some is intercepted by plants. The runoff equation bundles all of those losses into a single number — the curve number (CN) — and uses it to compute how much rain actually reaches the drainage system.

The equation is empirical. It was developed by the USDA Soil Conservation Service (now NRCS) from decades of runoff measurements on small agricultural watersheds, published in 1954 and refined through the 1970s. It is not derived from first principles of hydrology; it is a curve fit to observed data, simplified into a form usable by engineers without a hydrologist on staff.

In NJ stormwater practice, the NRCS Runoff Equation is the foundation of all major-development methodology. The Graphical Peak Discharge method, the Tabular Hydrograph method, and HydroCAD software all use this equation as their runoff generator. Rational Method (used for minor development) is a different approach entirely and does not involve CN.

The Equation

Where:

• Q = runoff depth (inches)
• P = rainfall depth (inches)
• S = potential maximum storage after runoff begins (inches)
• Ia = initial abstraction (inches) — rainfall absorbed before any runoff occurs

A high CN (say 98 for pavement) gives S = 0.204 inches — almost no storage, almost all rainfall becomes runoff. A low CN (say 30 for woods on sandy soil) gives S = 23.33 inches — the first 23 inches of rain would be absorbed before any runoff begins.

The Curve Number (CN)

The curve number is a dimensionless index from roughly 30 to 100 that captures how much rainfall a surface absorbs. It is a function of two things: surface cover (what’s on top) and Hydrologic Soil Group (HSG) (what’s underneath).

CN values come from TR-55 Table 2-2, which lists dozens of cover types crossed with the four hydrologic soil groups. The tool encodes all 31 of the cover types most common in NJ work across all four HSG columns — 124 values in total.

Hydrologic Soil Groups (HSG)

The same cover type — say, “open space in good condition” — has dramatically different CN values across the four groups: A=39, B=61, C=74, D=80. An open-space lawn on sand (HSG A) produces almost no runoff. The same lawn on clay (HSG D) produces substantial runoff. The surface looks identical; the soil underneath determines the hydrology.

Newark is predominantly HSG C and D. The tool’s address lookup pulls HSG from the NRCS Soil Data Access database automatically.

What Tc Is

Time of concentration (Tc) is the time required for water to travel from the hydraulically most-distant point on a drainage area to the outlet. It is a property of the site, not of the storm. The same Tc is used for the 2-year, 10-year, and 100-year storms.

Tc matters because it sets the peak discharge. A site with a short Tc delivers its runoff fast, producing a high sharp peak. A site with a long Tc delivers the same total volume spread over more time, producing a lower, broader peak. For a given rainfall distribution and CN, halving Tc can double the peak discharge.

The Three Segments

Sheet flow. Rain lands on a plane surface and moves as a thin sheet, typically less than an inch deep. Sheet flow is slow and capped at 100 feet.

Shallow concentrated flow. After sheet flow breaks down, water collects into small rivulets. Velocity is determined by slope and surface type (paved or unpaved).

Channel flow. Water enters a defined channel — a curb line, a swale, a storm sewer pipe. Velocity is computed by Manning’s equation.

Sheet Flow Formula

Where n = Manning’s roughness for sheet flow (much higher than channel flow values), L = length capped at 100 ft, P2 = 2-year 24-hour rainfall depth, s = slope (ft/ft).

Shallow Concentrated Flow

Channel Flow

On small Newark sites, channel flow is often negligible or absent — water reaches the public storm sewer directly from shallow concentrated flow across the sidewalk and curb.

NJ imposes a minimum Tc of 10 minutes regardless of computed value. On most Newark infill sites, the computed Tc lands below 10 minutes and the floor applies.

What It Is

The NRCS Graphical Peak Discharge Method is the standard peak-flow calculation for major development under NJAC 7:8. It comes from TR-55 Chapter 4 and converts CN, Tc, drainage area, and storm depth into a peak discharge value.

Where qu = unit peak discharge (from curve lookup), Am = area in mi², Q = runoff depth (inches), Fp = pond/swamp factor.

The Ia/P Ratio

The method uses Ia/P (initial abstraction over rainfall depth) to select the unit peak discharge curve. TR-55 publishes curves for Ia/P between 0.10 and 0.50. Outside that range, the tool clamps to the boundary value and flags it. Most Newark sites land in 0.10–0.20.

Distribution Type

The tool uses Type III coefficients as an approximation of NJ’s NOAA_C/D distributions for preliminary sizing. Final design requires HydroCAD or WinTR-55 with the actual NOAA distribution.

6-Storm Compliance

For major development, the tool runs six storms: current and projected 2/10/100-year. Post-construction peaks must not exceed 50%/75%/80% of pre-construction peaks respectively. A fail on any storm fails the full compliance check.

The tool produces the compliance check but not detention basin sizing for major development. For basin sizing, use HydroCAD or WinTR-55 with the parameters from the tool’s output summary.

What a Design Storm Is

A design storm is a specified rainfall event — a depth of rain falling over a defined duration — used as the reference input for stormwater calculations. NJ design storms are defined by total depth (inches) and duration (hours). The standard duration is 24 hours for quantity control and 2 hours for water quality.

Return Periods

A “10-year storm” has a 10% chance of being equaled or exceeded in any given year. Over a 30-year career, you’ll see multiple “10-year” storms.

Source — NOAA Atlas 14

Depths come from NOAA Atlas 14 Volume 2 Version 3. For Newark (PDS, 24-hour):

Current vs. Projected

Table 5-5 adjusts Atlas 14 for current NJ records (factors near 1.0). Table 5-6 projects forward for climate change (factors 1.15–1.25). Major development must pass both current AND projected storms — six storms total.

The 50/75/80 Targets

The asymmetric targets push a specific design hierarchy: infiltrate the small storms, detain the middle storms, pass the big storms with modest attenuation.

What the WQDS Is

The Water Quality Design Storm (WQDS) is 1.25 inches of rainfall over 2 hours, with a specific nonuniform distribution (NJAC 7:8 Table 5-4). It applies to all major development and sizes BMPs for pollutant removal and volumetric retention.

Roughly 90% of the annual pollutant load in NJ waters is delivered by storms of 1.25 inches or less. Designing BMPs around this storm captures the events that account for most pollution.

Runoff, Not Rainfall

BMPs treat runoff, not rainfall. The 1.25” rainfall is converted to runoff depth using the NRCS Runoff Equation. For impervious (CN=98):

For pervious surfaces on HSG C/D, runoff is substantially less. On HSG A/B, runoff may be zero for the WQDS because rainfall does not exceed initial abstraction.

The total WQDS runoff volume is what the BMP train must treat, retain, or pass through for pollutant removal.

What TSS Is

Total Suspended Solids (TSS) is the regulated pollutant target in NJ stormwater. TSS is a measure of suspended particulate matter — sediment, organic material, and the pollutants that adhere to those particles. Removing TSS reduces the broader pollutant load.

TSS Tiers

Series BMP Formula

Two 80% BMPs in series: R = 80 + 80 − (80×80)/100 = 96%. The second BMP treats what the first did not remove.

Three rules: same-mechanism BMPs cannot be in series; MTDs cannot follow MTDs or media filters; infiltrating BMPs cannot be in series.

Meeting 80% in Practice

Most Newark sites meet 80% with a single bioretention cell, infiltration basin, pervious paving, or sand filter sized to the WQDS runoff volume. For 95% (C1 riparian), a series arrangement is typically required — e.g., bioretention (90%) + sand filter (80%) = 98%.

What Retention Means

NJAC 7:8-5.6(d)1 (effective January 2026) requires that major development retain the full WQDS runoff using green infrastructure BMPs. “Retention” means infiltration, evapotranspiration, or reuse — no discharge to surface waters.

This applies to runoff from all surfaces in the disturbed area, not just motor vehicle surface.

BMPs That Count

Cistern, dry well, green roof, pervious paving (with infiltration), bioretention (with infiltration), infiltration basin, sand filter (with infiltration). BMPs with underdrains do NOT count — the underdrain discharges.

Impracticability Fallback

When retention is technically impracticable (Ksat < 1 in/hr, high SHWT, or high-pollutant runoff), two hydrograph tests must pass vs. a woods/good/HSG-D benchmark (CN=77):

• Test 1 — Peak: post-construction WQDS peak < benchmark peak
• Test 2 — Duration: post-construction hydrograph duration > benchmark duration

Both tests must pass. Test 2 tends to fail on high-CN sites.

Offsite Alternatives

When on-site fails: (1) remove existing impervious ≥ new impervious in same HUC-14, or (2) retain equivalent WQDS volume offsite in same HUC-14. Must be constructed prior to or concurrent with the development.

Two Sizing Drivers

Every infiltrating or filtering BMP is sized against two constraints. The governing design is whichever requires the larger footprint.

Storage-based area: The BMP must hold the full WQDS runoff from its sub-area.

Drain-time-based area: The BMP must fully drain within 72 hours (24 hours for pedestrian-accessible).

Design area = max(area_storage, area_drain)

Factor of Safety

All infiltration BMP sizing uses FOS=2 on tested permeability. A perc test returning 3.0 in/hr is used as 1.5 in/hr. This protects against clogging, compaction during construction, spatial variability, and seasonal variation.

CDA Limits

For typical Newark infill sites, CDA limits are rarely binding — small lots produce small CDAs well below the thresholds.

What Recharge Is

Groundwater recharge is the portion of rainfall that infiltrates past the root zone and reaches the saturated zone. Development reduces recharge by converting pervious surfaces to impervious. NJAC 7:8-5.4(b) requires major development to maintain pre-construction recharge volumes.

The GSR-32 Formula

Soil Factor and Soil Constant come from the NJ Geological Survey (208 NJ soil types × 14 land use codes). Climate Factor is municipality-specific (Newark = 1.31).

The Deficit

The deficit must be met by recharge BMPs (dry well, pervious paving, bioretention, infiltration basin, sand filter — all with infiltration, no underdrains).

URA Exemption

Newark is entirely within PA1 and qualifies as Urban Redevelopment Area. Any Newark project is exempt from recharge as a regulatory matter. The tool defaults this ON for Newark jurisdiction but allows override for voluntary compliance (LEED credits, client requests).

High-Pollutant Exclusion

Runoff from gas stations, solvent storage, hazardous materials areas, and industrial stormwater exposed to source material must NOT be recharged. Those areas are excluded from the recharge computation.

The 30% Standard

The safety factor on detention storage volume is 30%. This applies uniformly to all system types (both infiltration and detention) per the 2026 NJAC 7:8 update. The older practice used 20% for detention-only systems.

What It Protects Against

• Modeling uncertainty — actual runoff may exceed calculated by 10–20%
• Construction tolerance — as-built geometry varies from design
• Sediment accumulation — reduces effective capacity over time
• Pipe and orifice fouling — partial clogging alters release rates

What It Does NOT Protect Against

• Storms larger than the design storm
• Downstream flooding from storm coincidence
• Groundwater or structural failure

The 30% factor is a hydraulic buffer. It does not make the system resilient to events outside the design storm’s statistical scope.

For infiltrating detention systems (seepage pits), both the 30% volume safety factor AND the FOS=2 permeability factor apply in the same calculation chain. The combined effect is intentionally conservative.

The Equation

Where Q = flow (cfs), Cd = discharge coefficient, A = orifice area (ft²), g = 32.2 ft/s², h = head above orifice centerline (ft).

Discharge Coefficient Cd

The tool defaults to Cd = 0.60 for sharp-edged orifices in weir plates — the standard for NJ residential detention outflow.

Sizing to a Target

The 2.5” minimum diameter rule (Newark §41:17-6(1)(a)(ii)) applies to the intake opening, not the outflow orifice. The intake must be large enough that debris does not clog it.

The Equation

Where V = velocity (ft/sec), n = Manning’s roughness, R = hydraulic radius (ft), s = slope (ft/ft). The 1.49 constant converts from SI to US customary units.

Manning’s n by Material

Important: Manning’s n for channel flow is different from sheet-flow n. Dense grass is 0.24 in sheet flow but 0.035 in channel flow. Do not use sheet-flow n values in Manning’s equation for pipe or channel.

Hydraulic Radius

For a circular pipe flowing full: R = D/4. For a 12” pipe, R = 0.25 ft. For a 36” pipe, R = 0.75 ft.

Velocity Check

Newark §41:17-4-6(c)(iii)(6) requires velocity between 2.5 and 10 fps. Below 2.5 fps, sediment deposits. Above 10 fps, the discharge erodes downstream.

What a Distribution Type Is

The rainfall distribution type specifies when rain falls during a 24-hour design storm. A storm with all 5.03 inches concentrated in one hour produces very different peak runoff than one spread evenly. Distribution types are standardized so projects use consistent assumptions.

Type II and Type III

The original SCS distributions from TR-55 (1986). Type II covers most of the eastern US (sharper peak, mid-storm). Type III covers Atlantic coastal regions (broader, later peak). NJ historically straddled the boundary.

NOAA_C and NOAA_D

2015 NRCS updates based on NOAA Atlas 14 analysis. NOAA_C replaces Type II for most of NJ. NOAA_D replaces Type III for the DelMarVa coastal plain. The updates reflect more intense short-duration rainfall consistent with climate trends.

The NJ Ambiguity

The NJ BMP Manual cites NOAA_C/D as current practice for WinTR-55. NJAC 7:8 and most commercial software (HydroCAD) still reference Type II/III. The tool uses Type III coefficients as an approximation, flagged explicitly. For final design, run WinTR-55 with NOAA_C (most of NJ) or NOAA_D (southwestern/coastal).

When It Matters

On typical Newark sites, the distribution choice produces peak discharge values that agree within 5–15%. The differences become more pronounced on larger sites and where Tc is very short or very long. For water quality (WQDS 1.25”/2hr), the distribution is Table 5-4 — entirely separate from the 24-hour Type II/III/NOAA_C/D choice.

Content for this section is being drafted.