Stormwater Calculator v2

Rational method runoff, detention/infiltration sizing, orifice and overflow pipe sizing, 72-hr drain-down, and system elevation checks. NJAC 7:8 (Jan 2026) county-based rainfall.
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Site Location

Site Address (auto-fills county, township, soil & HSG)

County & Township

NJ County

Township / Municipality

County selection pre-populates 24-hour rainfall depths from NRCS Table 5-2 and applies NJAC 7:8 Tables 5-5/5-6 adjustment factors. Township auto-fills GSR-32 climate factor.

Jurisdiction

Newark §41:17 extends stormwater requirements to all site plans and subdivisions, including non-major (minor) development.

Development Type

Newark Ordinance §41:17. Minor development uses the 10-year storm; major uses the 100-year storm. Major development per NJAC 7:8 §7:8-1.2: ≥1 acre disturbance or ≥0.25 acre new impervious.

System Type

Minor development only

Storm Event

Rainfall Source

Intensity (in/hr)

Duration Tp (min)

Engineering Overrides

Safety Factor (%)

30% default per standard NJ infiltration practice (user-overridable)

TP Multiplier — Infiltration

TP Multiplier — Detention

TP vs TP/2: infiltration uses full storm duration (water accumulates while infiltrating over entire storm). Detention uses TP/2 (peak inflow to outflow lag, approximates triangular hydrograph). Standard NJ stormwater practice — exact source under verification.

Site Perc Rate

Tested Perc Rate

Raw tested value. FOS=2 applied automatically in sizing.

Unit

Site Areas

Total Lot Area (SF)

Existing

Building (SF)

Paved (SF)

Seeded (SF)

Leave blank = Lot - Bldg - Paved

Proposed

Building (SF)

Paved (SF)

Seeded (SF)

Leave blank = Lot - Bldg - Paved

Proposed — CN Composite (NRCS)

Auto-populated from proposed areas above. Adjust cover types and per-area HSG as needed. Areas should total to lot SF.

Runoff Coefficients

C — Building

C — Paved

C — Seeded

Site Conditions

Major development (NJAC 7:8)

Major Development — NJAC 7:8-5.6(b). Analysis uses NRCS methodology per §7:8-5.7(a)(1). Rational Method is not permitted for major development. Results are preliminary — final design requires full hydrograph routing (HydroCAD/WinTR-55).

Override distribution:

Hydrologic Soil Group (Dominant)

Per NRCS Web Soil Survey. For mixed soils, use area-weighted assignment in the CN composite sections below.

% Pond / Swamp Area

TR-55 Table 4-2 adjustment factor Fp. Leave 0 if none.

Tested Perc Rate

Raw tested value in inches/hour. FOS=2 applied automatically in BMP and detention sizing.

Perc Unit

Flags

Coastal Plain

Urban Redev. Area

Newark is entirely PA1 per §7:8-5.4(b)(2). Exempts GW recharge (Phase C).

Tidal Flood Hazard

§7:8-5.6(b)(4) — quantity check informational only.

Design Rainfall Depths (24-hr)

Pre-Construction Conditions

Per §7:8-5.7(a)(2), pre-construction is presumed woods/good condition unless site history documented for 5+ years without interruption.

Cover TypeArea (SF)HSGCN

Time of Concentration — Pre-Construction

Per NJ BMP Manual Ch. 5: no minimum Tc and no assumed Tc. Tc must be computed from actual flow path.

Time of Concentration — Post-Construction

Storm Analysis — 6-Storm Compliance

Water Quality & BMP Selection

Site Classification

Within 300 ft of C1 Waters?

Motor Vehicle Surface?

MVS Area (SF)

Public Trans Entity?

High-Pollutant Loading Area?

WQDS Runoff

Water Quality Design Storm: 1.25 in / 2 hr, nonuniform distribution per NJAC 7:8 Table 5-4.

TSS Removal Target

BMP Treatment Train

BMP Sizing

Volumetric Retention (§7:8-5.6(d)1)

Detention Storage Sizing (TR-55 Ch. 6)

Method: TR-55 Ch. 6 Storage-Indication, Types II/III (Table F-2 polynomial). Safety factor: % (NJAC 7:8 2026 default: 30%).

Groundwater Recharge (GSR-32)

URA Exemption

Site Parameters

Municipality

Auto-filled from County & Township above

Soil Type

Soil auto-filled from Site Address lookup above, or select manually from dropdown.

Pre-Development Cover

Impervious (SF) — from existing Site Areas

Pervious Cover Type

Post-Development Cover

Impervious (SF) — from proposed Site Areas

Pervious Cover Type

Recharge Analysis

Recharge BMP Availability

Seepage Pit — Peerless Concrete

Pit Model

# of Pits

Gravel Ring (ft)

Gravel Below (ft)

Void Ratio

Percolation

Perc Unit

Perc Rate

HDPE Detention Pipe

Pipe Diameter

Length per Row (LF)

# of Rows

SDR-35 Detention Pipe

Pipe Diameter

Length per Row (LF)

# of Rows

NDS Flo-Well Drywell

24" dia × 24" tall, 50 gal (6.68 CF) per unit. Stackable up to 4 high or side-by-side 18" apart. Gravel around at ~40% void.

# of Units

Gravel Ring (ft)

Gravel Below (ft)

Void Ratio

Stack Height (units)

Percolation

Perc Unit

Perc Rate

NDS EZflow French Drain

Bundle Size

Total Length (LF)

NDS StormChamber

Chamber Height

# Start

# Middle

# End

# Closed

ADS StormTech

Chamber Model

# of Chambers

Installed storage assumes 40% stone porosity per ADS specifications.

Outflow Pipe Sizing

Minor development only

Pipe Type

Slope (in/ft)

Selected Pipe

Outlet Control Structure

Orifice Type

Sharp-edged orifice in a thin plate — Cd = 0.60 per NEH Part 650, Ch. 3 (standard sharp-edged circular orifice).

System Elevations

Minor development only

Grade at System Location

Finish Grade at System (ft)

Inlet Pipe — Farthest Inlet to System

Grade at Inlet (ft)

Min. Cover (ft)

Pipe Length (ft)

Min. System Cover (ft)

Min. Pipe Slope (in/ft)

High Water Table

HWT Elevation (ft)

Outflow Connections

Enter available connection data below. The calculator will check gravity for each and show which options work.

Sewer Connection

Sewer Invert (ft)

Pipe Length to Sewer (ft)

Grade at Sewer (ft)

Sewer Depth Below Grade (ft)

Pipe Length to Sewer (ft)

Curb Connection

Top of Curb (ft)

Bottom of Curb (ft)

Pipe Length to Curb (ft)

Outflow Selection

Enter elevation data and calculate to see which outflow options are available.

Results

These calculations provide preliminary estimates for stormwater management design. Verify all outputs against current adopted codes, local amendments, and project-specific conditions. This tool does not replace professional engineering analysis. Green Stories LLC is a building design and consulting firm. All projects requiring a licensed architect are completed in collaboration with registered design professionals.