Stormwater Glossary

Rainfall data series that uses only the single largest precipitation event per year to construct return-period statistics. Produces slightly lower design storm depths than Partial Duration Series (PDS) at short return periods. NJ stormwater uses PDS, not AMS.

The National Weather Service’s precipitation frequency atlas. Publishes design storm depths (inches of rainfall for a given duration and return period) for every location in the US. NJ is covered by Atlas 14 Volume 2 Version 3.

A structural or non-structural stormwater control measure designed to remove pollutants, reduce runoff, or restore natural hydrologic function. Examples include bioretention, cisterns, infiltration basins, sand filters, and pervious paving.

NJDEP classification for waters of exceptional ecological, recreational, or water-supply significance. Sites discharging within 300 ft of C1 waters must meet a 95% TSS removal standard instead of the 80% standard.

The total area of land that drains runoff to a single BMP. Some small-scale BMPs have regulatory CDA limits (dry well: 1 acre; most others: 2.5 acres) to prevent overwhelming the treatment capacity.

An empirical factor (typically 0.60 to 0.82) applied in the orifice equation to reduce theoretical flow to real-world flow. Captures the effect of flow contraction and friction at the opening. NJ residential detention standard: Cd = 0.60 for sharp-edged orifices in weir plates.

The third flow regime in the Tc calculation. Water in a defined cross-section — curb, gutter, swale, pipe. Velocity computed by Manning’s equation. Often negligible or absent on small Newark sites.

A dimensionless index from 30 to 100 that captures how much rainfall becomes runoff from a surface. Depends on surface cover (pavement, lawn, woods) and soil type (Hydrologic Soil Group A/B/C/D). High CN (98 for pavement) means almost all rainfall runs off; low CN (around 30 for woodland on sand) means almost none does.

An area-weighted average CN for a site with multiple cover types and/or soil groups. The tool builds the composite CN from user-entered segments.

A specified rainfall event — defined by depth and duration — used as the reference input for stormwater calculations. NJ regulates the 2-/10-/100-year 24-hour storms (quantity) and 1.25in/2hr WQDS (quality).

A stormwater facility that temporarily stores runoff and releases it at a controlled rate through an outlet orifice, reducing peak discharge. Distinct from retention (which holds water permanently via infiltration, evapotranspiration, or reuse). Can be a surface basin (open pond, dry or wet) or underground (HDPE chambers, concrete vaults, seepage pits).

A multiplier applied to a calculated value to produce a larger design value, accounting for uncertainty. Stormwater uses two: 30% on detention storage volume, and a factor of 2 on soil permeability in BMP sizing.

A subset of stormwater BMPs that use vegetation, soil, and natural processes to infiltrate, evapotranspire, or reuse runoff on site. Includes cisterns, bioretention, pervious paving, dry wells, and green roofs. The 2026 volumetric retention standard requires GI specifically.

The NJ Geological Survey’s groundwater recharge evaluation method, published 1993, updated 2004. Computes annual recharge volume from soil factor x climate factor - soil constant. Basis of NJAC 7:8-5.4 groundwater recharge compliance.

A four-tier USDA classification (A, B, C, D) of soils by their saturated infiltration rate. Group A soils (sand, gravel) infiltrate faster than 0.30 in/hr; Group D soils (clay) slower than 0.05 in/hr. Same surface cover produces very different CNs on different HSGs.

A 14-digit Hydrologic Unit Code — a USGS watershed boundary identifier. NJ has roughly 150 HUC-14 watersheds. Used in NJAC 7:8 for offsite compliance: impervious removal or offsite retention must occur within the same HUC-14 as the development site.

The amount of rainfall absorbed before any surface runoff occurs. Includes interception by vegetation, ponding in surface depressions, and wetting of dry ground. Taken as 0.2 x S in the NRCS Runoff Equation by convention.

The ratio of initial abstraction to rainfall depth, used in the NRCS Graphical Peak Discharge method to select the unit peak discharge curve. Valid range is 0.10 to 0.50; the tool clamps outside this range.

A formula for computing water velocity in a pipe or channel flowing under gravity: V = (1.49/n) x R^(2/3) x s^(1/2). Used for pipe sizing, velocity checks (Newark requires 2.5–10 fps), and the channel-flow segment of Tc.

A proprietary, engineered stormwater treatment unit (hydrodynamic separator, filter vault, etc.) certified by NJDEP for a specified TSS removal rate (typically 50% or 80%). MTDs have specific siting and series-pairing restrictions.

2015 NRCS updates to the Type II and Type III rainfall distributions, based on newer NOAA Atlas 14 analysis. NOAA_C replaces Type II in most of NJ; NOAA_D replaces Type III in NJ’s DelMarVa coastal plain.

A formula for computing flow through a controlled opening under a water head: Q = Cd x A x sqrt(2gh). Used to size detention system outflow openings to achieve a target discharge rate.

Rainfall data series that includes all events above a threshold, which may mean multiple events per year. Used for NJ stormwater design. Produces slightly higher depths at short return periods than AMS.

NOAA’s online lookup tool for Atlas 14 rainfall depths at hdsc.nws.noaa.gov. Returns depth-duration-frequency tables for any US location. Newark values: 3.30/5.03/8.41 inches for 24-hour 2-/10-/100-year PDS.

A simple peak-flow calculation: Q = C x I x A. Used for minor development in Newark but not permitted for major development under NJAC 7:8, which requires NRCS methodology.

The statistical average time between storm events of a given size. A ‘10-year storm’ has a 10% chance of occurring in any given year — not that it occurs every 10 years.

A single number per surface type (0 to 1) specifying the fraction of rainfall that becomes runoff. Used in Rational Method. Roofs 0.95–0.99, pavement 0.95, lawn 0.20. Unlike CN, does not depend on soil type or storm size.

A parameter in the NRCS Runoff Equation representing the maximum storage a soil can hold after runoff begins. Computed from CN: S = 1000/CN − 10. High CN gives low S.

The second flow regime in the Tc calculation. Water has organized into small rills but has not yet entered a defined channel. Velocity computed by paved/unpaved slope lookup. Typical velocities 1–4 ft/sec.

The first flow regime in the Tc calculation. Thin, unconcentrated flow across a plane surface (lawn, parking lot), less than an inch deep. Length is capped at 100 ft; beyond that, flow concentrates into rivulets.

The highest expected groundwater elevation during the wet season, determined by site soil investigation. Infiltration BMPs must maintain a minimum 2-foot separation from SHWT (or 1 foot for some BMPs with underdrains).

The time required for water to travel from the hydraulically most-distant point on a drainage area to the outlet. Computed as the sum of three flow regimes: sheet + shallow concentrated + channel. NJAC 7:8 imposes a 10-minute minimum.

Regulated pollutant target in NJ stormwater. A measure of suspended particulate matter — sediment, organic material, and pollutants attached to those particles. The 2026 NJAC 7:8 requires 80% TSS removal for standard sites, 95% near C1 waters.

The USDA Technical Release 55 Chapter 6 Storage-Indication Method for estimating detention basin storage volume. Uses a polynomial equation (Table F-2) to compute the ratio of storage volume to runoff volume (Vs/Vr) based on the ratio of target outflow to peak inflow (qo/qi). Published as “adequate for final design of small detention basins” where ±25% storage variance is tolerable.

The 1986 SCS rainfall distribution types from TR-55. Type II covers continental interior and most of the eastern US; Type III covers Gulf of Mexico and Atlantic coastal regions. NJ historically used Type II for most of the state, Type III for southern coastal counties.

A state-designated zone where existing impervious surface is already extensive. Newark is entirely within PA1 and qualifies as URA. NJAC 7:8-5.4(b)2 exempts URA projects from the recharge requirement.

The ratio of required detention storage volume (Vs) to total runoff volume (Vr). Computed from TR-55 Table F-2 polynomial as a function of qo/qi and rainfall distribution type. For NJ (Types II/III), typical values range from 0.18 (marginal reduction) to 0.56 (aggressive reduction).

A 1.25-inch rainfall over 2 hours with a specific nonuniform distribution, used to size water quality BMPs and check volumetric retention. Separate from the 2-/10-/100-year quantity storms.