Glossary¶
Shared terminology for consistent understanding across all OPAL modules.
Reference template: Agents/03_component_templates.md — GLOSSARY_TERM_TEMPLATE
Each term follows the format: Definition → Engineering Context → Where It Appears
A¶
Atlas 14 : NOAA Atlas 14 — the authoritative reference for precipitation frequency statistics in the United States, providing point precipitation depth-duration-frequency (DDF) tables. : Engineering context: Required in NJ for all design storm hydrologic analysis under the 2026 BMP Manual and N.J.A.C. 7:8. Superseded older TP-40 and Huff-Angel data. Use Atlas 14 Volume 2 (Mid-Atlantic) for NJ. : Appears in: Design Workflow (Hydrologic Analysis), BMP Sizing
B¶
Best Management Practice (BMP) : A structural or non-structural practice designed to manage stormwater quantity and/or quality. : Engineering context: In NJ, BMPs are selected based on site constraints and must meet N.J.A.C. 7:8 performance standards. BMPs are divided into GI (Green Infrastructure) and Non-GI categories in the 2026 BMP Manual. : Appears in: BMP Library, Regulatory Explorer, Engineering Constraints
Bioretention : A shallow vegetated depression filled with engineered media designed to filter and infiltrate stormwater. : Engineering context: One of the most versatile small-scale GI practices; qualifies as GI when native soil infiltration is confirmed per Chapter 12. Media specification, ponding depth, and underdrain configuration are the key design variables. : Appears in: BMP Library — Bioretention
C¶
Curve Number (CN) : A dimensionless parameter from the NRCS Runoff Curve Number method that represents the runoff potential of a land surface based on land use, hydrologic soil group, and antecedent moisture condition. : Engineering context: Used in TR-55 and HEC-HMS to calculate design storm runoff volumes and peak flows. Pre-development CN is required for comparison with post-development CN to demonstrate no increase in peak discharge. Higher CN = more runoff (impervious surfaces: CN 98; forested A soils: CN 30–39). : Appears in: Design Workflow (Hydrologic Analysis), BMP Sizing
D¶
Detention : Temporary storage of stormwater runoff with a controlled release rate, used to attenuate peak discharge. : Engineering context: Detention BMPs (dry detention, extended detention, underground detention) meet N.J.A.C. 7:8-5.4 quantity control standards by matching pre-development peak discharge rates for design storms. Detention does not reduce total runoff volume — it only delays it. : Appears in: BMP Library (Ch. 10 and 11), Regulatory Explorer
Drawdown Time : The time required for a stormwater BMP to drain completely after reaching its design ponding or storage level. : Engineering context: NJ BMP Manual specifies maximum drawdown requirements by BMP type: bioretention and infiltration BMPs must drain within 48–72 hours to prevent anaerobic conditions, mosquito breeding, and plant stress. Drawdown is verified through drawdown calculations based on soil Ksat or underdrain flow rate. : Appears in: BMP Library, Engineering Constraints
E¶
Embankment : An engineered earthen fill structure that retains water in a detention or retention basin. : Engineering context: Embankments ≥ 6 ft in height OR impounding ≥ 0.5 million cubic feet of water trigger N.J.A.C. 7:20 Dam Safety jurisdiction. Embankment design requires compaction specification, emergency spillway, and stability analysis. Animal burrowing is the leading long-term embankment failure mechanism. : Appears in: BMP Library (wet pond, dry detention, extended detention), Regulatory Explorer
Evapotranspiration (ET) : The combined process of water evaporation from soil and surface water and transpiration from plant leaves returning water vapor to the atmosphere. : Engineering context: ET is the primary mechanism for volumetric reduction in green roofs, bioretention, and cisterns. VRC credit for these BMPs is based on the volume that is lost to ET between storms rather than infiltrated into groundwater. NJ design uses monthly ET rates from published references. : Appears in: BMP Library (Green Roof, Bioretention, Cistern), Design Workflow
F¶
Forebay : A small inlet-zone settling chamber or pool upstream of the main treatment area of a BMP, designed to trap coarse sediment before it reaches the primary treatment zone. : Engineering context: Required by NJ BMP Manual for wet ponds, constructed wetlands, and extended detention basins. Typically sized at 10–15% of WQV. Must be accessible by equipment for periodic cleanout. The forebay extends the maintenance interval of the primary treatment zone by 3–10 years. : Appears in: BMP Library (wet pond, constructed wetland, extended detention)
G¶
Green Infrastructure (GI) : Stormwater management practices that mimic natural hydrology by infiltrating, evapotranspiring, or harvesting and using stormwater, as defined in N.J.A.C. 7:8-5.3. : Engineering context: NJ requires all major development to meet 25% of the Water Quality Volume standard using GI BMPs (the GI Requirement). GI BMPs include bioretention, infiltration basin, pervious pavement (over native soil), green roof, cistern, and other Ch. 9 and qualifying Ch. 10 BMPs. : Appears in: BMP Library (Ch. 9 and 10), Regulatory Explorer
Groundwater Mounding : The rise in the water table below and around an infiltration BMP caused by the accumulation of infiltrated volume faster than the aquifer can laterally transmit it. : Engineering context: Required analysis per NJ BMP Manual Chapter 13 for large infiltration footprints (typically > 3,000 ft² or when SHWT < 4 ft below BMP base). Mounding analysis uses the Hantush equation or equivalent. Uncontrolled mounding can eliminate infiltration capacity and cause neighboring basement flooding. : Appears in: Engineering Constraints, BMP Library
Groundwater Recharge : The replenishment of subsurface aquifers through infiltration of surface water into native soil. : Engineering context: Required under N.J.A.C. 7:8-5.2; each major development project must recharge the volume of groundwater equivalent to the volume recharged pre-development. Volume is calculated based on pre-development impervious cover and NRCS HSG. : Appears in: Regulatory Explorer, Engineering Constraints
H¶
Hydrologic Soil Group (HSG) : USDA NRCS classification of soils into four groups (A, B, C, D) based on saturated hydraulic conductivity and infiltration potential. : Engineering context: HSG A (sandy soils, high Ksat) and HSG B (loamy soils, moderate Ksat) are most suitable for infiltration BMPs. HSG C and D soils require underdrain or non-infiltration design. Obtained from NRCS Web Soil Survey — but field verification per Chapter 12 is required for GI credit. : Appears in: Engineering Constraints, BMP Library
I¶
Impervious Surface : Any surface that prevents the natural infiltration of water into the soil, including rooftops, asphalt, concrete, and compacted gravel. : Engineering context: Percent impervious drives regulatory applicability under N.J.A.C. 7:8 (major development threshold = 0.25+ acres new impervious). Also drives runoff volume calculations (CN), TSS load estimates, and GI Requirement sizing. : Appears in: Regulatory Explorer, Design Workflow
Infiltration Exclusion Zone (IEZ) / Infiltration Exclusion Area : Geographic areas where groundwater infiltration from stormwater BMPs is prohibited or restricted due to contamination risk, karst geology, or known groundwater vulnerability. : Engineering context: NJDEP designates IEZs based on sole-source aquifer areas, contaminated site proximity, and karst terrain mapping. Infiltration-based BMPs (bioretention, infiltration basin, dry well) cannot be used in IEZ designated areas without special review. Check NJDEP IEZ map before designing GI. : Appears in: Engineering Constraints — Infiltration Feasibility
K¶
Karst : A type of landscape formed in soluble bedrock (limestone, dolomite) where subsurface dissolution has created voids, sinkholes, and conduits that can transmit water rapidly through the aquifer with minimal treatment. : Engineering context: Karst terrain in NJ (portions of Warren and Sussex Counties) is incompatible with infiltration BMPs: infiltrated stormwater bypasses the soil treatment process and moves directly through karst conduits to groundwater. Chapter 12 field investigation must identify karst risk before GI siting. : Appears in: Engineering Constraints — Infiltration Feasibility, BMP Library
Ksat (Saturated Hydraulic Conductivity) : The rate at which water moves through a fully saturated soil medium under a unit hydraulic gradient, typically expressed in inches per hour (in/hr) or cm/hr. : Engineering context: The critical soil parameter for all infiltration BMP design. NJ requires field-measured Ksat (double-ring infiltrometer or falling-head permeameter) per Chapter 12 for all GI credit claims. Minimum Ksat thresholds vary by BMP type but are generally ≥ 0.52 in/hr for GI qualification under the 2026 manual. Soil survey Ksat estimates are NOT acceptable for regulatory credit. : Appears in: Engineering Constraints — Soil Permeability, BMP Library
L¶
Land Disturbance : The physical alteration, grading, clearing, or excavation of land that disrupts existing vegetative cover and soil structure. : Engineering context: N.J.A.C. 7:8 is triggered when land disturbance ≥ 1 acre. Land disturbance also triggers soil erosion and sediment control requirements under N.J.A.C. 7:50 (NJDEP) and local approvals. Temporary sediment controls (silt fence, inlet protection, construction entrance) are required during BMP construction to prevent premature clogging of newly installed BMPs. : Appears in: Regulatory Explorer (Major Development Definition), Design Workflow
M¶
Major Development : Under N.J.A.C. 7:8, a project that disturbs 1 or more acres of land OR increases impervious surface by 0.25 acres or more. : Engineering context: The threshold that triggers full N.J.A.C. 7:8 compliance, including groundwater recharge, 80% TSS water quality standard, GI Requirement, and stormwater quantity control. Minor development projects below the threshold may still require local municipal stormwater ordinance compliance. : Appears in: Regulatory Explorer, Design Workflow
N¶
N.J.A.C. 7:8 : New Jersey Administrative Code, Title 7 (Environmental Protection), Chapter 8 (Stormwater Management). : Engineering context: The primary regulatory framework governing stormwater management design for all new major development in NJ. Key sections: 5.2 (groundwater recharge), 5.3 (water quality including GI requirement), 5.4 (stormwater quantity control). Implemented through the Stormwater Management Plan review process by reviewing agencies. : Appears in: Regulatory Explorer, all modules
Non-GI BMP : A stormwater BMP that does not meet the definition of Green Infrastructure under N.J.A.C. 7:8-5.3 — meaning it does not infiltrate into native soil, evapotranspire, or harvest and reuse stormwater as its primary treatment mechanism. : Engineering context: Non-GI BMPs (dry detention, extended detention, wet pond, underground detention, oil/grit separator, HDS, manufactured treatment device, blue roof) count toward water quality TSS compliance and quantity control but do not generate VRC credit toward the GI Requirement. Separate GI BMPs must be provided to meet the GI Requirement. : Appears in: BMP Library (Ch. 11), Regulatory Explorer
O¶
Orifice : A calibrated opening (circular hole or slot) in an outlet structure through which stormwater is discharged at a controlled rate governed by the head of water above the orifice. : Engineering context: Orifice sizing is the primary quantity control design task for wet ponds, dry detention, extended detention, and underground detention. Q = Cd × A × √(2gh) where Cd ≈ 0.6 for sharp-edged orifices, A = orifice area, g = gravity, h = head above orifice centerline. Multiple orifices in a riser provide multi-stage control for different design storm levels. : Appears in: BMP Library (quantity control BMPs), Design Workflow (BMP Sizing)
P¶
Peak Discharge : The maximum instantaneous flow rate of runoff at a specific point in a drainage system during or after a storm event. : Engineering context: N.J.A.C. 7:8-5.4 requires that post-development peak discharge does not exceed pre-development peak discharge for the 2-year, 10-year, and 100-year design storms. Calculated using NRCS TR-55, rational method, or HEC-HMS depending on drainage area size. : Appears in: Regulatory Explorer, Design Workflow (Hydrologic Analysis), BMP Library (quantity control BMPs)
Pre-treatment : A BMP or structural element installed upstream of a primary treatment BMP to remove coarse sediment, oil, and floatables before they enter the primary BMP. : Engineering context: Pre-treatment is critical for extending the maintenance interval and service life of filter media, stone fill, and constructed wetland inlets. Common pre-treatment measures: forebays (basins), OGS/HDS units, sump catch basins, and vegetated filter strips. Without pre-treatment, most engineered BMP media systems experience premature clogging. : Appears in: BMP Library (all filter and infiltration BMPs)
S¶
Seasonal High Water Table (SHWT) : The highest elevation reached by groundwater in a typical year, determined through field soil boring investigation and mottling identification by a licensed soil scientist. : Engineering context: A minimum 2 ft separation between SHWT and the bottom of any infiltration BMP is required by the NJ BMP Manual. SHWT must be determined by an LPSS-certified boring, not estimated from county soil maps. High SHWT is the most common infiltration BMP exclusion factor in NJ. : Appears in: Engineering Constraints, BMP Library
Stormwater Hotspot : A land use or facility that generates stormwater runoff with significantly higher concentrations of pollutants (metals, hydrocarbons, pathogens) than typical residential or commercial areas. : Engineering context: NJ Stormwater Management Rules designate hotspot land uses (fueling stations, vehicle maintenance, certain industrial uses, commercial nurseries, etc.) that require more stringent stormwater treatment. Infiltration BMPs generally cannot be used for hotspot drainage areas due to groundwater contamination risk. : Appears in: Regulatory Explorer, BMP Library (engineering constraints sections)
T¶
Time of Concentration (Tc) : The time required for runoff to travel from the hydraulically most distant point in a watershed to the point of analysis (typically the outlet or BMP inlet). : Engineering context: Tc governs the peak discharge for the rational method and TR-55. Shorter Tc = higher peak discharge for the same rainfall depth. For pre/post comparison, Tc is calculated separately for pre-development (natural sheet flow, shallow concentrated flow, channel flow) and post-development (pavement, curb and gutter, swales). Minimum Tc = 5 minutes for the rational method. : Appears in: Design Workflow (Hydrologic Analysis)
Total Suspended Solids (TSS) : Particles (mineral and organic) suspended in stormwater that are captured during filtration and settling. : Engineering context: NJ requires ≥ 80% TSS reduction for major development water quality compliance (N.J.A.C. 7:8-5.3). The 80% removal standard applies to the Water Quality Volume (WQV) design storm and must be based on a BMP with documented removal efficiency. TSS is the primary water quality metric for BMP compliance in NJ. : Appears in: Regulatory Explorer, BMP Library
Treatment Train : A series of two or more BMPs arranged sequentially to achieve progressively higher levels of stormwater treatment, or to address both quality and quantity compliance through complementary measures. : Engineering context: Common NJ treatment trains: (1) Oil/grit separator → bioretention (quality + quantity); (2) Vegetated filter strip → infiltration trench (pretreatment + GI); (3) Green roof (GI/quality) + dry detention (quantity). A treatment train approach reduces the size and maintenance burden of each individual BMP while achieving the same compliance outcome. : Appears in: Design Workflow (BMP Selection), BMP Library
U¶
Underdrain : A perforated pipe (typically 4–6 inch diameter, schedule 40 PVC or HDPE) installed at or near the base of an engineered BMP media system to collect filtered effluent and discharge it to the storm drain or a receiving infiltration structure. : Engineering context: Whether an underdrain is present or absent determines GI vs. Non-GI classification for many BMPs. Bioretention or sand filter with underdrain to storm drain = Non-GI. Bioretention with underdrain to infiltration sump or no underdrain over permeable native soil = GI (with Chapter 12 documentation). Underdrain sizing must not be the limiting hydraulic element during design storms. : Appears in: BMP Library, Engineering Constraints
V¶
Volumetric Reduction : Reduction of total stormwater runoff volume through infiltration into native soil, evapotranspiration, or harvest and reuse — pathways that permanently remove water from the storm drain system. : Engineering context: New volumetric reduction standards introduced in the 2026 BMP Manual (Chapter 14). Volumetric Reduction Credit (VRC) is a metric quantifying how much of the WQV is eliminated from the drainage system through GI BMPs. VRC credit is the core calculation supporting GI Requirement compliance. : Appears in: Regulatory Explorer, BMP Library
Volumetric Reduction Credit (VRC) : The specific quantity of stormmwater volume (in acre-feet or cubic feet) eliminated from the site drainage system through GI BMP infiltration, ET, or reuse, credited toward the GI Requirement. : Engineering context: VRC was formalized as a specific compliance metric in the 2026 NJ BMP Manual, Chapter 14. Each GI BMP type generates VRC based on its mechanism: infiltration BMPs generate VRC equal to the volume infiltrated per storm; green roofs and cisterns generate VRC based on ET and reuse rates. VRC must equal or exceed 25% of WQV for the GI Requirement to be met. : Appears in: Regulatory Explorer, BMP Library
W¶
Water Quality Volume (WQV) : The volume of stormwater runoff generated by 1.25 inches of rainfall from all impervious surfaces on the project site, representing the first flush of a typical NJ storm. : Engineering context: WQV is the primary design volume for all NJ water quality BMPs under N.J.A.C. 7:8-5.3. All primary water quality BMPs must treat the full WQV from the contributing drainage area. The 1.25-inch rainfall depth represents the 90th percentile storm event depth in NJ — treating this volume captures the majority of stormwater pollutant loading. : Appears in: Regulatory Explorer (Water Quality Design Storm), BMP Library, Design Workflow
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