Stormwater Glossary: Key Terms Defined

This glossary defines common terms used in stormwater management, hydrology, and watershed planning. It covers concepts related to stormwater treatment practices, site design, and local ordinances.

A

Anti-seep collar: An impermeable diaphragm, typically made of metal or concrete, installed around a spillway conduit to increase the seepage length and prevent water from leaking along the outside of the pipe.
Aquatic bench: A shallow, flat area located around the inside perimeter of a permanent pool in a stormwater pond. The bench is typically 10 to 15 feet wide and vegetated with aquatic plants to improve pollutant removal and enhance safety.
Aquifer: A permeable geologic formation that is capable of storing and yielding significant quantities of groundwater to wells and springs.
As-built: A drawing, plan, or certification that documents the final conditions of a site or structure as it was actually constructed.

B

Baffles: Guides, grids, or similar devices placed within a pond or other treatment practice to deflect or regulate flow. Baffles create a longer flow path to increase treatment time and improve settling of pollutants.
Bankfull flow: The streamflow condition that completely fills a stream channel up to the top of its banks, at which point water begins to overflow onto the floodplain.
Baseflow: The portion of stream discharge that originates from groundwater seeping into the channel. It represents the sustained, fair-weather flow of a stream.
Berm: A linear earthen embankment or dike, often used to contain or divert runoff, or a shelf that breaks the continuity of a slope.
Best Management Practice (BMP): A structural or non-structural practice used to manage the quantity and improve the quality of stormwater runoff. A wide variety of best management practices are used to reduce flooding, remove pollutants, and provide other environmental amenities.
Bioretention: A stormwater treatment practice that uses a conditioned soil bed and vegetation to filter runoff stored in a shallow depression. Bioretention areas are commonly installed in parking lot islands and residential yards.
Buffer: An area of natural or managed vegetation adjacent to a shoreline, wetland, or stream where development is restricted or prohibited to protect the water body from impacts.

C

Channel: A natural or artificial watercourse that conveys water.
Channel Protection Volume (Cpv): The runoff volume from a small, frequent storm event, typically the 1-year, 24-hour storm, that must be controlled to protect downstream channels from erosion. The methods for calculating sizing criteria like Cpv are defined by local or state authorities.
Conservation easement: A voluntary legal agreement that limits the type or amount of development on a property to protect its natural or open-space values. The easement is typically held by a government agency or land trust.
Curve Number (CN): A numerical representation of a drainage area’s runoff potential, based on its hydrologic soil group, land cover, and imperviousness. Developed by the Natural Resources Conservation Service, the CN is used to calculate runoff volume from a given rainfall amount.

D

Dam: A barrier constructed to confine or raise water for storage or diversion, create a hydraulic head, or retain sediment or debris.
Detention: The temporary storage of storm runoff in a facility with the goals of controlling peak discharge rates and allowing for gravity settling of pollutants.
Disturbed area: An area in which the natural vegetative soil cover has been removed or altered, making it susceptible to erosion.
Drainage area: All land and water area from which runoff flows to a common point. Also known as a watershed or catchment.
Dry pond: A stormwater basin designed to have no permanent pool of water. Stormwater is detained in the basin temporarily to control peak flows and settle pollutants before being released.
Dry swale: An open drainage channel designed to temporarily detain and filter stormwater runoff through an underlying bed of prepared soil media.

E

Emergency spillway: A secondary spillway designed and constructed to discharge flows that exceed the capacity of a dam’s principal spillway, preventing the dam from overtopping.
Energy dissipator: A structure, such as a riprap apron or concrete baffle, placed at a pipe outlet or in a channel to reduce the velocity and turbulence of flowing water and prevent erosion.
Erosion: The process by which the land surface is worn away by the action of water, wind, ice, or gravity. Accelerated erosion is often the result of human activities that disturb the soil.
Eutrophication: The process of over-enrichment of water bodies by nutrients, particularly nitrogen and phosphorus. Eutrophication often leads to excessive algal growth, low dissolved oxygen, and degradation of water quality.
Extended Detention (ED): A stormwater management design feature that gradually releases a captured volume of runoff over a period of 12 to 48 hours. This extended release time enhances pollutant settling and protects downstream channels from erosive flows.
Exfiltration: The downward movement of water from a stormwater practice through the soil into the underlying ground. Also referred to as infiltration.

F

Filter strips: A vegetated area, typically of grass or other dense vegetation, designed to treat sheet flow from adjacent impervious areas by filtering sediment and other pollutants.
First flush: The initial surface runoff of a rainstorm. The first flush typically contains higher concentrations of pollutants that have accumulated on impervious surfaces between storms.
Floodplain: The flat area of land adjacent to a stream or river that is subject to inundation during a major flood event, such as the 100-year flood.
Flow splitter: An engineered hydraulic structure designed to divert a portion of storm flow to an off-line treatment practice or to bypass baseflow around a practice.
Forebay: A small basin or chamber located near the inlet of a stormwater pond or wetland that serves to trap incoming coarse sediments before they reach the main treatment area.
Freeboard: The vertical distance between the maximum designed water surface elevation in a pond, channel, or other structure and the top of the adjacent banks or walls. Freeboard provides a margin of safety against overtopping.

G

Gabion: A rectangular wire-mesh basket filled with stones, used to build erosion control structures such as retaining walls, revetments, and check dams.
Geotextile fabric: A synthetic textile used in construction and erosion control. Permeable geotextiles are used to separate soil layers, provide filtration, or reinforce soil, while impermeable geotextiles are used as liners.
Grass channel: An open vegetated channel used to convey runoff and provide water quality treatment by filtering pollutants and promoting infiltration. Also known as a swale.

H

Head (hydraulics): The energy of a fluid expressed as a vertical height of water. It can refer to pressure head, velocity head, or elevation head.
Hot spot: A land use or activity that generates highly contaminated runoff, with pollutant concentrations in excess of those typically found in stormwater. Examples include gas stations, vehicle maintenance areas, and industrial sites.
Hydrograph: A graph showing the rate of flow (discharge) versus time at a specific point in a stream or channel.
Hydrologic Soil Group (HSG): A classification system used by the Natural Resources Conservation Service to categorize soils into one of four groups (A, B, C, or D) based on their runoff-producing potential. Group A soils have the highest infiltration rates, while Group D soils have the lowest.

I

Illicit discharge: Any discharge to a municipal separate storm sewer system that is not composed entirely of stormwater, except for discharges allowed under an NPDES permit.
Impervious cover: Any surface in the landscape that cannot effectively absorb or infiltrate rainfall, such as rooftops, roads, parking lots, and sidewalks. The amount of impervious cover in a watershed is a key indicator of stream health.
Infiltration: The process by which water on the ground surface enters the soil. Infiltration practices are designed to capture and percolate stormwater runoff into the underlying soils.
Infiltration basin: An impoundment designed to temporarily store stormwater runoff and allow it to infiltrate into permeable soils over a designated period.
Infiltration trench: A long, narrow, rock-filled trench that receives stormwater runoff. Runoff is stored in the void space between the stones and infiltrates into the surrounding soil.

K

Karst: A type of topography formed over soluble rock such as limestone, characterized by sinkholes, caves, and underground drainage systems. Infiltration of stormwater in karst areas can pose a risk to groundwater quality.

L

Level spreader: A device designed to distribute concentrated runoff uniformly over a wide area as sheet flow. Level spreaders are used to prevent erosion and promote infiltration at the outlet of channels or pipes.

M

Manning’s formula: An empirical equation used to estimate the average velocity of water flowing in an open channel based on the channel’s shape, slope, and roughness.
Micropool: A small permanent pool of water incorporated into the design of a stormwater pond or wetland, typically at the outlet, to prevent sediment resuspension and clogging of the outlet structure.

N

National Pollutant Discharge Elimination System (NPDES): A national program under the Clean Water Act that regulates point source and stormwater discharges to waters of the United States through a permitting system.
Non-structural stormwater practices: Stormwater management techniques that use natural measures or source control to reduce pollution and runoff, such as preserving natural areas, reducing impervious cover, and implementing pollution prevention plans.
Nutrient: A substance that provides nourishment for growth. In the context of stormwater, the primary nutrients of concern are phosphorus and nitrogen, which can cause eutrophication in receiving waters.

O

Oil/grit separator: A structural stormwater treatment device, typically an underground multi-chamber vault, designed to remove coarse sediment, oil, and grease from runoff.
Ordinance: A law or regulation enacted by a municipal government, such as a city or county. Ordinances often govern local matters such as zoning, land development, and stormwater management.
Outfall: The point where water flows from a conduit, channel, or drain into a receiving water body.

P

Peak discharge: The maximum instantaneous rate of flow during a storm event, typically expressed in cubic feet per second (cfs).
Perennial stream: A stream channel that has continuous water flow throughout the year, even during dry periods, as it is supported by baseflow from groundwater.
Permeability: The property of a soil or rock that enables it to transmit water. It is a measure of the rate at which water can move through a porous medium under saturated conditions.
Pervious: A material that allows water to pass through it. Pervious surfaces, such as vegetated areas and porous pavements, allow for rainfall infiltration.
Porous pavement: A permeable pavement surface with an underlying stone reservoir designed to temporarily store surface runoff before it infiltrates into the subsoil. It is used for low-traffic areas like parking lots and sidewalks.
Principal spillway: The primary pipe or weir structure designed to carry baseflow and storm flows up to a specific design storm frequency through a dam embankment.

R

Recharge rate: The annual amount of rainfall that contributes to groundwater, often expressed as a function of hydrologic soil group.
Retention: The process of capturing and holding stormwater runoff on-site with no surface discharge. Retention is typically achieved by infiltrating runoff into the ground or through evapotranspiration.
Riparian: Relating to the banks of a natural course of water, such as a river, stream, or lake. Riparian areas are the vegetated corridors along these water bodies that support unique ecosystems and protect water quality.
Riprap: A permanent, erosion-resistant layer of large, angular stones used to protect soil from erosion in areas of concentrated runoff or wave action, such as on streambanks and at pipe outlets.
Runoff: The portion of precipitation that flows over the land surface as either sheet flow or concentrated channel flow.
Runoff Coefficient (C): A dimensionless factor representing the fraction of rainfall that becomes surface runoff. The coefficient varies based on the characteristics of the drainage surface and is used in hydrologic calculations like the Rational Method and the Simple Method.

S

Sediment: Eroded soil and rock particles that are transported and deposited by water or wind.
Sheet flow: Runoff that flows over the ground surface as a thin, uniform layer, not concentrated in a channel.
Slope: The incline or gradient of a land surface, road, or channel, often expressed as a percentage or a ratio of vertical rise to horizontal run.
Spillway: A structure over or through a dam or other embankment designed to safely convey excess water flows.
Stormwater: Water that originates from precipitation events, including rain and snowmelt. In developed areas, stormwater flows over impervious surfaces, collecting pollutants before entering the drainage system.
Swale: A shallow, open vegetated channel designed to convey, treat, and sometimes infiltrate stormwater runoff.

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 interest. It is a key parameter in hydrologic analysis for calculating peak discharge rates.
Topography: The physical features of a land surface, including its relief and the position of natural and man-made features. Topography determines the direction and rate of runoff flow.
Trash rack: A screen or grate installed at the inlet of a spillway or culvert to prevent debris from entering and clogging the conduit.

U

Underdrain: A perforated pipe installed in a gravel layer at the bottom of a stormwater practice, such as a bioretention area or sand filter, to collect and remove filtered water.
Unified Soil Classification System (USCS): A standard system for classifying soils for engineering purposes based on particle size, gradation, and plasticity characteristics.

W

Water Quality Volume (WQv): The volume of runoff from a small storm event, often the 90th percentile storm, that must be captured and treated to remove a target percentage of pollutants. Local regulations define the specific sizing criteria for the WQv.
Water table: The upper surface of the zone of saturation in the ground, where the soil or rock is saturated with water.
Watershed: All land and water area from which runoff flows to a common outlet point. Watersheds can be as small as a few acres draining to a storm drain inlet or as large as thousands of square miles draining to an ocean.
Weir: A structure placed in an open channel to measure or control the flow of water. The rate of flow is a function of the water depth over the weir crest.
Wet pond: A stormwater pond that maintains a permanent pool of water, which improves pollutant removal through biological processes and settling. Also known as a retention pond.
Wetland: An area that is inundated or saturated by surface or ground water at a frequency and duration sufficient to support a prevalence of vegetation adapted for life in saturated soil conditions.