Stormwater Glossary

An algae bloom is a rapid increase of algae growth in water that creates thick green, blue-green, or reddish mats on the surface. It happens when nutrients like nitrogen and phosphorus build up in ponds, retention basins, or drainage systems, often turning clear water murky and smelly.

A bathymetric survey measures the depth and shape of underwater areas like stormwater ponds, retention basins, or waterways. It creates a detailed map showing where sediment has built up and how much storage capacity you've lost over time.

A Best Management Practice (BMP) is a proven method or structure designed to control stormwater runoff, prevent pollution, and protect water quality. BMPs include retention ponds, swales, filters, and other systems that capture and treat rainwater before it reaches streams, rivers, or groundwater.

A catch basin is a below-grade drainage structure that collects surface water runoff from paved areas, roads, and landscapes, then directs it into the underground storm pipe network. Most have a sump at the bottom to trap sediment and debris before they enter the pipe system.

A consent order is a legal agreement between a property owner and environmental regulators to fix stormwater violations within a specific timeframe. It's typically issued when your property isn't meeting stormwater compliance requirements, giving you a chance to correct problems without going to court.

A Construction General Permit (CGP) is a federal permit required for construction sites that disturb one acre or more of soil. It's issued under the Clean Water Act to prevent sediment and pollutants from construction activities from entering waterways during building projects.

A corrective action plan is a detailed document that outlines specific steps to fix stormwater management problems identified during inspections. It includes timelines, responsible parties, and methods to bring your property back into compliance with environmental regulations.

A culvert is a tunnel or large pipe that allows water to flow under roads, driveways, walkways, or other structures. It's designed to channel stormwater or natural drainage from one side of an obstruction to the other without flooding or washing out the surface above.

Dewatering is the process of removing water from construction sites, excavations, or stormwater systems so work can be completed safely. It involves pumping water out of trenches, retention ponds, or work areas and directing it to an appropriate discharge location while preventing erosion or contamination.

A drainage easement is a legal agreement that gives someone the right to use part of your property for water management purposes. This could include installing pipes, channels, or retention areas to handle stormwater runoff from neighboring properties or public areas.

An embankment is a raised structure made of soil, rock, or other materials built to hold back water, support roads, or create barriers. In stormwater management, embankments are commonly used around retention ponds, along waterways, and as part of erosion control systems to direct water flow and prevent flooding.

An erosion blanket is a temporary mat made of natural or synthetic materials that's rolled out over exposed soil to prevent rain and runoff from washing away sediment. Think of it as a protective cover that holds soil in place while grass or other vegetation gets established.

A forebay is a settling chamber at the inlet of a stormwater pond that slows down water flow and captures sediment before it reaches the main pond. It's designed to trap debris, trash, and coarse particles, protecting the pond from filling up with sediment too quickly.

A headwall is a concrete or masonry structure that supports the end of a culvert pipe where it meets an open channel, ditch, or waterway. It prevents erosion around the pipe opening and provides a stable transition point for water flow entering or leaving the pipe system.

Hydraulic dredging removes sediment and debris from ponds, drainage systems, and waterways using high-pressure water and suction equipment. The process pumps water to loosen accumulated material, then vacuums it out through pipes to a disposal area, leaving the water feature clean and properly functioning.

An illicit discharge is any unauthorized flow of polluted water into storm drains, ditches, or waterways. This includes anything other than clean rainwater entering your stormwater system, like soapy water from washing equipment, paint, oil, or sewage. It's illegal and can result in hefty fines.

Impervious surfaces are areas that don't allow water to soak into the ground. Think pavement, rooftops, concrete walkways, and pool decks. When it rains, water runs off these surfaces instead of absorbing naturally into the soil, creating stormwater runoff that needs to be managed.

Infiltration rate measures how quickly water soaks into soil, expressed in inches per hour. It determines whether your property's stormwater systems can handle rainfall without flooding or runoff violations. Poor infiltration rates often mean waterlogged areas, failed BMPs, and compliance headaches.

An inlet structure is the point where stormwater enters your drainage system, typically featuring a grated opening, catch basin, or pipe inlet. It's designed to capture runoff from parking lots, roads, and other surfaces and direct it into underground pipes or open channels for proper drainage.

Invasive species control is the active removal and management of non-native plants and animals that harm local ecosystems. In stormwater systems, this typically means removing aggressive plants like water hyacinth or kudzu that can clog drainage, destabilize slopes, and crowd out beneficial native vegetation.

Invert elevation is the lowest point inside a pipe, culvert, or drainage channel where water flows. Think of it as the bottom floor of your drainage system. When engineers measure pipe connections or design stormwater systems, they use invert elevations to make sure water flows downhill properly from one section to the next.

A government-issued permit required before disturbing soil on a property, typically for construction, excavation, or grading activities that affect more than one acre. The permit includes requirements for erosion control measures and stormwater management during and after the work.

The littoral zone is the shallow area around the edges of a pond or lake where aquatic plants grow. It's the transition zone between deep water and dry land, typically extending from the shoreline to where rooted plants can no longer grow due to depth.

Mechanical dredging removes sediment and debris from stormwater ponds, basins, and drainage systems using heavy equipment like excavators and vacuum trucks. It's the most common method for cleaning out accumulated mud, sand, and organic matter that reduces your facility's drainage capacity.

A Municipal Separate Storm Sewer System (MS4) is a publicly-owned drainage network that collects rainwater runoff and directs it to local waterways without treatment. Unlike sanitary sewers, storm drains don't process the water they carry, making pollution prevention critical for water quality protection.

NPDES (National Pollutant Discharge Elimination System) is a federal permit program that regulates what can be discharged into waterways. If your property has stormwater runoff, construction activities, or wastewater systems, you likely need NPDES permits and must follow specific water quality requirements.

A Notice of Intent (NOI) is a legal document that must be filed with environmental agencies before beginning construction activities that disturb one or more acres of land. It's your permit application to discharge stormwater runoff under the National Pollutant Discharge Elimination System (NPDES) program.

A Notice of Violation is an official document issued by a regulatory agency when your property fails to meet stormwater, environmental, or construction standards. It identifies specific violations and requires corrective action within a set timeframe to avoid fines or legal action.

Nutrient loading is the amount of nitrogen and phosphorus entering a water body from runoff, fertilizers, and organic matter. High nutrient levels cause algae blooms, fish kills, and water quality problems in ponds, lakes, and coastal waters.

The Office of Coastal Resource Management (OCRM) is South Carolina's state agency that regulates development and environmental activities in coastal areas. They issue permits for projects affecting wetlands, beaches, and waterways, and enforce coastal protection laws that can impact your stormwater systems.

An outfall is the end point where stormwater pipes discharge into natural water bodies like creeks, rivers, lakes, or wetlands. It's where your property's drainage system connects to the environment. You'll typically see it as a pipe or concrete structure where water flows out from your storm drains.

An outlet structure is the engineered exit point where water flows out of a stormwater pond, retention basin, or drainage system. It controls the rate and volume of water discharge, typically including pipes, weirs, or spillways that regulate flow during storms and prevent flooding downstream.

Peak flow is the highest rate of stormwater runoff that occurs during a storm event, measured in cubic feet per second. It's the maximum amount of water flowing through pipes, channels, or detention systems at any given moment during heavy rainfall.

Residence time is how long stormwater sits in a pond, basin, or treatment system before flowing out. Think of it as dwell time. Longer residence time means more opportunity for sediment to settle and pollutants to be removed before the water continues downstream.

A riparian buffer is a strip of vegetation along streams, ponds, or wetlands that helps filter pollutants from stormwater runoff. These planted areas catch sediment and absorb nutrients before they reach waterways, protecting water quality while providing habitat for wildlife.

Riprap is a protective layer of rocks, stones, or concrete blocks placed along shorelines, drainage channels, and slopes to prevent erosion. It's commonly used around stormwater ponds, outlet structures, and stream banks to absorb wave energy and slow water flow that would otherwise wash away soil.

A riser pipe is a vertical pipe that connects an underground storm drain system to the surface. It typically runs from a buried inlet or catch basin up through the ground, allowing stormwater to enter the drainage system while keeping debris from clogging the main pipes below.

A Section 404 permit is federal approval from the Army Corps of Engineers required before disturbing wetlands or waterways. If your property has wetlands, streams, or ponds, you'll likely need this permit before doing any excavation, dredging, or construction work that affects those areas.

Sediment removal is the process of clearing accumulated sand, dirt, and debris from stormwater ponds, drainage systems, and BMPs. This maintenance prevents clogs, restores proper water flow, and keeps stormwater systems working as designed to protect your property from flooding.

Slope ratio describes the steepness of a surface by comparing horizontal distance to vertical rise, expressed as ratios like 3:1 or 4:1. A 3:1 slope means for every 3 feet horizontal, there's 1 foot of vertical drop. Steeper slopes have smaller first numbers and create more runoff problems.

A spillway is a structure that safely releases excess water from a stormwater pond, detention basin, or dam when water levels get too high. It prevents flooding and erosion by providing a controlled path for overflow water to exit the pond during heavy rain events.

Storage capacity refers to the maximum volume of stormwater that a pond, basin, or tank can hold during rain events. It's measured in cubic feet or acre-feet and determines how much runoff your facility can manage before water overflows or backs up onto your property.

A storm drain is a system of pipes, grates, and channels that collects rainwater from streets, parking lots, and buildings and carries it away from developed areas. Unlike sewers, storm drains typically discharge untreated water directly into rivers, lakes, or coastal waters.

A legal document that outlines who's responsible for maintaining stormwater systems like ponds, bioswales, and drainage infrastructure on a property. It specifies what maintenance tasks must be done, how often, and who pays for them.

A Stormwater Pollution Prevention Plan (SWPPP) is a written document that identifies potential sources of pollution on your property and outlines specific steps to prevent contaminated stormwater from entering waterways during construction or industrial activities.

A Total Maximum Daily Load (TMDL) is the maximum amount of a specific pollutant that a water body can receive daily while still meeting water quality standards. It's essentially a pollution budget that determines how much contamination a lake, river, or stream can handle without becoming impaired.

A trash rack is a metal grate or screen installed at the inlet or outlet of stormwater systems to catch debris before it enters pipes, ponds, or drainage structures. It prevents leaves, branches, and trash from clogging your drainage system while allowing water to flow through freely.

Turbidity measures how cloudy or murky water looks. It's caused by tiny particles suspended in water that scatter light. Think of muddy water after rain versus clear water from your tap. Higher turbidity means more particles and cloudier water, which can indicate pollution or erosion problems.

An underdrain is a perforated pipe system installed beneath stormwater facilities like retention ponds, bioswales, or infiltration areas. It collects and removes excess water that doesn't naturally soak into the ground, preventing waterlogging and helping maintain proper drainage function.

Vacuum sweeping uses specialized trucks to suction sediment, debris, and pollutants from stormwater systems like catch basins, storm drains, and detention ponds. It's a maintenance technique that prevents clogs and keeps your drainage working properly while meeting environmental compliance requirements.

A weir is a low barrier built across a channel or pond outlet to control water flow and maintain specific water levels. It's essentially a small dam with a notch or opening at the top that allows water to spill over when it reaches the desired height, regulating drainage in stormwater systems.