Glossary of Terms

Aggregate

Rock or stone material used in green infrastructure to create storage space for stormwater. Washed #57 aggregate (¾”-1” in size) is popular for subbases. 

Backflooding

A somewhat unique occurrence for New Orleans where the grey infrastructure is overwhelmed by rainwater runoff, causing stormwater to come back out of pipes and catch basins and flood streets. 

Bioswale

A channeled depression or trench that retains, detains, and filters rainwater runoff (as from a parking lot) and uses native vegetation (such as grasses, flowering plants) to filter out pollutants from stormwater. These can control the flow and direction of stormwater.

Detention

The temporary holding of stormwater which helps prevent overwhelming grey infrastructure and reduce flooding.

Evapotranspiration

The process by which water is transported back into the air through evaporation (usually from the sun) and transpiration (like breathing for plants). 

Filtration

The removal of pollutants from stormwater by plants, soil, and/or aggregate. 

Flow-through Planter Boxes

Above ground planter boxes filled with native, water-loving plants, easily infiltrated soils, and aggregate. Can be directly connected to rain gutter systems. Planter boxes detain, retain, and filter stormwater.

Green Infrastructure

A water management design that mimics the natural water cycle to reduce flooding by retaining, detaining, and/or filtering stormwater. This can be a bioswale, rain garden, infiltration trench, and more.

Grey Infrastructure

Human-made structures, such as pipes, pumping stations, and levees, used to remove stormwater. 

Groundwater

Water beneath the surface of soil. It is important for soil stability, plant health, and the health of the surrounding bodies of water.

Impervious (non-permeable)

Surfaces that do NOT allow water to infiltrate, e.g. concrete, asphalt, roofs. 

Infiltration 

The process by which water soaks into the ground.

Infiltration Trenches

Long, narrow, aggregate-filled trenches used to capture, filter, and convey (move) stormwater. They work effectively when positioned at building downspouts and around building foundations. Also sometimes called ‘French drains.’ 

Perforated Pipe

Pipe with small holes that allow water to slowly seep into the ground. Usually used to increase stormwater holding capacity and to convey stormwater underground. 

Permeable (pervious)

Surfaces that allow water to infiltrate into the ground, e.g. permeable paving, rain gardens, infiltration trenches.

Permeable Paving

Paving that allows water to infiltrate into the ground. Usually supported by a deep aggregate base that can store stormwater. Pavers over an aggregate base, geogrid, and permeable concrete are examples of permeable paving systems. 

Rain Barrel

A barrel usually connected to rain gutters to collect (or harvest) rainwater runoff from roofs to be stored and used later. 

Rain Garden

A depression planted with native, water-loving and drought resistant plants that allows rainwater runoff from impervious urban areas like roofs, driveways, walkways, and compacted lawn areas to infiltrate into the ground, recharging our water table. 

Recharged Groundwater

A hydrologic process where water infiltrates the ground from the surface. In non-urban settings, this naturally occurs and is an important part of the water cycle.

Retention

The permanent holding of stormwater. This can be referring to a body of water such as a lake, or the infiltration of stormwater into the ground by green infrastructure. 

Stormwater Runoff

Rainwater that does not soak into the ground but rather flows over impervious areas or areas already saturated with water. Runoff will either be pumped out by grey infrastructure or flood streets and property. 

Subsidence

The sinking of land, typically from the compaction, decomposition, and/or erosion of soil.

Subsidence occurs when stormwater is not allowed to infiltrate the ground. Clay soils shrink when dried out, highly organic soils decompose when exposed to oxygen (i.e. when the water table lacks a sufficient amount of water) and sandy soils are easily washed away with  large amounts of runoff. Subsidence often causes foundation issues, potholes, and can make flooding worse. 

Urban Heat Island Effect

Urban areas can be  subjected to dangerously high temperatures due to over-pavement and a lack of tree canopy.