Key Terms


100-Year Storm

The phrase “100-year storm” refers to the estimated probability of a storm event happening in any given year. A 100-year event has a 1 percent chance (or 1-in-100 chance) of occurring in a year. The term “100-year flood” allows us to place a particular weather event in context with other similar events. These 100-year storms are defined by the severity of the winds - the large amount of rain and the intensity in which it falls and the flooding that results.

Climate Change

Climate change is the term used to describe the process by which increasing levels, or concentrations, of carbon dioxide (CO2) - primarily from the burning of fossil fuels - traps heat from the sun in the earth’s atmosphere. This is shown to lead to a gradual warming of our planet and its oceans, and consequently, significant changes to local weather patterns. In the case of North America, experts predict larger and more intense storms and extreme weather events. This results in more rain falling in a shorter period of time as well as longer gaps between storms. Also known as a flood-drought cycle.

Combined Sewer Overflow

Is a term used to describe the common problem that occurs when runoff from storms flow down the storm water drain but overflow or mix with the sanitary sewers - often making their way to the receiving rivers, streams, lakes and oceans untreated.


A pipe to carry rainwater from a roof to a drain or to ground level. They can be located on the outside of the building like in residential buildings on the inside of the buildings in case of flat roofed schools and commercial buildings.


Erosion is the process of breaking down and moving soil and other particles by wind, water, or other natural agents. In the case of soil erosion it often occurs as rain and runoff picks up soil particles and moves them downstream. Erosion of landscapes can lead to water pollution and lost soil.

Extreme Weather

Is a term used to describe unexpected, unusual, unpredictable, severe or unseasonal weather events. These weather events are considered extreme because they at the rare in the historical record - defined as lying in the most unusual ten percent.

French Drain

A trench filled with gravel or rock or containing a perforated pipe that redirects surface water and groundwater away from an area.

Green Infrastructure

Is a term used to describe trees and plants, marshes and ponds, green roofs and swales designed and incorporated to manges runoff without the need to expand costly centralized infrastructure like sewers and treatment plants.

Green Roof

A green roof or living roof is a roof of a building that is partially or completely covered with vegetation and a growing medium, planted over some kind of waterproofing material.

Inner Downspouts

Inner downspouts or drains are often found on larger buildings and many school roofs. The drains are usually placed near the center of the building. They attach to pipes that drain the water down through the building’s roof. This keeps the water safely away from the walls and foundation, but at the same time deposits the rain directly to the storm sewers. Unlike outdoor downspouts and gutter systems, inner drains will not freeze up and crack or fail during the winter. The building and walls naturally protect the pipes from the elements. Strainers for inner drains are critical. They assist in keeping any debris from clogging the drain.


Material (such as decaying leaves, bark, or compost) spread around or over a plant to enrich or insulate the soil.


A surface that does not allow liquids to pass through it (such as asphalt or concrete).


The measure of the ability of a substance to allow water or liquids to flow through it.


A surface that allows liquids to pass through it (such as grass or mulch).


Pollution is the introduction into the environment of a substance or pollutant that is harmful or has poisonous effects on wildlife, humans or the ecosystem as a whole. In the case of water pollution it can take the form of plastics, hazardous chemicals like oil, pesticides and fertilizers, industrial discharges or even human waste from sewer overflows.

Rain Barrel

A rain barrel is a system that collects and stores rainwater from your roof that would otherwise be lost to runoff and diverted to storm drains and streams.

Rain Gardens

A rain garden is a sunken garden designed to capture, absorb and filter stormwater during heavy rainfalls and after seasonal snow melts. In addition to reducing stormwater runoff and infiltration rainwater safely back into the ecosystem they can also provide habitat and food for a variety of birds and butterflies. If designed correctly Rain Gardens are considered green infrastructure, they slow down the flow of runoff, holding and filtering it to be infiltrated over a longer period of time. Strategically placed rain gardens are often used as a way to safely disconnect a downspout, reducing the flow of stormwater runoff and allowing water to slowly seeping into the ground -- rather than enter the storm drain.

Rainfall Intensity

Rainfall intensity is measured over a 5-minute period. It is recorded, in mm per hour, as the resulting accumulation as if the intensity remained constant for a full hour. The more intense the rainfall, the more likely that a sewer system will be overwhelmed and flooding and sewer overflows will occur.


Runoff is water - from rain, snowmelt, or other sources - that flows over the land surface. Runoff is a component of the water cycle. If runoff occurs on surfaces before reaching a drain it is also known as overland flow. An area which produces runoff draining to a common point is called a watershed. When runoff flows along the roof, pavement and ground, it will pick up contaminants such as deposited air pollution, plastic litter, cigarette butts, animal wastes, petroleum, pesticides and fertilizers that are then discharged to sewers, and/or, local streams, lakes and oceans. Urban areas have more surface runoff due to impervious surfaces such as pavement and roofs that do not allow percolation of the water down through the soil to the aquifer. Because of the impermeable surfaces runoff is funneled into storm sewers, ditches and streams, where pollutants, erosion and siltation can case beach closures, fish kills and threats to wildlife. Increased runoff reduces groundwater recharge, thus lowering the water table, as well as reducing water flow to local streams and rivers and making droughts worse, especially for farmers and others who depend on water wells.

Runoff Footprint

A footprint is an expression meaning the environmental impact made by something. Runoff footprint is the amount of water that runs off surfaces into sewers, drains, streams, etc.


A surface that allows some liquids to pass through it (such as semi-permeable pavement). These can have different percentages of liquid flow through them, but for the purposes of our Activities, we shall assume a semi-permeable surface is 50% porous.

Sewer Sheds

Sewer systems are the most common way of dealing with stormwater runoff in urban settings. In many cities, there is a single system for dealing with both sewage and stormwater. This one system collects waste and runoff and carries it to a sewage treatment plant, where it is all treated and discharged. In wet weather, or during spring thaws, the amount of water flowing through the combined sewer system can overwhelm the systems. When this happens the excess water, a mix of runoff and sewage, leaves the sewer system through emergency exits, which usually lead directly into rivers, streams and oceans. So on a very rainy day, untreated sewage can be dumped directly into our environment. However, even in a relatively light rain event, some cities treat the runoff as though it were sewage.

Sewer System

When we walk in our cities we seldom give a thought of what runs under our feet. Sewers! These underground tunnels take waste water from our showers and toilets to the treatment plant before it is discharged to our streams, rivers, lakes and oceans. When it rains, water that runs off roofs and roads and flows to that sewer system. Sewer systems are an essential part of the urban infrastructure and our cities depend on how well they work. But, across Canada sewer systems - and the amount of treatment they provide - differs greatly. In many cities, we have a single system for dealing with both sewage and stormwater. This one system collects waste and runoff and carries it to a sewage treatment plant, where it can be treated. In some cases the sanitary sewer is separated or, partially separated. In the case of partially separated sewers storms can cause the runoff to mix with the sewage. when sewer systems become overloaded they discharge the untreated water to the environment. Some separated storm sewers systems treat stormwater, and some discharge directly without treatment.

Storm Sewer

The storm sewer is a system designed to carry rainfall runoff and other drainage. It is not designed to carry sewage or accept hazardous wastes. The runoff is carried in underground pipes or open ditches and discharges untreated into local streams, rivers and other surface water bodies.


Stormwater is water that originates during precipitation events and snow/ice melt. Stormwater can soak into the soil, be held on the surface, or runoff and end up in nearby streams, rivers, or other water bodies (surface water). In natural landscapes such as forests, the soil absorbs much of the stormwater and plants help hold stormwater close to where it falls. In urban areas unmanaged stormwater can create two major issues: one related to the volume and timing of runoff water (flooding) and the other related to potential contaminants that the water is carrying (water pollution).

Stormwater Infrastructure

Stormwater infrastructure is essential to managing the impact of sudden and heavy rainfall or the gradual melting of snow and ice come springtime. This infrastructure can be designed and built -- like sewers and drains, or natural like streams, ponds and trees and plants.


A swale is a shallow channel with gently sloping sides. A swale may be either natural or human-made. Artificial swales are often designed to manage water runoff and filter pollutants.

Water Cycle

The water cycle is the continuous movement of water on, above and below the surface of the Earth. Oceans contain 97% of the world’s water. As heat evaporates this water it forms clouds. Some of these clouds are pushed over land. When it rains or snows, the precipitation falls onto the ground. It can soak into the ground, where it nourishes plants, soil and groundwater, but it can also runoff - especially in urban areas - and this can carry pollution. Precipitation can be evaporated directly from water bodies, or it can be used by plants, which then breathe it back into the air. Once the water vapour ends up back in the atmosphere it cools and condenses, forming clouds, which then return the water to the surface of the earth through rain. It’s all one big cycle!
John Evans and Howard Periman, USGS -


The surface of the Earth can be divided up into areas we call watersheds based on where water flows within that area. Any drops of rain that fall in a given watershed will eventually run downhill. Because all water is flowing downhill, towards streams, rivers and the oceans, our activities have an impact on everything that happens downhill of us. Watersheds are separated by hills or ridges, so the rain falling on one side of a hill may end up in a different watershed than rain falling on the opposite side.