Floodplain FAQ
The following questions have been compiled using resources from the Toronto and Region Conservation Authority. You can learn more about flood risk management on the TRCA website.
What is a floodplain?
A flood plain is the natural flat area next to a river or stream that is able to convey extra water that spills out of the river channel during a flood event. Normally this area is dry and not covered by water. High water levels in the river system that result in flooding within the flood plain are referred to as riverine floods. These floods are often the result of higher than normal river flows (or water volume) produced by extreme rainfall and snowmelt events.
Why is the flood plain important?
Knowing which areas will be inundated by flood waters is an important tool for land use planning and emergency management. This knowledge helps guide sensitive land use away from high-risk areas and identify ways to mitigate flood risk.
What is a watershed?
A watershed is an area of land drained by a river or creek and its tributaries into a body of water.
For example, Central Lake Ontario Conservation Authority (CLOCA) includes four major watersheds and eighteen smaller watersheds.
What is a Conservation Authority?
Conservation authorities are local watershed management groups. They work to conserve, restore and manage Ontario's water, land and natural habitats. There are 36 conservation authorities in Ontario and five conservation authorities within Durham Region:
What is a 100-Year Storm?
The term “100-year storm” refers to a rainfall event that statistically has a 1% chance of occurring in any given year, at any given place. It does not mean that such a storm will only occur once every 100 years.
Who creates the boundaries of the regulatory flood plain?
Flood plain mapping studies are technical assessments that use local information from each watershed in order to determine the extent of the flood plain. Flood plain mapping studies are prepared and approved by experts within Conservation Authorities.
Flood plain mapping studies use topographical information, surveys of infrastructure (such as the size of bridges and culverts), land use and land cover information, weather, and stream flow data to create detailed hydraulic and hydrologic models of each watershed.
Can flooding happen outside of the flood plain?
Yes. The flood plain defines the flood hazard for planning purposes based on the standard set out by the Ontario Ministry of Natural Resources and Forestry. However unlikely, it is still possible to have a larger storm or a different combination of factors (for example, debris blockages) that result in an even bigger flood
The flood plain relates only to riverine flooding. See the types of flooding page for more information.
