Watershed Health

Floodplain Mapping

The NPCA is a watershed management agency. Our goals include protecting our communities from natural hazards, conserving our natural environment, and supporting our partners in creating sustainable communities.  From a flood management viewpoint, the NPCA and our municipal partners rely on floodplain mapping for:

  • flood forecasting and warning,
  • emergency planning and response,
  • prioritization and planning for flood mitigation works,
  • community planning and land use decision making, and
  • identifying the extent of the flood hazard.
What Is a Floodplain?

A floodplain is an area of land near water bodies (rivers, lakes, etc.) that is often flooded when the water body is too full.  Examples of floodplains include low lying lands that are flooded or inundated with water when a river spills over its banks or when lake levels rise due to storm surge or significant precipitation.  Floodplains are natural features that allow flow to spread across the landscape.

What Is Floodplain Mapping?

Floodplain mapping is used to identify areas that may be susceptible to river or coastal flooding during large storm events.  Floodplain mapping relies on field surveys, engineering analysis and hydrologic and hydraulic modelling. Hydrologic modelling predicts how much stormwater will runoff the land during a rainfall event.  Flows generated by the hydrologic model are then input into the hydraulic model which routes these flows through the river system to predict the peak flood depth.  

What Standards Are Followed?

Analysis and floodplain mapping are undertaken in a manner consistent with the Ontario Ministry of Natural Resources and Forestry’s (MNRF) Technical Guide – River and Stream Systems: Flooding Hazard Limit (2002).  This guideline sets out provincial expectations on analysis approaches applied in mapping the regulated flood hazard.  The MNRF guideline is used by all Conservation Authorities undertaking flood hazard mapping.  

Why Is Floodplain Mapping Important?

In Canada, floods account for the largest portion of disaster recovery costs on an annual basis.  The first step to reduce the cost of flood damage within a community is to have mapping that accurately shows flood hazards.  These maps help people prepare for and respond to potential flooding and make informed decisions about their own emergency plans, property improvements, and insurance needs. 

Updating Floodplain Maps

The NPCA is working with our partners to reduce flood risk in our communities by updating floodplain maps, some of which are over 30 years old. This will be a multi-year program. These updates include comprehensive public consultation processes, when these are taking place, more information will be available at the link below. 

Get Involved

What Is the Regulated Flood Hazard?

In the NPCA’s jurisdiction, the regulated flood hazard is defined in provincial legislation by the greatest flood extent associated with the 1 in 100-year storm event. This is the anticipated limit of flooding that has a 1% chance of occurring in any given year.

Does Flood Risk Extend Beyond the Regulated Flood Hazard? 

While flood hazard mapping identifies the extent of the regulated floodplain associated with river or coastal flooding, it does not identify the full extent of flood risk.  Flooding may be experienced outside of the defined river and coastal flood hazard for a variety of reasons, including occurrence of extreme rainfall events (which are greater than the regulatory 1 in 100-year storm event standard), formation of significant ice or debris jams and large beaver dams, or due to other flooding mechanisms such as urban flooding caused by rainfall that exceeds the capacity of local storm sewer systems.   

Who is Responsible for Dealing with Urban Flooding? 

Urban flooding is caused by rainfall that exceeds the capacity of local storm sewer systems. The local municipality is responsible for studying and addressing issues caused by their storm sewer system.