Floodplain Mapping

Why Floodplain Mapping is 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

Conservation Halton is working with our partners to reduce flood risk in our communities by updating floodplain maps, some of which are over 20 years old.  This will be a multi-year program. 

Conservation Halton’s Role in Floodplain Mapping 

Conservation Halton 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, Conservation Halton 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. 

How You Can Participate

Conservation Halton is encouraging public engagement and will host a minimum of two Public Information Centres (PIC’s) as part of each floodplain mapping study. The first PIC will be held to share details about the study area and analysis approaches being considered.  The second PIC will present draft study results.  The PIC sessions will seek local knowledge from area residents and stakeholders to improve the quality of the project and to raise awareness of potential flood risks.  Depending on the extent of any revisions arising from feedback at the second PIC, a third PIC may be held to share revised draft mapping.  Following release of the draft mapping, the Floodplain Mapping Study will be finalized, and the report and mapping will be available to the public for review, prior to approval by Conservation Halton’s Board of Directors.  

Please monitor local papers, Conservation Halton’s website and social media feeds for more information about the time and location of upcoming PICs.  You may also request to be added to a Study’s Contact List to receive direct e-mail notifications of forthcoming PICs and review periods.   Following the PIC, presented materials will be posted on Conservation Halton’s website.  To access materials for current studies, please click on the study name below:

Grindstone Creek Flood Hazard Mapping Project

Morrison-Wedgewood Diversion Channel Flood Risk Study

Urban Milton Flood Hazard Mapping

Frequently Asked Questions 

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/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, limiting flooding and erosion potential. 

What Is Floodplain Mapping?

Floodplain mapping is used to identify areas that may be susceptible to riverine or coastal flooding during large storm events.  Floodplain mapping relies on supporting analysis, including hydrologic and hydraulic modelling. Hydrologic modelling predicts how much runoff will be generated by a rainfall event.  Flows generated by the hydrologic model are then input into the hydraulic model to predict the peak flood depth, elevation, and velocity of flood flows.  The flood elevation is mapped using topographic data (the natural features of the land) to show the limits of the floodplain and other critical information.  

There are many different types of information that may be shown with floodplain maps.  The most common form of floodplain mapping in Ontario is a Flood Hazard Map, which shows the limit of the regulated flood hazard in conjunction with natural features of the land, and human-made structures such as roads.  This type of map is required for land use planning purposes.  More detailed maps are typically developed to support emergency response planning.  

What Is the Regulated Flood Hazard?

In Conservation Halton’s jurisdiction, the regulated flood hazard is defined in provincial legislation by the greatest flood extent associated with either the:

  • 1:100 year floodplain, which is the anticipated limit of flooding that has a 1% chance of occurrence or exceedance in any given year, or
  • floodplain associated with the Regional Storm, Hurricane Hazel, which is a defined rainfall event (285 mm rainfall over 48 hours), that resulted in the death of more than 80 Ontarians and left thousands homeless.

Conservation Halton’s Regulatory Policies apply to lands within and immediately adjacent to the Regulated Flood Hazard. 

Does Flood Risk Extend Beyond the Regulated Flood Hazard? 

While flood hazard mapping identifies the extent of the regulated floodplain associated with riverine or coastal flooding, it does not identify the full extent of flood risk.  Flooding may be experienced outside of the defined riverine and coastal flood hazard for a variety of reasons, including occurrence of extreme rainfall events (which are greater than the regulatory standard), formation of significant ice or debris jams and large beaver dams, major channel adjustments, or due to other flooding mechanisms such as overland flooding caused by rainfall that exceeds the capacity of local drainage systems, sewer backup, seepage, etc.   

Will New Floodplain Mapping Studies Change Regulated Floodplain Limits?

Technologic advances, such as the use of LiDAR (Light Detection and Ranging) allow capture of highly detailed topographic data, which better describes the natural land features. LiDAR is a surveying method that measures distance to a target by illuminating the target with pulsed laser light and measuring the reflected pulses with a sensor. Increased computing power and more sophisticated software can apply detailed LiDAR data and model complex natural processes to better predict the path and nature of a flood. This will result in a more accurate flood hazard limit for regulatory purposes.The regulation limit also encompasses other types of hazards, including the erosion hazard and unstable bedrock/soils, and wetlands. 

What Data Is Used to Update Floodplain Mapping?

In 2018, Conservation Halton purchased LiDAR data, which provides highly detailed and accurate surface elevations. LiDAR data will be used to update computer models required to support floodplain mapping including hydrologic models (which determine overland and groundwater flows associated with a rainfall event) and hydraulic models (which determine flood elevations and velocities associated with a specified flow).  The extent of the flooding will then be mapped using the LiDAR data. Use of the LiDAR data in conjunction with modern software and analytic tools will provide a more accurate prediction and understanding of flood risk.

Stream flow gauges, precipitation gauges, and records from past flood events will be used where available to calibrate or ‘ground truth’ the models.  Other key data that will be relied upon includes soils mapping, orthophotos, local official plans, and past engineering reports. 

What Standard Will Be Followed?

Analysis and mapping will be 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.  Other guidelines and standard industry approaches will also be considered, including Natural Resources Canada and Public Safety Canada’s Federal Floodplain Mapping Guidelines Series, and Environmental Water Resources Group Ltd. et. al.’s Technical Guidelines for Flood Hazard Mapping (2017), which was developed in partnership with six Greater Golden Horseshoe Conservation Authorities.

Conservation Halton will engage industry experts through a Technical Advisory Committee (TAC), that will review and approve the final study.  The TAC will include representation from the project consultant, impacted Municipalities and Regions, and Conservation Halton.