Toronto and Region Conservation Authority (TRCA) defines flood risk in its jurisdiction through extensive mapping of the flood plain.
Flood plain maps identify the areas on normally dry land that may be covered by water during flood events. These maps can identify the specific risk of flooding to structures, people, and assets.
Mapping is key to fulfilling TRCA’s responsibility for flood plain management.
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What Is a Flood Plain?
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.
About TRCA Flood Plain Mapping
TRCA has a comprehensive flood plain mapping program. Qualified engineering staff prepare and approve flood plain mapping studies for TRCA’s watersheds.
Flood plain mapping studies are technical reports that use information from each watershed in order to determine the extent, or boundary area, of a flood plain.
Flood plain mapping studies are used to guide decisions related to land use, infrastructure, and emergency management planning.
As directed by the Province, the regulatory flood event standard applicable within TRCA’s jurisdiction is the greater of:
- The Hurricane Hazel storm event; or
- The 100-year storm event
Within TRCA’s jurisdiction the vast majority of our regulatory flood plain mapping is based on Hurricane Hazel event; as such it is also referred to as the Regional Storm.
What is a Regional Storm?
The Regional Storm is the rainfall event and soil conditions existing during Hurricane Hazel, which occurred within the Humber River watershed in Toronto in 1954, transposed over a specific watershed and combined with local conditions
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.
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Under What Authority Does TRCA Map the Flood Plain?
The Conservation Authorities Act (CA Act) provides the legal basis for TRCA’s mandate to study watersheds and deliver programs and services that prevent, eliminate, or reduce the risk to life and property from flood and erosion hazards.
Section 21.1 (1) of the Act lists programs and services related to the risk of natural hazards as mandatory.
TRCA undertakes flood plain mapping in accordance with its responsibility for implementing its Section 28 regulation under the CA Act: Ontario Regulation 166/06, as amended (Regulation of Development, Interference with Wetlands and Alterations to Shorelines and Watercourses).
What Are Flood Plain Mapping Studies?
Flood plain mapping studies are prepared and approved for TRCA by qualified engineers, using standards and criteria established by the Ontario Ministry of Northern Development, Mines, Natural Resources and Forestry.
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.
Within TRCA’s jurisdiction it is standard practice to complete comprehensive flood plain mapping updates on a 10-year cycle.
This ensures that flood plan maps and associated hydrology and hydraulic modelling incorporate the latest land use and land cover information, and incorporate technological advancements in modelling software and techniques to ensure the resulting flood plain maps remain current and state of the art.
TRCA’s Flood Plain Management Framework
The framework for flood plain management is TRCA’s Living City Policies (LCP). The LCP sets out development guidelines for properties influenced by valleys and stream corridors.
The LCP’s fundamental principles include:
- Protection of life and property from flooding and erosion hazards is dependent on natural system protection, restoration and remediation, inclusive of valley landforms, stream corridors, wetlands, watercourses and shorelines.
- Development and redevelopment should contribute to the prevention, elimination, and reduction in risk from flooding, erosion, and slope instability.
- Adaptive watershed management requires a preventative and proactive approach to address the potential impacts of urbanization and climate change.
Flood plain maps and studies are also the foundation of numerous other programs at TRCA, including:
- Flood Forecasting and Warning Program
- Flood Risk Assessment and Flood Vulnerable Area Ranking
- Flood Risk Reduction and Flood Protection Remedial Studies (including environmental assessments and feasibility studies)
TRCA’s flood plain mapping program also provides key information relating to:
- Special Policy Areas (SPA)
- Land Use Planning (official plans, block plans, and zoning)
There are many uses for flood plain mapping, including developing risk assessments, watershed planning, emergency management, flood remediation, and dam assessments.
Flood plain mapping improves flood management response and helps everyone to understand their flood risk. It is a critical component of emergency management plans.
Also, for areas where historical development has already occurred, floodplain mapping helps TRCA to identify ways to mitigate flood risk, all with the aim of better protecting people and property.
A History of Flood Plain Mapping
As the Greater Toronto Area, expanded and urbanized through the decades, the need for flood protection grew.
Over time, advancements in computing capabilities, and the development of sophisticated modelling software allowed TRCA to create state of the art mapping.
It all began in the 1940s, with the birth of Ontario’s Conservation Authorities (CAs)
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1940s
Conservation Authorities were established in 1946 under the Conservation Authorities Act. Initially, CAs were primarily tasked with reforestation.
1950s
On October 15, 1954, Hurricane Hazel struck Southern Ontario. Bridges and streets were washed out, homes and trailers were washed into Lake Ontario. Thousands were left homeless, and 81 people lost their lives.
This storm would change the Toronto landscape forever and highlight the need for managing watersheds on a regional basis.
Recognizing the importance of flood management, the provincial government amended the Conservation Authorities Act to enable an Authority to acquire lands for recreation and conservation purposes.
The province also developed flood plain regulations to restrict future development and land use within a flood hazard area, thereby reducing the potential damage and risk to human life.
In the 1950s, conservation authority staff would record high water levels manually. The first two images below are examples of recorded high water levels for TRCA in 1956.
High water marks are used to create a physical marker indicating one or more of the highest water marks for an area, usually with a line at the level to which the water rose (for documenting flooding extents). The use of high water marks from floods dates as far back as ancient Egypt.
Today the high water mark from Hurricane Hazel is still located on Bloor Street Bridge in Toronto, shown in the third image above.
1960s
TRCA’s first comprehensive flood plain mapping update took place in the 1960s.
It was undertaken for the purposes of meeting the requirements of Ontario Regulation 253/64 for regulating the construction of buildings or structures in areas below the high-water mark of rivers, creeks or streams; and regulating the placing or dumping of fill of any kind in areas defined by the Authority.
It should be noted that the term “below the high-water mark of rivers, creeks, or streams” was in reference to the fact that floodlines at the time were based on recorded high-water marks collected after the Hurricane Hazel event for each watershed.
Where water marks were not available, floodlines were based on manual hydrology and hydraulic calculations.
1970s
The next comprehensive flood plain mapping update occurred when federal funding was made available as part of the 1975 National Flood Damage Reduction Program (FDRP).
The FDRP was intended to coordinate federal and provincial strategies through defining flood risk areas, by discouraging continuing development in those areas, and by following up with appropriate measures to limit damage to existing development.
1990s
In the 1990s, TRCA moved towards digital mapping products. The first two-dimensional (2D) computer model of the Lower Don area was developed to understand flood risk in this complex and highly urbanized watershed.
TRCA flood plain mapping: 1960s vs 1990s.
2000s
The new millennium ushered in new technologies and fiscal commitments to bring floodplain mapping into the digital era.
TRCA initiated its third comprehensive flood plain mapping update. Leveraging funding from municipal partners, TRCA modernized the program, moving away from analog base mapping into a digital environment using advanced modelling software to establish floodlines.
The intent of this comprehensive flood plain mapping update was to convert analog base mapping to a digital format, ensuring that both mapping and modelling updates resulting from development applications could occur in real time.
TRCA also started to leverage GIS technology and Digital Elevation Mapping to produce better flood plain maps.
Today
With an ever-expanding landscape, constant land use changes, advancements in computing capabilities, and the development of sophisticated modelling software including Two-Dimensional (2D) Modelling, in 2016 TRCA initiated the most recent comprehensive flood plain mapping update, which was further accelerated through the availability of NDMP funding.
TRCA flood plain mapping: 2000s and today.
How Does TRCA Make Flood Plain Maps?
Flood plains are determined based upon information gathered through flood plain mapping studies, which is analyzed as part of a flood plain mapping update.
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Step 1: Gather Data
The first step is to gather information such as topographic data, surveys of infrastructure — such as the size of bridges and culverts — and land use information.
Step 2: Modelling Watershed Features
Hydrology studies are the first process that needs to be completed to undertake flood plain mapping updates.
Watershed hydrology is the study of how water moves through the water cycle. For flood plain mapping purposes, it is the study of how TRCA’s watersheds, with current and planned land-use changes, would respond to rainfall events like Hurricane Hazel as well as other hypothetical storms.
To help inform hydrology model updates, TRCA continually collects monitoring data (stream flow and precipitation), as well as information on land use, topography, land cover, and soil types.
Step 3: Modelling Physical Features
Open channel hydraulics is the study of how water moves through a river channel or valley corridor.
A hydraulic model is a representation of the physical characteristics of the valley and stream corridor, including:
- The channel and valley shape
- Slope and land cover (and the corresponding resistance to flowing water)
- Water crossings (bridges and culverts)
Hydraulic modelling defines the extent of the flood plain, based on these characteristics and the flow inputs for a given storm based on the hydrology model.
Hydraulic models provide detailed outputs of various model results, like water surface elevations and velocity, which are important for defining flood extents and flood risk.
Step 4: Creating the Final Maps
TRCA uses two different modelling approaches to define floodlines within our watersheds:
- The majority of TRCA’s jurisdiction uses the one-dimensional (1D) modelling approach.
- In select areas, where complex hydraulic conditions exist, TRCA uses the two-dimensional (2D) modelling approach.
- 2D modelling is used in specific areas where flood flows are not contained in a valley corridor and spill into adjacent urban areas, in areas with wide, shallow flood conditions, and in areas where multiple major channel confluences exist.
This information is mapped and distributed to the appropriate divisions within TRCA.
Updating Flood Plain Maps
Currently, the best practice at TRCA is to conduct flood plain mapping updates on a 10-year cycle to ensure our mapping remains current.
Historically, flood plain mapping updates have been critical for supporting municipal implementation of provincial legislation and policies for managing flood risk.
About TRCA’s Flood Plain Map Viewer
TRCA has created a web application to display the location of the regulatory flood plain in a user-friendly format. Residents and property owners can search by address to find out if a property is located within a TRCA flood plain.
Check Out Our Frequently Asked Questions
FOR EXPERTS: RESOURCES
About Flood Return Periods (Recurrence Intervals)
The concept of risk, reliability, and return periods are widely used in the examination of extreme weather events in the field of hydrology as well as for number of other natural hazards such as flooding, earthquakes and more.*
Purchasing Flood Plain Maps
If you’re interested in purchasing flood plain maps for the purposes of a development application, please contact TRCA Planning and Permits.
To purchase flood plain maps outside of a development application, visit First Base Solutions Map Warehouse. Each flood plain map displays contours, roads, buildings, as well as hydraulic data.
Modelling References
Flood Risk Assessments
| TRCA Flood Risk Assessment and Ranking Project |
|---|
| Flood Risk Assessment and Ranking Report Composed by IBI Group Final Report, October 2019 |
Storm Reports
| July 8, 2013 – Extreme Rainfall Event |
|---|
| Summary & Analysis Report Submitted by AMEC Environment & Infrastructure Final Report December 2014 |
| August 19, 2005 – Storm Event |
|---|
| Summary of Rainfall Analysis Submitted by Clarifica Water Resources and Environmental Solutions Final Report June 2006 |
Shoreline Hazard Mapping Reports
| TRCA Shoreline Hazard Mapping Report |
|---|
| This report contains information on the flood hazard, erosion hazard and dynamic beach hazard on TRCA’s Lake Ontario shoreline. Prepared by W. F. Baird & Associates Coastal Engineers Ltd. Final Report November 2022 |
Case Studies
| Communicating Flood Risk to Residents Living in Riverine Flood Vulnerable Areas Prepared by Rhianydd Phillips, Flood Risk Management Coordinator and Rehana Rajabali, MUDS, Associate Director, Engineering Services Development and Engineering Services Final Report September 1, 2020 |
Technical Guidelines and Considerations
| Technical Guideline: MIKE FLOOD Modelling Approaches for Floodplain Mapping Prepared by DHI Water and Environment Inc. April 2018 |
| Toronto and Region Conservation Authority Flood Protection Land Forming Technical Design Considerations Prepared by AECOM December 2018 |
Hydrology/Hydrologic Modelling Reports
TRCA’s Engineering Services department utilizes hydrologic and hydraulic models to aid in enforcing policies and regulations intended to ensure that new developments are located outside of the flood risk area.
You can download a copy of the Hydrology and Hydraulic reports for each of TRCA’s watersheds.
| Watershed | Hydrology Reports | Hydraulic Modelling Reports |
|---|---|---|
| Carruthers Creek | Carruthers Creek Watershed Hydrology Composed by Cole Engineering Final Report, October 2011 |
Please contact TRCA directly at (416) 661-6600. |
| Don River | Don River Hydrology Composed by AECOM Final Report, December 2018 |
Don River Flood plain Mapping Update Phase 1 Composed by KGS Group Final Report, April 2020 Don River Flood plain Mapping Phase 2 • Appendix A: Structure Data Sheets • Appendix B: TRCA Standard Manning’s Roughness • Appendix C: PCSWMM Modelling Memo • Appendix D: HEC-RAS Output • Appendix E: Flow Nodes Used for the Development of HEC-RAS Steady Flow Table • Appendix F: Modelling Approach by Crossing Structure Composed by WSP Canada Group Limited Final Report, August 2020 Hydraulic reports are available for the following areas: – Lower Don 2D model – Yonge and Elgin Mills Please contact TRCA directly at (416) 661-6600. |
| Duffins Creek | Duffins Creek Hydrology Composed by Aquafor Beech Limited Final Report, February 2013 |
Duffins Creek Flood Plain Mapping Update Composed by Wood Environment & Infrastructure Solutions Final Report December 2020 Pickering Ajax 2D Model Flood Plain Mapping Update Composed by Valdor Engineering Inc. Final Report, March 2018 |
| Etobicoke Creek | Etobicoke Creek Hydrology Composed by MMM Group Limited Final Report, April 2013 |
Etobicoke Creek Flood Plain Mapping Extension Composed by Toronto and Region Conservation Authority (TRCA) Final Report, October 2022 Etobicoke Creek Flood Plain Mapping Update Spring Creek 2D Modelling Extension and Flood Plain Mapping Update Spring Creek Flood Plain Mapping Update Study Flood Plain Mapping in Applewood and Dundas/Dixie Special Policy Area Downtown Brampton Flood Plain Mapping Update |
| Frenchman’s Bay | Frenchman’s Bay Watershed: Hydrologic Model Update Prepared by Matrix Solutions Inc., A Montrose Environmental Company February 2024 |
Frenchman’s Bay Watershed: Hydraulic Model Update Prepared by Matrix Solutions Inc., A Montrose Environmental Company February 2024 |
| Highland Creek | Highland Creek Hydrology Model Update Composed by Matrix Solutions Inc Final Report, May 2020 |
Highland Creek Watershed Hydraulic Model Update Composed by Matrix Solutions Inc Final Report, June 2020 |
| Humber River | Humber River Hydrology Composed by Civica Infrastructure Inc. Final Report, April 2018 Humber River Hydrology Composed by Civica Infrastructure Inc. Final Report, June 2015 |
Bolton Floodplain Mapping Composed by R .J. Burnside and Associates Limited Final Report, August 2014 Lower Humber 2D Modeling Composed by Valdor Engineering Inc Final Report, March 2017 Rockcliffe SPA Floodplain Mapping Update Composed by DHI Final Report, January 2020 Humber River in Peel Floodplain Mapping Update Composed by Cole Engineering Group Ltd. Final Report, June 2018 Humber River in York Floodplain Mapping Update Composed by Aquafor Beech Limited Final Report, December 2019 Humber River in Toronto Floodplain Mapping Update Composed by Wood Environment & Infrastructure Solutions Final Report, September 2018 Summary Report for Digital Floodplain Mapping for the East Humber River within the Town of Richmond Hill Prepared by R.J. Burnside & Associates Limited Revised July 2014 Floodplain Mapping in Jane and Wilson Special Policy Area, Black Creek: FINAL REPORT | FIGURES | APPENDICES Prepared by Valdor Engineering Inc Final Report, December 2016 |
| Mimico Creek | Mimico Creek Hydrologic Composed by MMM Group Limited Final Report, December 2009 |
Mimico Creek Flood Plain Mapping Update Composed by Valdor Engineering Inc Final Report, August 2020 |
| Petticoat Creek |
Petticoat Creek Hydrology Update Composed by WSP Canada Group Limited Final Report, December 21, 2020 Petticoat Creek 2020 Hydrology Update Revision Composed by WSP Canada Group Limited Final Report, September 10, 2021 |
Petticoat Creek Flood Plain Mapping Study Composed by Planning and Engineers Initiatives LTD. Final Report, November 2006 |
| Rouge River | Rouge River Hydrology Study Composed by Wood Environmental & Infrastructure Solutions, a Division of Wood Canada Limited Final Report, September 2018 |
Rouge River FPM Update Phase I Composed by Wood Final Report, March 2020 Rouge River FPM Update Phase II Composed by Wood Final Report, March 2020 Unionville SPA 2D Study and Floodplain Mapping Update • Appendices A-C • Appendix D Part 1 • Appendix D Part 2 • Appendices E-F Composed by Valdor Engineering Inc Final Report, February 2019 |
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