Purpose and Uses for Flood Plain Mapping
Flood plain mapping identifies the areas that may experience flooding due to rising water levels in a watercourse. This mapping is critical for the effective management of riverine flood risks through prevention/mitigation of risks, protection of people and property, and emergency preparedness and response planning. Effective hazard and risk management will often involve a combination of measures. The City, Conservation Authorities, and property owners all have a role to play in flood risk management.
Flood plain mapping in the City of Ottawa
The City of Ottawa uses flood plain mapping to:
- Direct land use planning through the Official Plan and Zoning By-Law
- Inform risk assessments and adaptation planning
- Inform emergency preparedness and response plans
Conservation Authorities use the maps to fulfill their mandates for:
- Regulation of development in the hazard area or alterations to the watercourse
- Implementing policies and requirements for flood proofing or flood protection where minor development is approved in a flood plain
- Flood forecasting and warning within their watershed
Visit Conservation Ontario or your local Conservation Authority [LA4] for more information on the role of conservation authorities.
Resources for residents
Property owners and residents can reference the flood plain maps to review the potential risks to their properties and take steps to protect them. This also helps build resiliency to future climate conditions. Steps may include:
- Physical changes to the property
- Adjustments to how belongings are stored in basements
- Ensuring they have the appropriate insurance for their property
Regulatory flood plain mapping
The Ministry of Northern Development, Mining, Natural Resources and Forestry defines the regulatory flood event standard for flood plain mapping . Regulatory event standards are defined by region and for Eastern Ontario it is the 1 in 100-year flood event. The location of the 1 in 100-year flood plain is identified in the flood plain overlay of the City of Ottawa Zoning By-law (Section 58). The 1 in 100-year flood plain is also regulated by the Conservation Authorities and subject to regulations and policies under the Conservation Authorities Act. Development is generally prohibited in the 1 in 100-year flood plain.
The 1 in 100-year flood event standard is lower than the regulatory flood event standards which apply in other regions of Ontario where historical flood events, such as Hurricane Hazel and the Timmins Storm, or other larger flood events apply. The regulatory flood plain mapping used to restrict development activity in Eastern Ontario also does not capture changes to flood magnitude or probability that are anticipated to occur with climate change.
Climate change regulatory context
The Ontario Provincial Policy Statement provides direction to municipalities under the Planning Act and states that they must prepare for the impacts of a changing climate.
In 2019 the City of Ottawa declared a Climate Emergency. Through this declaration, Council directed staff to embed climate change across all elements of City business. The Climate Change Master Plan is a framework for how Ottawa will mitigate and adapt to climate change over the next three decades. The vision of the Climate Change Master Plan is to take unprecedented, collective action that transitions Ottawa to a clean, renewable, and resilient city by 2050. The City is currently developing a Climate Resiliency Strategy to identify the top climate risks facing Ottawa and develop steps to reduce the harmful effects of these risks.
Ottawa’s climate is changing
Ottawa’s climate is already changing.
- Average temperatures have increased by 1.3 degrees Celsius since the mid 1940s.
- More severe weather events in recent years such as heat waves, tornadoes, and significant flooding along the Ottawa River.
- In 2017 flood levels on the Ottawa River reached the 1 in 50-year flood event, and in 2019, the flood levels approached the 1 in 100-year event.
Local climate projections for the National Capital Region indicate trends of warmer temperatures, increased amounts of rain as well as more intense rainfall and earlier springs. This can all affect river levels and flows. Projected changes suggest an increased risk of extreme riverine flooding which exceeds the current 1 in 100-year flood event.
The effect of climate change on the magnitude and probability of future flood events is difficult to predict given the uncertainty of climate projections, and the variability of climate conditions and their impacts on the watershed of each riverine system. The 1 in 350-year flood event was selected as an appropriate larger magnitude flood event for assessing potential changes in riverine flood hazards and risks due to climate change.
The 1 in 350-year Flood Plain Maps
Rationale for the 1 in 350-year flood event
The return period or probability of a flood event is defined based on historical data and does not reflect possible changes to the magnitude or probability of a flood event due to climate change. Predicting exactly how climate change will impact a flood event is difficult. A larger event with a lower probability of occurrence may be used as an estimate. The 1 in 350-year flood event was selected as an appropriate event for assessing changes in riverine flooding due to climate change. The 1 in 350-year event:
- Aligns with local climate projection trends for climate parameters that influence riverine flooding
- Is comparable to Hurricane Hazel and the Timmins Storm, which are already used to regulate development throughout Southern and Northern Ontario
- Is the Public Safety Canada recommended standard for creating flood hazard maps
Mapping of the 1 in 350-year flood plain is completed by the Conservation Authorities in partnership with the City as part of an on-going flood plain mapping update program. Models and analysis developed by the Conservation Authority for regulatory flood plain mapping are prepared in accordance with provincial technical guidelines and subjected to a third-party technical review by an engineering consultant retained by the City of Ottawa. These same models are used to generate the 1 in 350-year flood elevations and the corresponding flood limits are mapped using detailed topographic data.
Unlike the 1 in 100-year flood plain maps, the 1 in 350-year flood plain maps are not used to define limits of development. For this reason, the 1 in 350-year flood limits may be less refined than the 1 in 100-year flood limits.
Policies for the 1 in 350-year flood plain
The City’s new Official Plan defines a climate change flood vulnerable area as the area between the regulatory 1 in 100-year flood plain and the 1 in 350-year flood plain. In the climate change flood vulnerable area:
- The Official Plan will require new development applications in these areas to assess riverine flood risks and include mitigation measures to reduce or avoid identified flood risks.
- Official Plan policies will only apply where an approval under the Planning Act is required to permit new development, such as site plan control, plan of subdivision or when a new secondary plan is being prepared.
- The policies and provisions of the Zoning By-law (section 58) do not apply.
Climate change flood vulnerable areas may still fall within the Conservation Authority’s Natural Hazard Regulation Limit and require approval or permits for other reasons under the Conservation Authorities Act. Where 1 in 350-year flood plain mapping is not yet available, development is subject to any regulatory mapping policies until 1 in 350-year flood plain mapping is available.
Frequently Asked Questions
What is a flood plain?
A flood plain is land near a watercourse which may be subject to flooding due to rising water levels during a rainfall event or spring snow melt. The limit and/or depth of flooding over the flood plain area will vary depending on the magnitude of the flood event.
What is a return period?
The magnitude of a flood event is often described by its return period which represents the frequency or probability of a given flood event occurring or being exceeded in a given year. For example, a 1 in 5-year event (1/5) has a 20% chance of occurring or being exceeded in any given year and a 1 in 100-year event (1/100) has a 1% chance. It does not mean, for example, that a 1 in 100-year flood will only happen once every 100 years.
More frequent flood events have a higher probability of occurring or being exceeded in any given year and are smaller flood events. Less frequent flood events (e.g. 1 in 50-year or 1 in 100-year) have a lower probability of occurring or being exceeded in any given year and are associated with more extreme floods.
What is the difference between the 1 in 100-year and 1 in 350-year flood plain maps?
Generally, areas already vulnerable to flooding in a 1 in 100-year event will experience some increased depth of flooding in a 1 in 350-year event. A few areas that are not affected by a 1 in 100-year event could experience flooding under a 1 in 350-year event.
Why doesn’t my area have 1 in 350-year flood plain mapping yet?
1 in 350-year flood plain mapping is prepared as part of the overall flood plain mapping update program. Prioritization has been based on required 1 in 100-year flood plain mapping updates which, outside of the Ottawa and Rideau Rivers, has historically focused on areas of growing development. 1 in 100-year flood plain mapping must be completed or updated before the 1 in 350-year flood event can be mapped.
Will this mapping affect the value of my property?
Property owners are often concerned to learn that their land may be affected by a flood or other natural hazard. However, it is important to understand that these hazards already exist, and the mapping only documents them; it does not create the hazard.
Flood plain mapping can inform decision making for any future changes to a home or property and help ensure those investments are more resilient to possible future climate impacts.
The City of Ottawa will continue to work with the conservation authorities to update flood plain mapping and produce new 1 in 350-year flood plain mapping for watercourses that are not yet mapped. This data will be used to continue to inform:
- Risk assessment and adaptation planning, with the objective to achieve more resilient land use planning,
- infrastructure planning and design,
- asset management planning,
- emergency preparedness and response planning.
Floodplain Mapping Program and 1 in 350-year flood plain policies:
Amanda Lynch, P.Eng.
Senior Project Manager, Infrastructure Planning
Infrastructure and Water Services Department
City of Ottawa
Climate Risk Assessments and Climate Resiliency Strategy
To obtain copies of mapping data for a property, contact the appropriate Conservation Authority.
The interactive map may be used to identify known riverine flood hazards for a property or area.
The content of the map is for informational purposes only and represents results of on-going flood plain mapping work completed by the Conservation Authorities in partnership with the City.
- Property owners may contact the Conservation Authority for the most up to date information or data for an affected property.
- The mapping presented below includes multiple flood events and differs from the Zoning By-law floodplain overlay or viewed on GeoOttawa and reflects only the 1:100-year flood plain maps which have been approved by the Conservation Authority and adopted in the City’s Zoning By-Law.
Flood plain mapping of three different riverine flood events is included: a large flood event (1 in 50-year), regulatory flood event (1 in 100-year) and a more extreme flood event that could occur with climate change (1 in 350-year).
Tips for using the interactive map:
- Use the search bar to efficiently locate a specific property or area.
- Maps of all three flood events will be displayed by default. These may be turned on or turned off by selecting or deselecting the check box beside each flood event in the layers list.