Flood Plain Maps Now Available

The City of Ottawa now has made available online interactive flood plain maps to identify known riverine flood hazards for a property or area.

Flood plain mapping is completed by local Conservation Authorities in partnership with the City as part of the City’s on-going flood plain mapping update program to produce and update flood plain mapping for watercourses throughout the City. As directed by City Council, staff have posted an interactive flood plain map on the City of Ottawa website. The interactive map includes flood extents for three different riverine flood events: a large flood event (1 in 50-year), regulatory flood event (1 in 100-year) and a more severe event that could occur with climate change (1 in 350-year). The mapping presented in the interactive map includes multiple flood events and differs from the Zoning By-law floodplain overlay, which reflects only the 1:100-year flood plain maps that have been approved by the Conservation Authority and adopted in the Zoning By-Law, and are used to restrict development in flood plains. 

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. Conservation Authorities provide regular flood plain mapping updates to the City, which will be used to update the interactive mapping tool. The public is advised to contact the relevant Conservation Authority should they require or wish to obtain the most current information or details before proceeding with a project on their property. The web page also includes background information on flood plain mapping, details regarding the 1 in 350-year flood mapping and related policies, next steps, Frequently Asked Questions, and contact information. 

The interactive map can be viewed here. 

1 in 350-year Flood Mapping and Affected Areas 

To date, 1 in 350-year floodplain mapping has been generated for 23 watercourses, with others planned or in-progress. 

While the 1:350 flood plain mapping is not yet completed for all watercourses, there are some areas of the city that are potentially more affected than other areas. Generally, areas already vulnerable to flooding in a 1 in 100-year event will experience increased depth of flooding in a 1 in 350-year event. Consequently, flood extents may also increase in some areas and surrounding areas that are not affected by a 1 in 100-year event could experience flooding under a 1 in 350-year event. 

The areas with existing development that are most impacted by available 1 in 350-year flood plain mapping are along the Ottawa and Rideau Rivers. Many of these areas are already known to be vulnerable to flooding in a 1 in 100-year flood event and would experience an increase in both the depth and extent of flooding. Affected areas include: 

  • Ward 13 – Rideau Rockcliffe: New Edinburgh 

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. 

Why map flood plains? 

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 uses flood plain mapping to: 
    • Direct land use planning through the Official Plan and zoning policies 
    • 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 

What measures and resources are available for affected property owners? 

Property owners and residents can reference the maps to review the potential risks to their properties and take steps to protect it. This also helps build resiliency to future climate conditions. Steps may include, but are not limited to, physical changes to the property, adjustments to how belongings are stored in basements, and ensuring they have the appropriate insurance for their property. Useful resources for residents and property owners are available from the City of Ottawa and the Province of Ontario. 

Why use the 1 in 350-year flood event for climate change? 

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 

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 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. 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. 

Why don’t some areas 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. 

What policies apply in areas without 1 in 350-year flood plain mapping? 

Policies regarding 1 in 350-year flood plain do not apply until mapping for such areas is available. Development remains subject to existing policies based on available regulatory 1 in 100-year flood plain mapping. 

What about areas with flood control structures/berms? 

Some affected areas do have flood control structures that provide flood protection. The performance of these structures has not been assessed for a 1 in 350-year flood event. As such, flood hazards are identified for these areas on the assumption that flood control structures may not provide adequate protection. This is consistent with the approach for the 1 in 100-year event and follows Provincial guidelines. It may be appropriate to consider these structures when assessing flood risk and mitigation measures for a property in a climate change flood vulnerable area. 

Will this mapping affect property value? 

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. 

Will the mapping increase the cost of insurance? 

Most insurance policies have not historically covered overland or riverine flood damage. Increasingly, insurance companies are now offering optional water damage or flood damage coverage that may be purchased in addition to basic insurance policy coverage. Cost and availability of coverage will depend on the products offered by the specific insurance company. The Insurance Bureau of Canada provides a helpful overview: http://www.ibc.ca/on/disaster/water. 

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