How the City of Ottawa is Building Resiliency into their Flood Plain Mapping

PSD Citywide

Share on

As the frequency and severity of storms and climatic events continues to increase, Canadian municipalities are being tasked with coming up with innovative solutions to protect their assets. For the City of Ottawa, one of the ways that they are instituting resiliency into their processes is through their flood plain mapping.

While flood mapping doesn’t prevent flooding, it does define where flooding will take place, and has assisted the City with planning new developments and making assets more resilient to the impacts of flooding. PSD Citywide spoke with Kevin Cover, Senior Project Manager in the Infrastructure Policy Unit within Planning, Infrastructure and Economic Development for the City of Ottawa, about how the City is continuing to work towards building resiliency measures into their practices.

Can you give some context around flood plain mapping in Ontario?

Foremost, it is important to understand that flood plain mapping identifies where flooding will occur under defined conditions, and by prohibiting development within the flood plain avoids risks under these conditions. The idea of directing development away from flooding is indeed good planning and is required by the Provincial Policy Statements (PPS). It is a provincial direction to implement hazard lines, and among the hazard lines to include hazards due to flooding. We have an official plan that conforms to the PPS (i.e. regulating development out of the flood plain). Within the zoning by-law we have a flood plain overlay, which identifies any concerns from a zoning perspective.

Ontario was previously a leader in flood plain mapping, however, support in terms of funding has faltered since around 1995. Ontario does have regulations in place, which are established through the Ontario Ministry of Natural Resources and Forestry (MNRF), who determined the standard regulatory storms used for mapping flood hazards. Flood hazards are one of a series of hazards considered by the MNRF Hazard Guidelines. Other hazards are steep slopes, unstable slopes, and erosion hazards.

For Eastern Ontario, the regulatory event for flooding is the 100-year flood, and that is in contrast to Southern Ontario and the Greater Toronto Area (GTA), where it is Hurricane Hazel. The 100-year flood is the lowest level of flood hazard in Ontario (in contrast Hurricane Hazel would be more so ranked as a 300- or 400-year storm). To translate this a bit, a 100-year storm means there is a one percent (or one percent flood condition) risk of the event occurring each year. In context for community design, it has a 22 percent chance of happening in 25 years and a 63 percent chance of happening in 100 years if you think about flood risk in terms of the risk to a community over a few generations.

What has the City of Ottawa done to not only implement regulations under the PPS but to expand their flood plain mapping processes?

The City of Ottawa now has a multi-year program to create and update flood plain mapping. The City is working with the local conservation authorities, providing funding to do the mapping. In addition to using flood plain mapping for planning and decision-making within regulated limits, we have also added inundation or depth-of-flooding mapping. The mapping uses raster information rather than simple lines delineating flooding.

Raster data is like a colourized filter: a picture that has pixels with each pixel being a square meter in size that showcases the depth of what the flooding would be in that particular location. To contextualize this more, you have the surface of the flood plain – mapping shows this line – and then you have the water surface; subtract the elevation of the land underneath it and we have a depth of flooding for every square meter found within the flood plain.

We are able to do this by using high-resolution LiDAR data the city has acquired with ~ 18 billion measurements of the surface across the 2,800 km2 of the city. We have this not just for the regulated 100-year storm data, but for 5, 25, 50 and 350 return periods as well, both for the Ottawa and Rideau Rivers. The City will be collecting this data for all areas where we have floodplain mapping. This information is used for both emergency management planning and for infrastructure and community planning.

Using this information, we are able to turn on transparency with colorized indicators (i.e. yellow for shallower depths) for each return period, so you can overlay on photography and understand what is going on and flag risks from frequent 20 percent chance a year to rare 0.3 percent chance a year. We use 0.3 meters as a limit for access and egress (i.e. getting in and out of areas). If the risk is greater than 0.3, we don’t have great access, therefore, it is considered an isolated or more vulnerable area. We use this information to flag priorities, such as when we do roadwork. It also provides insight on how better to design roadways.

Understanding the variability in the climate and how it changes and how it has always changed is very interesting. Particularly in the context of precipitation, we see clear trends in temperature, but precipitation has varied considerably. There are a few theories on how that all evolves, and we are considering these things and trying to incorporate them the best we can.

In what ways has the flood plain mapping and multiple return periods the City is using assisted with resiliency measures and overall planning?

We regulate development within the 100-year flood plain per the PPS, the MNRF Technical Guidelines and the CA Regulations. However, the 350-year information can be used to ensure communities are resilient in response to the 350-year flood conditions. Because of this we want to ensure that the design does consider the conditions, so we plan to be able to recover from flooding under those conditions.

An example of these design measures would be ensuring that critical equipment, building entrances, etc., are above the flood level. We are using the 350-year as a surrogate for increasing risk. The 2014 PPS does say to consider climate change. However, the difficult thing about return periods for flooding is that in comparison to changes in temperatures being relatively well defined, changes in precipitation are far harder to predict for areas like Ottawa.

Understanding the variability in the climate and how it changes and how it has always changed is very interesting. Particularly in the context of precipitation, we see clear trends in temperature, but precipitation has varied considerably. There are a few theories on how that all evolves, and we are considering these things and trying to incorporate them the best we can. We use the 350-year almost as a stress test. While we don’t have a provincial regulation that requires us to do this, we are adding it as an extra best practice. The difficult part is trying to predict where the 100-year will likely be given climate change, especially since currently there is no clear standard of practice.

We have built this resiliency into our processes, insofar as considering a greater range as a surrogate for the increased risk in the variability in climate. We have been instituting these practices specifically for river flooding. However, we are also doing something similar for flooding related to urban stormwater. We are undertaking a citywide flood risk profile, reviewing the tolerance between water levels and sewers and buildings. In order to facilitate this, we use LiDaR, direction and ranging, that gives us 10 measurements per square meter of the urban areas of the City of Ottawa. We have surface and drainage pathways of the entire city modelled at that level, therefore, we understand our drainage very well, and one of the triggers for doing this was to update our flood plain mapping. We were a leader in Ontario, by using citywide LiDAR topography for floodplain and inundation mapping. Newfoundland and Labrador is taking a similar approach, although they have done further forecasting on what the 100-year floods will look like in the future.

The depth of flood mapping and the evaluation of critical assets and vulnerable areas, coupled with this information being used in our comprehensive asset management program, gives the City a more holistic understanding of risk and the assets that are at risk.

What are the next steps for the City of Ottawa?

Our next steps will be to continue to work on precipitation trends, spring melt, and variability of streamflow with climate change. We have done some work on this front, but establishing best practices and clearly documenting these measurements are imperative steps moving forward.

PSD Citywide icon.

Asset Management

Strengthen your community with our expertise.

More Articles