Flood Factors

While flooding is a natural phenomenon, there are many factors that can make the impacts of flooding more or less severe. Some of these contributing factors may feel out of our control, while others can be managed with both large and small changes at regional, municipal, community, and personal levels.

 Major Factors Influencing Flooding

Climate Change: we know that the climate is changing globally and here in Durham and that we are locked-in to significant change over the next century. These changes can be summarized as: “warmer, wetter, and wilder”. This means more frequent and more extreme floods in the future.

Urban Growth: Durham Region has grown rapidly and many vibrant communities have developed in recent decades. It is critical to ensure that this growth is sustainable and flood risk is not increased by constructing homes, businesses, and infrastructure in flood-prone areas.

Impervious Surfaces: the environment we live in now looks very different from what Durham looked like hundreds of years ago. We have increased the amount of impervious surfaces, like pavement, that cannot absorb water the same way as natural systems like soils and plants. When it rains, water moves faster and needs to be captured and stored to prevent flooding.

Design Standards: in recent decades, we've learned more about what causes flooding, where it might occur, and how we can prevent damage to our communities. With these improvements, we can acknowledge that some areas in the region that were built in the past were not built with the latest understanding and best practice of how to design for more flood-resilient communities. Sometimes, this means that these neighbourhoods can experience worse impacts from flooding.

For more resources on Climate Change

• Visit the Durham Region Adaptation and Resilience Page, including the Durham's Community Climate Adaptation Plan that sets out a vision, goals, and actions to help Durham Region adapt to climate change.
• Visit Canada's Changing Climate Report about how and why Canada's climate has changed and what changes are projected for the future.
• Learn about Canadian Cities and Climate Change at Climate Atlas of Canada
• Learn about Climate Change and Extreme Precipitation at Climatedata.ca by Environment and Climate Change Canada

Urban Growth and Land Use Patterns

Durham Region has grown rapidly, and many vibrant communities have developed in recent decades. It is critical to ensure that this growth is sustainable and flood risk is not increased by constructing homes, businesses and infrastructure in flood-prone areas. Conservation authorities work to ensure that new development occurs outside of flood hazard areas and encourages existing or legacy buildings and structures that are in higher hazard areas to be more resilient to flood events.

When reviewing plans, consideration is given to protect and restore the ecological and hydrological integrity of watersheds to protect people and their property, including:

• Natural Hazards: flooding and erosion processes associated with watercourses, valley lands, and the Lake Ontario shoreline
• Stormwater Management: water quality and quantity requirements for receiving watercourses
• Groundwater: protection of the quality and quantity of our groundwater resources
• Natural Heritage Features, Hydrologic Features, Systems and Environmentally Sensitive Areas: protection/conservation of physical and biological components of the landscape in order to maintain, enhance, and grow our natural heritage system

Example: The Great Wall of Durham

Durham Region has a 'Great Wall' somewhat like China’s, but in our case, the wall consists of the railway and highway embankments that run along the lakeshore parallel to the Highway 401 corridor. The largest wall includes the railway line built in the 1880s by the Grand Trunk Railway at a time when most of Durham was forest and farms, and flood runoff was modest. Today the rail lines are owned by CNR and CPR and are used extensively by freight trains, GO trains, and VIA Rail. The embankments are of substantial height in many areas and are punctuated by bridges and culverts that are in some cases too small for the much greater runoff caused by modern development and hard surfaces to the north. There are several well-known “pinch-points” where there is already occasional flooding on the north side of the corridor.

A future extreme rain event in Durham could trigger large-scale urban flooding at these 'pinch-points' and potentially flooding across the 401 in places, as well as the washout of the main rail lines. One well-known 'pinch-point' is where the Oshawa Creek crosses under the rail lines and 401 in south Oshawa. While complicated by the ownership of CNR and CPR, early expansion of these bridges is considered a priority and could avoid major transportation disruptions. Further analysis and costing of this and other sites along the Great Wall of Durham as pre-emptive investments are critical.

Source: Durham Community Climate Adaptation Plan 2016

Impervious Surfaces and the Water Cycle

The environment we live in now looks very different from what the Region looked like hundreds of years ago. When it rains in natural systems, water is slowed down and absorbed by soil, plants, and vegetation as part of the natural water cycle. Water is stored and used by plants and transferred to groundwater aquifers where it is cleaned, kept cool, and released to watercourses. When natural systems are altered, like by paving surfaces with asphalt, water can no longer reach the soil to be absorbed.

In a natural watershed, many raindrops never make it to the ground. Instead, they are taken up on leaf surfaces and by thirsty vegetation, to be released later through evapotranspiration. Also, the soil around healthy vegetation has the capacity to hold water in air spaces around root systems temporarily. These temporary storage pockets gradually transfer water to longer-term storage reservoirs within groundwater aquifers, where it is cleaned by natural filtration, kept cool, and eventually released into watercourses as baseflow to support aquatic life. When we replace natural pervious surfaces with impervious surfaces, like asphalt and concrete, water moves much faster as it cannot be absorbed. As a result, changes to land use have increased peak flows in storm events.

Design Standards

We have learned a lot about flooding in the past 50 years, and our technology to map and understand flood risk has also improved. With these improvements, we can acknowledge that some areas in the region that were built in the past were not built with the latest understanding and best practice of designing for more flood-resilient communities. Sometimes, this means that these neighbourhoods can experience worse impacts from flooding.

Many historians attribute the beginning of flood control in Ontario to the devastating impacts of Hurricane Hazel in 1954. Flood control plans began to be implemented in the 1950s and 1960s, including regulations to begin restricting development in flood hazard areas. In the 1980s, stormwater management began to take increasing importance in the Greater Toronto Area, and a better understanding of how urbanization was impacting the hydrologic cycle. Source: The History of Flood Control in the TRCA.

We cannot take the past as prediction for the future. As we work to upgrade and design flood-resilient communities, it is critical that practitioners working on design leverage the latest design standards that take into account increasingly variable, intense, or frequent storms. Infrastructure practitioners should account for the latest updates in data, what is changing, and advocate for forward-looking perspectives on historical and current infrastructure systems.

In recent years, it has become increasingly necessary for design standards to consider and incorporate the changing climate.

The Durham Community Climate Adaptation Plan 2016 included several adaptation actions to address urban flooding, including:

• Implementing Low Impact Development (LID) techniques, sites, and programs;
• Addressing infrastructure through several actions, including maintaining existing systems and incorporating climate change when designing new systems;
• Addressing existing development, including acquiring highly flood-vulnerable land, retrofitting buildings and creating more opportunities for flood storage;
• Addressing future development by managing any proposed development in the floodplain, updating planning policy and design standards, and revising stormwater management policies.

 Many of these actions will take considerable effort from all partners across Durham Region, including conservation authorities and local area municipalities. Together, we can build consistency and reduce the severity and frequency of flooding events.

Many organizations are interested in promoting and increasingly requiring Low Impact Development (LID) practices to reduce problems related to urban stormwater runoff, including erosion, sedimentation, and pollution.

The traditional approach to dealing with stormwater has been to move it away from city streets as quickly and efficiently as possible. This results in large volumes of water entering our waterways at high velocities, carrying the pollutants picked up along the way.

LID, by contrast, deals with stormwater by mimicking natural water cycles. It increases the infiltration of stormwater into the soil, where it can be filtered and/or absorbed by plants. LID is a lower-cost alternative to conventional grey infrastructure and provides a number of ecological, economic, and social benefits.

Examples of LID Include:

• Rain gardens
• Bioswales
• Infiltration trenches
• Permeable pavement
• Rainwater harvesting