Changes

Flood mitigation strategies that incorporate Low Impact Development (LID), traditional stormwater management, and hybrid infrastructure can manage stormwater effectively and reduce flood impacts.

Flood mitigation strategies that incorporate Low Impact Development (LID), traditional stormwater management, and hybrid infrastructure can manage stormwater effectively and reduce flood impacts.

==Types of flooding==

==Types of flooding==

! Pluvial (surface) flooding

! Pluvial (surface) flooding

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[[File:Reflecting-on-the-devastating-2013-storm-mississauga-takes-lead-in-municipal-flood-resilience-the-pointer-be39ea9b.jpg|300px|thumb|right|Street flooding in Mississauga (The Pointer, 2023)<ref>The Pointer. 2013. Reflecting on the devastating 2013 storm, Mississauga takes lead in municipal flood resilience. https://thepointer.com/article/2023-07-30/reflecting-on-the-devastating-2013-storm-mississauga-takes-lead-in-municipal-flood-resilience</ref>.]]

[[File:Reflecting-on-the-devastating-2013-storm-mississauga-takes-lead-in-municipal-flood-resilience-the-pointer-be39ea9b.jpg|400px|frameless|center]] Street flooding in Mississauga (The Pointer, 2023)<ref>The Pointer. 2013. Reflecting on the devastating 2013 storm, Mississauga takes lead in municipal flood resilience. https://thepointer.com/article/2023-07-30/reflecting-on-the-devastating-2013-storm-mississauga-takes-lead-in-municipal-flood-resilience</ref>.

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* Caused by intense rainfall that exceeds soil infiltration and storm sewer capacity, especially in urban areas with impervious surfaces.

* Caused by intense rainfall that exceeds soil infiltration and storm sewer capacity, especially in urban areas with impervious surfaces.

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! Fluvial (riverine) flooding

! Fluvial (riverine) flooding

| [[File:Screenshot 2025-09-22 100405.png|300px|thumb|right|Don River floods DVP (City News, 2024)<ref>City News. 2024. From the scene: Don Valley River floods section of DVP, stranding drivers. https://www.youtube.com/watch?v=fbyaYZy0d0A&t=68s</ref>]]

| [[File:Screenshot 2025-09-22 100405.png|400px|frameless|center]]Don River floods DVP (City News, 2024)<ref>City News. 2024. From the scene: Don Valley River floods section of DVP, stranding drivers. https://www.youtube.com/watch?v=fbyaYZy0d0A&t=68s</ref>.

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* Occurs when rivers exceed their capacity due to heavy rain or snowmelt, resulting in water overtopping the banks and flowing into adjacent areas.

* Occurs when rivers exceed their capacity due to heavy rain or snowmelt, resulting in water overtopping the banks and flowing into adjacent areas.

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! Coastal (shoreline) flooding

! Coastal (shoreline) flooding

|[[File:Screenshot 2025-09-19 115121.png|300px|thumb|right|Lake Ontario floods Toronto Island (Toronto Life, 2017)<ref>Toronto Life. 2017. Flooding on the Toronto Islands is terrible—but also weirdly beautiful. https://torontolife.com/life/flooding-toronto-islands-terrible-also-weirdly-beautiful/</ref>]]

|[[File:Screenshot 2025-09-19 115121.png|400px|frameless|center]]Lake Ontario floods Toronto Island (Toronto Life, 2017)<ref>Toronto Life. 2017. Flooding on the Toronto Islands is terrible—but also weirdly beautiful. https://torontolife.com/life/flooding-toronto-islands-terrible-also-weirdly-beautiful/</ref>.

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* Driven by storm surges and lake-level rise due to storm surges or seiches.   

* Driven by storm surges and lake-level rise due to storm surges or seiches.   

==Mitigative strategies==

==Mitigative strategies==

[[File:Western beaches storage tunnel.jpg|300px|thumb|right|The 4km long West Beaches Storage Tunnel in Toronto stores and treats combined sewer overflows and stormwater to prevent untreated sewage from entering Lake Ontario. It is a grey infrastructure solution which helps prevent localized flooding by collecting and storing water and uses [[sedimentation]] and UV disinfection to improve water quality (McNally, 2017) <ref>McNally. 2017. Western Beaches Tunnel – Toronto, ON. http://mcnally.ca/tunneling-projects/western-beaches-tunnel-toronto/#:~:text=Project%20Outline,pump%20station%20at%20Strachan%20Avenue.</ref>.]]

[[File:Western beaches storage tunnel.jpg|400px|thumb|right|The 4km long West Beaches Storage Tunnel in Toronto stores and treats combined sewer overflows and stormwater to prevent untreated sewage from entering Lake Ontario. It is a grey infrastructure solution which helps prevent localized flooding by collecting and storing water and uses [[sedimentation]] and UV disinfection to improve water quality (McNally, 2017) <ref>McNally. 2017. Western Beaches Tunnel – Toronto, ON. http://mcnally.ca/tunneling-projects/western-beaches-tunnel-toronto/#:~:text=Project%20Outline,pump%20station%20at%20Strachan%20Avenue.</ref>.]]

[[File:FEMA P-259 Engineering Principles and Practices for Retrofitting Flood-Prone Residential Structures Structure protected by levee and floodwall 520px (1).png|300px|thumb|right|Floodwall and levee protects building from flood water (Reduce Flood Risk, 2022)<ref>Reduce Flood Risk. 2022. Construct a floodwall barrier. https://www.reducefloodrisk.org/mitigation/construct-a-floodwall-barrier/</ref>.]]

[[File:FEMA P-259 Engineering Principles and Practices for Retrofitting Flood-Prone Residential Structures Structure protected by levee and floodwall 520px (1).png|400px|thumb|right|Floodwall and levee protects building from flood water (Reduce Flood Risk, 2022)<ref>Reduce Flood Risk. 2022. Construct a floodwall barrier. https://www.reducefloodrisk.org/mitigation/construct-a-floodwall-barrier/</ref>.]]

Effective flood mitigation strategies fall into three categories: grey infrastructure (traditional engineered solutions), green infrastructure (nature-based solutions), and grey-green hybrids. Cities typically combine measures based on local flood risks, scale, and desired co-benefits such as water quality improvement and urban cooling.

Effective flood mitigation strategies fall into three categories: grey infrastructure (traditional engineered solutions), green infrastructure (nature-based solutions), and grey-green hybrids. Cities typically combine measures based on local flood risks, scale, and desired co-benefits such as water quality improvement and urban cooling.

===Hybrid approaches===

===Hybrid approaches===

[[File:Corktown3-2048x-q60.jpg|450px|thumb|right|Corktown Common in Toronto is a levee designed with sustainable stormwater management, recreation, and biodiversity in mind. The previous brownfield site was a gateway for Don River floodwaters that put 500 acres of the city at risk. A series of trails lined with native vegetative, playground, and splash pad and were built on top of the 13 foot clay levee. A marsh was constructed on the west side of the levee which collects rainwater for park irrigation, conserving up to 145,000 gallons per day (Michael Van Valkenburgh Associates Inc, ND)<ref>Michael Van Valkenburgh Associates Inc. ND. Corktown Common. https://www.mvvainc.com/projects/corktown-common </ref>.]]

[[File:Corktown3-2048x-q60.jpg|400px|thumb|right|Corktown Common in Toronto is a levee designed with sustainable stormwater management, recreation, and biodiversity in mind. The previous brownfield site was a gateway for Don River floodwaters that put 500 acres of the city at risk. A series of trails lined with native vegetative, playground, and splash pad and were built on top of the 13 foot clay levee. A marsh was constructed on the west side of the levee which collects rainwater for park irrigation, conserving up to 145,000 gallons per day (Michael Van Valkenburgh Associates Inc, ND)<ref>Michael Van Valkenburgh Associates Inc. ND. Corktown Common. https://www.mvvainc.com/projects/corktown-common </ref>.]]

Combining green and grey infrastructure enhances flood resilience. Examples include:

Combining green and grey infrastructure enhances flood resilience. Examples include:

==Modelling Flood Mitigation Potential of Conventional LIDs==

==Modelling Flood Mitigation Potential of Conventional LIDs==

TRCA conducted [[modeling]] to evaluate the capacity of different stormwater management measures (LID and Ponds) to mitigate impacts of development on the peak flow and runoff volume. A sub-catchment in Humber River was selected that has an area of 35.7 ha. The existing land use in the sub-catchment is agriculture and the proposed future land use is employment land with 91% total imperviousness.

TRCA conducted [[modeling]] to evaluate the capacity of different stormwater management measures (LID and Ponds) to mitigate impacts of development on the peak flow and runoff volume. A sub-catchment in Humber River was selected that has an area of 35.7 ha. The existing land use in the sub-catchment is agriculture and the proposed future land use is employment land with 91% total imperviousness.

===Peak Flow===

===Peak Flow===

*The 25 mm on-site retention using LID measures reduced post-development peak flows generated from 2 to 5 year design storms by over 26%,

*The 25 mm on-site retention using LID measures reduced post-development peak flows generated from 2 to 5 year design storms by over 26%,

*For 50  and 100 year design storms it reduces only 4%  and 1% respectively.  

*For 50  and 100 year design storms it reduces only 4%  and 1% respectively.  

LID practices are typically designed to manage more frequent and lower magnitude [[Understanding rainfall statistics|rainfall]] events. They work by detaining runoff and releasing it slowly over time. However, larger events can overwhelm the capacity of LID practices.  Once their storage capacity is full, the [[overflow]] rapidly discharges excess water into storm sewers, thus limiting their ability to mitigate large flood events. LID designed for flood control should integrate large active storage volumes to temporarily store stormwater and slowly release it to streams or downstream sewer systems. The mechanisms by which conventional [[wet ponds]] and hybrid stormwater [[infiltration trench]]/[[bioretention]] facility provide this temporary storage are shown in the figures below:

LID practices are typically designed to manage more frequent and lower magnitude [[Understanding rainfall statistics|rainfall]] events. They work by detaining runoff and releasing it slowly over time. However, larger events can overwhelm the capacity of LID practices.  Once their storage capacity is full, the [[overflow]] rapidly discharges excess water into storm sewers, thus limiting their ability to mitigate large flood events. LID designed for flood control should integrate large active storage volumes to temporarily store stormwater and slowly release it to streams or downstream sewer systems. The mechanisms by which conventional [[wet ponds]] and hybrid stormwater [[infiltration trench]]/[[bioretention]] facility provide this temporary storage are shown in the figures below:

[[File:Screenshot 2025-09-22 114031.png|550px|thumb|left|Wet pond: The permanent pool provides water quality control, while the ‘active storage’ above the permanent pool provides temporary storage and slow release to reduce peak flows, stream channel erosion control, and flooding. Wet ponds do not provide runoff reduction or thermal mitigation benefits (MOE, 2003)<ref>Ontario Ministry of Environment. 2003. Stormwater Management Planning and Design Manual. https://www.ontario.ca/document/stormwater-management-planning-and-design-manual/stormwater-management-plan-and-swmp-design</ref>.]][[File:Screenshot 2025-09-22 161822.png|550px|thumb|right|Hybrid trench and bioretention system: Combines flood protection with water quality and water balance benefits. Active storage above the underdrain provides channel and flood control, while infiltration below the underdrain improves water quality and maintains water balance. The underdrain is positioned close to the trench bottom to maximize storage capacity and may be fitted with an orifice to regulate release rates, ensuring full use of storage even during the 100-year event. Because infiltration rates increase with hydraulic head, this design can achieve higher volume reduction than conventional LID practices not intended for flood control. Inlets consist of a distributed network of curb cuts connected to high-flow cobble/gravel columns (about 1 × 2 m). A similar concept can also be applied using stormwater chambers or underground infiltration trenches. Base Image: Schollen and Co<ref>https://www.toronto.ca/ext/digital_comm/pdfs/transportation-services/green-streets-technical-guidelines-document-v2-17-11-08.pdf</ref>.]]<br clear="all" />

    [[File:Screenshot 2025-09-22 114031.png|600px|thumb|center|Wet pond: The permanent pool provides water quality control, while the ‘active storage’ above the permanent pool provides temporary storage and slow release to reduce peak flows, stream channel erosion control, and flooding. Wet ponds do not provide runoff reduction or thermal mitigation benefits (MOE, 2003)<ref>Ontario Ministry of Environment. 2003. Stormwater Management Planning and Design Manual. https://www.ontario.ca/document/stormwater-management-planning-and-design-manual/stormwater-management-plan-and-swmp-design</ref>.]]

 

    [[File:Screenshot 2025-09-22 161822.png|600px|thumb|center|Hybrid trench and bioretention system: Combines flood protection with water quality and water balance benefits. Active storage above the underdrain provides channel and flood control, while infiltration below the underdrain improves water quality and maintains water balance. The underdrain is positioned close to the trench bottom to maximize storage capacity and may be fitted with an orifice to regulate release rates, ensuring full use of storage even during the 100-year event. Because infiltration rates increase with hydraulic head, this design can achieve higher volume reduction than conventional LID practices not intended for flood control. Inlets consist of a distributed network of curb cuts connected to high-flow cobble/gravel columns (about 1 × 2 m). A similar concept can also be applied using stormwater chambers or underground infiltration trenches. Base Image: Schollen and Co<ref>https://www.toronto.ca/ext/digital_comm/pdfs/transportation-services/green-streets-technical-guidelines-document-v2-17-11-08.pdf</ref>.]]

</div>

 

Kim & Han (2008)<ref>Kim, Y., & Han, M. (2008). Rainwater storage tank as a remedy for a local urban flood control. Water Science and Technology: Water Supply, 8(1), 31-36.</ref> and Han & Mun (2011)<ref>Han, M. Y., & Mun, J. S. (2011). Operational data of the Star City rainwater harvesting system and its role as a [[climate change]] adaptation and a social influence. Water Science and Technology, 63(12), 2796-2801.</ref> conducted studies in Seoul, South Korea, to assess the extent to which the installation of [[rainwater harvesting]] cisterns could help mitigate existing urban flooding problems without expanding the capacity of the existing urban drainage system. System operational data showed that 29 mm of rainwater storage per square meter of impervious area (3000 m3 cistern in this instance) provided sufficient storage for a one in 50 year period storm without the need to upgrade downstream sewers designed to 10 year storm capacity. Stormwater chambers, [[infiltration chambers]], [[bioretention]] and other LID systems designed with large volumes of temporary storage could have similar benefits, while also reducing runoff volumes and providing other co-benefits.

Kim & Han (2008)<ref>Kim, Y., & Han, M. (2008). Rainwater storage tank as a remedy for a local urban flood control. Water Science and Technology: Water Supply, 8(1), 31-36.</ref> and Han & Mun (2011)<ref>Han, M. Y., & Mun, J. S. (2011). Operational data of the Star City rainwater harvesting system and its role as a [[climate change]] adaptation and a social influence. Water Science and Technology, 63(12), 2796-2801.</ref> conducted studies in Seoul, South Korea, to assess the extent to which the installation of [[rainwater harvesting]] cisterns could help mitigate existing urban flooding problems without expanding the capacity of the existing urban drainage system. System operational data showed that 29 mm of rainwater storage per square meter of impervious area (3000 m3 cistern in this instance) provided sufficient storage for a one in 50 year period storm without the need to upgrade downstream sewers designed to 10 year storm capacity. Stormwater chambers, [[infiltration chambers]], [[bioretention]] and other LID systems designed with large volumes of temporary storage could have similar benefits, while also reducing runoff volumes and providing other co-benefits.

===Example 4. [https://sustainabletechnologies.ca/app/uploads/2020/12/SmartBlueRoofSTEPTechBrief_Dec2020.pdf Smart Blue Roof System at CVC Head Office]===

===Example 4. [https://sustainabletechnologies.ca/app/uploads/2020/12/SmartBlueRoofSTEPTechBrief_Dec2020.pdf Smart Blue Roof System at CVC Head Office]===

[[Blue roofs]] are emerging as an innovative rooftop stormwater management solution that provides flood protection and drought resistance. Instead of quickly conveying stormwater away from a property, blue roof systems temporarily capture rainwater until it either evaporates from the rooftop or is sent to rainwater harvesting storage tanks. A Smart Blue Roof was piloted at the CVC head office in Mississauga. Smart roofs are fitted with weather forecasting algorithms via internet connectivity and automated valves to regulate water discharge from the roof.  

 

[[Blue roofs]] are emerging as an innovative rooftop stormwater management solution that provides flood protection and drought resistance. Instead of quickly conveying stormwater away from a property, blue roof systems temporarily capture rainwater until it either evaporates from the rooftop or is sent to rainwater harvesting storage tanks. A [https://sourcetostream.com/2024-track-1-day-1-cowan/ Smart Blue Roof was piloted at the CVC head office in Mississauga]. Smart roofs are fitted with weather forecasting algorithms via internet connectivity and automated valves to regulate water discharge from the roof.  

{{#widget:YouTube|id=Fy0kEdoFH30}}

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# In addition to peak flow control, blue roof systems can facilitate runoff reduction through rainwater reuse and evaporative rooftop cooling

# In addition to peak flow control, blue roof systems can facilitate runoff reduction through rainwater reuse and evaporative rooftop cooling

'''Stormwater Management Criteria'''

'''Stormwater Management Criteria'''

*Quantity Control –

*Quantity Control – the rain garden was designed to capture runoff from a 27mm storm, covering up to the 90th percentile of the annual rain events in the area.

*Quality Control –

*Quality Control – reducing total suspended solids (TSS) by 80% before entering Fletcher’s Creek and providing thermal control by cooling runoff before discharging

*Water balance/Erosion Control –

*Water balance/Erosion Control – increased floodplain storage by a total of 800m3 reducing flooding potential during large storm events.

'''Stormwater Strategy'''

'''Stormwater Strategy'''

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# Swales direct surface runoff towards a rain garden.

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# The rain garden incorporated trees, shrubs and native plantings as well as native soil amendments and micro-topographic features to encourage adsorption, infiltration and support plant growth.

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# Drain pipes under the pathways allow the water level to equalize between garden cells

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# Perforated underdrain pipes, placed under the planted area, drained the facility

==Data Analysis/Modelling==

==Data Analysis/Modelling==