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.  

===Active storage volume===

===Active storage volume===

LID practices are typically designed to manage more frequent and lower magnitude [[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:

 

 

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.

STEP/TRCA conducted a hydrologic monitoring and modelling study of the site in 2012/13 with researchers from The Metropolitan University to assess runoff volume and peak flow reductions.  Results showed that, relative to a conventional stormwater approach without LID, runoff was reduced over the study period by between 30% and 35% for the entire site, and by between 58 and 62% in the catchment with a higher density of LID practices.  Peak flows were also reduced by 73 to 78%.  In the Northeast catchment, 20% of [[Rainwater Harvesting|rainfall harvested]] from the roof was stored and reused for irrigation during the summer months.  This reuse volume represented 6% of total site rainfall over 8 months.  A hydrologic model calibrated using monitored data showed that the stormwater management system met the design objective of providing quantity control for the post development 100 year storm.

STEP/TRCA conducted a hydrologic monitoring and modelling study of the site in 2012/13 with researchers from The Metropolitan University to assess runoff volume and peak flow reductions.  Results showed that, relative to a conventional stormwater approach without LID, runoff was reduced over the study period by between 30% and 35% for the entire site, and by between 58 and 62% in the catchment with a higher density of LID practices.  Peak flows were also reduced by 73 to 78%.  In the Northeast catchment, 20% of [[Rainwater Harvesting|rainfall harvested]] from the roof was stored and reused for irrigation during the summer months.  This reuse volume represented 6% of total site rainfall over 8 months.  A hydrologic model calibrated using monitored data showed that the stormwater management system met the design objective of providing quantity control for the post development 100 year storm.

===Example 2: [https://sustainabletechnologies.ca/app/uploads/2020/06/Wychwood-Report.pdf Wychwood Subdivision in Brampton]===

===Example 2: [https://sustainabletechnologies.ca/app/uploads/2020/06/Wychwood-Report.pdf Wychwood Subdivision in Brampton]===

*Event greater than 30 mm showed peak flow reductions of 74%, with a total volume reduction of 59%  

*Event greater than 30 mm showed peak flow reductions of 74%, with a total volume reduction of 59%  

*Modelling pre and post development peak flow rates indicated that peak flow targets were met for the 2 to 50 year storms but post development peak flows were *10% greater than pre development peak flows for the 100 year storm.

*Modelling pre and post development peak flow rates indicated that peak flow targets were met for the 2 to 50 year storms but post development peak flows were *10% greater than pre development peak flows for the 100 year storm.

===Example 3: Costco Distribution Centre===

===Example 3: Costco Distribution Centre===

#Final design required both LIDs to reduce the overall runoff volumes, but also sub-surface storage chambers to provide quantity control for rare storm events up to the 100-year design storm. Due to large area required for truck parking, limited opportunities for more landscaping to promote [[evapotranspiration]], runoff volumes increased beyond ability of LIDs to negate the need for quantity control.

#Final design required both LIDs to reduce the overall runoff volumes, but also sub-surface storage chambers to provide quantity control for rare storm events up to the 100-year design storm. Due to large area required for truck parking, limited opportunities for more landscaping to promote [[evapotranspiration]], runoff volumes increased beyond ability of LIDs to negate the need for quantity control.

===Example 4. West Gormley, Town of Richmond Hill===

 

Residential development consisting of low and medium density land-use is implemented on the site. Average site imperviousness is approximately 60%;

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

[[File:Blue roof schematic.png|400px|thumb|right|Smart blue roof schematic (City of Missisauga, 2024)<ref>City of Mississauga. 2024. Mississauga is home to Canada’s first CSA-compliant smart blue roof.https://www.mississauga.ca/city-of-mississauga-news/news/mississauga-is-home-to-canadas-first-csa-compliant-smart-blue-roof/</ref>]]

[[File:Screenshot 2025-11-14 140403.png|400px|thumb|right|Glendale Public School rain garden (STEP, 2020)<ref name = Glen>STEP. 2020. Glendale Public School Rain Garden: Design and Build Overview. https://sustainabletechnologies.ca/app/uploads/2020/09/CVC-Glendale-Rain-Garden-Case-Study.pdf.</ref>.]]

[[File:Screenshot 2025-11-14 140956.png|400px|thumb|right|Glendale Public School rain garden study area (STEP, 2020)<ref name = Glen></ref>.]]

Glendale Public School area in Brampton faced increased urbanization, limited stormwater controls, and on-site drainage issues that were harming aquatic health in nearby Fletchers Creek, particularly the endangered Redside Dace. To address these concerns, CVC designed a large-scale rain garden using a treatment-train approach, incorporating three swales, conveyance pipes, an underdrain system, and a flow-control valve.

 

[[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.

 

'''Stormwater Management Criteria'''

'''Stormwater Management Criteria'''

*Quantity Control – Rouge River – match post development peak flow rates to pre-development;

*Quantity Control – the rooftop can hold up to 180 mm of precipitation, thus capturing the 100-year storm

*Water Balance – water either evaporates from the rooftop, is sent to rainwater harvesting tank for reuse (can meet non-potable water demands of 8.84 m3/day), or gradually flows into the municipal stormwater system

*Water Balance –Match post development water budget to pre-development;

*Erosion Control – temporary detention and slow release

*Erosion Control – Southern portion of site discharging to a natural dry valley feature.  Feature and contributing drainage area consists of very sandy soil, producing no runoff until a greater than 25-year storm event.

'''Stormwater Management Strategy'''

'''Stormwater Management Strategy'''

#Use a combination of increased topsoil depths, perforated storm sewers, stormwater management facility, and an [[infiltration]] facility to provide [[water quality|quality]], quantity, and reduce runoff volumes to match pre-development.

# Smart blue roof systems can regulate rooftop runoff by storing and controlling the release of rainwater

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

 

===Example 5: 3775-4005 Dundas St West (includes 2-6 Humber Hill Ave), Toronto===

===Example 5: [https://sustainabletechnologies.ca/app/uploads/2020/09/CVC-Glendale-Rain-Garden-Case-Study.pdf Glendale Public School Rain Garden]===

'''Stormwater Management Criteria'''

'''Stormwater Management Criteria'''

*Quantity Control – not requirement as drains to Lower Humber River

*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 – 80% TSS Removal

*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 – Retention of 5 mm event on-site

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

'''Stormwater Strategy'''

'''Stormwater Strategy'''

#Large portion of the roof proposed as green roof and cistern proposed in underground parking to capture remaining volume to meet 5 mm target.  Water to be used for irrigation and carwash stations.

# Swales direct surface runoff towards a rain garden.

#Quality target achieved as majority of site is ‘clean’ roof water or directed to pervious area. Underground storage tank provided to satisfy municipal release rates to receiving storm sewer system.

# 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.

#Final design required both LIDs to reduce the overall runoff volumes, but also sub-surface storage chambers to provide quantity control to meet municipal requirements.

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

#Due to underground parking limited opportunities for infiltration LIDs but used [[green roof]] to promote [[evapotranspiration]], and [[Rainwater Harvesting & Cisterns: Life Cycle Costs|cistern]] to reduce runoff volumes.

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

# A flow control valve was installed at the underdrain outlet to control the amount of water draining into the municipal system located on. During normal operation, this valve is closed to maximize storage and infiltration. Under extreme rainfall events the valve can be opened to release water.

==Data Analysis/Modelling==

==Data Analysis/Modelling==