Flood mitigation

Insufficient drainage systems, climate change, high-impact development, and urban spread exacerbate flooding in cities (Kumar et al., 2021).

Overview

The above chart shows insurable losses each year caused by natural disasters, the most costly of these being flooding, as reported by the Insurance Bureau of Canada in 2022. “In today's world of extreme weather events, the new normal for yearly insured catastrophic losses in Canada has become $2 billion, most of it due to water-related damage. Compare this to the period between 1983 and 2008, when Canadian insurers averaged only $422 million a year in severe weather-related losses." (IBC, 2022)..

Flooding is a major environmental and economic challenge, particularly in urban areas where impervious surfaces prevent natural infiltration. Flooding can result in traffic interruption, economic loss, infrastructure damage, basement flooding and other undesirable consequences. Climate change is increasing the frequency and intensity of extreme rainfall events, exacerbating flood risks. This will be particularly severe in older areas where the minor system was not designed to today’s standards and/or major drainage system pathways have been altered or do not exist. Catastrophic losses from flooding have been steadily rising in Canada over the last two decades.

"Hydrological changes associated with urbanization are increased storm runoff volumes and peak flows, faster flow velocities and shorter time of concentrations. A reduction in infiltration generally leads to less groundwater recharge and baseflow. The flashy response results in tremendous stresses for the urban stream and downstream receiving areas" (Walsh et al., 2005)

In order to protect downstream properties from flooding due to upstream development, Conservation Authorities establish flood control for future stormwater management planning through regularly updated of Hydrologic and Subwatershed Studies that characterize flood flow rates, define the location and extent of flood-prone areas, and assess the potential impact of further urbanization.

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

Pluvial (surface) flooding	Street flooding in Mississauga (The Pointer, 2023).	Caused by intense rainfall that exceeds soil infiltration and storm sewer capacity, especially in urban areas with impervious surfaces. Poor drainage and outdated infrastructure worsen impacts.
Fluvial (riverine) flooding	Don River floods DVP (City News, 2024)	Occurs when rivers exceed their capacity due to heavy rain or snowmelt, resulting in water overtopping the banks and flowing into adjacent areas. Runoff from upstream land use changes can worsen flooding
Coastal (shoreline) flooding	Lake Ontario floods Toronto Island (Toronto Life, 2017)	Driven by storm surges and lake-level rise due to storm surges or seiches. More common in areas with poor shoreline protection.

Mitigative strategies

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 an 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) .

Floodwall and levee protects building from flood water (Reduce Flood Risk, 2022).

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.

Grey infrastructure solutions

Strategy	Flood Mitigation	Additional Benefits
Detention ponds & stormwater basins	Store excess runoff, release gradually	Can improve water quality if designed with wetlands
Underground stormwater storage	Prevents sewer overflows, reduces localized flooding	Can integrate water treatment (e.g., Toronto’s Western Beaches Storage Tunnel)
Levees & floodwalls	Protect against riverine/coastal flooding; also used around buildings for surface flooding	Can be temporarily erected around buildings in response to floods

Green infrastructure solutions

Strategy	Flood Mitigation	Additional Benefits
Bioretention	Reduces peak flow, increases infiltration	Improves water quality
Permeable pavements	Allows infiltration, reduces runoff	Reduces heat island effect
Green roofs	Absorb rainfall, delay runoff	Urban cooling, energy savings
Riparian buffers	Protect banks, reduce erosion	Habitat, water quality protection
Constructed wetlands	Retain/filter stormwater	Habitat, recreation
Rainwater harvesting	Collects precipitation, reduces runoff	Water reuse
Rain gardens	Reduces on-site runoff	Aesthetic value, pollinator habitat

Hybrid approaches

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).

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

Strategy	Benefits
Stormwater tunnels + LID	Large-scale storage integrated with distributed LID practices.
Green streets with subsurface storage	Maximizes infiltration while providing underground detention.
Floodable parks / multi-use public spaces	Parks or plazas designed to temporarily store stormwater during extreme rainfall.
River restoration with levee setbacks	Reconnecting rivers to expanded floodplains while using engineered levees for protection

Pluvial (Surface) flooding

LIDs are well suited to address pluvial flooding because they can be shoehorned into smaller areas that may have increased flood risk. Runoff reduction and temporary detention are the primary means by which LIDs can reduce flooding at the scale of urban drainage systems. Kim & Han (2008) and Han & Mun (2011) 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 (see section below on ‘designing for flood resilience’).

Infiltration facilities and low impact development practices (such as bioretention and rainwater harvesting) are typically designed to manage more frequent and lower magnitude rainfall events. However, should these practices be designed for year round functionality, with sufficient flood storage capacity, the volume reductions associated with these practices will be recognized where the local municipality has endorsed the use of these practices and has considered long term operations and maintenance.

Riverine Flooding

The most common stormwater practices for mitigating riverine flooding are wet ponds and dry ponds, typically located at the end of the urban drainage system near streams. LIDs are traditionally designed to manage more frequent and lower magnitude rain events. However, larger storm chambers, trenches and even bioretention can be designed with large temporary storage volumes to provide flood control functions similar to wet or dry ponds.

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.

Hydrological model runs were carried out by integrating different stormwater management measures (LID and SWM Pond) for 2-year and 100-year 6-hr AES design storms.

Scenarios evaluated include:

1. LID measures that provide 25 mm on-site retention
2. SWM pond to control post-development peak flows to pre-development peak flows.
3. Combination of scenario 1 and scenario 2

Runoff volume and peak flow reductions were calculated:

Peak Flow

Peak flow reductions of different LID types during frequent rain events. Top left: Grey and green roof at York University; bottom left: permeable pavement, bioretention and asphalt at Seneca College; right: Kortright permeable pavement and asphalt.

• 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.

This shows that LID designed for frequent flows will not significantly reduce the post-development peak flows generated from major storms. In order to meet flood control requirements, traditional LID need to be augmented by some flood storage measures such as dry ponds or underground storage. LIDs can be designed with increased temporary storage to increase detention times. This allows them to function more like underground end-of-pipe facilities (see Honda case study below).

Runoff Volume

• The 25 mm on-site retention using LID measures can reduce post-development runoff volume generated from 2 to 5 year design storms by over 52 %,
• For 50 and 100 year design storms, runoff volumes are reduced by 33% and 30%, respectively

This shows that the post-development runoff volume generated from major storms conveyed to receiving features can be reduced considerably by implementing LID designed to retain 25 mm.

LID Design for Flood Control

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).

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.)

Many studies show that LID practices help reduce peak flows during smaller, more frequent storms. 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 on the right.

When designing LID for flood control it is important to consider the need to not only provide extended detention storage but also a means for water to enter the storage reservoir quickly. Incoming flows should also be pre-treated to avoid clogging of media and drainage pipes. Such pre-treatment can be achieved through OGS, catchbasin inserts or high flow cobble inlets, among others. The storage media in the LID facility should have a high void ratio to reduce the potential for clogging with fine sediment that may bypass the inlet pre-treatment controls.

Keeping inlets clear via pre-treatment and maintenance helps take advantage of the full storage capacity of the LID feature

Literature Review

Review examples of where LID practices with quantity control components have been used for achieving flood control

Example 1: Costco Distribution Centre

Costco Distribution Centre located within Block 59, Vaughan. The site has 26.4 ha and the land use is commercial site with an average site imperviousness of approximately 90%;

Stormwater Management Criteria

• Quantity Control – meet Humber River Unit Release Rates;
• Quality Control – 80% TSS Removal;
• Water Balance – Best Efforts to match post to pre;
• Erosion control, 25mm erosion storm released over 72 hours, on-site retention of the first 5mm of rainfall

Stormwater Management Strategy

1. A series of sub-surface infiltration chambers providing on-site retention/infiltration of the 5mm storm, water balance to reduce runoff volumes, and storage of the 100-year storm;
2. Quality treatment provided using an oil/grit separator immediately upstream of each of the infiltration chambers, filtration through the infiltration chamber, and finally a stormwater management facility provided prior to discharging from the site.
3. Final erosion control provided within the stormwater management facility, controlling release rates to maintain the existing condition erosion exceedance values.
4. 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 2. 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%;

Stormwater Management Criteria

• Quantity Control – Rouge River – match post development peak flow rates to pre-development;
• Quality Control – 80% TSS Removal;
• Water Balance –Match post development water budget to pre-development;
• 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.

Therefore, development discharging to dry valley needed to match runoff volumes, or have no runoff from development area for storms less than 25-year design storm.

Stormwater Management Strategy

1. Use a combination of increased topsoil depths, perforated storm sewers, stormwater management facility, and an infiltration facility to provide quality, quantity, and reduce runoff volumes to match pre-development.
2. Even with favorable soils and maximum use of infiltration techniques, site still requires quantity control storage for large storm events.

Example 3: 3775-4005 Dundas St West (includes 2-6 Humber Hill Ave), Toronto

The size of the site is 0.53 ha. The site currently developed as commercial and residential. Proposed high rise (11-storeys) residential building with 3 levels of underground parking Proposed average site imperviousness is 90% (excluding uncontrolled buffer area 0.22 ha)

Stormwater Management Criteria

• Quantity Control – not requirement as drains to Lower Humber River
• Quality Control – 80% TSS Removal
• Water balance/Erosion Control – Retention of 5 mm event on-site

Stormwater Strategy

1. 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.
2. 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.
3. 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.
4. Due to underground parking limited opportunities for infiltration LIDs but used green roof to promote evapotranspiration, and cistern to reduce runoff volumes.

Data Analysis/Modelling

• Suggest detailed modelling to evaluate how source and conveyance controls could provide a flood control function
• Fieldwork
• Visit sites to record information/interview designer/landowner
• Technical Input/Design Considerations
• If flood control is your goal how does that impact other performance measures?
• How do other combinations of infrastructure impact effectiveness? For example, underdrains, ponding overflow drains, and inlets/outlets may significantly reduce the effectiveness of the practice to retain runoff and also increase costs?
• How would we have designed Elm Drive differently?
• Reducing the potential to mobilize and wash out soil media and erode the practice (this was a big concern raised by Mississauga with the LRT)
• Inlet design to accept the minor and major system
• How to incorporate emergency overflow structures into the design?
• What features can be incorporated to adjust the infrastructure?

Check for related content on Peak flow

A hard surface area (e.g., road, parking area or rooftop) that prevents or retards the infiltration of water into the soil.

The slow movement of water into or through a soil or drainage system.

Penetration of water through the ground surface.

That storm drainage system which is frequently used for collecting, transporting, and disposing of snowmelt, miscellaneous minor flows, and storm runoff up to the capacity of the system. The capacity should be equal to the maximum rate of runoff to be expected from the minor design storm which may have a frequency of occurrence of one in 2, or 5 years. The minor system may include many features ranging from curbs and gutters to storm sewer pipes and open drainage ways.

That storm drainage system which carries the total runoff of the drainage system less the runoff carried by the minor system (storm sewer). The major system will function whether or not it has been planned and designed, and whether or not developments are situated wisely with respect to it. The Major Drainage System usually includes many features such as streets, gullies, and major drainage channels.

Relating to the properties, distribution and effects of water on and below the earth’s surface, and in the atmosphere.

The changing of land cover and land uses from rural to urban; the growth of urban settlements.

The portion of water precipitated onto a catchment area, which then flows as surface discharge from the catchment area past a specified point.

Water from rain, snow melt, or irrigation that flows over the land surface.

Increases in groundwater storage by natural conditions or by human activity. See also artificial recharge

The inflow of surface water to a groundwater reservoir or aquifer.

Low impact development is a stormwater management and land development strategy applied at the parcel and subdivision scale that emphasizes conservation and use of on-site natural features integrated with engineered, small scale hydrologic controls to more closely mimic pre-development hydrologic functions.

A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

Low Impact Development. A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

The portion of water precipitated onto a catchment area, which then flows as surface discharge from the catchment area past a specified point.

Water from rain, snow melt, or irrigation that flows over the land surface.

The liquid waste from domestic, commercial and industrial establishments. City of Toronto Wet Weather Flow Management Guidelines November 2006 49

A process of disinfecting that involves subjecting the item, object, or instrument to ultraviolet radiation.

Natural vegetation and vegetative technologies in urban settings such as: urban forests; green roofs; green walls; green spaces; rain gardens; bioswales; community gardens; natural and engineered wetlands and stormwater management ponds; and porous pavement systems. These systems are designed to provide multiple benefits, such as moderate temperatures, clean air and water, and improve aesthetics.

Natural vegetation and vegetative technologies in urban settings such as: urban forests; green roofs; green walls; green spaces; rain gardens; bioswales; community gardens; natural and engineered wetlands and stormwater management ponds; and porous pavement systems. These systems are designed to provide multiple benefits, such as moderate temperatures, clean air and water, and improve aesthetics.

the maximum flow rate occurring during a flood event measured at a given point in a river, street, or pipe system.

(1) The wearing away of the land surface by moving water, wind, ice or other geological agents, including such processes as gravitation creep; (2) Detachment and movement of soil or rock fragments by water, wind, ice or gravity (i.e. Accelerated, geological, gully, natural, rill, sheet, splash, or impact, etc).

Surface runoff from at-grade surfaces, resulting from rain or snowmelt events.

Any form of rain or snow.

1. A mineral soil separate consisting of particles less than 0.002 millimeter in equivalent diameter. 2. A soil texture class. 3. (Engineering) A fine-grained soil (more than 50 percent passing the No. 200 Sieve) that has a high plasticity index in relation to the liquid limit. (Unified Soil Classification System).

Human application of water to agricultural or recreational land for watering purposes. City of Toronto Wet Weather Flow Management November 2006 47

The temporary storage of stormwater to control discharge rates, and allow for sedimentation.

The practice of intercepting, conveying and storing rainwater for future use. Captured rainwater is typically used for outdoor non-potable water uses such as irrigation, or in the building to flush toilets.

Tank used to store rainwater (typically roof runoff) for later use.

A system flow of gully inlets, pipes, overland flow paths, open channels, culverts and detention basins used to convey runoff to its receiving waters. City of Toronto 45 Wet Weather Flow Management November 2006

Tank used to store rainwater (typically roof runoff) for later use.

A shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation.

A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

Toronto and Region Conservation Authority

The land draining to a single reference point (usually a structural BMP); similar to a subwatershed, but on a smaller scale.

Stormwater Management

A body of water smaller than a lake, often artificially formed.

A body of water smaller than a lake, often artificially formed.

refers to the characteristics and functions of a system prior to urban development.

the maximum flow rate occurring during a flood event measured at a given point in a river, street, or pipe system.

A thin layer of vegetation and growing medium installed on top of a conventional flat or sloped roof, also referred to as living roofs or rooftop gardens.

An alternative practice to traditional impervious pavement, prevents the generation of runoff by allowing precipitation falling on the surface to infiltrate through the surface course into an underlying stone reservoir and, where suitable conditions exist, into the native soil.

A mixture of mineral aggregates bound with bituminous materials, used in the construction and maintenance of paved surfaces.

A natural waterway, formed by fluvial processes, that conveys running water.

Includes the protection of soil from dislocation by water, wind or other agents.

The accounting of inflow and outflow of water in a system according to the components of the hydrologic cycle.

A perforated pipe used to assist the draining of soils.

A stormwater design features that provides for the gradual release of a volume of water in order to increase settling of pollutants and protect downstream channels from frequent storm events.

An underlying bed filled with aggregate or other void-forming fill material that temporarily stores stormwater before infiltrating into the native soil or being conveyed by an underdrain pipe.

Systems designed to remove trash, debris and some amount of sediment, oil and grease from stormwater runoff based on the principles of sedimentation for the grit and phase separation for the oil.

Box like underground concrete structure with openings in curb and gutter designed to collect runoff from streets and pavement.

The void ratio (e) of a mixture is the ratio of the volume of void-space to the volume of solids. It is closely related to the concept of porosity (n) where porosity is the ratio of the volume of void-space to the total or bulk volume of the mixture. e = Volume of voids/Volume of solids = n/(1-n)

Soil, sand and minerals washed from land into water, usually after rain. They pile up in reservoirs, rivers and harbors, destroying fish-nesting areas and holes of water animals and cloud the water so that needed sunlight might not reach aquatic plans. Careless farming, mining and building activities will expose sediment materials, allowing them to be washed off the land after rainfalls.

Total suspended solids

Includes the protection of soil from dislocation by water, wind or other agents.

The technique of removing pollutants from runoff as it infiltrates through the soil.

A rainfall pattern for use for the design of a hydraulic structure or system. A design storm is a specified amount of rainfall from a storm1 with its spatial and temporal distribution used to estimate a design discharge.

The mathematical expression of the water balance.

The total surface area upstream of a point on a stream that drains toward that point. Not to be confused with watershed. The drainage area may include one or more watersheds.

The accounting of inflow and outflow of water in a system according to the components of the hydrologic cycle.

The quantity of water transpired (given off). Retained in plant tissues, and evaporated from plant tissues and surrounding soil surfaces. Quantitatively it is usually expressed in terms of depth of water per unit area during a specified period. e.g. mm/day

The combined loss of water to the atmosphere from land and water surfaces by evaporation and from plants by transpiration.

Refers to the conditions in which an organism lives and survives or the conditions in which an organism resides. These conditions can be described as aspects of a “physical”, “social” or an “economic” environment, depending on the perspective perceived by the observer.