Flood mitigation

Pluvial (Surface) flooding

Pluvial flooding occurs when a heavier storm exceeds the urban drainage capacity and causes flooding in some low-lying areas. This results in traffic interruption, economic loss, and other issues. As the climate changes, the incidence of extreme weather events in Ontario is expected to increase and the urban drainage capacity may be overwhelmed more often. LID’s effects urban flooding at a scale of urban drainage systems Kim & Han (2008);and Han & Mun (2011) conducted studies to assess if the installation of a rainwater harvesting cisterns could help solve existing urban flooding problems without expanding the capacity of the existing urban drainage system.

Riverine Flooding

Urbanization increases impervious surfaces and the increased impervious surface will result in an increase in runoff, as a result the flows exceed the capacity of the receiving downstream section of river and this may cause flooding.

“Hydrological changes associated with urbanisation are increased storm runoff volumes and peak flows (Qp), 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 flood increases due to upstream development, CVC and TRCA have established flood control targets (2012 Stormwater Management Criteria Document) for future SWM planning through the process of updating of Hydrologic Studies and Subwatershed-level Stormwater Management Studies that characterize flood flow rates, define the location and extent of Flood Damage Centers and assess the potential impact of further urbanization.

Examples of SWM practices that can be applied to provide stormwater quantity control include:

• wet ponds;
• dry ponds;
• infiltration facilities and other low impact development practices with quantity control component.

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.

Background research

TRCA conducted modeling to evaluate different stormwater management measures (LID and Ponds) in mitigating impacts of development on the peak flow and runoff volume. A sub-catchment in Humber River was selected that has an area of 35.71 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 run 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

• 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 will not reduce significantly the post-development peak flows generated from major storms. In order to meet flood control requirements, LID need to be augmented by some flood storage measures such as dry ponds or underground storage.

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 it reduces only 33% and 30% respectively.

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

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

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.

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

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

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

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.

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

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.

Credit Valley Conservation

Toronto and Region Conservation Authority

Stormwater Management

The drainage area of one or more contributing watercourses to a river.

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

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

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 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.

Total suspended solids

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.

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

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

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.

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.

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

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

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.