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==Pluvial (Surface) flooding==  

==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.  

Pluvial flooding occurs when larger storms exceed the capacity of the urban drainage system to convey water, resulting in flooding of some low-lying areas. This may result in traffic interruption, economic loss, infrastructure damage, basement flooding and other undesirable consequences. As the climate changes, the incidence of extreme weather events in Ontario is expected to increase, causing  urban drainage system capacity to be exceeded more frequently. 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.

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]] tanks could help solve existing urban flooding problems without expanding the capacity of the existing urban drainage system.

 

LIDs are well suited to address localized 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)<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 (see section below on ‘designing for flood resilience’).

==Riverine Flooding==

==Riverine 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)."''

[[File:2022-01-18-severe-weather-2021-21-billion-damage-image2.jpeg|thumb|750px|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).<ref>Insurance Bureau of Canada (IBC). 2022. Severe Weather in 2021 Caused $2.1 Billion in Insured Damage." News & Insights. Accessed: https://www.ibc.ca/news-insights/news/severe-weather-in-2021-caused-2-1-billion-in-insured-damage</ref>.]]

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.

Riverine flooding occurs when rivers and streams exceed the capacity of their channels to convey flows, resulting in water overtopping the banks and flowing into adjacent areas. In urban areas, this typically occurs where there has been an increase in upstream impervious cover that is not adequately mitigated by stormwater management practices.

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

''“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)."''

*[[infiltration]] facilities with quantity control component; and,

*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 only be recognized where the local municipality has endorsed the use of these practices and has considered long term operations and maintenance.

Catastrophic losses from flooding have been steadily rising in Canada over the last two decades. 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, as mentioned above, 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.

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, as mentioned above, larger storm chambers, [[infiltration trench|trenches]] and even [[bioretention]] can be designed with large temporary storage volumes to provide flood control functions similar to wet or [[dry ponds]].

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.

 

 

 

 

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:

Scenarios evaluated include:

*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.  

This shows that LID will not reduce significantly the post-development peak flows generated from major storms.

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 [[infiltration chambers|underground storage]]. As noted below, 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)

In order to meet flood control requirements, LID need to be augmented by some flood storage measures such as [[dry ponds]] or [[infiltration chambers|underground storage]].

===Runoff Volume===

===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 %,

*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.  

*For 50 and 100 year design storms, runoff volumes are reduced by 33% and 30%, respectively

 

 

 

A large number of studies have shown the beneficial effect of LID on reducing peak flows for more frequent events. They do this by detaining flows and releasing them over longer time periods (see figure below).  However, as discussed in the previous section, larger events overwhelm the capacity of these LID practices to provide significant flood mitigation because most practices are designed with overflows that rapidly discharge incoming runoff to storm sewers once the design capacity of the practice has been exceeded.

 

==Literature Review==

==Literature Review==

#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.

#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.

#Final erosion control provided within the stormwater management facility, controlling release rates to maintain the existing condition erosion exceedance values.

#Final erosion control provided within the stormwater management facility, controlling release rates to maintain the existing condition erosion exceedance values.

#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.

<h3>Example 2. West Gormley, Town of Richmond Hill</h3>

<h3>Example 2. West Gormley, Town of Richmond Hill</h3>

*Inlet design to accept the minor and major system

*Inlet design to accept the minor and major system

*How to incorporate emergency overflow structures into the design?

*How to incorporate emergency overflow structures into the design?

*What features can be incorporated to all you to adjust the infrastructure?

*What features can be incorporated to adjust the infrastructure?

Check for related content on [[Peak flow]]

Check for related content on [[Peak flow]]