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==Pluvial (Surface) flooding==
Pluvial flooding occurs when a heavier storm may exceed the urban drainage capacity, and it causes flooding in some low-lying areas and results in traffic interruption, economic loss, and other issues.As the climate warms up, the incidence of extreme weather events will increase, as a result the urban drainage capacity will be overwhelmed.
LID’s effects on 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 storage tank can 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.
The MOE SWMPD Manual (2003) and the CVC/TRCA LID Manual (2010), describe a number of practices that can be implemented to provide quantity control treatment of stormwater runoff as part of urban development. Examples of SWM practices that can be applied to provide stormwater quantity control include:
wet ponds;
dry ponds;
infiltration facilities with quantity control component; and,
low impact development practices with quantity control component.
The criteria document provides specific guidance on the planning and design of SWM infrastructure within the TRCA and CVC watersheds.
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.
Include a new chapter that provides design guidance for LID practices with quantity control components.
Background Research
TRCA undertook modeling excises to evaluate effectiveness of 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. The figures and tables below show hydrographs extracted and the calculated change runoff volume and peak flow .
Post-development mitigated using LID-25mm
Post-development hydrograph mitigated using pond
Post-development mitigated using pond+LID
Reduction of Peak flow & Runoff Volume for 2-year design storm
Reduction of Peak flow & Runoff Volume for 100-year design storm
Peak Flow
25 mm on-site retention using LID measures can reduce post-development peak flows generated from 2 to 5 year design storms by over 26%, whereas 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.
Runoff Volume
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 %, whereas 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.
Finally, LID as standalone cannot control post-development peak flows to pre-development peak flows under major storm events.
Therefore, in order to meet flood control requirements, LID need to be augmented by some flood storage measures such as dry/park ponds, underground storage.
Literature Review/Identify examples
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 for the site include:
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
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;
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 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 for the site include:
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
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.
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 site is located in 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
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.
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.
Overall Conclusions
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.
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 all you to adjust the infrastructure?
Operation & Maintenance considerations
Reporting
Literature Review Summary
TOC/Outline
First Draft
Second Draft
Final
Meeting with Executive Planning Committee (@TRCA)
5 meetings
Webinar with CO
Planning Webinar
Attend Webinar
Internal CVC
Chapter coordination etc.
Check for related content on [[Peak flow]]
Check for related content on [[Peak flow]]