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Line 22: − + − 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. − + + + Line 39:
Line 39: − + − − + + + + + + + + − 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.
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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.
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==
==Modelling Flood Mitigation Potential of Conventional LIDs==
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.
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:
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
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==
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==