Changes

[[File:Screenshot 2025-09-22 113355.png|600px|thumb|right|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.]]

[[File:Screenshot 2025-09-22 113355.png|600px|thumb|right|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.]]

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 flooding. Designing LID for flood control functions requires integrating large volumes of active storage to temporarily store stormwater while it is released slowly to streams or downstream sewer systems.

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 (left) and hybrid stormwater infiltration trench/bioretention facility (right) provide this temporary storage are shown in the figures below:

 

The mechanisms by which conventional wet ponds and hybrid stormwater infiltration trench/bioretention facility provide this temporary storage are shown in the figures below.

 

<gallery mode="packed" widths=200px heights=200px>

<gallery mode="packed" widths=200px heights=200px>

File:Screenshot 2025-09-22 114031.png|400px|thumb|left|Wet pond: The permanent pool provides the 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)<ref>Ontario Ministry of Environment. 2003. Stormwater Management Planning and Design Manual. https://www.ontario.ca/document/stormwater-management-planning-and-design-manual/stormwater-management-plan-and-swmp-design</ref>.   

File:Screenshot 2025-09-22 114031.png|400px|thumb|left|Wet pond: The permanent pool provides the 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)<ref>Ontario Ministry of Environment. 2003. Stormwater Management Planning and Design Manual. https://www.ontario.ca/document/stormwater-management-planning-and-design-manual/stormwater-management-plan-and-swmp-design</ref>.   

File:Screenshot 2025-09-22 120027.png|600px|thumb|left|Flood, water quality and water balance control in a hybrid trench and bioretention system.  Water quality and water balance storage are provided through infiltration below the underdrain. Active storage above the underdrain provides channel and flood protection. The underdrain may be fitted with an orifice to meet release rate requirements. The infiltration rate increases with hydraulic head, resulting in potentially higher rates of volume reduction through infiltration than conventional LID not designed for flood control. Inlets consisted of a distributed network of curb cuts draining to high flow through cobble/gravel columns (roughly 1 x 2 m). Source of base image: Schollen and Co.)

File:Screenshot 2025-09-22 120027.png|600px|thumb|left|This 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. Source of base image: Schollen and Co.)

</gallery>

</gallery>

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.

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.