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Line 120: − + − 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. The mechanisms by which conventional wet ponds provide this temporary storage is shown in the figure below. The permanent pool represents the water quality volume and the ‘active storage’ above the permanent pool provides temporary storage and slow release to reduce peak flows, stream channel erosion control, and flooding.+ − The figure below shows how large volume active storage has been integrated into a hybrid stormwater infiltration trench and bioretention facility. The underdrain is close to the bottom of the trench to maximize active storage availability. Orifices on underdrains help temporarily hold back flows to ensure full utilization of available storage during the 100 year event. Infiltration of stormwater below the underdrain provides water quality and water balance control. A similar concept can be achieved with stormwater chambers and underground infiltration trenches. + − − [[File:Screenshot 2025-09-22 114031.png|500px|thumb|left|Flood and water quality control in stormwater ponds. Water quality control is provided by the permanent pool, Channel and flood protection are provided by the temporary or active storage above the permanent pool. Source: (MECP 2003). Wet ponds do not provide runoff reduction or thermal mitigation benefits.]] − [[File:Screenshot 2025-09-22 120027.png|500px|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.) ]] + + + +
→LID Design for Flood Control
[[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.
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.
The mechanisms by which conventional wet ponds and hybrid stormwater infiltration trench/bioretention facility provide this temporary storage are shown in the figures below.
The underdrain is close to the bottom of the trench to maximize active storage availability. Orifices on underdrains help temporarily hold back flows to ensure full utilization of available storage during the 100 year event. Infiltration of stormwater below the underdrain provides water quality and water balance control. A similar concept can be achieved with stormwater chambers and underground infiltration trenches.
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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.)
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