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.]]

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.

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 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.

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.

[[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 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.]]

Figure x 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.   

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.   

 

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.