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→LID Design for Flood Control
[[File:Screenshot 2025-09-22 161822.png|650px|thumb|right|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. Base image: Schollen and Co.<ref>https://www.toronto.ca/ext/digital_comm/pdfs/transportation-services/green-streets-technical-guidelines-document-v2-17-11-08.pdf</ref>)]]
[[File:Screenshot 2025-09-22 161822.png|650px|thumb|right|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. Base image: Schollen and Co.<ref>https://www.toronto.ca/ext/digital_comm/pdfs/transportation-services/green-streets-technical-guidelines-document-v2-17-11-08.pdf</ref>)]]
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 and hybrid stormwater infiltration trench/bioretention facility provide this temporary storage are shown in the figures on the right.
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. LID practices are typically designed to manage more frequent and lower magnitude rainfall events. 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 and hybrid stormwater infiltration trench/bioretention facility provide this temporary storage are shown in the figures on the right.
LIDs are well suited to address flooding because they can be shoehorned into smaller areas that may have increased flood risk. Runoff reduction and temporary detention are the primary means by which LIDs can reduce flooding at the scale of urban drainage systems. Kim & Han (2008)[10] and Han & Mun (2011)[11] conducted studies in Seoul, South Korea, to assess the extent to which the installation of rainwater harvesting cisterns could help mitigate existing urban flooding problems without expanding the capacity of the existing urban drainage system. System operational data showed that 29 mm of rainwater storage per square meter of impervious area (3000 m3 cistern in this instance) provided sufficient storage for a one in 50 year period storm without the need to upgrade downstream sewers designed to 10 year storm capacity. Stormwater chambers, infiltration chambers, bioretention and other LID systems designed with large volumes of temporary storage could have similar benefits, while also reducing runoff volumes and providing other co-benefits (see section below on ‘designing for flood resilience’).
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.