Changes

[[File:Screenshot 2025-09-12 113952.png|500px|thumb|right|Recommended regional 90th percentile volume targets for Ontario (MECP, 2022) <ref>MECP. 2022. Low Impact Development Stormwater

[[File:Screenshot 2025-09-12 113952.png|500px|thumb|right|Recommended regional 90th percentile volume targets for Ontario (MECP, 2022) <ref>MECP. 2022. Low Impact Development Stormwater

Management Guidance Manual Draft. https://ero.ontario.ca/public/2022-01/Draft%20LID%20Stormwater%20Management%20Guidance%20Manual%202022.pdf</ref>]]

Management Guidance Manual Draft. https://ero.ontario.ca/public/2022-01/Draft%20LID%20Stormwater%20Management%20Guidance%20Manual%202022.pdf</ref>]]

[[File:Pearson_Graph.png|thumb|Typically designed to handle the smaller, most frequent storm events, LID practices in Southern Ontario are usually sized according to the 90th percentile event]]

Typically designed to handle the smaller, most frequent storm events, LID practices in Ontario are usually sized according to the 90th percentile event. The rainfall depth of the 90th percentile event varies geographically. Use the map on the right to determine the typical 90th percentile event based on the site location. Controlling this amount of runoff may provide stormwater engineers with control of over 90 % of the mean annual pollutant load <ref>Pitt, R. 1999.  Small Storm Hydrology and Why it is Important for the Design of Stormwater Control Practices. In: Advances in Modeling the Management of Stormwater Impacts, Volume 7. Computational Hydraulics International, Guelph, Ontario and Lewis Publishers/CRC Press. 1999</ref>.

Typically designed to handle the smaller, most frequent storm events, LID practices in Ontario are usually sized according to the 90th percentile event. In many areas this translates into events that are < 30 mm in depth. Note that 25 mm is considered to be a suitable representation of the ‘first flush’ volume, and that controlling this amount of runoff may provide stormwater engineers with control over 90 % of the mean annual pollutant load <ref>Pitt, R. 1999.  Small Storm Hydrology and Why it is Important for the Design of Stormwater Control Practices. In: Advances in Modeling the Management of Stormwater Impacts, Volume 7. Computational Hydraulics International, Guelph, Ontario and Lewis Publishers/CRC Press. 1999</ref>.

===Runoff Volume Control Target (RVC_T)===  

===Runoff Volume Control Target (RVC_T)===  

'''Volume capture and treatment''' - Also referred to as `treatment and release`, where the volume capture and treatment directly aims at reducing surface water impairment through treatment of the specified volume, often referred to as a “water quality volume”.<ref>MOECC (2018)</ref>}}

'''Volume capture and treatment''' - Also referred to as `treatment and release`, where the volume capture and treatment directly aims at reducing surface water impairment through treatment of the specified volume, often referred to as a “water quality volume”.<ref>MOECC (2018)</ref>}}

==See also==

See [[Understanding rainfall statistics]] to learn how to calculate the 90th percentile event using rainfall observations for a specific site.

*[[Understanding rainfall statistics]]

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