Changes

[[File:Subsurface draw outlet.PNG|500px|thumb|Schematic of a reverse sloped subsurface draw outlet to help ensure cooler outflows occur from stormwater ponds (MOE, 2003).<ref>Ministry of the Environment. 2003. Stormwater Management Planning and Design Manual. March, 2003. ISBN 0-7794-2969-9. PIBS 4329e. https://dr6j45jk9xcmk.cloudfront.net/documents/1757/195-stormwater-planning-and-design-en.pdf</ref>]]

[[File:Subsurface draw outlet.PNG|500px|thumb|Schematic of a reverse sloped subsurface draw outlet to help ensure cooler outflows occur from stormwater ponds (MOE, 2003).<ref>Ministry of the Environment. 2003. Stormwater Management Planning and Design Manual. March, 2003. ISBN 0-7794-2969-9. PIBS 4329e. https://dr6j45jk9xcmk.cloudfront.net/documents/1757/195-stormwater-planning-and-design-en.pdf</ref>]]

There are several opportunities to mitigate thermal impacts both within the pond itself and/or implemented within the upstream drainage area (and the lands surrounding) of the pond.  Options shown in past studies to provide appreciable thermal mitigation benefits include (Van Seters and Dougherty, 2019)<ref>Van Seters, T., Graham, C., Dougherty, J., Jacob-Okor, C., and David, Y. 2019. Data Synthesis and Design Considerations for Stormwater Thermal Mitigation Measures. Sustainable Technologies Evaluation Program. Ontario. https://sustainabletechnologies.ca/app/uploads/2019/04/Thermal-Synthesis-Final.pdf</ref>]]

There are several opportunities to mitigate thermal impacts both within the pond itself and/or implemented within the upstream drainage area (and the lands surrounding) of the pond.  Options shown in past studies to provide appreciable thermal mitigation benefits include (Van Seters and Dougherty, 2019)<ref>Van Seters, T., Graham, C., Dougherty, J., Jacob-Okor, C., and David, Y. 2019. Data Synthesis and Design Considerations for Stormwater Thermal Mitigation Measures. Sustainable Technologies Evaluation Program. Ontario. https://sustainabletechnologies.ca/app/uploads/2019/04/Thermal-Synthesis-Final.pdf</ref>

===Subsurface Draw Outlets===

===Subsurface Draw Outlets===

====Design Considerations====

====Design Considerations====

*Subsurface outlet inverts should be a minimum 1.2 m below the permanent pool water level to avoid influence from solar induced diurnal fluctuations.   

*Subsurface outlet inverts should be a minimum 1.2 m below the permanent pool water level to avoid influence from solar induced diurnal fluctuations.   

*Subsurface outlets need to elevated from the pond bottom by between 1 and 1.5 m to allow space for sediment to accumulate.  Reducing the separation distance may mean that the pond will need to be cleaned prior to its normal life cycle maintenance schedule, as per MECP guidance (link to our stormwater inspection and maintenance guide).   

*Subsurface outlets need to elevated from the pond bottom by between 1 and 1.5 m to allow space for sediment to accumulate.  Reducing the separation distance may mean that the pond will need to be cleaned prior to its normal life cycle maintenance schedule, as per MECP guidance (link to our stormwater inspection and maintenance guide).   

*Meeting outlet to bottom separation requirements by over excavating around the reverse slope outlet pipe will not provide the same temperature reduction benefits as a uniformly deep pond because the scour pool promotes vertical mixing of warmer top water with bottom water and there will be less cool bottom water below the outlet invert.   

*Meeting outlet to bottom separation requirements by over excavating around the reverse slope outlet pipe will not provide the same temperature reduction benefits as a uniformly deep pond because the scour pool promotes vertical mixing of warmer top water with bottom water and there will be less cool bottom water below the outlet invert.   

*Deeper ponds may require larger footprints to meet the MECP side slope requirements of 5:1 above the permanent pool and 3:1 elsewhere.   

*Deeper ponds may require larger footprints to meet the MECP side slope requirements of 5:1 above the permanent pool and 3:1 elsewhere.   

 

*Reverse sloped pipes should not be solid (not perforated) to ensure water is drawn from the deeper part of the pond.

*Reverse sloped pipes should not be solid (not perforated) to ensure water is drawn from the deeper part of the pond.<br>

[[File:Subsurface outlet depth.png|500px|thumb|right|The figure above highlights the optimal subsurface outlet depth above the sediment accumulation zone and below the zone most influenced solar radiation.]]<br>

 

[[File:Subsurface outlet depth.png|700px|thumb|right|The figure above highlights the optimal subsurface outlet depth above the sediment accumulation zone and below the zone most influenced solar radiation.]]<br>

 

 

 

 

 

 

 

 

 

 

 

 

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====Expected Performance====

====Expected Performance====

*Relative to a surface draw outlet, the expected 95th percentile temperature reduction provided by a 2 m deep draw outlet for years with similar air temperatures is between 3 and 5⁰C.  

*Relative to a surface draw outlet, the expected 95th percentile temperature reduction provided by a 2 m deep draw outlet for years with similar air temperatures is between 3 and 5⁰C.  

===Night Time Release Outlets===

===Night Time Release Outlets===

Employs real time control on pond outlets to automatically close outlets during the day when surface outflow from ponds is warmer, and release water during the night when outflow temperatures are cooler.  The outlets are configured and programmed to maintain release rates below threshold values for stream erosion prevention and match pre-development peak flow rates.

Employs real time control on pond outlets to automatically close outlets during the day when surface outflow from ponds is warmer, and release water during the night when outflow temperatures are cooler.  The outlets are configured and programmed to maintain release rates below threshold values for stream erosion prevention and match pre-development peak flow rates.

====Design Considerations====

====Design Considerations====