Changes

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==Within the Pond Block==

===Within the Pond Block===

[[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., and Dougherty, J. 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></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., and Dougherty, J. 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></ref>

===Subsurface Draw Outlets===

====Subsurface Draw Outlets====

A reverse sloped outlet pipe drains cooler water from below the pond surface to a control manhole that is accessible from the bank for ease of maintenance.  Deeper outlet result in cooler [[Overflow|outflows]] than shallow outlets because pond water temperatures decrease with depth.  The figure below shows an example of thermal stratification during the summer in a deep pond using said subsurface outlets.

A reverse sloped outlet pipe drains cooler water from below the pond surface to a control manhole that is accessible from the bank for ease of maintenance.  Deeper outlet result in cooler [[Overflow|outflows]] than shallow outlets because pond water temperatures decrease with depth.  The figure below shows an example of thermal stratification during the summer in a deep pond using said subsurface 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).   

[[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.]]

[[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.]]

====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.  

*Water drained from outlets 2m below the surface will typically be less than 24°C at least 95% of the time.

*Water drained from outlets 2m below the surface will typically be less than 24°C at least 95% of the time.

===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.

[[File:Temp. depth pond.png|600px|thumb|The graph above depicts the temperature of a monitored stormwater pond within TRCA's watershed between Southern Ontario's two hottest months (July and August). Depths are in meters below the normal water level.  Note strong diurnal fluctuations at the 0 m , 0.5 m and to a lesser extent at the 1.0 m depths.]]

[[File:Temp. depth pond.png|600px|thumb|The graph above depicts the temperature of a monitored stormwater pond within TRCA's watershed between Southern Ontario's two hottest months (July and August). Depths are in meters below the normal water level.  Note strong diurnal fluctuations at the 0 m , 0.5 m and to a lesser extent at the 1.0 m depths.]]

====Design Considerations====

'''Design Considerations'''

*Optimal 8 hour duration for night time release outlets was found to be between 3 AM and 10 AM inclusive based on data from 4 ponds  

*Optimal 8 hour duration for night time release outlets was found to be between 3 AM and 10 AM inclusive based on data from 4 ponds  

*Optimal 4 hour duration release times were found to be between 6 and 9 AM inclusive (Van Seters and Dougherty, 2019).<ref>Van Seters, T., and Dougherty, J. 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>]].

*Optimal 4 hour duration release times were found to be between 6 and 9 AM inclusive (Van Seters and Dougherty, 2019).<ref>Van Seters, T., and Dougherty, J. 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>]].

*Electrical supply and back-up power are typically needed at the outlet to reduce operation and maintenance requirements

*Electrical supply and back-up power are typically needed at the outlet to reduce operation and maintenance requirements

====Expected Performance====

'''Expected Performance'''

*Based on modelling of four ponds, the 95th percentile temperature reduction for ponds with night time release outlets was calculated to be approximately 1.6⁰C for surface draw outlets, and 0.6⁰C for an outlet 1.4 m below the permanent pool level.  For deeper outlets, the benefits of night time release are very small as temperatures from these outlets do not exhibit strong diurnal variations.

*Based on modelling of four ponds, the 95th percentile temperature reduction for ponds with night time release outlets was calculated to be approximately 1.6⁰C for surface draw outlets, and 0.6⁰C for an outlet 1.4 m below the permanent pool level.  For deeper outlets, the benefits of night time release are very small as temperatures from these outlets do not exhibit strong diurnal variations.