Changes

*''C'' = heat capacity of water (4187J/kg°C)}}  

*''C'' = heat capacity of water (4187J/kg°C)}}  

Since urban runoff volumes often increase by 2 to 5 times after development, and stormwater pond effluent temperatures are between 4 and 11°C warmer than pond influent temperatures in the summer, the overall thermal load increases to streams can be very significant ([https://sustainabletechnologies.ca/app/uploads/2019/04/Thermal-Synthesis-Final.pdf 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>

Since urban runoff volumes often increase by 2 to 5 times after development, and stormwater pond effluent temperatures are between 4 and 11°C warmer than pond influent temperatures in the summer, the overall thermal load increases to streams can be very significant ([https://sustainabletechnologies.ca/app/uploads/2019/04/Thermal-Synthesis-Final.pdf Van Seters and Dougherty, 2019]).<ref name="example2">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>

==Selecting a Stream Temperature Target==

==Selecting a Stream Temperature Target==

===Upstream of the Pond===

===Upstream of the Pond===

Any measure that decreases runoff volumes or temperatures or both can help mitigate thermal loads to streams or downstream treatment facilities.  Examples include [[bioretention]], [[infiltration trenches]] or [[infiltration chambers|chambers]], [[enhanced swales]], [[permeable pavements]], [[rain gardens|absorbent landscaping]] and increased canopy cover.  The figures below show the temperature (event mean temperature) and thermal load reduction results from several LID practices monitored in the Greater Toronto Area (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>] The most effective practices were deeper systems such as trenches, some deep bioretention facilities, and practices that promote significant runoff volume reductions. Beyond reducing temperatures and runoff volumes, enhancing [[infiltration]] also helps re-establish the natural baseflow regime that existed prior to development.<br>

Any measure that decreases runoff volumes or temperatures or both can help mitigate thermal loads to streams or downstream treatment facilities.  Examples include [[bioretention]], [[infiltration trenches]] or [[infiltration chambers|chambers]], [[enhanced swales]], [[permeable pavements]], [[rain gardens|absorbent landscaping]] and increased canopy cover.  The figures below show the temperature (event mean temperature) and thermal load reduction results from several LID practices monitored in the Greater Toronto Area (Van Seters and Dougherty, 2019).<ref name="example2" /> The most effective practices were deeper systems such as trenches, some deep bioretention facilities, and practices that promote significant runoff volume reductions. Beyond reducing temperatures and runoff volumes, enhancing [[infiltration]] also helps re-establish the natural baseflow regime that existed prior to development.<br>

</br>

</br>

[[File:Influent Effluent EMT.PNG|350px|thumb|right|Influent and effluent event mean temperatures (EMT) for common LID practices. Source: (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>]]

[[File:Influent Effluent EMT.PNG|350px|thumb|right|Influent and effluent event mean temperatures (EMT) for common LID practices. Source: (Van Seters and Dougherty, 2019).<ref name="example2" />]]

[[File:1EMT reduction LID.PNG|380px|thumb|left|Event mean temperature (EMT) reduction for common LID practices. Source: (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>]]

[[File:1EMT reduction LID.PNG|380px|thumb|left|Event mean temperature (EMT) reduction for common LID practices. Source: (Van Seters and Dougherty, 2019).<ref name="example2" />]]

[[File:Thermal load reduction LID.PNG|380px|thumb|center|Thermal load reductions for common LID practices. Source: (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>]]<br>

[[File:Thermal load reduction LID.PNG|380px|thumb|center|Thermal load reductions for common LID practices. Source: (Van Seters and Dougherty, 2019).<ref name="example2" />]]<br>

[[File:STEP MONITORING.jpg|thumb|500px|Example of TRCA/STEP staff conducting monitoring tasks associated with a bioretention cell at Kortright Centre in Vaughan, ON. Staff are downloading water level data from the feature using a [[Digital technologies|data logger]] placed within a monitoring well.<ref>TRCA. 2017. Furthering the State of Knowledge on Stormwater Management - News. October 4th, 2017. Accessed: April 18th, 2022. https://trca.ca/news/furthering-the-state-of-knowledge-on-stormwater-management/</ref>]]

[[File:STEP MONITORING.jpg|thumb|500px|Example of TRCA/STEP staff conducting monitoring tasks associated with a bioretention cell at Kortright Centre in Vaughan, ON. Staff are downloading water level data from the feature using a [[Digital technologies|data logger]] placed within a monitoring well.<ref>TRCA. 2017. Furthering the State of Knowledge on Stormwater Management - News. October 4th, 2017. Accessed: April 18th, 2022. https://trca.ca/news/furthering-the-state-of-knowledge-on-stormwater-management/</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>]]

[[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><br>

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 name="example2" />.<br>

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'''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 name="example2" />.

*Robust automation technology is critical to avoid excessive repairs and down time.  

*Robust automation technology is critical to avoid excessive repairs and down time.  

*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

[[File:Cooling Trenches.jpg|450px|thumb|Close up view of a 'cooling trench'. These trenches work by outflows of warm pond water coming into contact with cooler [[stone]]

[[File:Cooling Trenches.jpg|450px|thumb|Close up view of a 'cooling trench'. These trenches work by outflows of warm pond water coming into contact with cooler [[stone]]

media (along the top of the trench) and side walls, which in turn promotes heat transfer between the two, resulting in a reduction in outflow water temperatures beign discharged to a receiving waterbody (i.e., stream) (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>. Photo Source: [[Acknowledgements|Azimuth Environmental​]]]]

media (along the top of the trench) and side walls, which in turn promotes heat transfer between the two, resulting in a reduction in outflow water temperatures beign discharged to a receiving waterbody (i.e., stream) (Van Seters and Dougherty, 2019) <ref name="example2" />. Photo Source: [[Acknowledgements|Azimuth Environmental​]]]]

'''Design Considerations'''

'''Design Considerations'''

The volumes infiltrated may also be used to meet site water balance requirements.  Proponents will need to consult with local approval agencies to determine whether the typical 1 meter separation depth to groundwater may be waived given that the infiltrated water has been treated through the pond.   

The volumes infiltrated may also be used to meet site water balance requirements.  Proponents will need to consult with local approval agencies to determine whether the typical 1 meter separation depth to groundwater may be waived given that the infiltrated water has been treated through the pond.   

Analysis of temperature and thermal load data from three monitored ponds showed that the bottom area of the infiltration area would need to be up to 7% that of the pond area depending on the infiltration capacity of the native soils.  These areas could be reduced by 40% if the outlet draws water from 0.25 m below the surface, and further reduced for deeper outlets (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>.  

Analysis of temperature and thermal load data from three monitored ponds showed that the bottom area of the infiltration area would need to be up to 7% that of the pond area depending on the infiltration capacity of the native soils.  These areas could be reduced by 40% if the outlet draws water from 0.25 m below the surface, and further reduced for deeper outlets (Van Seters and Dougherty, 2019)<ref name="example2" />.  

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