Changes

*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.

==Cooling Trenches==

===Cooling Trenches===

Cooling trenches typically consist of one or more geotextile wrapped perforated pipes embedded in a clear stone filled trench that is buried underground.  Water temperatures are reduced through heat transfer from the water passing through the trench to the stone and surrounding soils.  Cooling trenches may be installed downstream of the primary pond outlet or draw from a secondary orifice controlled outlet draining water from the pond at or below the permanent pool water level (e.g Van Seters and Graham, 2013<ref> Van Seters, T., Graham, C. 2013. Evaluation of an Innovative Technique for Augmenting Stream Baseflows and Mitigating the Thermal Impacts of Stormwater Ponds. Sustainable Technologies Evaluation Program, Toronto and Region Conservation Authority, Toronto, Ontario. https://sustainabletechnologies.ca/app/uploads/2013/08/Cooling-trench-final-2013a.pdf</ref>.; TRCA, 2020<ref>Toronto and Region Conservation Authority (TRCA) 2020. Evaluation of a Thermal Mitigation System on the Heritage at Victoria Square Pond in Markham. Toronto and Region Conservation Authority, Vaughan, Ontario. https://sustainabletechnologies.ca/app/uploads/2021/01/TM-Heritage-report-2021R.pdf</ref>). Further information about these innovative cooling trench features installed as part of the stormwater pond

Cooling trenches typically consist of one or more geotextile wrapped perforated pipes embedded in a clear stone filled trench that is buried underground.  Water temperatures are reduced through heat transfer from the water passing through the trench to the stone and surrounding soils.  Cooling trenches may be installed downstream of the primary pond outlet or draw from a secondary orifice controlled outlet draining water from the pond at or below the permanent pool water level (e.g Van Seters and Graham, 2013<ref> Van Seters, T., Graham, C. 2013. Evaluation of an Innovative Technique for Augmenting Stream Baseflows and Mitigating the Thermal Impacts of Stormwater Ponds. Sustainable Technologies Evaluation Program, Toronto and Region Conservation Authority, Toronto, Ontario. https://sustainabletechnologies.ca/app/uploads/2013/08/Cooling-trench-final-2013a.pdf</ref>.; TRCA, 2020<ref>Toronto and Region Conservation Authority (TRCA) 2020. Evaluation of a Thermal Mitigation System on the Heritage at Victoria Square Pond in Markham. Toronto and Region Conservation Authority, Vaughan, Ontario. https://sustainabletechnologies.ca/app/uploads/2021/01/TM-Heritage-report-2021R.pdf</ref>). Further information about these innovative cooling trench features installed as part of the stormwater pond

operation design in two sites located in Markham, ON. visit the [https://sustainabletechnologies.ca/home/urban-runoff-green-infrastructure/thermal-mitigation/thermal-mitigation-system-evaluation/ STEP project page]. The permanent pool of stormwater management ponds acts as a heat sink during the summer, resulting in warmer summer discharges during both storm and baseflow conditions.  

operation design in two sites located in Markham, ON. visit the [https://sustainabletechnologies.ca/home/urban-runoff-green-infrastructure/thermal-mitigation/thermal-mitigation-system-evaluation/ STEP project page]. The permanent pool of stormwater management ponds acts as a heat sink during the summer, resulting in warmer summer discharges during both storm and baseflow conditions.  

[[File:Rock pad energy dissipator.PNG|450px|thumb|An energy dissipater, also known as a "bed friction outlet" that utilizes coarse rip-rap or jagged concrete blocks to ford a rough bed to help slow higher velocity flows leaving the feature. By increasing the roughness of the dissipator the flow of water is spread out, energy is decreased and as a result reduces the chance of downstream erosion or bed scour.<ref>Catchments & Creek Pty Ltd. 2010. Energy Dissipaters, Drainage Control Technique. ED-1.doc. Version 2 - May 2010. International Erosion Control Organization (IECO), Australia. https://www.austieca.com.au/documents/item/307</ref>]]

[[File:Rock pad energy dissipator.PNG|450px|thumb|An energy dissipater, also known as a "bed friction outlet" that utilizes coarse rip-rap or jagged concrete blocks to ford a rough bed to help slow higher velocity flows leaving the feature. By increasing the roughness of the dissipator the flow of water is spread out, energy is decreased and as a result reduces the chance of downstream erosion or bed scour.<ref>Catchments & Creek Pty Ltd. 2010. Energy Dissipaters, Drainage Control Technique. ED-1.doc. Version 2 - May 2010. International Erosion Control Organization (IECO), Australia. https://www.austieca.com.au/documents/item/307</ref>]]

===Design Considerations===

'''Design Considerations'''

*Built-in overflows bypass high flows to help enhance thermal function and prevent excessive sediment build-up in the trench.   

*Built-in overflows bypass high flows to help enhance thermal function and prevent excessive sediment build-up in the trench.   

*Available research shows that the trench storage volume should be equal to or greater than 5% of the runoff volume discharged from the pond during the 25 mm event. Trenches with smaller storage volumes cannot be relied on to provide any cooling benefits, although under favourable conditions, these may still provide some cooling.   

*Available research shows that the trench storage volume should be equal to or greater than 5% of the runoff volume discharged from the pond during the 25 mm event. Trenches with smaller storage volumes cannot be relied on to provide any cooling benefits, although under favourable conditions, these may still provide some cooling.   

*Trenches may incorporate an infiltration function to help further reduce thermal loads if native soils and groundwater levels are suitable.   

*Trenches may incorporate an infiltration function to help further reduce thermal loads if native soils and groundwater levels are suitable.   

===Expected Performance===

'''Expected Performance'''

*Performance of primary outlet cooling trenches is highly variable due to differences in cooling trench sizing, design, initial water temperature and degree of groundwater interaction   

*Performance of primary outlet cooling trenches is highly variable due to differences in cooling trench sizing, design, initial water temperature and degree of groundwater interaction   

*Based on case studies reviewed, primary outlet trenches without groundwater interaction may provide summer temperature cooling of the warmest flows by roughly 1 to 3⁰C if the trench storage volume is equal to or greater than 5% of the runoff volume discharged from the pond during the 25 mm event.

*Based on case studies reviewed, primary outlet trenches without groundwater interaction may provide summer temperature cooling of the warmest flows by roughly 1 to 3⁰C if the trench storage volume is equal to or greater than 5% of the runoff volume discharged from the pond during the 25 mm event.