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→Cooling Trenches
==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; TRCA, 2020). https://sustainabletechnologies.ca/home/urban-runoff-green-infrastructure/thermal-mitigation/thermal-mitigation-system-evaluation/
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.
The technology, known as the Groundwater Emulation Management System (GEMS), was designed to maintain cool stream discharge from the SWM pond catchment at a level similar to that experienced prior to development. This is accomplished by draining water at a controlled rate from the permanent pool of a stormwater pond to a cooling/infiltration trench. The cooling trench lowers the temperature of pond water through below ground heat transfer and discharges to the receiving watercourse at a rate and volume mimicking the natural discharge of groundwater. Estimated reductions in groundwater recharge caused by the conversion of land from agriculture and open space to residential use provided the basis for setting continuous flow rates released into the system.
[[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===
*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.
*Flush-out pipes, high flow bypasses and sediment pre-treatment mechanisms should be provided to reduce maintenance.
*Pre-treatment filters, energy dissipators or isolator rows are useful to improve overall functionality but will require regular inspection and maintenance to ensure they are fulfilling their intended function.
*Secondary pond outlets that drain water from 0.5 m below the permanent pool elevation to cooling trenches can help to reduce or eliminate the long duration and very warm interevent flows.
*Trenches may incorporate an infiltration function to help further reduce thermal loads if native soils and groundwater levels are suitable.
===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
*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.
==References==
==References==