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====Subsurface Draw Outlets====
→Within the Pond Block
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)
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)
===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.
[[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>]]
===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 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.
*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.
==In the Receiving Water==
==In the Receiving Water==
==References==
==References==