• Open Surfaces and Outlet Design Contribute to Thermal Pollution

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• Open surfaces and outlet design contribute to thermal pollution • Wetlands Marginally Better than Wet Ponds – Increase of temperatures significantly less for wetlands – Due to Shading (we suspect) 4

30 3.5

25 3

20 2.5

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Surface Rainfall (cm.)

Temperature (°C) 1.5 30 cm below 10 60 cm below 1 Rainfall 90 cm below 5 0.5 Deeper depths: Cooler Effluent 0 0 05/15/06 05/17/06 05/19/06 05/21/06 05/23/06 05/25/06 05/27/06 05/29/06 05/31/06

Surface 30 cm 60 cm 90 cm Rainfall

Concerns: • Water Quality Issues • PAHs? • Maintenance Issues • State of NC Decided – Essentially Stop using Wet Ponds in WNC – If a large scale practice needed, stormwater wetlands preferred – Require Intake to be placed lower in water column • No Decision Made on – Whether to promote the use of piped conveyance • Specific Plant Selection Guidance Provided: No “Floaters!” • Let’s Play a Game: Name Two Ways a BMP Can Reduce Thermal Load: (1) Reduce the Temperature of Effluent (2) Reduce the Volume of Effluent

% of Underdrain Vegetative Watershed Location Depth Cover Area Asheville 135 cm 55% 16%

Lenoir 95 cm 79% 4% Brevard 48 cm 43% 7% East Brevard 43 cm 43% 11% West • Soil temperature at greater depths are: – Coolest during the summer months – Buffered from daily weather changes • Temperatures at bottom depths warmer than 21°C for much of the monitoring period 40

20 Temperature (°C) Temperature

10 05/01/06 08/09/06 11/17/06 02/25/07 06/05/07 09/13/07 Median Inlet Median Outlet Max Inlet Max Outlet 40

20 Temperature (°C) Temperature

10 05/01/06 08/09/06 11/17/06 02/25/07 06/05/07 09/13/07 Median Inlet Median Outlet Max Inlet Max Outlet % of Reduced Reduced Watershed Median Maximum Area Temperature Temperature Asheville 16% X

Lenoir 4% XX

Brevard East 7% XX

Brevard West 11% X 25 10

9 24 8 23 7 22 6

21 5

4 20 Temperature (°C) 3

19 Cumulative Rainfall (cm) 2 18 1

17 0 6/14/07 6/14/07 6/15/07 6/15/07 6/16/07 6/16/07 6/17/07 0:00 12:00 0:00 12:00 0:00 12:00 0:00

30 cm 60 cm 90 cm 120 cm Rain “Big” Cell: Soil Temps Unaffected by Storms 25 10

9 24 8 23 7 22 6

21 5

4 20 Temperature (°C) 3

19 Cumulative Rainfall (cm) 2 18 1

17 0 6/14/07 6/14/07 6/15/07 6/15/07 6/16/07 6/16/07 6/17/07 0:00 12:00 0:00 12:00 0:00 12:00 0:00

30 cm 60 cm 90 cm Rainfall “Small” Cell: Soil Temps Affected by Storms

• Drain depths of 90 cm (3 ft) or deeper generally yielded coolest effluent • At shallow bioretention, effluent temperatures correlated with time of day • Influent temperature variance greater than effluent temperature variance • Ability of bioretention to exfiltrate water leads to reductions in thermal load • Larger bioretention areas better able to reduce runoff volumes Percentage of Events with Watershed Area Outflow Asheville 16% 12% Lenoir 4% 79% Brevard East 7% 76% Brevard West 11% 27% 40 16% of Contributing Watershed 35

20 Temperature (°C) Temperature

10 05/01/06 08/09/06 11/17/06 02/25/07 06/05/07 09/13/07 Median Inlet Median Outlet Max Inlet Max Outlet 40

20 Temperature (°C) Temperature

15 4% of Contributing Watershed 10 05/01/06 08/09/06 11/17/06 02/25/07 06/05/07 09/13/07 Median Inlet Median Outlet Max Inlet Max Outlet • Bioretention able to reduce the effects of initial temperature spikes • For larger bioretention areas, consistent soil temperatures controlled effluent temperatures • Smaller bioretention areas impacted by the temperature of the influent runoff • Effluent temperatures more predictable than stormwater wetlands and wet ponds Available on BMPs & Thermal Loads website: www.bae.ncsu.edu/topic/thermal