Mitigating the Thermal Characteristics of Stormwater

by Charlene LeBleu, FASLA

Data logging thermistors are located inside and outside material testing mesocosms. / Image: Britton Garrett

The Auburn University Green Infrastructure Team is studying thermal inputs to stormwater systems.  The team includes faculty researchers Amy Wright, Horticulture, Mark Dougherty, Biosystems Engineering, Keith Rahn, Building Science, and Charlene LeBleu, FASLA, Landscape Architecture. Graduate Research Assistants include MLA students Andres Orjuela, Student ASLA; Britton Garrett; Rui Wang, Student ASLA; and Ryan Bowen, MLA & Master of Building Science. The research is conducted in the Green Infrastructure Laboratory at the Mike Hubbard Center for Advanced Science, Innovation and Commerce (CASIC) Building. The laboratory provides a controlled environment, and is designed for both wet and dry research. The center was built with funding provided by a grant from the U.S. Department of Commerce’s National Institute of Standards and Technology, and the Alabama Agricultural Experiment Station. The main purpose of this research is to develop design models for standard stormwater control measures that can be used to meet specific effluent temperature standards and to maintain the required thermal regime in a receiving stream. This project hypothesizes that pervious surfaces, turfgrass and rain gardens can be used to mitigate ground level thermal loads in stormwater runoff.

The team’s interest in thermal pollution began when one of LeBleu’s MLA students detected high temperatures of stormwater runoff while doing fieldwork in Mobile, AL. Mobile Bay is home to Alabama’s seafood industry and provides estuary service for many fish, crustaceans, and wildlife. The main stormwater outfalls for downtown Mobile flows directly into Mobile Bay. The temperature of stormwater runoff (first inch of flow) moving over impervious surfaces in downtown Mobile, AL, has been recorded as high as 48 degrees Celsius to 50 degrees C during the month of July.  Fish and other aquatic life in particular are most sensitive to thermal pollution. Many dead juvenile fish and crustaceans have been observed at Mobile’s downtown stormwater outfall due to this thermal pollution.

In the Green Infrastructure Lab, controlled laboratory test are used to assess low impact development (LID) stormwater control measures to reduce the impact of the thermal characteristics of stormwater runoff.

Configuration of material mesocosms being tested in Green Infrastructure Lab. / Image: Rui Wang, Student ASLA

Following a four hour heating cycle, the mesocosm samples (pervious pavement, impervious pavement, permeable pavers, and turf grass) are sprayed with water from an irrigation system for 30 minutes, simulating the common short rain showers experienced in Mobile, AL. Water running off the impervious or infiltrating through the pervious is collected in a pan bellow the stands where a temperature probe captures the water’s final temperature.

Automated data acquisition system makes recording data accurate and easy. / Image: Keith Rahn

The goals of this research include:

  1. Establish benefits of pervious pavements in mitigating thermal heat removal
  2. Establish a baseline measurement of heat removal effectiveness of pervious paving as a stormwater control measure
  3. Establish role of pervious paving in mitigating thermal heat removal when used in combination with other stormwater control measures.

Current trends that the researchers have found include:

  • Pervious pavements heat up faster than their impervious counterparts due to their higher porosity.
  • Pervious paving also tends to cool down faster, giving up its stored thermal energy due to its porosity.
  • Even with the pervious samples heating up, stormwater is stored in a much cooler base whereas impervious concrete simply transfers the heated stormwater runoff.
  • Permeable pavers heat up slower and cool down faster than impervious and pervious pavement.

This research has the potential to establish the positive role pervious and permeable pavements have in mitigating the thermal pollution encouraged by stormwater runoff. As testing continues, the role of rain gardens and bioswales may be better understood in reducing stormwater temperatures when acting in conjunction with other low impact development strategies such as pervious pavement. To find out more about this research, please visit http://www.greeninfralab.com/

Charlene LeBleu, FASLA, AICP, is an Associate Professor of Landscape Architecture at Auburn University, Auburn, Alabama. She is a Fellow of the American Society of Landscape Architects (FASLA), and a member of the American Institute of Certified Planners (AICP).

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