by Thomas P. Healy
When it rains as little as a quarter of an inch, the city’s antiquated drainage system combines stormwater with wastewater (sewage). The U.S. Environmental Protection Agency (EPA) considers such combined sewer overflows (CSO) a public health threat.
As part of a consent decree with the EPA, Citizens Energy Group (CEG) designed the $2 billion Dig Indy tunnel project to capture combined runoff for treatment and reduce the amount of sewage released into waterways by up to 97 percent.
While that construction project continues “out of sight, out of mind” 250 feet underground, a complementary aboveground project is also under way.
Ten Thousand Trees is a partnership between CEG, Keep Indianapolis Beautiful (KIB), and the Indianapolis Department of Public Works (DPW) that aims to plant 10,000 trees in CSO areas around the city.
Civil engineer Olivia Hawbaker coordinates the effort for CEG. “Our whole intent is to provide some additional benefits to reduce combined sewer overflows while providing visible community and neighborhood benefits that are a little more tangible and green than the tunnel system, which is capturing a huge amount of overflow but isn’t visible,” she said.
“Most of what people experience with Dig Indy is the disruption from construction and, once it’s done, they’ll forget about it,” Hawbaker said. “Whereas with trees, in 10 to 15 years you’ll be able to see the benefits continue to accrue.”
The Purdue grad works on CEG’s wastewater capital projects related to the Dig Indy project. “I manage the consent decree program and the design for a couple of large tunnel projects,” she said.
Initial plantings occurred in 2016, and all 10,000 trees should be planted by 2024. Seven hundred trees were planted this spring and another 700 are scheduled to go in the ground this autumn.
KIB helps organize volunteers to do the plantings in the public right-of-way and assumes responsibility for watering trees for three years to get them established. KIB’s high school Youth Tree Team handles pruning after the fifth year. After that, the trees are DPW’s responsibility.
Hawbaker said choosing the number 10,000 was intentional. “Using different sources for the potential rainwater capture of one mature tree, we came up with a reasonable number of 100 gallons,” she said. “Ten thousand trees equates to 1 million gallons captured every time it rains.”
With an estimated 40 rain events annually, she said the amount of stormwater captured could total up to 40 million gallons. “When it rains, there’s a peak flow that comes into the combined sewer system,” she explained. “By capturing 40 million gallons of rainwater, you reduce the peak flow that goes into the CSO and reduce potential overflows.”
Hawbaker said that recognizing the direct benefit to combined sewer overflow reduction, CEG funds the program as part of the Dig Indy project. That’s why the focus of tree planting efforts is within the city’s CSO area—predominantly Center Township, but also in areas Dig Indy affects, including Basin 156.
“Basin 156 is the area in Midtown identified for tree-planting efforts,” she said. “A basin would be anywhere we have combined sewer overflow. A flow comes out of a pipe into a waterway and every point is associated with a basin—the area upstream that flows into that pipe and into the overflow.”
Hawbaker said CEG looks for areas where planting trees could have the greatest impact. “We look at areas with CSOs and where we know at the end of Dig Indy, there’s still potential for CSOs.” She said CEG plans about one year ahead and that the 20 areas of greatest impact have been identified.
Nearly 600 trees have already been planted in Midtown with the potential for more trees on College Ave. once Red Line construction is complete. “We want to stay out of the way now. We’ll go back and plant in that corridor and even more in the future.”
A version of this article appeared in the June/July 2019 print edition of the magazine.