Page 18 - Jan-Feb 2020

TCAPWA 2019 MARK MILLER PROJECT OF THE YEAR By Colline Miller MPA

Chattanooga’s Big Dig:
Protecting People and Their Water

Background a plan to re-route the existing storm cal schools, and events for the timing of
The historic district of St. Elmo in Chat- infrastructure around the landfill. The the closure of Broad Street (a primary
plan consisted of 1,360 feet of 10'x10' box roadway).
tanooga, Tennessee is located in the valley culvert at depths reaching over 30 feet Energy Dissipation
of Lookout Mountain near the Tennessee deep, 200 feet of 8'x8' box culvert, 630 feet
River. Storm drainage for the St. Elmo area of 60" reinforced concrete pipe (RCP), 230 A major design challenge was the
outfalls into Chattanooga Creek at the curl feet of 54" RCP and 400 feet of 36" RCP, accommodation of a significant peak
in the Tennessee River referred to as Moc- including 3 tunnels (2-78" tunnels under stormwater flow (1,900 CFS) and
casin Bend. Norfolk Southern Railroad and 1-96" dissipating the energy of such a flow
tunnel under the parking lot of a dialysis reaching Chattanooga Creek. A 100-year
The existing area storm infrastructure clinic). The project coordinated with the peak storm event would decimate the
consisted of different pipe sizes and materi- St. Elmo Riverwalk project to include existing banks of the creek at the outfall
als that had been added on to and extended installation of sidewalks, bike lanes and without dissipation of the flow velocity.
over many years. Area storm water (ap- streetscape features. Civic’s engineers had to balance slowing
proximately 1,100 acres) fed into a 10'x7' Uniqueness/Innovation: New or Existing the flow to an acceptable velocity while
corrugated metal pipe that extended over Technology maintaining hydraulic grades upstream.
30 feet below the now capped Wheland
Foundry Landfill. The Chattanooga “Big Dig” provides the The solution included an array of
storm drainage backbone to accommodate uniquely-designed reinforced concrete
The City of Chattanooga desired to a 100-year storm event for a 1,100-acre energy dissipation blocks at the outfall
perform a hydraulic study and structural drainage area encompassing the historic St. followed by a 120' long gabion block
evaluation of the existing storm convey- Elmo District, large parts of Alton Park and lined channel with gabion baffles. The
ance to evaluate existing conditions and South Chattanooga with one outfall into combined utilization of different materials
determine requirements to redirect storm Chattanooga Creek, which is a tributary to and dissipation methods resulted in an
conveyance around the landfill. The City the Tennessee River. Harnessing the power outfall that can handle both maximum
contracted Civic Engineering and Informa- of the potential water volume from this and minimum flows.
tion Technologies, Inc. (Civic) to perform large of an area created extreme challenges. Maintenance Considerations
the study.
This project was not only massive in In order to make sure the City can
The findings indicated that existing size and volume, it was equally massive in maintain the system and keep it operating
infrastructure was undersized for existing its required planning and coordination. properly, the reinforced concrete energy
stormwater flows as suspected, and that the The impact to vehicular and pedestrian dissipaters at the outfall were spaced to al-
pipe under the landfill was extremely dete- traffic had to be managed and required low a skid steer loader to enter at the out-
riorated and needed to be addressed in an coordination with property owners, lo- fall, to assist with the removal of sediment
expeditious manner. A collapse of this pipe
would not only cause flooding throughout
the St. Elmo area, but it could also leach the
landfilled foundry sand into the Chatta-
nooga Creek and Tennessee Rivers.

To prevent a catastrophic failure of their
stormwater infrastructure and poten-
tial water contamination from a former
landfill, the City of Chattanooga took
action to redesign the storm conveyance
in the historic St. Elmo area of the city. The
discovery of contaminated soils compli-
cated the project and brought in additional
stakeholders.

The new storm drainage is designed to
accommodate a 100-year storm event. The
new infrastructure has been designed with
expansion in mind to pave the way for
future development of the area.
The Plan

At the City’s request, Civic developed

18 TPW January/February 2020