Page 7 - July-Aug22Vol40No2

a competitive process to select an Pumps are a specialized design, and chose to use the new HEC-RAS 6.0 2D
engineering consultant,” WTRBA pumping at a larger scale requires a modeling software for a more accurate
Executive Director David Blackwood said, thorough team.” representation of the discharge from this
“so [WTRBA] solicited proposals through The new system would soon offer flat drainage basin.”
a public process to evaluate the skills and Tiptonville the ability to operate The team planned to install multiple
capabilities of interested consultants. It is flood control automatically, providing large pumps, three-phase electricity, and
always important that the most qualified immediate response to stormwater events enclosed facilities for the pump stations.
proposal is selected to execute a project.” with no need of volunteer labor from
Barge Design Solutions, Inc. (Barge) local farmers and their tractor pump. At Optimizing the Design
joined the competitive pursuit based on last, a permanent solution for overcoming Although the challenge of funding
the design team’s specialized experience flooding in the Tiptonville area was in the was out of the way, the team still faced a
with stormwater pump station design, works. major hurdle in planning: how to develop
flood plain modeling, benefit-cost analysis, an accurate inflow hydrograph from
and working knowledge of the proposed Initial Planning Gets Underway the upstream drainage area to optimize
Tennessee project sites. Once funding and a notice to proceed the design for the community’s flood
“Our team has a history of supporting were in place, Barge’s design team began mitigation system.
West Tennessee, so pursuing the the important process of securing permits “Accurate discharge estimates are critical
Tiptonville project was an effort we related to impacts to the adjacent levee and in determining sizing of the large pump
believed in doing as a service to the stream. Coordination with the Tennessee station,” Barge Project Engineer Clayton
community,” Barge Vice President and Department of Environment and Foster said. “Given the flat topography
Senior Project Manager Trevor Cropp said. Conservation (TDEC) and the US Army of the land, even a small miscalculation
“Our knowledge of West Tennessee and the Corps of Engineers (USACE) was critical. in stormwater inflow could result in an
team’s in-house capabilities put us in a solid Barge identified utilities and excessive increase in inundation area,
position to effectively complete the project infrastructure that would be required flooding residential homes.”
from start to finish.” to support the site’s new pump station. Traditional hydrology methods were first
Three NDRC Grant projects were Since the watershed was flat, the team used and resulted in a large peak inflow
awarded to Barge by WTRBA, including also confirmed the need to use HEC-RAS of around 1,400 cubic feet per section
the Tiptonville pump station, a permanent 2-Dimensional (2D) modeling to develop during the 100-year rainfall event due to
stormwater pumping solution to mitigate inflows required for input into the pump limitations in the modeling approach. The
flood risk in the community. station design. projected inundation elevations seemed to
“While there were multiple qualified “Using 2D models instead of traditional exceed those of the existing conditions. As
consultants, Barge was elevated because of TR-55 models is relatively new and is a result, a new 2D model was developed.
their specific experience with stormwater generally chosen for very flat terrain and New LiDAR data was used to build a 2D
and stormwater pump station projects,” areas where large amounts of overland flow computational mesh, and excess rainfall
Blackwood said. “They also had the staff are expected,” Cropp said. “To produce amounts were applied to that mesh. As
and experience to deliver the projects. a more realistic hydrologic response, we excess rainfall was applied to individual
cells within the mesh, water accumulated
was conveyed downstream from one cell to
the next until it reached the outfall.
Each cell had its own set of hydraulic
properties and allowed the software
to compute a combined discharge
hydrograph based on the response of each
individual cell.
The results showed that the existing flat
topography had a large attenuation effect
on the discharge hydrograph. The result?
To optimize the design for the greatest
benefit to the community and the least
financial impact, the team concluded that
a total pumping capacity of 30,000 gallons
per minute would be needed for the pump
station.
Pump Station Design Takes Shape
WTRBA selected a 10-year rain event
for the basis of the final design, meaning
two 21,000 gpm pumps would be presented
in Barge’s plan. This design threshold
Designers worked with the U.S. Army Corps of Engineers to protect the levee from excavation, translates to an overall risk below the one
choosing to build up and over the structure to maintain its integrity, while also making it drivable. Continued on page 8

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