Page 7 - May-June2019 Vol37 No1

SITE LAYOUT
is understanding the site’s utilities prior
to beginning the design. The same rang
true with HHS, mostly due to the age
of the campus, the number of previous
renovations and the lack of historic civil
design documents.
Our team began with an in-depth
due diligence research effort to better
understand the site’s existing flows to
public storm infrastructure and how
it related to the site’s existing drainage
basins. During our initial due diligence,
we discovered three major utility design
challenges affecting the site.
First, a public sanitary sewer line
snaked its way through the center of
the site under the current football field.
We relocated a portion of the line and
it caused minimal interruption to our
design.
Our second surprise came when
Nashville Metro Water Services (MWS)
informed us that a major stormwater
trunk line sewer was running through
the campus. It was believed to be a piped
stream that was constantly flowing in a
50-square-foot riparian area where the
stream daylighted, which meant we had
to get additional permitting from the
U.S. Army Corp of Engineers and the
Tennessee Department of Environmental
Conservation (TDEC) for a stream
relocation. While the project’s timeline
wasn’t affected by the added permit
process, we did have to account for a
shallow riddle pool in our final design.
Finally, our largest surprise came when
our team discovered storm drainage in
the right-of-way. Even after multiple
discussions with MWS and the Tennessee
Department of Transportation (TDOT),
as well as pulling every historical
document the
project team could
find, there wasn’t
a clear answer as
to the location of
the water’s final
drainage area. It
seemed to drift off now be occupied by bioretention ponds.
into no man’s land
where the most
logical downstream
catch basin was
full of concrete
from a previous
construction
project.
However, after
multiple trips to
the site, TDOT was able to chisel away
concrete and use a camera to explore
the stormwater network. To the surprise
of the entire project team, there was
a trunk line sewer running down the
center of Hillsboro Pike that was blindly
connected to every inlet. Ultimately,
our team redesigned the trunk sewer
to increase the capacity of the storm
network.
If stormwater surprises weren’t
enough, our team also discovered
unusual site conditions while excavating
for a small parking lot. The fill never
had been compacted, and it contained
blocks and stumps, which were parts
of basements from houses that were
demolished there in the early 1950s and
1960s.
The Best Bet: Bioretention Ponds
In 2016, MWS implemented
mandatory development regulations that
require all site designs to utilize green
infrastructure to meet stormwater runoff
water quality (unless a hardship, such as
karst, can be demonstrated). Although
low impact development practices were a
requirement, our design team saw HHS
as the perfect project to demonstrate
different options for stormwater
infiltration practices.
Initially, our design called for 20,000
square feet of pervious paving, 15,000
square feet of bioretention ponds, 9,500
square feet of green roof, and 3,000 square
feet of bioswale. After a pricing estimate
from an independent contractor, we
needed to make cuts to stay within the
original $73 million dollar budget. We
decided to eliminate the green roof, as
well as most of the pavers and bioswale, in
favor of an all bioretention pond system,
ultimately saving the client approximately
$1 million.
While the cost savings was welcomed
by MNPS, this stormwater management
solution meant that every available open
space on the tight, landlocked site would
In fact, space was at such a premium
on the HHS campus that bioretention
ponds often had to be placed closer to
structures than MWS’ recommended
offset distances.
Due to the design challenge, our team
coordinated often with the project’s
structural engineer to ensure the angle of
repose for the buildings’ footings didn’t
fall within the bioretention ponds’ loose,
uncompacted area. The final design of
HHS encompassed seven bioretention
ponds with a total treatment volume of
40,000 cubic feet, 10,000 linear feet of
pipe, more than 150 storm structures and
3,500 man hours of planning, design and
plan production.
Going Back to School
HHS students weren’t the only
ones learning on that campus—our
project team learned a thing or two
as well. When updating existing sites
to comply with current stormwater
regulations, it can be challenging, costly
and full of surprises. Engineers and
developers frequently find themselves
updating infrastructure that has become
insufficient due to growth and age.
Land is often at such a premium that
buildings are built-out to the fullest
extent possible, allowing minimal
room for green space and stormwater
infrastructure. Additionally, due to
updated standards and regulations,
sometimes only a fraction of an existing
site will be suitable for redevelopment
without substantial additional
permitting.
While small urban spaces often
present a slew of site challenges, they
are often the perfect place to implement
low impact development practices.
As Nashville continues to expand and
redevelopment projects become more
prominent, complex site problems will
likely become the new normal.
Close, careful coordination between
the project team and public works
can address the unique needs of our
cities, improving public infrastructure
while limiting the impact on our built
environment and enhancing quality of
life for residents and visitors alike.
View online at tnpublicworks.com 7