Stormwater Retrofits at Nolensville
High School
PRESENTED BY STEVE CASEY, P.E., CPESC
RESILIENT WATERSHEDS SHORT COURSE
NASHVILLE, TN
OCTOBER 11, 2018
Nolensville High School
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Source: https://www.bing.com/maps/
Introduction
►High school site selection
►Specific practice site selection
►Goals for project
►Details of project including some lessons learned
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High School Site Selection
▪ CRC approached CEC with site
▪ Mill Creek watershed
o Increase baseflow
o Divert from detention basin
▪ Rapidly developing
o Residential development around school
▪ Education opportunity
o Residential development around school connected via greenways
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Detention Basin at Nolensville High School
Plan Development
► Kevin Fortney with Williamson County Schools – great partner
▪ CEC was provided school designer’s plans in .dwg format
▪ No field survey performed
► Vena Jones with Tennessee Department of Environment and
Conservation (TDEC) reviewed plans
▪ She knew history of site
▪ Riparian mitigation area – RSC in particular viewed as benefit
► Project was viewed as maintenance
▪ Relatively small footprint retrofit project
► Project would lessen burden on existing stormwater
management infrastructure
▪ Less focus on “design storm” and more on “maximizing benefit”
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Stormwater Retrofit Practice Layout
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Bioretention Area 1
Bioretention Area 2
RSC
Bioretention Areas
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Bioretention Area 1
Bioretention Area 2
Regenerative Stormwater Conveyance
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RSC
Specific Practice site selection
▪ Maximize diversion
from detention basin
▪ Good accessibility for
construction
▪ Visibility for education
▪ Intercept impervious
surface runoff (triple
catch basin is a good
sign )
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Specific Practice Site Selection
RIPARIAN MITIGATION
AREA
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TRIPLE CATCH BASIN
MILL CREEK BEYOND
RIPARIAN MITIGATION AREA
Specific Practice Site Selection
REGENERATIVE
STORMWATER
CONVEYANCE (RSC)
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RIPARIAN MITIGATION
AREA
FLUME TO ALLOW FLOW
THROUGH SIDEWALK ADJACENT
TO TRIPLE CATCH BASIN
Regenerative Stormwater Conveyance
► Four cell RSC was constructed rather than just two cells
based upon field conditions being different than what was
represented in the original school design plans –
importance of field survey
► Geotextile was not placed under the infiltration areas
► Flow limited to what could pass through flume. Excess
would flow into triple catch basin.
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Specific Practice Site Selection – Bioretention 1
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BEFORE
AFTER
Specific Practice Site Selection – Bioretention 2
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Bioretention Areas
► Placed where grass had been but with questionable perviousness
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Bioretention Areas
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Timeline
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2/13 Proposal
2/14 Notice to Proceed
3/8 – Concept Plan
4/6 – Meeting with Kevin Fortney, WCS
4/16 – 65% Plans
5/30 –Meeting to discuss 65% Plans
6/14 – Voted on by WCS Board
7/23 – Sent bids to Cumberland River Compact (CRC)
8/1 – Start Construction
8/10 – School Starts – goal was to be finished before school started
8/21 – Observed RSC Blowout
8/29 – Repaired Blowout
9/19 – 2 Additional Cells were added to RSC
10/9 – Planting Plan Delivered to CRC
Stormwater Practice Pontifications
► Stormwater retrofit projects are usually bound by more constraints than new projects
► Existing stormwater management infrastructure deemed sufficient due to recent design/construction of school (within last 4-5 years)
▪ However, these retrofits effectively increased the ability of the existing infrastructure to accommodate larger and more intense storms
► Opportunity to maximize available footprints in practice locations to promote infiltration and diversion of water from existing detention basin that had been partly excavated out of rock
► Engineering budget was shifted to spend more on field time than office time compared to original scope
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More stormwater practice thoughts…
► Using principles of bioretention and RSC design, rather than strict adherence to design criteria
▪ Checking 100 year, 24-hour design storm flow through RSC, in particular
▪ Redundancy: Bioretention Area 1 in watershed of RSC thereby reducing burden on RSC (and triple catch basin)
▪ Use same engineered media (sand, compost, fine soil) in bioretention areas as in RSC for simplicity but add hardwood mulch around RSC plantings before placing river gravel mulch
▪ Geotextile only on sides of excavations
▪ Allowing approximately 6” of pooling within bioretention areas
▪ Pooling as deep as 18” in RSC based on field conditions
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Now for some Math!
► Next few slides provide some hydrology & hydraulics
information that was calculated in support of the design
► Watershed delineations were based upon the original
school design plans and observations in the field
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Bioretention in Parking Island
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► Drainage Area = 1.49 acres
► CN = 95
► Tc = 6 minutes
Storm
Event
Peak
Flow (cfs)
Volume
(ac-ft)
2-yr 7.1 0.353
10-yr 10.1 0.513
100-yr 14.9 0.772
Bioretention near Tennis Courts
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► Drainage Area = 1.47 acres
► CN = 87
► Tc = 6 minutes
Storm
Event
Peak
Flow (cfs)
Volume
(ac-ft)
2-yr 5.6 0.259
10-yr 8.7 0.409
100-yr 13.6 0.662
Regenerative Stormwater Conveyance (RSC)
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► Drainage Area = 1.70 acres
► CN = 95
► Tc = 6 minutes
Storm
Event
Peak
Flow (cfs)
Volume
(ac-ft)
2-yr 8.1 0.403
10-yr 11.5 0.585
100-yr 17.0 0.880
Flume Through Sidewalk
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► Max Flow = 2.9 cfs
Flow over rock weir
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► Max Flow = 14 cfs
Planting Plan
►Clumping plants
►Drought tolerance
►Withstand flowing
water energy in RSC
►No plantings in
middle of final RSC
cell due to depth, but
on edges
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Coreopsis sp.
Hydrangea sp.
Echinacea sp.
Planting Plan
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Cost Information
► Construction cost not including planting
▪ Total construction cost ~$75,000
▪ Material costs:
o Raingarden Soil Mix: $36.50/CY ($65/load delivery fee)
o Class A & B Rip Rap: $23.50 per ton
o Dump Truck fee to haul away excavated material:
$100.00/hour plus $75.00 dump fee per load
This equated to about $275.00 per truck load
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Key points to keep in mind for retrofit projects
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► Field survey is recommended
► Look for triple catch basins!
► Try to include redundancy in your design:
Bioretention Area 1 in watershed of RSC and triple catch
basin conveys flow that does not pass through flume
► Create hydraulic jump (drop) at entrances to practices if
possible; otherwise, sediment will likely accumulate
upstream
► Plan for maintenance
► Safety: a railing is planned for sidewalk adjacent to the RSC
► Similar practices could save cost of traditional
infrastructure on new projects
Closing Remarks
► Special thanks to the Cumberland River Compact with
whom I have had the pleasure of working on cool projects!
► Kevin Fortney, Williamson County Schools Facilities and
Construction Director
► Vena Jones, TDEC
► Contractor: Viking Products, Mt. Juliet, TN
► Justin Bryan, P.E., CEC Project Manager for this project
► Jay Cameli, RLA, CEC Landscape Architect
Civil & Environmental Consultants, Inc.
325 Seaboard Lane, Suite 170
Franklin, TN 37067
615-333-7797