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

scasey@cecinc.com31