2022-04-27_BR-0076_DeckDrains_Summary

BR-0076: NCDOT/Deck Drains Discussion – 4/01/22 Meeting Summary
Bridge No. 9 on NC 28 over Little Tennessee River
Division 14 – Graham/Swain County Line, NC

5/13/2022

This summary covers a deck drain discussion between NCDOT and NCDWR.

PARTICIPANTS:
Josh Deyton NCDOT - Division 14- Project Development Team Lead
Andrew McDaniel NCDOT – Hydraulics Unit
Brian Lipscomb NCDOT – Hydraulics Unit
Stacy Oberhausen NCDOT/TGS – Division 14 – Project Manager

Kevin Mitchell NC DEQ, Division of Water Resources – Environmental Specialist
Robert Patterson NC DEQ, Transportation Permitting – Stormwater Engineer

Randy Henegar TGS - GESC Hydraulic Review
Jay Twisdale TGS – GESC Hydraulic Review
Paul Fisher TGS – GESC Hydraulic Review

Cathy Houser RK&K – Roadway / Project Manager – Design Phase
Jon Whittington RK&K – Hydraulics and Erosion Control
John Williams RK&K – Planning / Project Manager – Planning Phase

EXPLANATION OF NCDOT HYDRAULIC POLICY AND NC SELDM
NCDOT’s Brian Lipscomb and Andy McDaniel began by providing an overview of NCDOT Hydraulic Policy
and a newer tool called NC SELDM Catalogue.

NCDOT’s policy remains that we will try to avoid direct discharge if the conveyance can be done via the
bridge deck. The hydraulics unit does not support the use of closed drainage systems for various reasons.
The Department does have tools to support the evaluation of direct discharge if it is the best solution for
the site. One of the tools we can use is the USGS SELDM program which is a mixing dilution program.

NCDOT worked with USGS by adding Water Quality information from NC and running scenarios (75,000 of
them) to generate the NC SELDM Catalogue. To use it on a specific site, NCDOT will input information from
USGS Stream Stats for that water body along with roadway project data. That model is for everything
draining to that stream, not limited to just the bridge deck. The development of the overall model
included DWR’s Robert Patterson and Amy Chapman. The tool is not limited to bridge replacements. The
NC SELDM Catalogue was designed to help set stormwater treatment goals at a planning level for all NCDOT
projects early in the process.

NC SELDM APPLIED TO BR-0076
The NC SELDM Catalogue was applied to this particular project with the outcome that discharge might be
okay at this location. DWR raised the question whether it was appropriate to use in a regulated stream
flow. NCDOT approached USGS with the question and the conclusion is that it is not appropriate because

BR-0076: NCDOT/Deck Drains Discussion – 4/01/22 Meeting Summary
Bridge No. 9 on NC 28 over Little Tennessee River
Division 14 – Graham/Swain County Line, NC

the NC SELDM model is based on unregulated stream flows. USGS and NCDOT discussed what could be
considered. The suggestion was to consider a worst-case scenario where there is no flow released from the
Fontana Dam and just evaluating the upstream contributions on the drainage area that was below the dam
(so not impacted by the dam). Carrying out that analysis, the upstream drainage area below the dam is
4.33 square miles. We ran the analysis again for two scenarios: the bridge deck alone and the bridge deck
with the contributing roadway drainage area which was 1.44 acres of drainage. The analysis returned a
result of “discharge may be okay for this stream crossing.” As a matter of interest, the model was run to
determine the threshold of it not being okay. The result was that the drainage area would have to be
reduced to less than 10 acres to get a result discharge not being okay for this site. We also looked at what
roadway drainage area would be needed to require additional measures. It would take well over 10 acres
just to get to a toolbox BMP being needed.

Brian Lipscomb stated that in his opinion there would not be a negative impact to water quality with open
deck drains/direct discharge from the bridge deck and the roadway work given shoulder sections and
vegetated conveyance. It was also noted that NCDOT’s BMP Toolbox manual does recognized that a
distributed direct discharge can be a BMP in certain cases but in order to do that it does require a planning
level cost analysis to determine that there is not a feasible alternative. The stormwater management plan
would need to address cost.

Brian noted that we have a general permit condition that requires avoidance of a direct discharge unless a
written exemption is provided.

Kevin Mitchell requested the inputs and outputs of the NC SELDM Catalogue analysis plus the cost analysis.
The information from NC SELDM was shared on screen for general information and will be provided to DWR
as part of a justification package.

Robert Patterson responded that the information presented along with the cost analysis is what they are
looking for. He further noted that the updated stormwater management plan has a stronger emphasis on
written justification for anything that deviates from standard.

John Williams noted that to keep the water on the bridge is an additional 2 feet of bridge width on a 500+
foot long bridge at current costs is around $220,000. The meeting concluded with John indicating that the
justification package would be sent in a subsequent e-mail. Robert Patterson suggested they would be able
to review the information within a week’s time.