20110718 Ver 1_More Info Received_20111014

SUMMIT
CONSULTING
October 10, 2010
Karen Higgins / Annette Lucas
NCDENR-DWQ Central Office
1650 Mail Service Center
Raleigh, NC 27699-1650
ENGINEERING ¦ ARCHITECTURE ¦ SURVEYING
k\- 01 12
Re: University of North Carolina Hospitals Hillsborough Campus
Un-named Tributary to Cates Creek [030401, 27-2-8, C; NSW]
Waterstone Drive, Town of Hillsborough
Orange County
To Whom It May Concern::
On behalf of our client and property owner, UNC Hospitals at Chapel Hill, Summit
Consulting would like to address the concerns/items listed in the "REQUEST FOR
MORE INFORMATION" letter issued by NCDENR-DWQ on September 1, 2011. After
multiple correspondences with Ms. Witherspoon of the DWQ staff and other approving
authorities via email and telephone, we would like to clarify the issues surrounding the
proposed pipe discharge and corresponding concentrated flow that is being released at the
head of an un-named tributary that flows to Cates Creek.
With regards to item #1 of the aforementioned letter, it has been established through
previous calculations provided and current supplemental calculations that the design of
the post development conditions has been conducted in a manner to meet both quantity
and quality restrictions for direct discharges into streams.
a. To exhibit the quantity control (i.e. attenuated flows) we had previously submitted
information showing that the discharge quantities at the outfall location were at or
below the pre-development rates. This was achieved by drastically reducing the
drainage area at the point of discharge from Pre-Dev to Post-Development. The
flows as submitted and approved by Orange County Erosion Control and
Stormwater reviewer, Mr. Terry Hackett are as shown table below.
PreDev PreDev PreDev PreDev PostDev PostDev PostDev PostDev
Basin Name
Area(Ac) lyr 2yr 10yr
Area(Ac) lyr 2yr 10yr
Discharge Discharge Discharge Discharge Discharge Discharge
DA #1 17.37 25.30 cfs 31.94 cfs 40.65 cfs 4.95 12.65 cfs 16.04 cfs 20.27 cfs
As is evident above, the reduction in drainage areas inherently affected the Post-
Development flows such that they were reduced by approximately 50%. Under
the original design, there was only a wall in this location and all pipe flows from
Waterstone were being conveyed to the west along the shoulder of Waterstone
where they would enter stormwater conveyance system and subsequently the
main tributary flowing Northeast to Southwest as shown on the drainage maps.
J:\09-0200\Civil\Submittals\2011-10-07 NCDENR CREEK NITROGEN REDUCTION INFO\2011-10-11 ResponseLTR.doc
504 Meadowland Drive Corporate Office 919.732.3883 • Fax [phone]
Hillsborough, NC 27278 www.summit-engineer.com Geotechnical/CMT 919.732.8500 • Fax [phone]
Due to utility conflicts along the roadway and upon reviewing the stream
internally and with Mr. Terry Hackett from an overall stormwater approval, we
pursued a different avenue whereby we would direct some of the site drainage
back to the original stream such that it would not be cut off completely from
water sources and essentially cease to being a stream. Therefore we revised the
wall design to incorporate a pipe and rip rap pad as is currently proposed.
b. During the current review by DWQ it was realized that we had not proved that we
had also reduced the nitrogen loading in this stream by the requested 30%. In the
solutions above the exact point of discharge was used as an analysis point since
the quantity of flow directly at the outfall could contribute erosive flows to the
existing stream, however with respect to water quality is the goal of the rules to
provide a level of nutrients in the watershed/basin. This being the goal an analysis
of the Un-named Tributary as it exits the subject property was evaluated such that
the overall export from the entire drainage area either from the subject property or
conveyed through the subject property would meet the goals of the Neuse River
Basin. The results revealed that all water flowing from or through the subject
property (all areas to the east of the main tributary flowing Northeast to
Southwest) would export approximately +/- 32% less nitrogen during the post
development conditions. Again, this reduction is directly attributed to the +/-
71.5% change in total drainage area contributing to the stream. See attached
calculations, charts and maps.
We look forward to discussing these matters further with your office and providing and
additional clarification to the items noted in your letter dated September 1, 2011.
Sincerely,
SU IT C
Chad E. Abbott, PE
Civil Department
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J:\09-0200\Civil\Submittals\2011-10-07 NCDENR CREEK NITROGEN REDUCTION INFO\2011-10-11 Responsel-TR.doc
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