20100110 Ver 1_More Info Received_20100614

v McKIM&CREED
v
E N G I N E E R S
S U R V E Y O R S
P L A N N E R S
June 9, 2010
M&C 00592-0037 (40)
Ian McMillan
Acting Supervisor
401 Oversight/Express Review Permitting Unit IJUN 1 _;v
2321 Crabtree Blvd. ' -
Raleigh, NC 27604 V ENR - WATE ., r
WETLANDS AND SPGR%,,, njE j BRqdCt(
RE: City of Raleigh - Poplar Creek Sewer Line (DWQ Project # 10-0110)
Dear Mr. McMillan:
In reference to your letter of June 7, 2010, "Request for more information", please
find our answer to your comment contained in paragraph 1, referencing the non-
perpendicular stream impact depicted on Exhibit 18.
There are several contributing factors that impact the position of the sewer line at
the location in question on Exhibit 18. These are discussed in the paragraphs
below to help justify the location of the sewer line in question and explain the
non-perpendicular crossing of the branch leg stream.
Whenever possible, the City of Raleigh and the Engineer try to keep the utility
alignment and associated easement as close to property boundaries as possible to
minimize the land acquisition impact to the private land owner. The property
being crossed on Exhibit 18, like many others along the easement corridor, is
bound by Poplar Creek; being the most eastern property line. Therefore, the
sewer alignment has been placed as close to the creek as possible (as in most
cases) without encroaching on the Poplar Creek riparian buffers and adjacent
Venture IV Building (parallel) wetlands. Unfortunately, many of the small tributaries to Poplar Creek
along the alignment cannot be avoided. This is the case with the stream on
Suite 500 Exhibit 18.
1730 Varsity Drive In addition to designing the alignment to minimize impact to the property owner
R a l e i g h , NC 2 7 6 0 6 and the wetland/buffer areas, the alignment is also positioned to minimize the
required depth of the sewer line. This is especially important with a gravity
sewer line where a continuous downward slope is required and the line cannot
9 19.2 3 3. 8 0 9 1 rise and fall as with a pressure line. Issues with installing sewer force mains at
higher elevations include increased static head on the pumps, the need for
Fax 919,233.8031 additional air release manholes which many times result in odor problems and
complaints by nearby citizens, and provide more areas in the force main for
www.mckimcreed.com
\ \ cry \ cad \ 0592 \ 0037 \ 40-Permit \ USACE-DWQ 404 401 \ 060910iMcMillan.doc
Mr. Ian McMillan
June 9, 2010
Page 2
hydroxen sulfide gas to collect and corrode the inner lining of the pipe. For these
reasons, force mains should be designed to avoid intermediate high-spots.
In the specific case of the sewer at the location shown on Exhibit 18, the
jurisdiction stream being crossed has a short branch leg that creates a situation
where it is impossible to get a perpendicular crossing (85 -105 degrees) of both
channels with the permanent easement. Therefore, we designed the alignment to
cross as the best possible combination of angles that we could determine while
maintaining the design objectives mentioned above. The main stream channel is
a perpendicular crossing, but the smaller branch leg crossing is not. We thought
it was most appropriate to cross the main channel at a perpendicular angle to
minimize the buffer and stream impact areas.
In conclusion, the sewer alignment and permanent easement have been designed
at the current location to minimize the land acquisition impact to the property
owner, while keeping the alignment out of the riparian buffers of Poplar Creek.
The design also places the easement at lower grade elevations along the corridor
to minimize depth of bury, static head on the pumps, and potential for corrosion.
This alignment methodology is consistent with the rest of the alignment on the
project. We have attached photos taken during the field inventory by F&R of
Streams 14 and 15 to clarify the difference between the two stream channels and
to exemplify why the decision was made to limit the non-perpendicular impact
to the lesser of the two channels.
If you would like to discuss this matter, I am more than welcome to do so and
will be glad to provide additional documentation if necessary.
Sincerely,
McKIM & CREED, PA
Chris L. Windley, PE
Senior Project Engineer
Enclosures: Pictures of Streams 14 and 15
cc: Aaron Brower - CORPUD
Thomas Colson - F&R
Amy Chapman - DWQ
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