20030179 Ver 6_More Info Received_20090724

Erg @ N 0 W A Uup
JUL 2. 4 2009
DENR - WATER QUALITY
WETLANDS AND STOMATER WAKH
F I DT ,
July 22, 2009
Ms. Lori Beckwith
U.S. Army Corps of Engineers
151 Patton Ave
Room 208
Asheville, NC 28801-5006
Subject: Duke Energy Carolinas, LLC - Dillsboro Dam and Powerhouse Removal
Project - Permit Modification Additional Information Request
Action ID: 200701194
Dear Ms. Beckwith:
Per our July 9, 2009 telephone conversation, you requested clarification on several issues
associated with the Duke Energy Carolinas, LLC (Duke) Dillsboro Dam and Powerhouse
Removal Project Permit Modification Request. This letter is intended to provide a summary of
your information requests and HDRIDTA's responses.
Request:
Although the exact elevation of the ordinary high water mark (OHWM) after dam removal is
unknown, please provide an estimate of the volume of rip-rap below the final waterline.
Response:
The estimated OHWM after dam removal and dewatering of the Dillsboro Reservoir will be
2 feet above the river bed. This estimate is based on the OHWM above the reservoir and the
OHWM below the dam. Given the elevation of 2 feet, the estimated volume of rip-rap to be
placed below the OHWM is 400 cubic yards.
Request:
As identified in drawings, the rip-rap placement at the upstream edge forms almost a right
angle with the bank. Generally, in-stream rip-rap is tapered to the bank to avoid creating
eddies that might compromise bank stability in that area. Please modify the drawing to show
the upstream edge tapered back to the bank.
Response:
Please refer to the attached drawing for the requested change.
HDR DTA 400 S Tryon Street Phone: [phone]
H D R Engineering, Inc.oftheCarolinas Suite 2401 I Fax: [phone]
Charlotte, NC 28285-0106 www.hdrinc.com
Ms. Lori Beckwith
July 22, 2009
Page 2
Request:
The placement of rip-rap in the channel will displace the water towards the opposite bank.
Please provide an estimate of how far the water is expected to shift.
Response:
As mentioned, the estimated volume of rip-rap below the OHWM is 400 cubic yards.
Although the width at the base will vary depending on bank and bed slope, this amount will be
spread out over 300 linear feet of river bed. This should cause a very minimal shift of water
due to displacement. Additionally, the shift will be onto land currently owned by Duke.
Please note that Duke is committed to removing at least 80,000 cubic yards of sediment from
the Dillsboro Reservoir prior to removing the dam. This includes 70,000 cubic yards removed
hydraulically during the dredging phase and an additional 10,000 cubic yards removed
mechanically as the construction access pad is removed.
HDRIDTA appreciates the opportunity to provide this information for your review. Should you
have any additional questions or concerns regarding this submittal, please do not hesitate to
contact either me at [phone] or Mr. Scott T. Fletcher at [phone].
Sincerely,
HDRIDTA
Jason N. Isbanioly S tt Wj, gPWScientist Environmental and Regulatory Manager
JNUcef
Enclosures
cc w/enclosures: John Domey, North Carolina Division of Water Quality
Hugh Barwick, Duke Energy Carolinas, LLC
File
HDRI DTA
HDR Engineering, Inc. of the Carolinas
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