NCS000546_Sediment Loss Report_20170822

f� DUKE
ENERGY
Mr. Zahid Khan
NC Division of Water Resources
Mooresville Regional Office
610 East Center Ave., Suite 301
Mooresville, NC 28115
Subject: Duke Energy Carolinas LLC.
Allen Steam Station
5 Day Follow-up Notification (NCGO 10000)
Gaston County
Dear Mr. Zahid Khan,
Duke Energy Carolinas, LLC
526 S. Church Street
Charlotte. NC 28202
August 22, 2017
As discussed with Ross Hartfield with Duke Energy on 8/16/17, the Allen Steam Station in
Belmont, NC experienced heavy rains late on 8/15/17 that resulted in sedimentation outside the
project limits of disturbance. This letter is to provide the 5-day follow up information on the
event as requested by your office. The rain event at Allen Steam Station measured
approximately 0.7 inches and occurred in approximately 30 minutes beginning around 18:00.
Prior to the rain event Allen Steam Station grading contractors had been working on the
GASTO-2017-015 project in both the Lined Retention Basin (LRB) and the Temporary Piping
Re-route (TPR) areas.
The grading contractors, as well as Duke Energy personnel were aware of the potential for rain
in the forecast on Tuesday evening, and erosion and sediment control measures (BMPs) had been
installed at both work areas per GASTO-2017-015. At the TPR area, a soil berm had been
installed on the east side for the area exposed for soil testing prior to installation of the ABC
stone foundation. At the recently initiated LRB area, silt fence and silt fence outlet had been
installed, and work had ceased in this area earlier in the afternoon. When a lightning stand -down
was ordered at 17:45, the contractor working in the TPR area, installed a soil berm on the west
side of their active area and left the site.
The Duke Implementation team was preparing to leave the station for the day, when they
observed water flowing around the south side of the east soil berm at the TPR area. A walk
down of the LRB area showed over -topping of a section of silt fence and silt fence outlet. The
grading contractors were called back to the site to shore up the existing erosion control measures
at both locations. The grading contractors arrived at 19:00 and a site walk was performed to
review the site conditions. During the site walk, discharge carrying sediment was observed at
SW-016 and SW-018 outfalls.

The team instructed the grading contractor to make repairs to the BMPs for the Temporary
Piping Re-route and Lined Retention Basin sites. A temporary dam was built for the TPR
project, and silt socks and sand bags were placed at the LRB project area where the silt fence
failed. The TPR area repairs were completed at approximately 20:20. After repairs were
completed on the TPR area, both grading contractors worked to stabilize the LRB site location.
Work was completed by 21:00.
During the after -rain event inspections on 8/16/17, both areas were inspected to determine the
extent of the sedimentation. For the LRB area, sediment deposition was observed in the area
immediately upstream and downstream of the silt fence that was overwhelmed during the storm
event. Sediment deposition was also observed immediately downstream of the existing culverts
under the FGD haul road. The conveyance from the culverts to SW-18 is a series of earthen
ditches, and sediment deposition in this area was observed to be both from the recent storm event
and historic natural deposition. No sediment deposition was observed in the discharge canal near
the SW-018 outfall. From these observations, it was determined that sediment deposition
occurred outside the project limits of disturbance and traveled to ditches that drain from the
project area to the SW-018 outfall. Due to the lack of observable sedimentation at the SW-018
outfall and the distance of the stormwater conveyance, it is likely that most of the sedimentation
was deposited in the conveyance and minimal sedimentation reached the discharge canal.
For the TPR area, sediment deposition was observed in the area immediately upstream and
downstream of the check dam that was overwhelmed during the storm event. No deposition was
observed at the SW-16 outfall; however, during the storm event storm water containing sediment
was observed discharging into the discharge canal from the SW-016 outfall pipe. From these
observations, it was determined that sediment deposition occurred outside the project limits of
disturbance, and it is likely that sediment laden waters traveled from the project area to SW-016.
Due to the lack of observable sedimentation at the SW-016 outfall and the observed sediment
captured by BMPs on site, it is likely that most of the sedimentation was captured by the E&SC
BMPs and minimal sedimentation reached the discharge canal.
Duke Energy took immediate action during the storm event to restore BMPs and limit the off -site
sediment loss. On Wednesday and Thursday, August 16-17, Duke Energy took the following
steps to further mitigate the off -site sedimentation experienced at the Allen Steam Station on
8/15/17:
• Restored all site BMP measures including silt fences and check dams that had been
overtopped at both the LRB and TPR areas.
• Cleaned the sediment observed in the culverts leading to SW-16 and SW-18.
• Any sediment remaining in front of the silt fences and check dams was dug out and
removed at both the LRB and TPR areas.

• Installed rip rap armoring at the end of the outlet pipe from the old sediment basin at the
LRB area where stormwater flow had been concentrated during the storm event.
• Installed a large check dam up -slope of the silt fence and silt fence outlet at the LRB area
where the stormwater runoff had overwhelmed these measures during the storm event.
• Installed gravel berm at lower end of existing gravel lot at the LRB area to control runoff.
If you have any questions regarding this notification please contact Anne Pifer at [phone]
or [e-mail].
1 certify, under penalty of law, that this document and all attachments were prepared
under my direction or supervision in accordance with a system designed to assure that
qualified personnel properly gather and evaluate the information submitted. Based on my
inquiry of the person or persons who manage the system, or those persons directly
responsible for gathering the information, the information submitted is, to the best of my
knowledge and belief, true, accurate, and complete. I am aware that there are significant
penalties for submitting false information, including the possibility of fines and
imprisonment for knowing violations.
Sincerely,
o4 E
Larry Hatcher
VP Environmental
526 S. Church Street
Charlotte, NC 28202