20210204 Ver 1_Crabtree Creek Interceptor Replacment - Add'l Responses 06-09-2023_20230711

systems
June 9, 2023
Mr. Lyle Phillips
USACE — Regulatory Division
3331 Heritage Trade Drive, Suite 105
Wake Forest, North Carolina 27587
Re: Crabtree Creek Interceptor Replacement Phase III Project (SAW-2017-00014)
Raleigh, Wake County, North Carolina
Dear Mr. Phillips,
TranSystems
1 Glenwood Avenue
Suite 600
Raleigh, NC 27603
www.transystems.com
The following includes responses to the additional information request received May 11, 2023 from the U.S. Army
Corps of Engineers:
1) The proposed activities include the permanent discharge of fill material within stream channels. The
need for sewer service is not disputed; however, it is unclear if the project at the locations of
permanent fill within stream channels have demonstrated avoidance and minimization to the
maximum extent practicable as required by NWP General Conditions 23(a) and (b). For example,
could the proposed sewer infrastructure be installed without permanent fill material (rip rap) within
the stream bed?
Avoidance was initially executed by revising proposed alignment to reduce the total number of stream
crossings required. Avoidance was also achieved by eliminating impacts at one crossing with the use of
jack and bore.
Minimization practices at these locations include:
Stockpiling wetland soils of disturbed wetlands and then using those same soils to restore to original grades
and elevations after installation.
Stockpiling existing stream bed material to be used to restore the disturbed stream bed with its original
material for the majority of the disturbed area.
The steps of chinking and covering the imported material with the stockpiled existing stream bed material to
the original stream bed elevation were included in the construction detail that was developed.
Placement of permanent fill was minimized to occur within only approximately 113 of the construction
corridor disturbance width.
Within the stream and wetland crossing sites, the need for permanent fill material (rip rap) was determined
necessary for at least one of the reasons below.
Establish and secure long-term accessibility through wetlands or across channels for inspection and
emergency maintenance
Stabilize streambed and streambanks at crossing locations that already exhibit scour, erosion or other
instability in their current condition
Provide immediate and long-term stabilization of streambed and streambanks after pipe installation
disturbance.
Provide additional protection for pipes at stream crossings with minimal coverage (<3 ft)

Systems
Provide a more natural grade control transition at minimal coverage pipe crossings to prevent the scour
holes and bank erosion from redirected flows that are typically found at exposed pipes and or pipe
encasement crossings.
The installation of the proposed pipe at stream crossings requires complete disturbance of the existing
stream bed and stream banks. To determine what was necessary to ensure the immediate and long-term
protection of the proposed pipe as well as the stability of the stream bed and banks, multiple factors were
considered including the height of coverage over existing and proposed pipes, the size and mobility of the
existing stream bed material, the steepness of existing banks, as well as channel velocities taken from the
hydraulic model. In the case of many of the stream crossings, the coverage between the existing steam bed
elevation and the top of the proposed pipe was less than ideal. In these instances of shallow coverage,
concrete encasements were used to add protection to the pipe. From professional experience, a shallow
coverage of highly mobile sand and gravel steam bed material on a flashy urban stream the size of Crabtree
Creek would most likely result in the exposure of the pipe encasement. Once the encasements are
exposed, the abrupt hydraulic disturbance typically begins to scour holes immediately upstream and
downstream of the encasement. Additionally, most destructive are the flow forces that are redirected by the
exposed perpendicular and diagonal crossings that send erosive forces directly into the stream bank. The
proposed rip rap would provide a more gradual and less abrupt grade control transition and prevent
encasement exposure. The compromise of placing proposed rip rap permanent fill material that is chinked
and covered with existing stockpiled streambed material was determined a necessary step in ensuring that
the disturbance from the proposed pipe alignment installation does not cause future erosion and instability
within Crabtree Creek.
2) Per general condition 32, the description of the proposed activity and any proposed mitigation
measures should be sufficiently detailed to allow the district engineer to determine that the adverse
environmental effects of the activity will be no more than minimal and to determine the need for
compensatory mitigation or other mitigation measures. The nature of the proposed permanent fill
material within the stream channels is unclear. Based on a review of the plans it appears that rip rap
stone is proposed within the stream channel, within the cover letter the permanent fill is described
as salvaged stream bed material and rip rap, and within the PCN impact table the impacts are listed
as culvert. Please provide an undated description of the areas that are proposed to be permanently
impacted by a discharge of fill material.
As noted above, permanent fill material proposed includes rip rap that is chinked and covered with existing
stockpiled streambed material in order to ensure that the disturbance from the proposed pipe alignment
installation does not cause future erosion and instability within Crabtree Creek.
A revised table for the PCN is included. The "Impact Type" has been updated to bank stabilization or
stabilization for temporary and permanent impacts.
2

Systems
3) Per NWP 58, there must be no change in pre -construction contours of waters of the United States
for the installation of utility lines. Per NWP 58, access roads must be as near as possible to pre -
construction contours and elevations (e.g., at grade corduroy roads or geotextilelgravel roads).
Based on a review of the plans provided it appears that stream bed elevations at crossing location
will result in a change of pre -construction contours of waters. However, based on the application it
is unclear if these changes occur for an access road or for stabilization. Please provide clarification.
Changes to the pre -construction contours to the stream bed elevations at the proposed crossings are
necessary for streambank stabilization. No access roads are proposed in these areas.
4) Please provide any updated drawing that result from the comments above.
A revised set of impact maps is provided in the attached.
5) In order to verify compliance with General Condition 18, the Corps will initiate informal consultation
with The US Fish and Wildlife Service and will not be able to render a decision until ESA Section 7
consultation is complete.
Aquatic and Michaux's sumac surveys were completed for the project in 2021. Additional aquatic surveys
and Michaux's sumac surveys are underway and will be completed in June, 2023, during the USFWS
recommended survey window.
Please submit written comments to Ms. Anna Reusche at One Glenwood Avenue, Suite 600, Raleigh, 27603 or email
[e-mail]. If you have any questions, please contact me at [phone].
Sincerely,
1� V, �V'Io�
Anna Reusche
Environmental Project Manager
3

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