2023-05-02 Response Letter to NC DWR

May 2, 2023

Mr. Joey Winston
North Carolina Department of Environmental Quality
512 N. Salisbury Street
Raleigh, NC 27604
Subject: Response to Request for Additional Information on 401 Water Quality Certification
Application, Reservoir Drain and Geotechnical Subsurface Investigation,
Lake Lure Dam (RUTHE-003), Rutherford County, North Carolina
Dear Mr. Winston:

SCHNABEL ENGINEERING SOUTH, P.C. is pleased to present responses to your request for additional
information regarding the 401 Water Quality Certification permit application for the proposed Reservoir
Drain Project and geotechnical subsurface investigation at Lake Lure Dam. Listed below are the requests
from your email dated April 4, 2023, followed by our response. These responses have been prepared in
coordination with the Town of Lake Lure and Mr. Gerald Pottern of Mogensen Mitigation Inc (MMI), our
environmental subconsultant and the submitting applicant for the electronic Pre-Construction Notification
for a Nationwide Permit 3 and Preliminary Jurisdictional Determination Request package (PCN-PJD
Package) dated February 6, 2023.

1. Please provide plans, with dimensions, for the following impacts described in the application: the
temporary access road and culverts, the new dam drain gate, and an example accessway for the
geotechnical test borings. The Division requires plans of all proposed impacts to determine the
project’s effects on water quality. [15A NCAC 02H .0502(a)(9); 15A NCAC 02H .0506(b)]

The temporary access road and temporary culvert stream crossing is shown on Figures 5a and
5b from the PCN-PJD Package prepared by MMI. The access road and stream crossing will be
used for both the reservoir drain gate installation and access to select borings. We have included
the reservoir drain construction drawings and design report as Attachments 1 and 2 to this letter
in an effort to provide additional details on the reservoir drain and aforementioned impacts.

2. Please provide a construction sequence and timeline for the installation and removal of the
temporary access road, including a discussion of the measures taken to ensure water quality
standards will be met throughout the process. Please include any necessary details with project
plans. [15A NCAC 02H .0502(a)(4); 15A NCAC 02H .0502(a)(9); 15A NCAC 02H .0506(b)(2)]

The temporary access road and stream crossing will be used during both the reservoir drain
construction and the geotechnical subsurface investigation. It will be removed upon completion
of both of these tasks. The reservoir drain project is currently in the bid phase. We expect the
Town of Lake Lure to issue a notice to proceed no later than June 1, 2023. We expect the
Response to Request for Additional Information on Permit Application
Lake Lure Dam (RUTHE-003) – Reservoir Drain and Geotechnical Subsurface Investigation

Project 18C21024 / May 2, 2023 Page 2 Schnabel Engineering South, P.C.
reservoir drain construction phase to be approximately 6 months long. Installation of the
temporary access road and stream crossing will need to be one of the first major work items
performed by the contractor to establish access to Bay 5 of the existing dam where the rest of the
reservoir drain work will be performed. The geotechnical borings are part of a subsurface
investigation to support the design of a future replacement dam downstream of the existing dam.
We expect the subsurface investigation to being in the third quarter of 2023. The duration of the
investigation field work is expected to be about 3 to 4 months, after which the stream crossing will
be removed.

Construction sequence notes are provided on sheets 10 and 11 of the reservoir drain
construction drawings. Notes 1 and 2 on sheet 4 of the drawings discuss requirements related to
the stream crossing. In general, ground disturbance in the area of the stream crossing will be
minimized, with the majority of the stream crossing being founded on bedrock. The stream
crossing will be constructed of clean, non-erodible materials (washed riprap and stone) placed
over geotextile to minimize impacts to water quality during construction, operation/use, and
removal. Best management practices (e.g., silt fence, turbidity curtain) will be utilized as
necessary in accordance with the North Carolina Erosion and Sediment Control Planning and
Design Manual. Upon removal of the access road, vegetation will be re-established along any
disturbed areas of the river banks, though disturbance is expected to be minimal as indicated
above.

3. Please provide a construction sequence and timeline for the installation of the dam drain gate,
including a discussion of the measures taken to ensure water quality standards will be met
throughout the process. The Division does not allow uncured or curing concrete to come in
contact with surface waters; please include in the discussion measures taken to prevent this.
Please include any necessary details with project plans. [15A NCAC 02H .0502(a)(4); 15A NCAC
02H .0502(a)(9); 15A NCAC 02H .0506(b)(2)]

The estimated project timeline is included in the response to Request 2. Construction sequence
notes are provided on sheets 10 and 11 of the reservoir drain construction drawings.

The Contractor is required to create a surface water diversion around the downstream work area.
See note 2 on sheet 10 of the reservoir drain construction drawings. During the majority of the
construction period, the footprint for the reservoir drain work area will be below the existing
surrounding grade. The current sediment and overburden materials accumulated in Bay 5 will be
removed to exposed bedrock and hauled to an approved off-site disposal facility. Water that
accumulates in the excavation will be removed through a sump pumped through a filter bag to
minimize impacts to water quality. Additional best management practices will be utilized as
necessary in accordance with the North Carolina Erosion and Sediment Control Planning and
Design Manual. Upon completion of construction, the reservoir drain area will primarily consist of
concrete and washed stone placed on either side of the concrete encasement. These materials
are non-erodible and will result in improved quality compared to the existing sediment and
overburden materials currently within Bay 5.

4. Please provide a discussion of how the drain gate will be operated, including the frequency of
discharge, and how it will impact the water quality downstream of the dam. The source of the
Response to Request for Additional Information on Permit Application
Lake Lure Dam (RUTHE-003) – Reservoir Drain and Geotechnical Subsurface Investigation

Project 18C21024 / May 2, 2023 Page 3 Schnabel Engineering South, P.C.
discharge is indicated to be near the lake bottom, where water is naturally colder and has lower
oxygen levels. Please provide reasonable assurance that the gate will not be operated in a way
that will cause a water quality violation. [15A NCAC 02H .0502(a)(4); 15A NCAC 02H .0506(b)(2)]

The invert of the reservoir drain opening on the upstream face of the dam will be at elevation 910
feet (NAVD88), approximately 80 feet below the normal pool elevation of the lake, and the pipe
diameter will be 72 inches. The reservoir drain is sized to pass approximately 400 cfs when fully
open if the lake is drained. This exceeds the average winter baseflow of the watershed which is
approximately 325 cfs. At full pool (80 feet of head), the fully open reservoir drain system will
pass about 930 cfs. At the downstream end, it will discharge through a jet flow gate valve at high
velocity creating a turbulent “shower” effect onto the boulders below prior to flowing into the river
channel, which will help with aeration and minimize impacts to water quality in the event that
lowering the pool is necessary when the lake is stratified.

For the next two to three winter seasons (December through February), the reservoir drain will be
used to lower and maintain the lake level 20 feet below normal pool for planned improvements to
the sanitary sewer system in the lake. The reservoir drain valves will also be exercised annually,
which will consist of limited discharge for a period of less than one hour and could be scheduled
outside the April through October usual stratification period. Otherwise, the reservoir drain will
only be operated in the event of a dam safety emergency.

5. Please provide a discussion of the design choices made regarding turbidity and sedimentation.
Sediment has the potential to accumulate at the base of dams, where the new drain gate is
proposed. Please include in the discussion measures taken to ensure the gate will not release
sediment from the lake bottom when it is operated. If available, please also include any
bathymetric surveys that indicate the sediment depth at the location of the proposed drain gate.
The drain gate is also expected to release high velocity flow when opened. Please include in the
discussion measures taken to prevent erosion and sedimentation on the downstream side of the
gate. [15A NCAC 02H .0502(a)(4); 15A NCAC 02H .0506(b)(2); 15A NCAC 02H .0506(b)(3)]

A survey of the dam and surrounding area was performed in 2019, and the results of that survey
are shown on the reservoir drain construction drawings. The upstream face of the dam and the
lake area immediately upstream of the dam were surveyed using multi-beam sonar scanning
equipment. The data from the scan indicates that the sediment at the center of the arch at Bay 5
is at an elevation of approximately 898 feet, 12 feet below the invert of the reservoir drain
opening. Additionally, the reservoir drain penetration and pipe are oriented perpendicular to the
upstream face of the dam to further limit the potential for any accumulated sediment to be
discharged downstream over the service life of the drain system. Furthermore, portions of the
reservoir drain work on the upstream side of the dam will be performed by divers. Any minor
sediment accumulation on the concrete arches in the area of the reservoir drain will be removed
by divers prior to making the penetration through the dam.

As discussed above, the reservoir drain flows will be discharged onto the downstream, highly
erosion-resistant bedrock resulting in aeration and energy dissipation of the flows prior to their
release into the river channel. Therefore, erosion and sedimentation as a result of reservoir drain
discharges is not expected to be an issue.
Response to Request for Additional Information on Permit Application
Lake Lure Dam (RUTHE-003) – Reservoir Drain and Geotechnical Subsurface Investigation

Project 18C21024 / May 2, 2023 Page 4 Schnabel Engineering South, P.C.
6. Please provide a construction sequence and timeline for the installation and removal of the
temporary accessways for the geotechnical test borings, including a discussion of the measures
taken to ensure water quality standards will be met throughout the process. Please include further
discussion of how wastewater from the drilling operation will be contained and removed from the
site. Please include any necessary details with project plans. [15A NCAC 02H .0502(a)(4); 15A
NCAC 02H .0502(a)(9); 15A NCAC 02H .0506(b)(2)]

We assume “temporary accessways” is in reference to the temporary access road and stream
crossing. Please see response to Request 2 above for discussion on sequence and timeline. In
addition to the temporary access road and stream crossing, installation of temporary stone
working platforms may be required to level the work area for the drill rig at select boring locations.
However, the intent is to minimize the need for these working platforms and adjust boring
locations slightly, as needed, to reduce the footprint of or the need for these working platforms. If
necessary, these working platforms will consist of washed stone placed over geotextile on
foundation bedrock. These working platforms will be removed to restore existing conditions upon
drilling completion. As a result, no negative impacts to water quality are expected.

A specification will be prepared for the subsurface investigation which states requirements for
how the work is to be performed. The specification will include a section on pollution prevention
and control and use of best management practices and will directly address control of drill water.
Drill water will not be directly discharged to any surface waters. Drill water will be contained to
allow settlement of solids, and clarified drill water will be discharged through two or more best
management practices (e.g., filter bag and hay bales). Any drill cuttings will be collected and
disposed of at an approved off-site facility.

Please contact the undersigned if clarification is needed for any aspect of this letter.

Sincerely,

SCHNABEL ENGINEERING SOUTH, P.C.

Jonathan M. Pittman, PE
Project Manager / Principal

BFS:JMP

Attachments:
Attachment 1: Reservoir Drain Construction Drawings
Attachment 2: Reservoir Drain Design Report

cc: Gerald Pottern, MMI
Michael Dydula, Dean Lindsey, and Hank Perkins, Town of Lake Lure
Brandee Boggs, US Army Corps of Engineers
Andrea Leslie, NC Wildlife Resources Commission