20110410 Ver 3_NEI Letter From Town of Wrightsville Beach_20160728

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SEPI
July 28, 2016
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CONt♦NOCiION
Tim Owens, Town Manager
Town of Wrightsville Beach
1025 Wade Avenue
Raleigh, NC 127605
321 Causeway Drive
919.789.9977
Wrightsville Beach, NC 28480
11020 David Taylor
Subject: CAMA Major Permit Application
Drive I Suite 115
Bailey & Associates, Inc., Applicant
Chorion., 8 128262
7oa.71a.asao
Grand View Community Boating Facility
tY 9 Y
202 Summer Rest Road, Wilmington, NC
5030 New Centre
Drive I Suite B
Dear Mr. Owens,
Wilmington, NC
28403 1910.523.5715
SEPI Engineering &Construction (SEPI) has been asked by the Town of Wrightsville
10800 Midlothian
Beach (Town) to provide an engineering review of the proposed marina project,
Turnpike I Suite 100
Grand View Community Boating Facility at 202 Summer Rest Road, with regard to
Richmond, VA
the impact that project will have on the Town's sanitary sewer system. Of specific
23235 1804.594.0181
concern to the Town is the location of the proposed project relative to the Town's
sanitary sewer force main.
The force main was constructed in the early 1980's as part of the Northeast
Interceptor (NEI) project and represents the sole means of conveyance for sewer
generated by the Town to reach the wastewater treatment facility operated by the
Cape Fear Public Utility Authority (CFPUA). The force main is 14 inch Ductile Iron
Pipe (DIP) which is utilized to pump raw sewage at a current maximum rate of 1.3
Million Gallons Per Day (MGD) across the Atlantic Intracoastal Waterway (AIWW) to
CFPUA's Bradley Creek pump station located on Oleander Drive.
The force main crossing the AIWW is located in an easement granted by the State
of North Carolina and is approximately 3 to 4 feet below the mudline along the
easement corridor. The line is anchored with concrete counterweights which are
buried with the pipe at varying intervals. The existing and proposed project is
located on, over and adjacent to this easement and the force main itself.
Additionally, the Town's AIWW force main crossing runs parallel to underground
power and phone to the South. The proposed project is also located over and
adjacent to these utilities.
The force main crossing an active navigable channel, running parallel to other
existing utilities, and being obstructed by an existing pier and dock, presents
several operational challenges to the Town. In our opinion, the proposed project
will increase the difficulties in addressing these existing challenges as well as create
a situation where the operational integrity of the system is reduced.
The following represents a listing of concerns with regard to the information
included in the construction plans and for the impacts to the Town's force main and
utility operations;
1. Existing Condition of the Force Main: The 14 inch Ductile Iron Pipe (DIP)
force Main is over 30 years old and has been in continuous service over that
timeframe. Due to the location of the force main, inspection of the line is difficult
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to perform and therefore the current structural integrity of the pipe is unknown.
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SEPI
The routing of the force main places the pipe in various coastal environmental
conditions including installation below mean low water, within the tidal zone and in
an upland environment. Ductile Iron Pipe can experience corrosion as a result of
the interior and exterior operational environment the material is exposed to.
Raw sewage can generate corrosive chemicals and gasses that can degrade pipe
integrity. It is common to see the production of hydrogen sulfide gas within a
sewer collection system and this gas will collect in areas where a trap is formed and
the gas cannot escape. This occurs in manholes and in high spots along pressure
lines such as a force main. In the case of a force main that may be installed with a
varying vertical elevation it is common practice to install air release valves at
identified high points to vent these gasses. The Town's force main crossing was
installed across the bottom of the AIWW through means of excavation, either
hydraulic or mechanical, to a depth of 3 - 4 feet below the mudline. Due to
localized settlement along the force main route, it is anticipated that areas do exist
within the crossing that could trap hydrogen sulfide gas. Additionally, the force
main transitions in elevation by approximately 6 feet from the east side of the
AIWW to the West side which could also generate conditions suitable for the
trapping of hydrogen sulfide gas. It is in these areas where the traps are located
that deterioration of the DIP would be seen.
External deterioration of the underground DIP due to environmental factors is
another concern for the Town's existing force main. There are numerous factors
that contribute to this deterioration. Soil conditions that favor anaerobic or aerobic
bacteria, soil resistivity, galvanic corrosion of dissimilar metals, corrosion due to
dissimilar soils, and stray current corrosion that may be generated from buried high
voltage power lines are examples of the mechanisms of external corrosion on
ductile iron pipe.
Another source of pipe degradation is physical damage. The location of the force
main, crossing an active navigational channel, presents an opportunity for damage
from boating traffic as well as channel maintenance.
Due to the age, environmental and operational conditions for the force main
crossing, there is an expectation that the DIP has experienced some deterioration.
The extent of this cannot be determined without extensive testing and
investigation. However it is certain that the proposed project will not improve the
operational integrity and it is our opinion that the project represents a threat to the
force main and a hindrance to the ability of the operations staff to maintain the
force main or to respond to emergency situations.
The proposed project includes the installation of piles and the dredging of material
through mechanical means. The installation of piles can impact pipe integrity
through direct or indirect means. Pile installation can result in a pipe breach
through direct contact and also by means of indirect stress being placed on the
existing pipe. Soil consolidation can create changes in the material surrounding the
force main resulting in point stresses.
2. Exposure of the existing force main:
The proposed construction plans developed by Land Management Group (LMG)
Dated January 6, 2016 and provided to SEPI by the Town, indicate the NEI to be
approximately 3 - 4 feet below the "Mud line" near the target area for
dredging. Mechanical (Clamshell) dredging has been identified as the method of
removing material. This method is primarily successful at removing solid material
as the physical properties of silts and mud limit the amount of that material that
can be removed from a subsurface environment. The LMG drawings indicate the
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intent is to lower the existing subsurface elevation by 2 - 4 feet within the target
dredge area. Additionally, the horizontal location of the existing force main to the
target dredge area varies by approximately 7 - 15 feet.
As defined in the LMG drawings, the intent is to lower the bottom or mud line by
approximately 2 - 4 feet in the target dredge area. This can only be accomplished
by undercutting the solid material below the mud by this amount. Once the solid
material is removed, the mud and silt will fill in the void created through an
equalization process. Equalization through sluffing and settlement will also result in
a side profile of approximately 5 to 1. This process could expose the force main
near the dredge area removing any protection from cover that may exist now.
Furthermore, daily operation of the marina will likely subject the force main to
exposure from boating traffic and to an increased danger of damage from prop
wash, vessel strikes, anchors and miscellaneous debris coming from a general
increase in surface activity where limited activity should exist.
3. Existing Conditions Indicated on LMG Construction Drawings: The LMG
Drawings contain information with regard to the location and depth of the 14 inch
DIP force main as reported by Johnnie J. Williams Land Surveying PC. Based on
the information shown, the joint deflection of the 14 inch force main may have
exceeded the allowable deflection angle for restrained joint DIP as defined by the
Ductile Iron Pipe Research Association.
The cause of the joint deflection is unknown however the only influential factor that
has been introduced in this area since the installation of the force main
approximately 30 years ago is the construction of the existing pier and dock. It is
reasonable to expect that the construction of this structure and the increased
surface activity around the structure could have impacted the vertical location of
the force main. Prop wash from surface activity concentrated in a specific area and
changes made to current flows as a result of the piles could have contributed to
settlement of the force main.
The accuracy of the survey is not known therefore it is difficult to know if this
situation represents a need for immediate concern by the Town. However, given
the importance of this connection, the age of the force main, the potential for
continued settlement and the environment in which it is located, further
investigation to accurately determine the condition of the DIP force main is strongly
recommended.
4. Maintenance and Emergency Response: Protection of public health and
safety is the primary operational requirement for the Town of Wrightsville Beach.
The ability of the Town's operational staff and/or Contractors hired by the Town to
have clear unobstructed access to the sanitary sewer system is necessary for the
success of this effort. The Town's easement from the state defines the corridor in
which all operational activities must occur. The existing and proposed projects
have presented several physical restrictions that obstruct access and will restrict
access further in the future.
a. The permit application for the existing dock (State Permit No. 84.01)
indicated that a 50' section of the authorized access pier and floating
dock in the location of the NEI were designed to be bolted for removal
for required maintenance. Based on a visual inspection, there were no
bolted sections observed. Even if this access has been provided, the
process of staging equipment and setting up work platforms to remove
the bolted sections presents a significant delay in the ability of the
Town to access the force main. At a pumping rate of 1.3 MGD, this
delay represents approximately 54,000 gallons per hour of raw sewage
that could potentially be released into the coastal environment.
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b. The permit condition (State Permit No. 84.01) that removable sections
are needed in the pier and floating dock represents a maintenance
scenario that is very specific and unlikely. In the event that the NEI
were to breach, the likelihood that maintenance would only involve
access in this manner is very small, if at all. A breach would require
the town to float a new temporary pipe across the AIWW and across the
marsh, likely for most of the length of the existing floating dock and
access pier. This would require complete removal of the pier, pilings
and dock as these would hinder or obstruct access for equipment and
material. Additionally, the boats that were attached to the pier/dock
would need to be removed. If owners were not available then Town
would have to impound or relocate the vessels. The location of the
temporary and final replacement force main is fixed based on the
easement agreement with the State as well as the existence of the
other utilities in the corridor. The need to remove the pier, piles and
dock will also increase the environmental impact by delaying the Town's
response to this emergency.
5. Planned improvements for a new force main crossing by Wrightsville
Beach: The Town of Wrightsville Beach is currently developing a scope of work
for the replacement of the existing force main crossing the AIWW. The existing
pier and floating dock, as well as the planned project, represents a complication
to this process. The existing and proposed structures are directly impacting the
Town's access to the easement crossing the AIWW. Though the easement
documents do not specifically identify a width to the easement, it does define a
location for a corridor which is between the navigational access for Motts Creek
to the North and an underground/submerged high voltage line to the South. In
practice, easements are defined to provide sufficient width to allow for access,
equipment and manpower to perform the necessary tasks associated with the
intended use of the easement.
The force main is oriented such that the existing pier and floating dock are
directly obstructing horizontal and vertical access for a significant portion of the
route along the western side of the AIWW. This obstruction will likely impact
any proposed force main replacement both through schedule and budget. A
plan to either dismantle or remove the existing (or proposed) structure or to
avoid the structure entirely will need to be developed. Further complications
and impacts should be expected if there is a plan to reconstruct the pier and
dock facility after installation of the new force main is completed.
6. The boundary of the Riparian Corridor and Setback as shown on plans is
incorrect: The construction plans for the planned improvements identifies a
riparian boundary. A letter from Wessell & Raney, L.L.P. addressed to Mr.
Braxton Davis, Mr. Doug Huggett, and Mr. Robb Mairs, dated March 28, 2016,
provides information with regard to the delineation of the riparian area for the
subject property therefore the following comments are intended to clarify the
riparian boundary for the NCDOT.
The boundary line identified on the South side of the proposed project, north of
the USHWY 74-76 Bridge, has been oriented to intersect the navigation at an
angle of 90 degrees. This orientation is incorrect as it ignores and is in conflict
with the northern riparian boundary defined for the NCDOT. Standard practice
for the survey of riparian boundary is to identify or define boundaries such that
the adjoining boundaries are not adversely impacted. The northern NCDOT
riparian boundary originates at the intersection of the northern right-of-way
boundary for US74-76, located 150 feet from the centerline of the roadway as
defined by NCDOT, and the Mean High Water Line on the western side of the
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AIWW. This riparian boundary then runs parallel to the road centerline until it
intersects with the Mean High Waterline on the eastern side of the AIWW. The
riparian buffer along this boundary runs parallel to the northern riparian
boundary at a distance of 15 feet from the boundary.
This boundary is in place to allow NCDOT sufficient room to conduct operation
and maintenance activities for the support of their facilities crossing the AIWW.
Similar to the need for the Town of Wrightsville Beach to keep the easement
crossing the AIWW clear from obstructions and encumbrances that would
prevent the proper maintenance and operation of the force main crossing, it is
necessary for the corridor defined for NCDOT to remain clear. There are
processes in place that would allow for NCDOT to grant permission for an
encroachment into this corridor. However, no information has been found to
indicate that any approval has been granted for the proposed project.
In conclusion, it is the opinion of SEPI that the proposed project will expose the force
main to potential damage or breach due to the increased activity over and around the
force main as well as create an additional burden and encumbrance to the proper
operation and maintenance of the force main crossing. The Town's need to restrict
activity directly over or adjacent to the force main and for the protection of
operational access has a direct relationship to the operational integrity of the sanitary
sewer system and to the ability of the Town to protect the health and safety of the
public.
Additionally, the issues identified based on the force main existing condition data
provided on the Land Management Group drawings potentially represent a significant
problem. The only known physical changes since the installation of the 14 inch
Ductile Iron Pipe force main in the area around the existing pier and dock structure
and the proposed project is the construction and operation of the existing structure.
If the joint deflection issue was created by the construction and ongoing surface
activity around the existing pier and dock it is reasonable to expect the continued use
of the existing facility to exasperate this issue. The continued overstressing of the
pipe joints will ultimately result in a failure. Consideration should be given to the
removal of the existing pier and dock structure in order to improve the current
operational condition of the system.
Respectfully Submitted,
SEPI Engineering & Construction, Inc.
i�+' �/ —
Gregory R Thompson, PE, PLS
Site/Civil Department Manager
Enclosures:
Exhibit A
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DATE:
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RALEIGH, NORTH CAROLINA 704.714.4880
SEPI919.789.9977 5030 NEW CENTRE DR., SUITE B
SCALE: E N G I N E E R I N G & www.sepiengineering.com
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