NCS000508_SCM Restoration Plan_20180607

MEMORANDUM
CH2M HILL NORTH CAROLINA, INC. 1
Restoration of Stormwater Management Basins
PREPARED FOR: Piedmont Triad Airport Authority
(PTAA)

COPY TO: Recipients at the discretion of PTAA
PREPARED BY: Matt Wright, P.E.
DATE: June 7, 2018
CH2M Hill, Inc. is currently working with the Piedmont Triad Airport Authority to develop airport
property north of Runway 5L/23R. In conjunction with this project, the Authority requested a review of
existing stormwater management basins that were constructed to satisfy water quality requirements at
the time the Runway was built. Current legislation may allow for the removal of these basins. This
technical memo will summarize CH2M’s understanding of the reason the basins were installed initially,
and present a case for their removal under current water quality requirements.
Existing Stormwater Management Basins
The Basin Location Overview exhibit attached shows thirteen (13) basins that were installed during the
construction of Runway 5L/23R at Piedmont Triad International Airport. The United States Army Corps
of Engineers issued Permit No. 200021655 in the early 2000’s for the construction of Runway 5L/23R. A
Section 401 water quality certification (#00-0846) was issued by the NC Division of Water Quality on
November 10, 2003.
The USACE permit allowed for the discharge of fill into jurisdictional waters of Brush Creek and its
unnamed tributaries for the runway construction. The water quality certification called for the Authority
to submit a Stormwater Management plan that satisfied the requirements of the certification.
It appears that the design engineers satisfied these requirements through the creation of these 13
basins. All of these basins are located in close proximity to the runway and parallel taxiway and receive
runoff solely from the runway and taxiway areas.
These basins have been in place and functioning as intended. They are maintained and inspected
annually in accordance with the certification requirements.
Justification to Remove Stormwater Management Basins
There is a serious safety concern with these basins being so close to an active runway. Even extended
dry detention basins present a wildlife attractant which can present a threat to aircraft operations on
the runway. These 13 basins in question have been witnessed by airport staff to routinely attract
waterfowl, among other wildlife.
In light of the wildlife hazard, the General Assembly of North Carolina passed Session Law 2012-200 to
accept alternative measures of stormwater control at public airports. This legislation amended General
Statute 143-214.7 to prohibit DEQ from requiring airports, as well as other development projects
located within five miles from an airport air operations area, to use stormwater control measures that
promote standing water. (N.C.G.S. 143-214.7 (c3)) The statute permits replacement of these measures
with alternatives addressed in the Division of Water Quality’s Best Management Practice manual
chapter on airports.
RESTORATION OF STORMWATER MANAGEMENT BASINS
2 CH2M HILL NORTH CAROLINA, INC.
Additionally, N.C.G.S. 143-214.7 (c4) states that runways, taxiways, and other areas that provide for
overland stormwater flow that promote infiltration and treatment of stormwater into grassed buffers,
shoulders, and grass swales shall be deemed to be permitted pursuant to the State post-construction
stormwater requirements.
In 2017 the General Assembly amended both of these provisions so that the alternative measures
referenced in the statutes would also meet the requirements of local water supply watershed
regulations.
It is our understanding that based on studies referenced in the Division of Water Quality’s BMP manual
runways and taxiways create very little TSS while providing large disconnected spaces and vegetated
areas. We believe that the current BMP Manual and statutory changes referenced above justify the
removal of the 13 basins identified in this memo.
Water quality treatment would still be provided for runoff from the Runway and Taxiways through the
following measures:
1. Disconnected Impervious Surfaces (C-10, NCDEQ Stormwater BMP Manual, 15A NCAC
02H.1060)
A disconnected impervious surface is identified as a vegetated surface that is a
minimum of 10 feet wide with a maximum slope of 8%. This surface can treat up to 100’
of adjacent impervious sheet flow. Additionally, the vegetated receiving area shall not
contain any built-upon area except for incidental areas such as utility boxes, signs, and
lamp posts.

In the case of Runway 5L/23R there will be as much as 110’ of paved surfaces, however
the width of the disconnected surfaces on each side of the runway exceed the minimum
by over a factor of 17, with 175 feet. Additionally, the slopes of these disconnected
surfaces are much flatter than the 8% maximum, with an actual slope of 3%.

This vegetated area is designated as a “safety area” by FAA design standards (FAA
Advisory Circular 150/5300-13A), and also prohibits any built-upon area, with the
exception of edge lights and signs which comply with 15A NCAC 02H.1060 (2)(c).

Lastly the vegetated receiving area complies with 15A NCAC 02H.1060 (3) with respect
to pH compaction, grass species planted, and mulching during planting.

2. Treatment Swales (C-11, NCDEQ Stormwater BMP Manual, 15A NCAC 02H.1061)
A treatment swale is a grassed swale with side slopes of 3:1 or less and a longitudinal
slope not more than 7%.
In the case of Runway 5L/23R, surface water outside of the disconnected impervious
surfaces is currently conveyed in swales with 4:1 side slopes and longitudinal slopes
generally not exceeding 1.5%.

The treatment swales will also meet the requirements of 15A NCAC 02H.1061 (4) and (5)
which specify the grass species required, maintenance of swales, and requirement to
non-erosively pass the ten-year storm.

It is important to note that the removal of these basins would have no impact on any jurisdictional
waters.
RESTORATION OF STORMWATER MANAGEMENT BASINS
CH2M HILL NORTH CAROLINA, INC. 3
Approach to Remove Stormwater Management Basins
The removal of the 13 basins identified in the attached exhibit will be achieved through the following
steps:
• Identify fill material that is clean and free of any pollutants except in trace quantities. Metal
product, organic materials, or unsightly debris will not be used pursuant to USACE Permit
200021655
• Create plans to re-grade and re-vegetate basins to be flush with existing ground elevations.
• Maintain existing outfalls
• Provide appropriate sedimentation and erosion control practices that meet or exceed the
requirements of the NC Sediment & Erosion Control Planning and Design Manual, latest
edition.
• Apply for a land disturbance permit through NCDEQ
• Fill in basins
• Seed and mulch basins in accordance with NCDEQ BMP manual
• Establish and maintain permanent ground cover
If you have any further questions or concerns please do not hesitate to contact our office.
Sincerely,
CH2M Hill North Carolina, Inc.

Matthew R. Wright, P.E.
Project Manager
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