20120747 Ver 1_DevilsRC_95021_SMUDiscussion_2017-04-20_20170511

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WILDLANDS
E N G I N E E R I N G
April 20, 2017
Mr. Todd Tugwell
US Army Corps of Engineers
Regulatory Division, Wilmington District
3331 Heritage Trade Drive, Suite 105
Wake Forest, NC 27587
RE: Devil's Racetrack Mitigation Site — SMU & WMU Status
Contract Number 003989; RFP Number 16-003624
DMS Project No. 95021
Neuse River Basin Cataloging Unit 03020201; Johnston County, NC
Dear Mr. Tugwell:
This letter follows up on the discussion at the IRT credit release meeting related to the increase in SMUs
& WMUs from the Mitigation Plan to the monitoring year 1 report. The IRT -approved Mitigation Plan
for Devil's Racetrack identified 18,216 SMUs and 55.2 WMU's. The Mitigation Plan stated the following.
"Mitigation credits presented in Table 11 are projections based upon site design. Upon
completion of site construction the project components and credits data will be revised
to be consistent with the as -built condition."
During monitoring year one several errors were discovered in the as -built SMU credit calculations.
These errors were corrected in the monitoring year one report therefore the year one numbers better
represent the as -built condition SMUs. The approved monitoring year one report identified 18,381
SMUs on the project site, based on the as -built thalweg survey. This represented a net increase of 165
SMUs or 0.9% of the project from the Mitigation Plan credits reported at 18,216 SMUs. The additional
length (and therefore mitigation credit) was gained due to small differences between final construction
lengths and design lengths on most of the project reaches. There were no changes in design approach
or mitigation credit ratios that led to this increase. Specific adjustments in SMUs and WMUs from the
design to as -built phase are detailed below.
Stream alignments had minor design changes between the Mitigation Plan (60% design) and the
final set of construction plans. These design changes were very minor, but deemed necessary
by the designer to ensure appropriate buffer widths were achieved on all streams, and to ensure
streams would be stable.
The method used to construct the stream channels on the west side of the project had some
minor inaccuracies. The contractor on the west side of the project used traditional stakeout
methods which weren't as accurate as the GPS method used on the east side. This difference is
evident when comparing Devil's Racetrack (East) to the streams on the west side of the project
(Devil's Racetrack (West), Southwest Branch, Middle Branch, Southeast Branch, and North
Branch). On Devil's Racetrack (East), the heavy equipment was equipped with GPS equipment
and a 3D model. This method of construction only added 1 linear foot of stream length on a
channel that was designed to be 5,363 linear feet long. On the west side of the project there
Wildlands Engineering, Inc. phone [phone]W. Millbrook Road, #225 Raleigh, NC 27609

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WILDLANDS
E N G I N E E R I N G
were minor variations in length between the design channels and how the channels were
actually constructed.
• Minor construction variations were performed in the field during construction. These
construction changes were deemed necessary for a variety of reasons. A few examples are the
availability of materials in the field to build certain types of structures, avoiding certain features
in the field, and stream stability concerns.
• There was a small section of Southeast Branch that was calculated as a power line easement
during monitoring year one. This was a mistake and should not have been counted as stream
credit because it was an easement break and not a power line easement. This has been
corrected.
• The as -built plans were completed using the design alignment instead of the as -built centerline
alignment. This was done to keep stationing the same when comparing the as -built profile with
the design profile. If the as -built stationing had been used, the riffles and pools would not have
lined up on the profile comparison since most of the channels were built longer in length than
designed. On this project, the as -built plans do not show the true as -built stream lengths.
• WMUs were miscalculated as 55.2 at the Mitigation Plan stage due to a processing error in
Autocad. No wetland areas were added between the Mitigation Plan and as -built report. The
as -built and subsequent monitoring reports show 62.1 WMUs. This was noted in the as -built
report however no IRT coordination was completed since it was just a calculation error. If
needed, we can provide an overlay map comparing Mitigation Plan wetlands to as -built
wetlands showing they are the same areas.
Per IRT request, Wildlands evaluated the as -built credit calculations using the centerline rather than the
thalweg alignment. This centerline evaluation would result in a total of 18,216 SMUs which is the same
as the IRT -approved Mitigation Plan.
Please let me know if additional documentation is needed. Please contact me at 919-851-9986 x102 if
you have any questions.
Sincerely,
John Hutton
Vice President
Wildlands Engineering, Inc. phone [phone]W. Millbrook Road, #225 Raleigh, NC 27609