20021883 Ver 1_More Info Received_20040204

Soil & Environmental Consultants, PA
11010 Raven Ridge Road • Raleigh, North Carolina 27614 • Phone: [phone] • Fax: [phone]
www.SandEC.com
January 23, 2003
S&EC Project No. 6761.D9
FEB. 0 4 2004
Ms. Kristen Miguez
NCWRP - NCDENR
P.O. Box 29535
Raleigh, NC 27626-0535
WETLANDS 1401 GROUP
WATER QUALITY SECTION
Reference: Preliminary Total Nitrogen Removal Evaluation
Hatchet's Grove Stream Restoration Project
Wake County, North Carolina
Dear Ms. Miguez:
As requested, Soil & Environmental Consultants, PA (S&EC) has performed a
Preliminary Nitrogen Removal Evaluation for the Storm Water Pipe Network and
Drainage Swale proposed for construction of the referenced project site. The purpose of
this letter is to provide a preliminary indication of potential Total Nitrogen (TN) removal
effectiveness. This level of effectiveness is will allow the North Carolina Wetlands
Restoration Program (NCWRP) to evaluate project funding alternatives.
Project Background
The Hatchet's Grove Stream Restoration Project is located within the Prestonwood Golf
Course in Wake County, North Carolina. Stream restoration plans call for the
construction of a new channel with a more appropriate dimension, pattern, and profile.
While portions of the existing channel alignment will remain (with dimension
modifications) other portions will be backfilled.
On February 2, 2003, Soil & Environmental Consultants, PA, prepared a Preliminary
Storm Water Evaluation for the referenced project for NCWRP. For further detail on site
storm water conditions and background hydrology, the interested reader should refer to
this letter report. Final construction plans for the stream restoration and the proposed
storm water amenities were prepared on July 29, 2003 by Soil & Environmental
Consultants, PA, and submitted to NCWRP for approval.
Currently, plans call for the backfilling of approximately 1000 linear feet of existing
stream channel. During this process, a 24-inch diameter pipe will be installed (along the
general alignment of the existing stream channel) to convey storm water which currently
discharges directly to the existing stream. Along the length of the new pipe, a series of
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Hatchet's Grove Stream Restoration Project S&EC Project No. 6761.D9
Preliminary Total Nitrogen Removal Evaluation January 23, 2004
six catch basins with surface inlets are to be installed to allow for the drainage of ponded
surface water (at graded low points). These inlets will also allow access for necessary
pipe network maintenance. Additionally, existing golf course drains (including PVC and
HDPE pipes) which currently outfall to the existing stream channel will be extended as
necessary to allow for connection to the installed pipe network.
In addition to the installation of the pipe network, a drainage swale will be graded above
the pipe (and generally along the existing stream channel alignment) to allow for capacity
beyond what the pipe will provide. The swale will be graded such that it will be
incorporated into the play area within the golf course. While the Swale will have a rolling
appearance, it, like the pipe network will discharge into an existing pond near the eastern
end of the project site. We understand that this pond is currently used for irrigation
purposes in addition to serving as a site amenity.
At the upstream end of the pipe network, an additional length of pipe (also 24-inch
diameter) is to be installed to connect an existing drainage swale (the eastern swale as
described in our previous letter report) which currently collects runoff from portions of
both the existing apartment complex (The Legends at Preston) and the golf course along
the southern property boundary. A connection swale of similar configuration will be
graded above this pipe, here again to allow for surface flow beyond the capacity of the
installed pipe.
It should be noted that preliminary planning (reference our February 2 2003 letter)
called for the pipe network and drainage swale to receive drainag_e from both the eastern
and western drainage swales which parallel the property line However, based on site
topography constraints current plans call for only drainage from the eastern swale to be
conveyed through thepipe/swale system Approximately 30 acres draining from the
western swale (including a significant portion o The Legends storm water collection
system and local play area) will be drained directly to the new stream channel through a
similarlyQraded swale And while it is likely that some nutrient removal will occur as
storm water is convened through this swale based on its short leng-th (approximately 100
feet its e ect is likely insignificant Accordingdy this swale has been ignored in our
evaluation of Total Nitrogen removal.
Evaluation of Total Nitrogen Removal
Currently storm water runoff from a significant impervious area (street, parking lots,
rooftops, and sidewalks) is discharged (via pipe or overland) directly into the existing
stream channel. Additionally, storm water from a significant portion of the golf course,
which is likely nutrient-rich relative to other storm water sources, drains (either through
pipes or overland) and discharges directly into the stream.
With the installation of the proposed pipe/swale system, a significant portion of runoff
(both off-site and on-site) will be directed to the existing pond. According to the Neuse
River Basin: Model Stormwater Program for Nitrogen Control, dated August 30, 1999,
wet detention ponds are currently considered to remove 25 percent of the TN from storm
water inflow. Open channel practices (ie. Grassed Swales) are considered to have TN
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Hatchet's Grove Strearn Restoration. Project S&EC Project No. 6761.139
Prelirninary Total Nitrogen Removal Evaluation January 23, 2004
removal efficiencies of 30 percent. Similar removal efficiencies are described in City of
Raleigh Storm Water Design Manual and the Wake County Storm Water Design Manual.
Our Total Nitrogen Removal Evaluation focused on removal by three means; the existing
swale (previously described as the eastern swale), the proposed swale/pipe system, and
the existing pond. Each of these "devices" was evaluated based on topographic site
information, field observations, and the proposed construction drawings. Our analyses
evaluated each of these "devices" against design standards as outlined in the BMP
Manual. Assumptions were made regarding lesser removal efficiency if a given device
was determined not to meet necessary design standards. The design storm used in our
calculations was the 1-inch design storm. Runoff volumes associated with the 1-inch
design storm were calculated based on the contributing drainage area.
While not constructed according to any apparent design standard, the eastern swale
appears to serve as a combination of both a storm water wetland and a grassed swale.
Based on a comparison of storm water volume and available swale volume, the heavily
vegetated invert and sides of the swale, and an assumed short detention time, we
calculated a removal efficiency of roughly 1 percent of the incoming TN.
For the proposed swale we evaluated not only the proposed design characteristics
(Longitudinal Slope, Side Slopes, 2-Year Velocity, Non-erosive 10-Year Discharge,
Length, Vegetation, and Stabilization) but also potential "short circuiting" of the system
due to the proposed underlying pipe network. Our evaluation revealed that the swale
design met or exceeded all the necessary design standards to allow for the typical
assumption of 30 percent TN removal. However, based on a comparison of both inlet
and pipe capacity as compared with the expected discharges associated with higher
frequency events, we determined that a removal efficiency of roughly 10 percent was
likely more appropriate for the "device."
Our evaluation of the existing pond showed that the pond surface area and temporary
storage volume (water quality pool) was well in excess of that required for the
contributing drainage area (including the additional area to be drained by the swale/pipe
system). Accordingly we assumed a removal efficiency of 25 percent of the TN from the
incoming storm water. Please note that no specific evaluation o the pond outflow
structure was performed and modifications to the structure to allow for temporary
detention of storm water to meet BMP Manual standards may be necessary.
Efficiency removals are not additive in nature and must me calculated in a serial fashion.
Upon calculation of the combined removal efficiency we find a total removal efficiency
of approximately 33 percent for the combined series of "devices." Using methods
described in the Neuse River Basin: Model Stormwater Program for Nitrogen Control
guidance we calculated a Nitrogen Export Coefficient of 5.061bs./ac./yr. for existing site
conditions (direct discharge into the existing stream channel). With the calculated
reduction of 33 percent, we then calculated a post construction Nitrogen Export
Coefficient of 3.39 lbs./ac./yr. From these coefficients, we calculated an estimated value
of the nitrogen removal based on the proposed storm water design. Using a nitrogen
reduction value of 1.67 lbs./ac./yr. (5.06 lbs./ac./yr. - 3.39 lbs./ac./yr.), a removal value of
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Hatchet's Grove Stream Restoration Project S&EC Project No. 6761.D9
Preliminary Total Nitrogen Removal Evaluation January 23, 2004
$11.00/lb., and a drainage area of 14.51 acres we determine a value of $266.55/yr.
Multiplied over a period of 30 years we find a value of $7,996.46.
While further analyses would be required to fully evaluate the existing pond (assuming it
functions as a wet detention basin) against these assumed treatment efficiency for TN, the
overall quality of storm water leaving the site is likely to be significantly improved with
the installations of the proposed storm water system. Additionally. upon completion of
construction all assumptions re ag rding performance of the pipe/swale system should be
confirmed to validate our assumptions Should the constructed system vary from that
depicted in the construction drawings further evaluation may be necessaa.
Additional Water Quality Benefits
The North Carolina Department of Environment and Natural Resources (NCDENR),
Division of Water Quality (DWQ), Best Management Practices (BMP) manual outlines
design assumptions on pollutant removal of Total Suspended Solids (TSS) for various
BMP's. Based on the BMP manual, a wet detention basin is assumed to have a TSS
removal efficiency of 85 percent while a grassed Swale is assumed to have a TSS removal
efficiency of 35 percent. Accordingly, even without detailed examination, it can be
assumed that the swale/pipe system as proposed will provide a significant reduction in
TSS for the contributing drainage area.
Limitations
It should be remembered that this letter report is preliminary and that our evaluations,
calculations, conclusions, recommendations, and opinions are based on project and site
information available to us at the time of this letter report. Recommendations presented
in this letter may require modification if there are any changes in the project or site
conditions, or if additional data about the project or site becomes available in the future.
Additionally, decisions regarding any outstanding permit issues or policies issued by
governing regulatory agencies, or Prestonwood Golf Course are beyond our control, and
as such modifications to our evaluation, conclusions, and/or recommendations may be
necessary.
Thank you for giving Soil and Environmental Consultants, PA, the opportunity to
propose on this project for NCWRP. We look forward to continued work with you on the
Hatchet's Grove Stream Restoration Project. If you have any questions concerning this
submittal, please do not hesitate to call us.
Sincerely, 0a®1beo0?oA???1
Soil & Environme
Patrick K. Smith,
N.C. License No.
SEAL
/23 n
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