20140843 Ver 1_More Info Received_20150712

DocuSign Envelope ID: E46EA78C- 7049- 49C0- 964D- 6BFFB4820504
Wanda H. Austin, PE
Engineering Services
828 508 -2639
828 342 -5079
July 12, 2015
RE: DWR #14 -0843
401 & Permitting Unit
Attn: Ms. Karen Higgins
1617 Mail Service Center
Raleigh, NC 27699 -1617
Dear Ms. Higgins:
45 Homespun Road
Sylva, North Carolina 28779
Mr. Russell McLean has requested I respond to the question in your correspondence
dated June 26, 2015.
Question 1
A. All the stormwater features of the plan dated May 13, 2015 are designed for
the runoff volumes using the Simple Method described in Chapter 3 of the
NCDENR Stormwater BMP Manual. The basin should not overflow at this
design runoff volume. It is not economical to size the basin for a greater
storm event than the guidelines provided in the BMP Manual due to the
property constraints and our goal to preserve as much of the wetland as
possible. Wetland mitigation and minimizing the impacts is important to Mr.
Harmon.
B. The area in which the stormwater measures will be constructed is a
proposed fill area. In -situ soil analysis is not feasible; as the fill has not been
constructed. Engineering judgment has been applied to the available
information provided by Altamont Environmental, Inc. and ECS Carolinas,
LLP. Appendix G on page 38 of the report provided by Altamont
Environmental includes soil information for 5 soil borings. ECS Carolinas in
their WAM report dated December 11, 2014 confirmed the finding of the
earlier soil report. Note 2 on plan sheet SW2 states that the contractor
should test the media after placement for drainage rate.
C. The Ksat for the soil DsB Dillsboro Loam is in the range of 0.57 to 1.98 in /hr.
This range does meet the required 1 to 6 in /hr and 1 -2 in /hr preferred. Site
specific Ksat will be determined once the soil is in place as required in note 2
on plan sheet SW2. As a side note the Bioretention Cell Supplement
spreadsheet provided on the BMP website does indicate the value of 0.75 is
ok. If this indicator had been false, the design would have been modified to
correct any deficiency.
D. The manual actually states "An underdrain shall typically be installed...."
page 12 -3 item 18. The interpretation of "typically" is not required. Should
DWR wish to add this requirement to the permit approval, Mr. Harmon will be
able to accommodate this requirement.

DocuSign Envelope ID: E46EA78C- 7049- 49C0- 964D- 6BFFB4820504
Harmon Graham Jule Noland Road 2
E. Figure 12 -4 of the BMP manual was used as a guide in the design of the
bioretention cell. This figure shows grassed side slopes with a mulch bottom.
The plan can accommodate an all grassed cell or a grass side slope /mulch
bottom cell as indicated in figure 12 -4. This clarification should be addressed
in the permit approval for the contractor.
F. The requirements of the BMP manual on page 12 -2 item 4, states either a
bypass or an internal overflow is required for bypassing storm flows in excess
of the design flow for the bioretention cell. The vegetative filter strip is used
to meet this requirement.
Question 2
A. The dry detention basin is not intended to be the sole measure for the removal
credit of the design flow. As noted on the plan sheet SW2, natural conveyance
into the wetland will also be utilized to remove the sediment and to sustain the
vitality of the wetland. The volume provided by the basin; 2007 cf, as well as
the volume of the 2.2 acre wetland is adequate for the minimal required volume
as indicated on plan sheet SW3 in the Dry Detention Basin table located in the
center of the sheet. Also indicated in this table is the required 25% sediment
storage. The addition of 945 cf (25% sediment storage) to 3779 cf (minimal
required storage) equals 4724 cf. The one foot required freeboard is provided
from the top of the 3x3 box to the top elevation of the basin as indicated in the
Basin Cross Section Detail on SW2. Note the elevation of the parking lot is 2582
at the curb outlet and the top elevation of the basin is 2581.
B. Detail added to SW3
Question 3
A. See response above.
Question 4
A. See response of question 1 A -F
B. See response of question 1 A -F
C. See response of question 1 A -F
D. See response of question 1 A -F
E. See response of question 1 A -F
F. The storm water will sheet flow across the parking lot to the bioretetion cell
which is at elevation 2583. The bottom of the weir structure is at elevation 2584
as indicated in the profile view and by the elevations on the plan sheet. This
would make the top elevation of the weir at elevation 2589. The inlet bottom
elevation according to the weir diagram on SW 3 is 18" below the top of the
structure or elevation 2587.5. Overflow water will back into the overflow weir
along the edge of the parking lot and flow into overflow ditch into the level
spreader which is at elevation 2583 according to the plan. The only ditch on the
southside of the cell is the overflow ditch which has an west -east directional
arrow. One foot elevation contours are provided on the plan. Smaller elevation
contours will make the plan difficult to read. The overflow ditch detail is
provided on SW3.
G. As stated above, the weir is at elevation 2584; one foot above the cell top
elevation of 2583; with the inlet elevation at 2587.5. The contour lines in the
parking lot are directed towards the northern portion of the parking lot. Storm
water will be directed more to the north of the cell as indicated by the contours

DocuSign Envelope ID: E46EA78C- 7049- 49C0- 964D- 6BFFB4820504
Harmon Graham Jule Noland Road
instead of the south end of the cell, ensuring water will not bypass the cell into the
emergency overflow.
Question 5.
A. Soil bore information is found in Appendix G page 38 of the report by Altamont.
In these reports, the water table is noted to be a specific distance below the
existing elevations. Therefore water table elevations can be found by subtracting
this distance at the bore location indicated in Appendix G from the existing
known elevations.
B. All areas are calculated automatically from the AutoCad measuring tool. The
shapes of the basins do not lend themselves to conventional measuring practices.
C. Drawdown calculations are provided on SW3 at the center bottom of the page.
D. The Basin Cross Section Detail; which contains the outlet detail in the same box;
is a generic basin detail that describe all basins on the plan. This particular plan
only contains two basin in drainage area 2 and 3. The detail; therefore, would
pertain to both basins. The plan clearly indicates a 15" culvert in both detail
drawing for the dry detention basins in drainage area 2 and 3 on SW2.
The additional drawing for the 24" culvert is attached.
Sincerely
DocuSignell 6y:
gri$ep 2z l��rstin, PE CPM