20140843 Ver 1_Draft response_20150617

June 17, 2015
We have listed the concerns that we have with the stormwater plan by each Stormwater Control Measure
(SCM).
1. SCM for Drainage Area #1 — Bioretention Basin 1
a. The stormwater will overflow the berm at lamer storm events. The contour lines around
Bioretention cell 91 show the top of the bioretention cell walls at 2580' elevation. Since
the drawing of the emergency overflow structure on SW 2 shows the "emergency
overflow elevation" at 2581', the stormwater level would rise above the walls of the
bioretention cell and would overflow before the stormwater reached the elevation of the
emergency overflow weir. The structural integrity of the cell would be compromised and
you could expect a breach in the berm in the future.
b. There is no documentation on the K -sat rates of the media in the bioretention cell nor the
insitu soils beneath the added media. The `Bioretention Cell Supplement" sheets for
drainage area 91 indicates that the "Planting media soil" and the "In -situ soil" soil
conductivity tests show a soil permeability of 0.75 " /hr. It certainly seems unusual that
the purchased media would be the same infiltration rate as the existing soils. We did not
see any documentation on the infiltration tests for the in -situ soils as is specified in our
"Required Item Checklist" for a bioretention cell (httD: / /Dortal.ncdenr.ora /web /Ir /bmb-
manual). It specifies that "A soils report that is based upon an actual field investigation"
is required. It also specifies that "County soils reports are not an acceptable form of soils
information ". Please provide us with soils testing data supporting the in -situ K -Sat
numbers included in the plan. Also, please reconsider the media permeability numbers
and check with the supplier to see if a more - porous mixture is available.
c. The 0.75 " /hr K -sat rate does not meet the criteria specified in the Stormwater BMP
Manual ( "The Manual "). The Manual provides in the "Major Design Elements" for
bioretention cells on page 12 -2 that "Media permeability of 1 -6 in/hr is required, 1 -2 in/hr
is preferred ".
d. The Stormwater BMP Manual specifies that underdrains are needed for bioretention cells
in soils such as those found on this site at the 0.75 " /hr infiltration rate. Chapter 12 of the
Manual indicates that "An underdrain shall typically be installed if in -situ soil drainage is
less than 2 in/hr." This was added because of the drainage problems experienced in areas
with marginal soils. It's a very unusual situation where underdrains are not a part of the
design for bioretention cells in the western part of the state.
Parts of the plan indicate that sod will be used as the cover for the bioretntetion cell and
Parts show that hardwood mulch will be used. Drawing SW 2, under `Bioretention
Notes ", indicates that the surface would be covered with" Fescue or Bluegrass sod."
However, those same Notes also provide that "only triple shredded hardwood mulch"
should be used. The `Bioretention Cross Section" on that page indicates that there will
be "24 inches Hardwood Mulch" on the surface of the bioretention cell. The

June 17, 2015
Bioretention Cell Supplement shows "depth of mulch" at 4 inches but also indicates a
"Y" (yes) for the question "Is this a grassed cell ?" If mulch and plants were the plan, a
deeper media and plant specifications would be required. What was the intent of the
designer?
f. Some practices included in the design are not required in the Manual. Although we are
not advising against adding more structural practices than are required to provide greater
treatment, we felt you should be reminded that the Manual on page 12 -2 makes it clear
that "A vegetated filter strip is not required for overflows, bypass flows, or discharges
from a bioretention cell." Your plan includes these on the overflow discharge from
Bioretention Cell 91. These will add considerable additional expense to costs of the
stormwater plan. If this was added to make up for some deficiency in the bioretention
cell design, please let us know. We also observed that the 20' outlet structure for the
outlet weir seems rather large. It would seem the 3x3 box would work. Please explain
why the large weir was chosen.
2. SCM for Drainage Area #2 - Extended Dry Detention Basin 2
a. The basin is undersized to treat the design storm and the calculations were done
improperly. From the engineering drawings provided for basin 92, there is not enough
storage to meet the applicant's calculated "Minimum required volume ". The applicant's
Supplement states that 3779 cu. ft. is the "Minimum required volume." That seems
fairly accurate. However, on that sheet, the applicant indicates that the "volume
provided" is 5279.00 cu.ft. This number is incorrect and the actual "volume provided" is
probably near the 2007 cu. ft. stated as "Incremental Volume" on the "Stage Storage
Volume Calculation Table." This is insufficient to detain the design storm volume of
3779 cu.ft.
The Manual shows in Figure 17 -4 that the Temporary Pool Volume (or water quality
volume) is measured from the "Sediment Removal Elevation" to the "Temporary Pool
Elevation." The available temporary (or water quality) storage should be calculated from
the elevation of the orifice at 2579' up to the elevation of the emergency overflow at
2580. There is only one foot of storage available, not two as it indicated in the plan.
As shown in the BMP Manual (page 17 -2), one foot of freeboard must be provided
above the emergency overflow. The applicant's "Stage Storage Volume Calculation
Table" on sheet SW 3 shows the one foot of freeboard as being "over 2581" feet
elevation. That would seem to indicate that the foot of freeboard is provided above the
top of the 2581' elevation of the berm when actually the one foot of freeboard is
provided above the emergency overflow at 2580'. However, as shown above, if the
orifice is at 2579' and the emergency overflow is at 2580', then there is only one foot of
storage, not two.

June 17, 2015
b. Please provide more details on the pipe to the enm4v dissipator. Please provide
elevation details on the 24" culvert that leads to the energy outlet along with elevations
on the dissipator.
3. SCM for Drainage Area #3 - Extended Dry Detention Basin 3
The basin is also undersized to treat the design storm and the calculations were done
improperly. From the engineering drawings provided for basin 93, there is not enough
storage to meet the applicant's calculated "Minimum required volume ". The applicant's
Supplement states that 2737 cu. ft. is the "Minimum required volume." That seems
fairly accurate. However, on that sheet, the applicant indicates that the "volume
provided" is 5108.00 cu.ft. This number is incorrect and the actual "volume provided" is
probably near the 1806 cu. ft. stated as "Incremental Volume" on the "Stage Storage
Volume Calculation Table." This is insufficient to detain the design storm volume of
2737 cu.ft.
As explained in 92 above, the storage should be calculated from the elevation of the
orifice at 2579' up to the elevation of the emergency overflow at 2580'. There is only
one foot of storage available, not two as it indicated in the plan.
Also as stated in 92 above, the one foot of freeboard must be provided above the
emergency overflow. The applicant's "Stage Storage Volume Calculation Table" on
sheet SW 3 shows the one foot of freeboard as being "over 2581" feet elevation. Again,
that would seem to indicate that the foot of freeboard is provided above the top of the
2581' elevation of the berm when actually the one foot of freeboard is provided above
the emergency overflow at 2580'.
4. SCM for Drainage Area #4 — Stormwater Bioretention 4
a. There is no documentation on the K -sat rates of the media in the bioretention cell nor the
insitu soils beneath the added media. The ` Bioretention Cell Supplement" sheets for
drainage area 94 indicates that the "Planting media soil" and the "In -situ soil" soil
conductivity tests show a soil permeability of 0.75 " /hr. As discussed in 91 above, it
certainly seems unusual that the purchased media would be the same infiltration rate as
the existing soils. Also as in 91 above, we did not see any documentation on the
infiltration tests for the in -situ soils as is specified in our "Required Item Checklist" for a
bioretention cell.
b. The 0.75 " /hr K -sat rate does not meet the criteria specified in the Stormwater BMP
Manual "The Manual ". As discussed in 91 above, the manual provides in the "Major
Design Elements" for bioretention cells on page 12 -2 that "Media permeability of 1 -6
in/hr is required, 1 -2 in/hr is preferred ".

June 17, 2015
The Stormwater BMP Manual specifies that underdrains are needed for bioretention cells
in soils such as those found on this site at the 0.75 " /hr infiltration rate. As in 91 above,
no underdrains are specified in the details for bioretention cell 94.
d. Parts of the plan indicate that sod will be used as the cover for the biorentetion cell and
Darts show that hardwood mulch will be used. As in 91 above, Drawing SW 2, under
"Bioretention Notes ", indicates that the surface would be covered with" Fescue or
Bluegrass sod." However, those same Notes also provide that "only triple shredded
hardwood mulch" should be used. The `Bioretention Cross Section" on that page
indicates that there will be "24 inches Hardwood Mulch" on the surface of the
bioretention cell. The Bioretention Cell Supplement shows "depth of mulch" at 4 inches
but also indicates a "Y" (yes) for the question "Is this a grassed cell ?" If mulch and plants
were the plan, a deeper media and plant specifications would be required.
e. Some practices included in the design are not required in the Manual. Although we are
not advising against adding more structural practices than are required to provide greater
treatment, we felt you should be reminded that the Manual on page 12 -2 makes it clear
that "A vegetated filter strip is not required for overflows, bypass flows, or discharges
from a bioretention cell." Your plan includes these on the overflow discharge from
Bioretention Cell 91. These will add considerable additional expense to the costs of the
stormwater controls. If this was added to make up for some deficiency in the
bioretention cell design, please let us know.
f. Can't determine the destination for stormwater discharge from Bioretention cell 94. On
that portion of the plan, there is a "Bypass weir" designed for emergency overflow as is
shown on SW 3. The drawing does not provide adequate detail, such as its location or
elevation, to enable us to determine how and where the stormwater, high -flow bypass
will leave the bioretention cell. Furthermore, it is not at all clear how the stormwater
will leave the filter strip and travel off the site. Please revise the drawings to make it
clear where stormwater flows from the cell 94 area and eventually leaves the site. There
appears to be a ditch flowing in an east -west direction on the south side of drainage area
94 but it is not clear where it goes. There are no elevation points to enable us to
determine if the stormwater will travel by gravity across the site to its discharge point on
the west side or does it exit the site on the east side? There seems to be a ditch along the
southwest corner of drainage area 94. Is that ditch part of the stormwater discharge
system? Without those specifications on the plan, there is a good chance that the
channels will not be installed correctly.
g. We are unable to document the actual storage capacity of 94 bioretention basin. The
Stage Storage Volume Calculation Table shows a cumulative storage volume of 8980 cu.
ft. at the 2583' elevation. We know this is significantly over the actual storage capacity.
If the bypass weir is in a part of the dam as the "Cross Section Bypass Weir" drawing
seems to indicate, then we know that storage at 2583' elevation is not possible. We see
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June 17, 2015
from the drawing that there is a V -weir shaped rock weir where emergency flow exits
and which must be lower than the 2583' elevation. As we see it, the collected
stormwater will flow through the stone structure and much of it will bypass the intended
infiltration into the bioretention cell. Unfortunately, there are no elevations on the
drawings regarding that weir so it's difficult to document what will happen. However,
with a pervious, stone weir that seems to be 4 -5' in depth, we know that the stormwater
will not be stored at 2583' as is shown on the plan. It will flow to the base of the rock
structure and not be infiltrated as was intended. Please explain the intent of that design.
5. Miscellaneous Issues
a. The "General Notes" on plan sheet SW 1 indicate that cores taken by Altamont
Environmental in drainage areas 92 and 93 show SHWT elevations at 2574.5' and 2575'.
We could not find those references in the February 14, 2014 report. Please help us find
them.
b. Please check all of the dimensions and volumes of the four basins. It is possible that the
drawings are a little off - scale. We only checked basin 94 and according to the drawing,
the surface of the top of the cell seems to be about 4,500 sf not the 5,390 sf as was
entered into the Supplement.
c. As is specified in the "Required Items Checklist ", please provide the calculations for the
Orifice Outlet Drawdown Time as presented in SW 3. What is the K rate provided and
how was it used in the calculations?
d. It is not clear which "basin" the `Basin Cross Section Detail" on SW 2 applies or the
"Outlet Detail" on that same page. It would seem that they are both meant to apply to
the extended dry detention basins but they both drawings have a 15" outlet culvert which
is only specified for the Bioretention Basin.

Strickland, Bev
From: Devane, Boyd
Sent: Wednesday, June 17, 2015 5:33 PM
To: Price, Zan (George); Higgins, Karen
Cc: Fox, Tim; Davidson, Landon; Strickland, Bev
Subject: Draft response on the Jule Noland Drive project # 14 -0843
Attachments: Comments sent to AROv2.docx
Zan, after showing Karen my draft document, she suggested that it might be easier to understand if I described the
problems under the heading of each stormwater control measure (SCM). She acknowledged that there could be some
duplication but to give it a try. I did and I believe that this is a much better presentation. I would appreciate your
comments on this version. Since I'm going to be out tomorrow, I've rushed to get this completed and you may see more
errors than my norm. I will be checking my email tomorrow so if you have questions or comments, just send me an
email or a text at 919 - 931 -8631.