20100251 Ver 1_Staff Comments_20100421

10-0251 - Harnett Health Hospitals, Harnett Co.
RDP and AML Comments
April 21, 2010 (RDP) and May 2, 2010 (AML)
Brightwater Biotechnology and Medical Center:
Please clarify which parts of the proposed development are Phase I and Phase 11
on a full-sized plan sheet that shows surface waters, topographic lines and all
anticipated impervious surfaces.
2. You must provide the Required items Checklist associated with each of the Wet
Detention Basin Supplement Forms and all required items listed therein at the
correct scales. The checklist includes a signed and notarized Operation and
Maintenance Agreement for the wet detention ponds as well as many other items.
If all required items are not included in the next submittal, your application
will be returned.
Please provide a table of elevations, areas, incremental volumes and accumulated
volumes for the ponds and forebays. This must include rows for key elevations
(bottom, sediment cleanout, permanent pool, temporary pool, etc.).
4. Please show the following items on the pond profiles:
a. The forebay invert, ponds invert and sediment clean-out elevations.
b. The correct side slopes: 10:1 for the vegetated shelf and 3:1 elsewhere.
The permanent and temporary pond elevations as well as any other
relevant storage elevations.
d. The size, length and slope of the outlet barrel.
The average depths provided are less than the required minimum of 3 feet. This
depth cannot include the F of sediment storage. Please revise the ponds to
provide the minimum depth or more. Please correct the errors in the calculation
of average depth in your resubmittal.
6. The forebay volumes must comprise 10 percent of the total volume of the BMP.
None of the four ponds have an appropriate forebay volume.
7. Planting plans for the ponds must be provided on the plans as per 10.3.7 of the
BMP manual.
8. Please provide the drawdown calculations for each pond.

9. Please provide a soils test based on in-field borings conducted in the footprint of
the proposed wet detention ponds to document the SHWT elevation. Liners may
be required if it is well below the permanent pool elevations. See also 10.3.2 of
the BMP manual.
10. Please show details of all the pond outlets and associated energy dissipation
devices.
11. Per the BMP Manual, the emergency spillway must be hardened with concrete,
rip rap or other appropriate material.
12. Please be sure that any changes are consistently reflected on all documents (plans,
calculations, supplement forms, etc.)
13. For the Phase II portion of the site, you must provide a conceptual stormwater
management plan that includes the following:
a. A depiction of the drainage areas on the site.
b. An estimation of the percentage imperviousness associated with each of
the drainage areas.
'c. A calculation of the water quality volume associated with each drainage
area.
d. A depiction of the proposed BMP (state the type of BMP you plan to use)
on the site plan that is correctly sized based on the water quality volume.
Harnett Health Hospitals:
1. Please address the following issues with Wet Detention Basin #2:
a. The pond appears to be short circuiting. Please shift the outlet location to
lengthen the flow path length to width ratio. A ratio of at least 3:1 (L:W)
is recommended.
b. The 18" pipe discharging into the pond needs to enter the pond via a
forebay.
c. Sizing of the pond must take into account the build-out of the offsite
drainage area to the north. Another option would be to bypass that area
around the pond, and treat it separately if it is ever developed.
2. Please address the following issues with Wet Detention Basins #1 and #2:
a
The double 6-inch diameter orifices appear to draw down in less than 2
days. Also, please provide drawdown calculations that show that the
minimum required treatment volume draws down in 2 to 5 days.
b. Please provide calculations that show how the treatment volume was
calculated using the Simple Method.
The stage storage output provided does not provide all the information
needed. Please provide a table of elevations, areas, incremental volumes
and accumulated volumes for the overall pond and forebay so that we can
verify volumes, areas, and elevations provided on the plans and
supplement forms. This must include rows for key elevations (bottom,
sediment cleanout, permanent pool, temporary pool, etc.).
d. On page 3 of the O&M Agreements, the elevations for when sediment is
to be removed are incorrect.
e. The average depths provided are less than the required minimum of 3 feet.
This depth cannot include the F of sediment storage. Please revise the
ponds to provide the minimum depth or more.
f. Please provide the SHWT elevations. A liner may be required if it is well
below the permanent pool elevation. See also 10.3.2 of the BMP Manual.
g. Planting plans for the ponds must be provided on the plans as per 10.3.7 of
the BMP Manual.
h. Please provide outlet details on the plans showing orifices, weirs, trash
rack, etc.
i. We recommend adding an emergency spillway in the event the outlet
structure gets clogged. That way the flow is released in a controlled
manner.
The outlet structures should be moved closer to the bank of the ponds to
allow for easier inspection and maintenance of the outlets.
3. Please be sure that any changes are consistently reflected on all documents (plans,
calcs, supplement forms, O&M forms, etc.)