20030989 Ver 2_Staff Comments_20070219

AML Comments on Farmington Woods, 03-0989
February 19, 2007
1. Under Condition Number 4 of GC 3402, the DWQ can require stormwater
management plans for developments that have, or are anticipated to have, 30% or
greater of impervious surface area within an NSW watershed. Furthermore, it is the
DWQ's policy that areas of concentrated development within a project must treat
stormwater runoff. Please provide calculations showing the percent imperviousness
of the drainage areas to each of the proposed dry detention basins. The impervious
surface coverage given in the Diehl & Phillips, PA letter of January 31, 2007 allows
only an average of 1,225 square feet per lot for houses and patios and only 450
square feet per lot for driveways or walks. If these limits are necessary to keep
impervious surface below 30 percent, then each lot will be required to have
impervious surface limits formalized in a deed restriction.
2. If any of the drainage areas to the proposed dry detention basins exceed 30 percent
imperviousness, then the DWQ will require those drainage areas be treated by
stormwater BMPs appropriate for NSW watersheds in accordance with GC 3402.
This means that the dry detention basin will need to be replaced by either an
extended detention stormwater wetland, a bioretention area, or a wet detention pond
followed by a level spreader and forested filter strip.
3. Please show detailed plans and profiles for all proposed stormwater management
devices.
4. Please submit worksheets for all dry detention basins that are proposed on the site.
The worksheet forms can be found on the DWQ's web site at:
h~://h2o.enr.state.nc.us/ncwetlands/dr de~pdf.
5. The slopes in the vicinity of the scour holes proposed at the outlets of dry detention
basins 5 and 6 are too steep for this BMP. Please replace these scour holes with
properly designed level spreaders.
6. The slopes in the vicinity of the level spreaders proposed at the outlets of dry
detention basins 7, 8 and 9 are too steep for a single level spreader. Please either
place level spreaders in series to achieve diffuse flow or provide a properly
designed stormwater management facility to control nitrogen and attenuate flow
before stormwater is discharged.
7. The existing buffer conditions for the level spreaders proposed at the outlets of dry
detention basins 7 and 9 appear to be wooded, requiring 65 feet of level spreader
length per cfs of flow. No photo of existing conditions has been included for level
spreader 8. The DWQ cannot complete its review of level spreader 8 without a
photo showing the existing condition of the buffer.