HomeMy WebLinkAboutWPO201000072 Review Comments Stormwater Management Plan 2011-02-11ALg�,��
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COUNTY OF ALBEMARLE
Department of Community Development
401 McIntire Road
Charlottesville, Virginia 22902 -4596
Phone (434) 296 -5832 Fax (434) 972 -4126
Project:
Plan preparer:
Owner or rep.:
Date received:
Date of Comment:
Engineer:
WPO- 2010 - 00072, Lockwood Townhomes
Mr. Scott Collins, PE; Collins Engineering
Route 29 LLC
25 October 2010
(Rev. 1) 5 January 2011
17 December 2010
(Rev. 1) 11 February 2011
Phil Custer
The first resubmittal of the SWM plan for Lockwood Townhomes, received on 5 January 2011, has been
reviewed. Comments from the review of the site and ESC plans have been provided in a separate
comment letter. The SWM plan can be approved after the following corrections are made:
1. Please removal all notes referencing the Towncenter Drive Phase II Stormwater Management
plans. I could find no records of such a SWM plan being approved. The current application
(WPO- 2010 - 00072) will be the first Stormwater management approval for this facility.
(Rev. 1) Comment has been addressed.
2. Proffer 9D states that the owner shall provide additional stormwater management to achieve a
removal rate 20% better than would otherwise be required by the Water Protection Ordinance up
to an eighty percent (80 %) removal rate for each phase. The modified simple spreadsheet
provided by the applicant was completed for just the development proposed with the current site
plan application (and previously constructed roadways) and not the total impervious area as shown
on the rezoning plan. If this is the applicant's intent, please provide notes throughout the plan
stating that this stormwater application is only acceptable for the post - development condition
proposed with this plan. In this case, a removal rate between 55 % -65% will likely be required.
Therefore, the stormwater facility must be designed as a Type III Retention Basin (4 x WQV with
an aquatic bench).
(Rev. 1) After a detailed review of calculations provided by the applicant for Storm water
Facility 6 and a consultation with the county engineer, engineering staff is concerned that the
facility is too small for the watershed originally planned for it and that if the current plan were
to be approved as proposed, we would be creating future issues with regard to facility expansion
and other supplemental SWM measures. Therefore, the post- development hydrologic, routing,
and water quality calculations must assume full build out conditions. Detention for the]]. 05
acre watershed will likely need to be completely contained within facility 6. Regarding water
quality, the facility should be sized to treat the full impervious load with at least a 50% removal
rate, knowing that future developments (excluding this site plan) will need to provide facilities
with varying removal rates of 50 1%, 65 1%, and 73% to achieve an overall rate of 80 %.
The drainage area limits for the stormwater facility seems to be larger than the 13.92 acres shown
on the plan. At the very least, the drainage area line should be relocated to the center of the roofs
of the existing townhomes on Lockwood Drive. Also, the survey file shows that at Abington Dr.
the drainage system is being diverted into this basin. It appears that a significant portion of the
Abington development is being routed to this facility. Please modify all calculations as necessary.
(Rev. 1) The survey file has been modified and the drainage area has been confirmed.
Comment has been addressed.
4. In the modified simple spreadsheet provided by the applicant on SWM -1, the value entered in the
post - development pasture /grass cell is greater than the total drainage area. Please correct.
(Rev. 1) Comment has been addressed.
5. The retention basin detail correctly shows the construction of an impervious core and cutoff trench
for the embankment. However, the applicant also has provided notes on a few sheets that state the
embankment was not to be disturbed except for grading on the east side. Please clarify. Is the
applicant certifying that the facility was constructed to the detail shown in this plan when it was
constructed two years ago as a sediment basin?
(Rev. 1) On the detail sheet SWM -3, please include in the notation referencing the cutoff trench
and impervious core that you certify this was installed when the basin was first constructed.
Otherwise, provide a note stating that a geotechnical engineer will certify that the embankment
is sufficiently constructed for a permanent SWM facility and provide it to the ESC inspector
before release of the SWM bond.
6. The downstream slope of the embankment for the facility appears to be graded at 2.5:1. The
Virginia Stormwater Management Handbook states that, overall, embankments should be a total of
5:1, with a 3:1 downstream slope. Since this slope has already been constructed and grass
established, please propose the upstream slope to be 2.5:1, so that the total embankment slope is
5:1. Please update the calculations accordingly.
(Rev. 1) Comment has not been addressed.
7. Please move the forebay spillway to the north side of the forebay embankment to increase the
length of the flow path.
(Rev. 1) Comment has been addressed.
8. Please provide riprap channel from the forebay spillway to the water surface elevation of the main
cell of the pond.
(Rev. 1) Comment has been addressed.
9. Please specify the elevation and dimensions of the forebay spillway.
(Rev. 1) Comment has been addressed.
10. The pre - development curve number seems much too high. What year was the aerial photograph
provided by the applicant taken? The 2002 orthophotograph, which I have provided below
(though Meeting Street is mislabeled as Towncenter Drive), shows the majority of the drainage
area as wooded (with —10% as pasture) in B soils. The aerial photograph provided by the
applicant appears to have captured the site in the initial stages of land preparation for the
development. A curve number close to 56 is more appropriate. Please update the calculations
(including time of concentration) accordingly.
(Rev. 1) The predevelopment hydrologic calculations are acceptable. However, please refer to
comment 2.
11. Please confirm that the structural integrity of the riser is not compromised by the four 12" orifices.
(Rev. 1) Comment has been addressed.
12. Please provide the deedbook and page number for the permanent SWM facility easement
referenced on the plan.
(Rev. 1) Comment has not been addressed. Please provide the deedbook and page number for
this easement throughout the plan.
13. A stormwater facility maintenance agreement will need to be recorded prior to plan approval.
(Rev. 1) Comment has not been addressed.
14. A SWM bond will be provided after all technical comments have been addressed and the applicant
has provided a bond request form to the county engineer.
(Rev. 1) Comment remains unchanged.
15. (Rev. 1) The four recently proposed rain gardens do not meet any state standards and no
removal rate credit can be given to them. County staff would not object to the removal of these
facilities for this plan, but notes that future developments need more substantial secondary
facilities to meet the above and beyond water quality proffer.
File: E2_swm_PBC_wpo- 2010 -00072 Lockwood Townhomes.doc