HomeMy WebLinkAboutWPO201100059 Review Comments Erosion Control Plan 2012-04-12ALg�,��
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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: Stoneiield Towncenter SWM, ESC, and Minor Site Plan Amendment;
SDP - 2012 -00005 and WPO- 2011 -00059
Plan preparer: Mr. Herb White, PE; W & W Associates
Owner or rep.: Albemarle Place EAAP LLC
Date received: 28 December 2011
(Rev. 1) 26 March 2012
Date of Comment: 22 February 2012
(Rev. 1) 12 April 2012
Engineer: Phil Custer
The first revision to the minor site plan and stormwater management amendments for Stoneiield Town
Center (SDP- 2012 -00001 and WPO- 2011 - 00059), received on 26 March 2012, have been reviewed.
These plans can be approved after the comments below have been addressed. The amended ESC sheets
are hereby approved, though please refer to comment A.1.
B. Stormwater Management Amendment Review Comments (WPO- 2011 - 00059)
1. Please refer to the separate SWM memo provided at the same time as this comment letter.
(Rev. 1) The stormwater management concept for total project has been amended to make this
comment unnecessary.
2. The new biofilter is considerably undersized for watershed proposed to it. By my estimation there
is about 31 acres draining to structure 39.1 and the offline pipe system (please see following
comment as well). About 20 acres of this watershed is onsite drainage @ 90% impervious. The
remaining 11 acres is offsite area at 50% impervious. In total, there will be about 23.5 acres of
impervious surfaces in this facility's watershed which requires a biofilter bed of about 25,000sf to
treat at a removal rate of 50 %. The current bed area of 9,720sf is not sufficient. The biofilter bed
will need to almost triple.
(Rev. 1) Biofilter has been removed from the plan. This comment is no longer necessary.
3. Biorention facilities, as well as most BMPs in the Virginia Stormwater Management Handbook,
are volume -based facilities and achieve their specific removal rates by capturing the first flush
(1/2" or I", depending on RR) of storm events. The full first flush volumes cannot be caught with
current configurations of structures 39.1 and 46.1. Please design the pipe system to the northern
biofilter so that the full flow can pass through it without diverting untreated.
(Rev. 1) Biofilter has been removed from the plan. This comment is no longer necessary.
4. Sediment forebays are required on all SWM quality facilities. Biofilter forebays should be
between 10% and 20% of the biofilter floor area.
(Rev. 1) Biofilter has been removed from the plan. This comment is no longer necessary.
5. Please provide details of the retaining wall in this set. The set of details should include a typical
cross - section and specific details for the pipe outlet through the wall.
(Rev. 1) Biofilter has been removed from the plan. This comment is no longer necessary.
6. The wall must be watertight up to the 100 -year storm elevation. No water should be able to escape
M
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from the biofilter, above or below the bed material, through the drain beneath the wall's footer.
Please compare the 100 -year storm discharge coming into the facility with the stage /discharge
graph of DI -7 inlets in the Road Drainage Manual.
(Rev. 1) Biofilter has been removed from the plan. This comment is no longer necessary.
A planting plan for the biofilter is needed. The biofilter should have at least one plant for every
100sf of bed area and should have a variety of species of trees and shrubs.
(Rev. 1) Biofilter has been removed from the plan. This comment is no longer necessary.
This application will require that a new Stormwater Management Facility Maintenance Agreement
be recorded. Please complete this form and submit it to Ana Kilmer with a $17 recordation fee
after reading the instructions online. The applicant should wait until they are sure they are going
forward with this application to record this agreement.
(Rev. 1) Comment remains unchanged.
A revised bond amount will be needed before the plan can be enacted. Please provide a completed
Bond Estimate Request Form to the county engineer to receive a new SWM bond.
(Rev. 1) Comment remains unchanged. This will be evaluated after the revision is provided to
address the following comment.
(Rev. 1) The stormwater plan has been revised since the initial submittal to address county
concerns with the removal of supplemental swm measures from the southern watershed. Those
concerns were detailed in a separate stormwater memo dated 22 February 2012. One of the
solutions mentioned in that memo was to remove the northern biofilter and revert back to the
previous SWM concept plan, taking into account for the revised grading and drainage plan. In
the meeting to discuss county comments held a month ago, this was the direction decided by the
applicant. After comparing the approved plans and the most recent proposal, it seems the latest
plan is short. Please refer to the table below:
So, an additional 0.91 acres must be treated by supplemental measures in order to be equivalent
to the previously approved plan. On top of that, county engineering will not give credit to the
Initial Approval
I Latest Proposal
Effect
Hotel
unchanged
0
The Haven
unchanged
0
Towncenter (N of Bond St.)
unchanged
0
Cinema Lot (W of District)
1.76acre @74% with Filterras
2.16acre @65% with biofilter
0
Cinema
(Filterras- acres)
54.1
0.15
54.1
0.15
54.2
0.09
54.2
0.09
69
0.17
-
0
70
0.15
0
total
0.56
total
0.24
-0.32
Towncenter (S of District)
(Filterras- acres)
54.5
0.25
54.5
0.25
81.3
0.25
81.3
0.25
79.2
0.56
79.2
0.47
81.1
0.38
79.3
0.29
81.2
0.38
81.4
0.4
82.1
0.4
82.1
0.37
82.2
0.4
-
0
total
2.62
total
2.03
-0.59
total
-0.91
So, an additional 0.91 acres must be treated by supplemental measures in order to be equivalent
to the previously approved plan. On top of that, county engineering will not give credit to the
drainage area for 79.2 without an adjustment to the grading around it. Currently, the grading
plan does not direct water to this facility. If the grading cannot be adjusted to make this work,
a 4ft wide, face -of -curb to face -of -curb, concrete island will be needed from structure 80 to the
grassed island around 79.2. If neither option works for the applicant, 1.38acres must be treated
by another measure.
An updated Filterra approval letter will also be necessary.
Please provide an updated "SWM Treatment Areas Exhibit" when the plan is resubmitted
A. Minor Site Plan Amendment Review Comments (SDP- 2012 - 00001)
1. An ESC amendment will need to be provided to keep the ESC plan updated with the current
proposed changes. None of these changes will drastically change the ESC plan. Please keep the
same ESC concept and submit revised sheets 25 and 26 and $180 fee at the time of the next
submittal.
(Rev. 1) Amended ESC sheets are consistent with previous approved concept. These amended
sheets will be provided to the ESC inspector once the site plan is approved. Please update as
necessary after addressing comment B.10.
2. Street furniture must be shown along District Ave.
(Rev. 1) Comment has been addressed.
File: E2_mia esc swm_PBC _ sdp- 2012 -00001 wpo- 2011 -00059 Town Center Amendment.doc