HomeMy WebLinkAboutWPO201000023 Review Comments Stormwater Management Plan 2010-05-20ALg�,��
�'IRGINZ�
COUNTY OF ALBEMARLE
Department of Community Development
401 McIntire Road
Charlottesville, Virginia 22902 -4596
Phone (434) 296 -5832 Fax (434) 972 -4126
Project: WPO- 2010 - 00023, Albemarle Place
Plan preparer: Mr. John Beirne, Jr., PE and Mr. Herb White, PE; W & W Associates
Owner or rep.: Albemarle Place EAAP LLC
Date received: 5 April 2010
Date of Comment: 20 May 2010
Engineer: Phil Custer
The SWM plans for Albemarle Place, received on 5 April 2010, have been reviewed. The plans cannot be
approved as submitted and will require the following corrections before approval can be granted:
1. The Code of Development states that "to the extent feasible, the basins will also be designed to
retain and treat portions of the untreated stormwater runoff from the adjacent Sperry and Comdial
properties, as well as the upstream drainage area located to the west of the site." Engineering
review agrees with the applicant that it is not feasible to capture, retain, and treat any area draining
to the 60" bypass pipe. However, we feel it is reasonable that the North facility detain and treat
the runoff from the Comdial property and the northern sector of the Sperry Marine site, in addition
to the properties to the west currently being treated. Please update the drainage systems and
calculations accordingly.
2. The modified simple spreadsheets should include all existing and proposed impervious areas
draining to the detention and water quality systems. There should be no separate drainage area
determined for BMP analysis. For instance, the spreadsheet for the interim facility should include
offsite drainage areas 20, 22, and 23.
3. The proposed underground water quality treatment "train" will be given a 60% removal rate if the
entire water quality volume (0.5" over all impervious areas) for each drainage area is routed
through each Vortechs /Stormfilter system. This can be achieved by simply raising the second
orifice to the elevation that correlates to the water quality volume in each pipe detention system.
First instance, in the southern facility, if the orifice on the riser was raised to 455.5, the water
quality volume would be guaranteed to be routed through the stormfilter. If the applicant wishes
to use a routing to show that the orifice can be lowered slightly, the modified rational method must
be used, not SCS, for just this calculation; SCS may remain the method used to determine
detention compliance.
4. An analysis of each downstream channel will be required. The downstream channel analysis
should follow the standards as outlined in the VESCH. In addition to county approval, the City of
Charlottesville will need to approve the downstream channel analysis since nearly all of the
downstream channels are within the city limits. The applicant should work with the City of
Charlottesville independently and, once approval is given, provide the county with proof of city
approval. I would be happy to walk the downstream channel with the applicant if requested. What
kind of condition are the two 42" pipes under Route 29 in? Have either of these lines been
inspected recently?
5. In the note on the cover sheet, please clearly state that this plan does not provide the complete
water quality requirements for Albemarle Place and that future site plans will require additional
BMP's to meet minimum SWM requirements.
6. The point of analysis for determining detention requirements of the interim facility should be its
release point into the stream channel. The release rates of the facility must be no more than the
pre - development rates of area 18 on sheet C -12 in order to meet detention requirements [17 -314].
7. The current proposed volume of the interim pond will adequately treat only 12.97 acres. Please
provide the total impervious area offsite in watersheds 20, 22, and 23. This offsite area should be
subtracted from 12.97 acres to find the total area of imperviousness allowed to be developed
within Block D. This impervious area allowed in Block D should be clearly noted in the plan.
8. Please provide the composite curve number calculations for the post - development drainage area to
each of the three stormwater management facilities so that impervious areas proposed with future
site plans can be checked that they remain under the limits of the approved BMP.
9. The 10 -year 24 -year storm for Albemarle County is 5.6 inches. Hydrographs 5 and 6 of the
Southern Facility uses 5.5 inches. Please update the calculations accordingly.
10. All access points into water quality and detention facilities must meet VDOT Standard SL -1 with
regard to the spacing and staggering of the slabs. Please either detail this on the plan or provide
clear notes on all SWM detail sheets.
11. Access from the surface must be provided into the riser structure in each detention facility for
maintenance and inspection purposes.
12. All pipes from each facility must be at least 15" in diameter. For pipes from the detention
facilities to the Vortechs structures, a plate can be welded on the inlet end to reduce the flow as
necessary.
13. Please specify the design of each trash rack on all orifices within the detention systems.
14. Much more of the Westfield Duplexes are within area 23, as opposed to area 11, than delineated.
Please update the post development drainage area and the calculations after the adjustment has
been made.
15. Approval from the manufacturer of the BMP system must be provided before SWM plan approval.
The approval letter should reference the revision date of the approved plan. The approval letter
should specify a maximum discharge each Stormfilter structure could treat assuming average
operating conditions, as opposed to brand new filters.
16. Prior to plan approval, a Stormwater Facility Maintenance Agreement must be recorded for each
parcel with a proposed Stormwater Facility.
17. A SWM bond will not be required until a grading permit is requested to construct improvement
shown on an approved site plan (which would require an amendment to the ESC plan to show all
the drainage structures and refined grading esc measures). At that time, the project will be required
to post the entire Stormwater Management Bond unless the northern and southern facilities are
broken up into separate stormwater management applications, which is recommended.
File: E1_swm_PBC_wpo- 2010 -00023 Albemarle Place.doc