HomeMy WebLinkAboutWPO201100008 Review Comments Erosion Control Plan 2011-06-27ALg�,��
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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: WPO- 2011 - 00008; NGIC ESC Amendment (Sediment Basin 2)
Plan preparer: Mr. Scott Collins, PE; Collins Engineering
Owner or rep.: Next Generation, LLC.
Date received: (Rev. 1) 19 May 2011
10 February 2011
Date of Comment: (Rev. 1) 27 June 2011
25 March 2011
Engineer: Phil Custer
The latest ESC amendment for the NGIC site has been reviewed. The original ESC plan for this project
was approved on February 14th 2008. On July 7t'' of that year, an amendment to the plan was approved
which moved sediment basin 2 to a more natural position within the existing gully immediately uphill of
the headcut of the intermittent stream. At the same time, an amendment to the Stormwater Management
Plan was submitted, and later approved, that proposed a biofilter in the general footprint of sediment basin
2.
After discussions with the County and DCR inspectors for this site, it is my understanding that the current
situation in the field does not match the latest approved plan (July 2008). The county inspector believed
the intention of this ESC plan amendment was to provide an analysis of the as -built condition in the field
to assure compliance with state regulations. After a visit to the site this morning, I noticed that the latest
submittal does not match condition in the field. I also observed the July 2008 plan also didn't match the
as -built situation in at least two areas: the footprint of the constructed facility appeared to be 80 -85% of the
footprint of the approved plan and there is a berm in the middle of the facility further reducing the volume
of the pond. I did not carry any surveying equipment on my site visit so I was not able to confirm the
elevation of the dewatering orifice and crest of the spillway. If the intention of this plan is to prove that the
existing condition is adequate, please provide an as -built survey showing all contours of the pond and of
the riser structure with calculations showing all standards of VESCH 3.14 have been met.
The revised design of this sediment basin appears to match the footprint of the permanent stormwater
facility (approved as an amendment to the original plan) almost exactly (please see comment 3). Since this
work has to ultimately be done, I support the concept to revise the ESC plan and reconstruct this sediment
basin to the newest proposed contours (though, please see the comments to follow). If this is the
(Rev. 1) The applicant has informed me that this latest submittal contains as -built contours of the pond
and an update to the calculations according to what was constructed. The following comments must be
addressed prior to approval of this amendment.
1. The set that was submitted to the county includes an amendment to just phase 1, not phase 2,
despite the fact that the site is halfway built. Instead of revising sheet ESC -2, please revise sheet
ESC -3 and show all as -built characteristics of the site accurately. If the ESC plan does match the
approved site plan, a site plan amendment may be needed (see the following comment).
(Rev. 1) The sheet currently provided to the county was described as an as -built plan which
shows an intermediate step between ESC Phase I and Phase H. To construct building two, the
contractor will need to modify the basin in the field further to establish the basin designed as
approved in July 2008 or submit an amendment to the site plan to add a wall around the access
aisle behind building 2 to make the existing basin contours work with approved site plan.
Nothing is required to address this comment at this time; this comment was provided for
applicant to consider when he plans on proceeding to constructing building 2.
2. The ultimate drainage area for this basin will be 13.92 acres according to sheet SWM -2 of the
approved site (SDP- 2008 - 00112) and stormwater plan amendment. The sediment basin must be
sized for this drainage area, not 10.36 acres.
(Rev. 1) Comment has been addressed. The plan now acknowledges a phase 2 drainage area of
13.92acres.
3. For the most part, the grading of the facility appears to match the approved SWM plan amendment
showing the biofilter behind building 2 except that the embankment of the ESC pond is 2ft higher.
Please clarify. The applicant should attempt to use the same embankment dimensions for the ESC
and SWM plans if possible.
(Rev. 1) This comment is no longer applicable because no proposed contours are provided.
4. Please provide the calculations for the dewatering orifice. It appears that the orifice is oversized.
(Rev. 1) The calculations for the dewatering orifice were note provided. However, the orifice
specified on the plan has been reduced in this submittal and appears to match the size
constructed in the field. Based on the calculations and survey provided by the applicant, this
orifice is sized appropriately.
After visiting the site following a few days without rain, I'm not sure the dewatering orifice is
operating as intended. When I visited the site the dewatering orifice was submerged, but
looking through the top of the structure, the orifice was flowing with a depth of close to a I /z" in
the pipe. The tubing around the orifice maybe drastically limiting the flow to the dewatering
orifice which is problematic if the frequency of storms using the crest of the riser is too high. I
will pass this information along to the ESC inspector. No changes to the plan are necessary for
this item.
If the embankment and the riser structure are to remain unmodified, please provide an as -built
survey of these structures. If either of these aspects need to be changed, please note the required
changes on the plan. (The anti - vortex specified on the plan does not appear to have been used in
the field.)
(Rev. 1) Please provide a note on the plan concerning the topographic information of the
sediment basin as included in the email sent to me on June P, specifically that a survey was
performed on April IP by Commonwealth Surveying. It seems the survey matches the
footprint of the facility but the distance between the top of the embankment and the crest of the
riser appears to be greater than 3ft. In the field, this was measured by county staff to be 4ft.
The diameter of the riser was also measured to be 48'; not 36" (this actually makes the
6. A baffle will likely be needed for the outlet of the drainage system that comes from behind
building 1.
(Rev. 1) Comment has been addressed.
File: E2_esc_PBC_wpo201100008 NGIC SB2 amendment.doc