HomeMy WebLinkAboutWPO201000057 Review Comments Erosion Control Plan 2011-01-11� OF AL
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COUNTY OF ALBEMARLE
Department of Community Development
401 McIntire Road, Room 227
Charlottesville, Virginia 22902 -4596
Phone (434) 296 -5832 Fax (434) 972 -4126
Project:
Avinity ESC Plan [WP02010000571
Plan preparer:
Scott Collins [scott @collins- engineering.com]
Owner or rep.:
Avon Properties, LLC
Plan received date:
9 September 2010
(Rev. 1) 29 November 2010
Date of comments:
24 October 2010
(Rev. 1) 11 January 2011
Reviewer:
John P. Diez — Engineering Technician
Phil Custer — Civil Engineer
(Rev. 1) Phil Custer
Engineering has reviewed the first revision to the ESC plan associated with the Avinity site plan (WPO-
2010- 00057), received 29 November 2010. The review of the site, road, and stormwater plans will be
provided in a separate comment letter. The ESC plan can be approved after the following comments are
addressed:
1. It appears that E &S measures are not within the limit of disturbance for Phases 2 and 3.
(Rev. 1) Comment has been addressed.
2. The height of the embankment is acceptable as long as it designed to meet the dimensions of
VSMH 3.01.
3. The invert out of the principal spillway barrel does not match the SB Details on Sheet ESC -4.
Please revise.
(Rev. 1) Please refer to comment 12.e.
4. Please rotate the baffle 10 degrees clockwise in Phase III to lengthen the flow path.
(Rev. 1) Comment has not been addressed. My original comment probably could have been
clearer. Since the majority of the watershed in phase III will be entering the basin through the
pipe from structure 12, lengthening the distance this discharge must travel is important. The
baffle should be modified to provide a longer flow path by placing the northend of it between
the riser and outlet of the pipe from structure 12.
5. Please add a construction entrance to the Phase III ESC sheet. The construction entrance should
be placed on the east side of the crest in the entrance road so it can drain back to a sediment
trapping measure. Show construction road stabilization from the west side of the crest to Avon
Road and note that vehicle washoff will occur so runoff will drain to the sediment basin.
(Rev. 1) Comment has been addressed.
6. Avinity Court and Lots 87 -93 should not be constructed in Phase III. As proposed, there will not
be enough area for the staging, stockpiling of soil, and parking to construct all of the work
proposed in Phase IV.
(Rev. 1) This comment is being withdrawn only because the applicant has assured me that the
property is roughly balanced with regard to the cut and fill at the end of Phase III. The county
inspector has the ability to modify phasing lines if the cut and fill calculation was in err and a
stockpile is necessary adjacent to the basin.
7. In order for the project to continue on to Phase III of the ESC plan, the upstream watershed must
be stabilized. Therefore, in Phase IV, please reduce the limits of construction to the remaining
Engineering Review Comments
Page 2 of 4
portion of the site (east of lots 44 and 74 and between Avinity Loop). All ESC measures in the
other sectors of the project should be removed JP, CE, etc.). A construction entrance for the
Phase IV work should be provided as well as a diversion on the north end of the limits of
construction.
(Rev. 1) Comment has been addressed.
The ESC measures proposed to protect Phase IV construction (IP and SF) are not adequate for the
watershed and the amount of work required in this phase. I recommend breaking Phase IV into
two phases. In Phase IVA, Avinity Court, lots 87 -100, and the detention facility would be
constructed. Below this work the sediment basin would be filled in and converted to a sediment
trap sized for the upstream watershed (-2 acres). Once this area has been stabilized, the rest of the
work to finish the site can be performed using silt fence and inlet protection for the smaller
watershed.
(Rev. 1) This sheet is acceptable except that a CE is still required in phase V.
9. Since this is presumably the final WPO application for Avinity, please remove the final sentence
of the note at the top of sheet ESC -313 and step 13 in the construction sequence.
(Rev. 1) Comment has not been addressed. Please remove this note wherever it exists.
10. In Phase III and Phase IV, please provide diversion dikes east of lots 56 and 86 directing runoff
into the sediment basin and trap.
(Rev. 1) Comment has been addressed.
11. Please update the standard county ESC notes that can be found in the latest edition of the design
manual, available online.
(Rev. I) Comment has been addressed.
12. The county offers the following comments regarding the MS -19 analysis:
a. The discharge numbers used in the MS -19 analysis are based on the hydrologic data of the
drainage and SWM plans that will be reviewed at a later date. Whenever a necessary
correction to these hydrologic assumptions or calculations is noticed during the review of
the MS -19 analysis, it will be commented on. However, a more detailed review will be
undertaken later for the site drainage and swm plans. If issues are discovered during the
review of these plans that affect the MS -19 calculations, updates to these calculations will
be needed and additional comments may be required.
(Rev. 1) Corrections to the SWMplan are still necessary.
b. If the area used for the time of concentration from post - development drainage area I is
excluded will a greater discharge be the result? If yes, please exclude this area from the
analysis.
(Rev. 1) The flow path for the post- development scenario begins in a grass channel
sloped at >100/c, not sheet flow. It appears the minimum time of concentration of 6min
should be used.
c. The drainage area limits and flow path for the south end of drainage areas 2 and 3 do not
appear to be realistic. Both of these lines should be drawn perpendicular to contour lines
(DA limits on the ridge and the flow paths in the swales). But in many instances, these
lines create acute angles with topography. Please correct and update the time of
concentration calculations.
(Rev. 1) Comment has been addressed.
d. The calculations indicate that the existing culvert under Avon Street Extended may not be
adequate to convey the 10 -year storm. This existing structure will not be acting as a
culvert, but a drainage pipe as calculated on sheet DP -7. In these calculations, the
hydraulic capacity of this pipe is -- 22cfs. Also, the provided topography shows that the
outlet to this culvert is around 559, not 554, which would significantly reduce the capacity
of the pipe. It appears that the existing culvert and some of the other proposed pipes in
this area must be increased in diameter.
Engineering Review Comments
Page 3 of 4
(Rev. 1) Comment has been addressed. However, please remove any reference to
proposed work at the base of this culvert or provide proof of permission that this work is
authorized. An existing utility easement to ACSA will not suffice.
e. The barrel for the sediment basin has been set with the mass grading plan and appears to
be over a foot below the lowest elevation at the property line. Grading offsite will be
necessary to achieve positive drainage. Please provide a detail of the proposed
downstream channel.
(Rev. 1) There cannot be a sump at the outlet of the sediment basin /swm facility. This
will create a mosquito habitat that will negatively affect the public's health, safety, and
wellness. Please remedy this condition with this plan.
Please provide an analysis between the stream and the property line at the outlet of the 24"
pipe. There appears to be a very poorly- defined, shallow riprap channel that may not be
able to convey the 10 -year storm within its banks. This channel terminates at a 30" tree
—1 Oft from the stream. Where the riprap stops before the stream is a critical area and will
likely erode with the increased volume of water due to development. The western portion
of TMP 90 -35L appears to be filled in at some time since the survey.
(Rev. 1) A riprapped channel has been installed by the contractor under WPO -2005-
00001. However, this channel stops at least 20ft short of the stream and is not
indicative of the remainder of this reach. Please analyze the section between the stream
and the end of the riprapped channel. When the SWM facility is modified per SWM
comment 7, adequacy of the existing channel above its confluence with the stream will
be easier to prove.
Please also correct any reference to the riprap channel at this outlet to specify it as
being existing, not proposed.
g. The existing condition in the field at the front of the site, specifically at the southwest
corner, does not match the survey.
(Rev. 1) Comment has been addressed.
h. Cross - section 7 is not typical of the downstream condition of the outlet of the 15" VDOT
culvert west of Avon Street Extended. There is no defined channel.
(Rev. 1) The applicant has claimed that the volume and peak discharge is being
reduced in the post- development situation and therefore MS -19 is no longer a
requirement. I have checked the applicant's calculations comparing the pre and post -
development hydrology to the west and confirmed that the post- development volume
and discharge for the 2 and 10 year storm are lower than the pre- development, given
the applicant's assumptions for these drainage areas.
However, the grading at the front of the site does not concentrate any part of this area
to the yard drains. Modifications to the grading plan to assure that the drainage area
delineations of SWM -2 and DP -1 are established in the field are necessary. Please
provide spot elevations throughout the area establishing a deep enough sump condition
for each grate on the yard inlets and specify the type of inlet. VDOT will need to
approve this private drainage system within their ROW
The drainage areas shown by the applicant on sheet SWM -2 and DP -1 does not match
the drainage area shown on ESC -4. Also, please provide a profile of this system to
confirm it can be constructed to structure 90.
The front of lots 1 -10 differ between ESC -4 and S -1, S -2, etc. Please clarify.
i. For both the pre - development and post - development drainage area sheets, please specify
Engineering Review Comments
Page 4 of 4
the acreage and hydrologic coefficient for the portion of drainage area 2 that is on site.
Please send Phil updated pdfs of these drainage area sheets before addressing 121.
(Rev. 1) Please see the previous comment.
13. The ESC bond will be computed at the time of plan approval.
(Rev. 1) To receive an ESC bond estimate, please submit the Bond Estimate Request Form to
the County Engineer after the ESC plan is approved.
14. (Rev. 1) ESC measures and limits will need to be adjusted based upon the requirement of
constructing a paved walking trail to Cale Elementary School per Proffer 3, unless waived by
the Albemarle County School Board.
CDE2- esc_PBC_Avinity WPO- 2010- 00057.doc