HomeMy WebLinkAboutWPO201000011 Review Comments Erosion Control Plan 2010-04-03ALg�,��
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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: Treesdale Park; WPO- 2010 - 00011; SDP - 2010 -00013
Plan preparer: Mr. Scott Collins, PE; Collins Engineering
Owner or rep.: Albemarle Housing Improvement Program, Inc.
Date received: 16 February 2010
(Rev. 1) 1 April 2010 (MS -19 Calculations received 22 March 2010)
Date of Comment: 8 March 2010
(Rev. 1) 3 April 2010
Engineer: Phil Custer
The second submittal of the ESC plan for Treesdale Park Project has been reviewed. The following
comments remain.
D. WPO- 2010 -00011 Erosion and Sediment Control Plan Comments
1. Per Water Protection Ordinance text amendments that were approved by the Board of Supervisors
last year, the WPO application for the Stonewater Development, WPO- 2007 - 00045, has been
voided because approval was given to the plans more than 12 months ago. Treesdale must include
all of the design graphics and calculations for the shared facility in this set for re- review.
(Rev. 1) The Treesdale - Stonewater Sediment basin calculations have been provided in this set
and are adequate.
All offsite work necessary for the Treesdale Park project to be fully functional must be included
in the set and protected by the ESC plan (namely, the greenway /SWM access path and the road
connection to the Treesdale property line). Please refer to the email and comment letter sent to
the applicant April 2 "d
2. According to the existing contours, the diversion dike is going uphill in the northwest corner of the
site. The applicant has provided spot elevations but it is unclear how the grading is being
accomplished. Because the reliability of this diversion is critical to the success of the esc plan,
please show the grading and provide calculations for this diversion from the 422 contour to the
sediment basin. (Silt fence downhill of this diversion is not necessary.)
(Rev. 1) Please refer to comment 10.
3. In the northeast corner of the site, please extend the diversion running east to west to the water
meter adjacent to Rio Road and provide a note stating that all runoff from the VDOT ditch is to be
rerouted with the diversion dike.
(Rev. 1) Comment has been addressed.
4. The drainage area to the sediment basin must be increased to account for VDOT ditch runoff south
of the development.
(Rev. 1) Comment has been addressed.
5. Please show the drip lines of all trees on each ESC sheet. Please also show the location of the tree
preservation area as approved by Planning in the latest variation on each sheet.
(Rev. 1) Comment has been addressed.
6. Please provide silt fence for the construction of the trail from the clubhouse to the Stonewater
greenway trail/maintenance road. (Please note the greenway and swm access path must be
included in this set.)
(Rev. 1) The silt fence has been provided for the connection to the Stonewater trail, but the
greenway trail has not been included in the Treesdale set. Please see Comment I as well.
7. Please draw the 395 contour line in the basin if it was intended that it would be the bottom.
(Rev. 1) The basin has been redesigned. The 395 contour is no longer within the basin.
8. Please provide calculations for the dewatering orifice size. My calculations resulted in a diameter
of 3.5in which should not be rounded up to 4in.
(Rev. 1) Comment has been addressed.
9. There is a not a defined channel at the outlet of the 4.2 acre sediment basin. Discharging into this
area will likely cause rapid erosion. Please direct the outlet to an existing adequate channel or
attach a temporary slope drain to the barrel outlet and run overland until reaching the existing bed
of riprap within the stream channel.
(Rev. 1) Comment has been addressed.
10. The height of sediment trap 2 is taller than the maximum allowed per VESCH. Please redesign.
Considering comment #9, the outlet of the trap should be directed into the stream and not the tree
preservation area.
(Rev. 1) The sediment trap has been removed in Phase III. However, a sediment trap is needed
in the location where it was previously approved. Considering the amount of fill required, the
depth of the disturbed area, and the existing topography's general downward slope to the
southwest, silt fence is not adequate for the construction of buildings 3 and 4 in phase III.
A sediment trap below the fill should be added designed in Phase III. A diversion should be
run north to the northernmost 20" oak. A diversion should be run east to the silt fence
protecting the trail's silt fence. The note referencing the northern diversion in Phases I and H
should be modified to state that the diversion is to remain as a fill diversion (FD) in Phase III
for as long as possible. The silt fence in the northwest corner should be closed and not open
where the sanitary sewer line goes through.
11. A USC symbol is needed at the pipe crossing of the intermittent stream.
(Rev. 1) Please provide this symbol on the plan. On my visit to the site, I encountered running
water east of springhouse. The USC on the plan will alert the contractor that water MAY be
encountered, in which case a measure covered by the USC symbol will be required. Corps
approval of this crossing will not be needed per the delineation which they approved.
12. Please provide the method use to calculate the intermediate volumes in the sediment basin when
confirming the wet and dry storage elevations. In both instances, my calculation with the stage
area table provided by the applicant indicates a higher elevation is needed for both the dewatering
orifice and top of riser.
(Rev. 1) Comment has been addressed.
13. The MS -19 calculations should be updated. The drainage area shown in the Stonewater set seems
to underestimate the total watershed to this channel. The topography in the vicinity shows the
drainage area including land north of the Waldorf School up to House # 770 on Rio Rd. Please
also be sure to use the 24 -hour storm as well as identify the n- value, permissible velocity and
computed velocity for each segment of each cross - section in the calculations (these are common
comments that adequate channel analyses receive). A site visit identified potential erosion at the
outlet of the facility (on the adjacent property —an easement may be needed to extend outlet
protection to stream) and eroded downstream side slopes.
(Rev. 1) The following comments are provided regarding the MS -19 calculations:
-The 2-year discharge used in the computation is acceptable.
-The channel slopes used in calculations appear to be underestimated. By checking the
topography shown on SWM -1, I found no slope in the channel from the outlet to edge of the
survey flatter than 2.4 %. Please update the calculations accordingly.
-The method used to calculate the velocity in each segment is incorrect. The different segments
that need to have a separate calculated velocity are made by vertical cuts of the cross section.
Please see the accompanying pdf for an example. Flowmaster should have the ability to
provide this analysis. If not, I can provide you with a program that does so.
-The permissible velocity for alluvial silts (noncolloidal) is referenced as 3.5 fps in Table 5 -22,
not 5.0 fps as referenced frequently the analysis of cross section 1.
- "Graded, Loam to Cobbles" and "Graded, Silt to Cobbles" should be used rather than "coarse
gravel "for the bottom of these cross - sections.
-The USDA soil survey describes the two soil groups this channel runs through as Sandy Loam
(47D) and Silt Loam (16). The soil survey does not describe these as soils as a clay, so the
banks of each channel should not have a permissible velocity of 5fps.
-The edge of the riprap at the basin outlet terminates at the property line which is short of the
stream channel. On all applicable ESC and SWM sheets, please extend the rip rap to the
stream channel and add a note stating: "The offsite work shown on this plan to extend the
outlet riprap to the stream channel does not need to be performed if the property owner refuses
to grant temporary access to this developer for the installation. However, the developer must
provide proof to the Coutny that consent was sought before this work is waived."
14. An ESC bond will be computed once the WPO plan is ready for approval.
(Rev. 1) Comment remains unchanged.
15. (Rev. 1) Please extend the silt fence protecting the sewer line connection to the pavement of
Towne Lane.
File: E2_ esc _PBC_wpo20 100001 1Treesdale.doc