HomeMy WebLinkAboutWPO201000058 Review Comments Erosion Control Plan 2010-10-28ALg�,��
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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: Wickham Pond Phase II Block 1 (SUB- 2009 -00062 and WPO- 2010 - 00058)
Plan preparer: Mr. Frank Pohl, PE; Pohl Consulting, LLC
Owner or rep.: Wickham Two Holdings LLC
Date received: 13 September 2010
Date of Comment: 28 October 2010
Engineer: Phil Custer
The SWM, ESC, and road plans for Wickham Pond Phase I1 Block 2, received 13 September 2010, have
been reviewed. The plans cannot be approved as submitted and will require the following corrections
before approval can be granted:
A. General Review Comments
1. The preliminary plat included all information necessary to review the overlot grading plan
Engineering offers the following comments on the overlot grading plan:
a. A copy of the overlot grading plan will be kept in the preliminary subdivision file. Any
revision per Proffer 6J will also require an amendment to the ESC plan.
b. There is a missing 632 contour line around Lot 116. Please show a depression between
the houses for lots 116 and 117 to meet Proffer 6C.
c. To meet Proffer 6E, please provide a drainage inlet on Lot 121.
d. To meet Proffer 6F, please provide another drainage inlet in the alley between Lots 147
and 131. Engineering review also has a policy to keep concentrated flows greater than
lcfs out of travelways.
2. This construction plan is contingent on consent from the owners of TMP 56 -91B and TMP 56J -A.
All properties within the limits of disturbance will need to be party to the WPO bond. All
easements and ROW dedication from these properties will need to be included on the first phase of
the final subdivision plat. Also, the existing property line for TMP 56J -A along Route 240 is
unclear.
3. The existing conditions sheet indicates a stream feature in the southwest corner of the site into the
limits of construction. A site visit confirms that an intermittent stream exists within the limits of
disturbance, though the beginning of the stream is closer to the property line than the survey
exhibits. A permit from the Army Corps of Engineers should be provided to the county before a
grading permit is given. A mitigation plan for buffer disturbance will be required to make up for
the loss of a stream buffer. The buffer disturbance will be authorized per 17- 321.6. The
mitigation plan should fill in the voids within the buffer on the east side of the property.
4. Please update the existing conditions sheet to indicate underground utilities along the northside of
the property.
5. The pedestrian trail cross - section implies considerable grading to achieve the lft deep ditch and
flat 6ft path given the existing topography. Please show the design of this trail on the grading and
ESC plans matching the current cross - section or modify the detail to meet the primitive standard
for Class B Type 1 trails from the Design Manual, unless another standard of trail is required for
this development.
6. Please keep the drainage profiles, inlet drainage area maps, and calculations with the road plan and
not divide these sheets into two sets. Please also keep all Stormwater sheets within the same set.
B. Road Plan Review Comments (SUB- 2009 - 00062)
1. VDOT approval is required. Once VDOT approval is received, please forward the letter to county
engmeermg.
2. Proffer #4 from ZMA- 2005 -00018 states that if the western facing facades of the Phase I
townhouse units adjacent to Block 2 of Phase II are not evocative of a front entry elevation, a
landscape buffer on Block 2 will be required. The western facing facades of these units are not
evocative of a front entry elevation and a buffer will be required. Because the design of the buffer
was not identified during the rezoning application, it must meet the minimum requirements of 18-
32.7.9.8. The road must be moved to the west so this 20ft of landscape screening may be provided
in the future (when the site plan for Block 2 is submitted for ARB review). The matter is further
complicated by the —loft of ACSA easement centered on the eastern property boundary since
screening vegetation is prohibited within most ACSA easements. The road ROW for Wickham
Place must be placed at least 20ft off of the eastern boundary of Block 2. The ROW may need to
be moved 30ft if ACSA will prohibit the screening vegetation from the sewer easement. If the
applicant wishes to modify the facades of the existing houses, a letter of intent from TMP 56J -A
will be required prior to approval of the preliminary plat.
3. The stations corresponding to the design speeds beneath the profile for Henry Drive are incorrect.
I presume that the intended design speed for Henry Drive is 20mph.
4. The vertical profiles for the streets and alleys appear to be currently acceptable. However, the
future sight distance line from the garage entrance onto Wickham Place looking south may be
obstructed by the crest of the road. I recommend lowering the roadway at least a foot to avoid
possibly needing to perform a vertical correction when Block H is constructed.
5. Please provide the marks on the plan indicating the start and end point of the horizontal curves.
6. The stop sign at the end of Chesi Circle should be moved to the south of the "Do Not Enter" sign.
7. Please add speed limit signs to each street.
8. If this development will be constructed in phases, the road plans must provide greater detail
regarding the temporary turnarounds and other infrastructure (drainage pipes, sewer, and water).
9. Wickham Place from Wickham Way to Route 240 and the Route 240 intersection must be
constructed in the first phase of development. [18- 32.7.2.4, 18- 32.7.2]
10. The north side of Wickham Way and west side of Wickham Place require a sidewalk unless
waived by the Planning Commission.
11. The 634 contour disappears at the front of the site. Please correct.
12. The concrete ribbon is described as 0.5ft wide in the alley detail but 2ft width in the concrete
ribbon detail. Please clarify.
13. Many of the street trees are spaced at distances greater than 50ft. Please add more trees to the
planting strips so that the spacing is between 30ft and 40ft as stated on page 23 of the Code of
Development. The spacing should strive to be as uniform as possible.
14. Is a Splendens Scarlet Oak an acceptable street tree to VDOT? It does not appear to be on their
acceptable species list.
15. Engineering review will note at this time that it appears the modification to the VDOT street
standards in July of 2009 would allow many of the subdivision streets (denoted by I in Figure 4 of
the rezoning plan) to be reduced in width. County engineering review has no objection to the
reducing of the pavement widths to the latest VDOT standards. Please confirm with VDOT that
they would consent to a reduction in cross section width before modifying the design.
C. SWM Plan Review Comments (WPO- 2010 - 00058)
1. A signed Stormwater Management Facility maintenance agreement and recordation fee will need
to be updated for each parcel with a new stormwater management facility. Please contact Ana
Kilmer at 296 -5832 x3246 for questions regarding this procedure after consulting the materials
available online. A separate, modified agreement may be required for the facilities within the
VDOT ROW.
2. The Lickinghole Pro -Rata fee for this project has already been paid in June of 2007. No
adjustment to the fee is necessary because the amount of impervious area is essentially equal.
3. The Wickham Pond Phase I Water Protection Ordinance plan designed the retention pond
assuming 252,000sf of imperviousness in a total area of 12.5 acres on the Phase II property. The
current plan supplies 197,327sf of imperviousness within a drainage area of 11.3 acres to retention
pond. The stormwater quality and quantity requirements for this watershed will be considered met
if the following adjustments are made:
a. Please provide a trash rack on the orifices of the existing riser in the retention pond.
b. The flow path within the facility for the new impervious area is unacceptable. The
discharge point should be closer to the north end of the pond or a permanent baffle must
be constructed within the pond.
c. A sediment forebay is needed for the new discharge point.
d. The trail must double as a maintenance path for access to the sediment forebay and outlet
of Phase H development.
e. Please provide a summary of the total drainage and impervious areas referenced in the
beginning of this comment within the plan. Please reference the previous application
WPO- 2006 -00004 when doing so.
4. In the biofilter cross - section, please remove the reference to the Luckstone Mix and replace with
"state- approved mix."
5. Please provide a forebay for the biofilter. The forebay can simply be a stone ring around the
inflow point.
6. Please correct the biofilter detail so that the 3" of pea gravel is exclusive of the biofilter mix.
7. The maintenance path to the biofilter must be gravel. The 3" of topsoil must be removed.
8. Because the applicant is performing detailed hydrologic analyses for the watersheds leaving the
southwest and northwest corners of the project to meet MS -19 requirements, comments regarding
detention for these watersheds will be provided in the ESC section.
9. The SWM portion of the WPO bond will be computed once all comments have been addressed.
A ESC Plan Review Comments (WPO- 2010 - 00058)
1. Inlet protection is not a sufficient primary measure for sediment control in Phase II of the ESC
plan. Inlet protection is intended as a supplemental measure with a sediment trapping measure at
the end of the pipe. Please modify the plan so that an appropriately sized sediment trapping
measure is provided at the end of pipe in phase 2 of the ESC plan. The existing stormwater pond
cannot be converted back to the ESC basin. I recommend moving the outlet pipe to the property
line between lots 105 and 106 and providing a basin here sized for —3.5 acres (the houses on these
lots would likely need to be constructed after the site is stabilized and the basin can be removed).
This would require a temporary pipe from structure 109 into the maintenance path ditch and
sediment basin 1 to be designed for —7.5acres. The grading of this facility would need to be
shown on the phase 2 sheet as well and the construction of a few lots in the area may need to be
delayed until the site is stabilized to direct runoff to the retention pond permanently.
2. The construction entrance must drain to a sediment trap. A "settling basin" is not an approved
state standard. I recommend relocating the sediment trap so that it is farther downhill and
providing a diversion from the top of the temporary roadside culvert to the trap. The construction
entrance should be graded approximately 2% towards the diversion. This sediment trap should
also be shown in Phase I1 of the ESC plan.
3. Please move the diversion dike on the west side of the Block 2 disturbance to immediately
adjacent to the tree protection fencing, eliminating the need for silt fence in the area. The silt
fence would be acting as a diversion because has not been placed parallel to contour lines.
4. Similar to the previous comment, please move the diversion dike south of sediment basin 1 to the
end of the limits of construction to direct as much runoff as possible to the sediment basin. Silt
fence in this area would not operate successfully because it is proposed as being perpendicular to
contour lines.
The following comments are provided regarding the Minimum Standard 19 (adequate channel) for
this project's three discharge points:
a. Because the Phase II development is staying within the hydrologic parameters assumed
during the approval of the Phase I SWM and ESC plan (WPO- 2006 - 00004), an adequate
channel downstream of the existing pond will not need to be proved.
b. For the railroad culvert at the southwest corner of the property, the applicant provided an
analysis of pre and post development peak flow rates and volumes for the portion of the
property within the railroad culverts watershed. The applicant provided this analysis using
the rational method while Minimum Standard 19 requires the use of a hydrologic method
using a 24 -hour storm. County Engineering has performed this analysis using the SCS
method given the applicant's drainage areas and found the following results:
Assumptions: Pre CN =58, Post CN =64.7, Pre Tc= 16min, and Post Tc =6min
Because the flow rates and volume decrease for both the 2 and 10 year storms, Minimum
Standard 19 does not require an analysis of the railroad culvert or beyond (detention
requirements are addressed for this watershed as well). However, the channel downstream
of the biofilter to the existing stream does experience higher volumes of water and must be
analyzed. The 2 -year storm, 24 -hour storm must be contained within the banks of the
channel and non - eroding. Also, the railroad culvert is currently obstructed by a fallen tree.
Since construction will be occurring in the area, this tree should be cleared out with this
plan. I recommend that the applicant consider a trash rack or guard on this culvert due to
its proximity to the trail.
c. Rather than proving or providing an adequate channel downstream of development, the
applicant appears to be attempting to provide detention to the standard referenced in 10.1-
561 of the Code of Virginia for the front watershed of the parcel. Engineering review has
the following comments for the adequate channel requirements of this watershed:
i. The facility at the front of the site is larger than anticipated and seems impractical.
The county engineer will visit the site next week to inspect the downstream
channel when he performs an inspection of Phase I for a bond reduction request.
ii. If it is determined that detention is required at the front of the site, the analysis of
the 1.5, 2, and 10 -year discharges and volumes must be made using the 24 -hour
storm. The analysis provided by the applicant uses the Modified Rational
Method. Because the soils within the development are classified as B, please use
a CN of 55 for pre - development forested conditions.
iii. If it is determined that detention is required at the front of the site, please provide
an elevation -area table for the Stormtech 740 chambers.
6. The County Engineer has granted a variance for the use of Super Silt Fence as proposed as long as
the diversion above the stream buffer is modified to capture as much of the upstream lots as
possible.
7. Please update the elevations in the calculation for Sediment Basin 1.
8. Please provide the calculations for Sediment Trap 3.
9. Please provide outlet protection on the temporary culvert in phase 2 of the ESC plan.
10. Please update the County's General Erosion and Sediment Control Notes. A copy of the latest set
of notes can be found in the most recent edition of the Design Manual, available online.
11. Please revise the dates of construction within the ESC narrative.
12. The ESC portion of the WPO bond will be computed once all comments have been addressed.
Pre -Dev Vol
Post -Dev Vol
Storm
(cf)
(cf )
Pre -Dev Q cfs
Post -Dev cfs
2-year
20703.528
15733.584
5.665
5.478
10-
ear
58645.944
38033.856
16.048
13.244
Assumptions: Pre CN =58, Post CN =64.7, Pre Tc= 16min, and Post Tc =6min
Because the flow rates and volume decrease for both the 2 and 10 year storms, Minimum
Standard 19 does not require an analysis of the railroad culvert or beyond (detention
requirements are addressed for this watershed as well). However, the channel downstream
of the biofilter to the existing stream does experience higher volumes of water and must be
analyzed. The 2 -year storm, 24 -hour storm must be contained within the banks of the
channel and non - eroding. Also, the railroad culvert is currently obstructed by a fallen tree.
Since construction will be occurring in the area, this tree should be cleared out with this
plan. I recommend that the applicant consider a trash rack or guard on this culvert due to
its proximity to the trail.
c. Rather than proving or providing an adequate channel downstream of development, the
applicant appears to be attempting to provide detention to the standard referenced in 10.1-
561 of the Code of Virginia for the front watershed of the parcel. Engineering review has
the following comments for the adequate channel requirements of this watershed:
i. The facility at the front of the site is larger than anticipated and seems impractical.
The county engineer will visit the site next week to inspect the downstream
channel when he performs an inspection of Phase I for a bond reduction request.
ii. If it is determined that detention is required at the front of the site, the analysis of
the 1.5, 2, and 10 -year discharges and volumes must be made using the 24 -hour
storm. The analysis provided by the applicant uses the Modified Rational
Method. Because the soils within the development are classified as B, please use
a CN of 55 for pre - development forested conditions.
iii. If it is determined that detention is required at the front of the site, please provide
an elevation -area table for the Stormtech 740 chambers.
6. The County Engineer has granted a variance for the use of Super Silt Fence as proposed as long as
the diversion above the stream buffer is modified to capture as much of the upstream lots as
possible.
7. Please update the elevations in the calculation for Sediment Basin 1.
8. Please provide the calculations for Sediment Trap 3.
9. Please provide outlet protection on the temporary culvert in phase 2 of the ESC plan.
10. Please update the County's General Erosion and Sediment Control Notes. A copy of the latest set
of notes can be found in the most recent edition of the Design Manual, available online.
11. Please revise the dates of construction within the ESC narrative.
12. The ESC portion of the WPO bond will be computed once all comments have been addressed.
File: E1_esc swm rp_PBC_Wickham Pond Phase II Block Ldoc