HomeMy WebLinkAboutWPO201000058 Review Comments Stormwater Management Plan 2011-03-23ALg�,��
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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
(Rev. 1) 8 February 2011 (WPO plan received 16 February 2011)
Date of Comment: 28 October 2010
(Rev. 1) 23 March 2011
Engineer: Phil Custer
The first revision to the SWM, ESC, and road plans for Wickham Pond Phase H Block 2, received 8
February 2011 (WPO plans received 16 February 2011), have been reviewed. The plans can be approved
after the following corrections are made:
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.
(Rev. 1) Comment has been addressed.
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.
(Rev. 1) Comment has been addressed.
c. To meet Proffer 6E, please provide a drainage inlet on Lot 121.
(Rev. 1) Comment has been addressed.
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
1 cfs out of travelways.
(Rev. 1) Comment has been addressed.
e. (Rev. 1) The 638 contour between Lots 101, 102, and 103 is missing.
f. (Rev. 1) The 644 contour is not needed between Lots 95 and 96.
g. (Rev. 1) A 634 contour is needed between Lots 121 and 122.
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.
(Rev. 1) I have reviewed the plat recorded in DB 3324 on pages 540 through 543 referred to by
the applicant. This plat reserves, but does not dedicate, the ROW for the development on
parcels 91 and 91B. The necessary ROW on 56J -A does not appear to be reserved in this plat
since it was not signed by the owner of that lot. The stormwater management easement on
parcel 91B will be needed on the first plat submitted to the county. All three property owners
will need to be included on the WPO bond and grading permit unless all easements are
recorded prior to issuance of the permit. Please note that it appears that construction
easements and ROW dedication will be needed from the property owner of 56J -1 -1 as well.
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.
(Rev. 1) The county will honor Tamara Ambler's earlier assessment of this drainage feature as
NOT being an intermittent stream. No modification or mitigation plan is necessary.
4. Please update the existing conditions sheet to indicate underground utilities along the northside of
the property.
(Rev. 1) Comment has been addressed.
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.
(Rev. I) Comment has been addressed.
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.
(Rev. 1) Please include the inlet drainage area maps and the landscape sheets in the road plan
set as well. The "preliminary plat" set is no longer necessary. The county planner has given
approval to the preliminary plat already.
7. (Rev. 1) This comment is only advisory because there are no requirements within the COD or
subdivision ordinance that mandates the application of this standard now. Do all houses in this
block have garages? I will note at this time there is not enough room in most driveways for
vehicles to park without obstructing the sidewalk. I recommend moving some of the buildings
farther from the property line so vehicles may park on the driveway without overlapping the
property line. Again, this is not a requirement.
B. Road Plan Review Comments (SUB- 2009 - 00062)
1. VDOT approval is required. Once VDOT approval is received, please forward the letter to county
engineering.
(Rev. 1) VDOT approval appears to be imminent.
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 - 10ft 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.
(Rev. 1) A copy of the plan has been forwarded to the Planning Department for their approval.
I will forward their response to the applicant once it is received. The landscaping plan appears
to be what was agreed upon in the meeting held between the Planning Department and the
applicant in December. However, I do not see anywhere within the sets a callout for a white
picket fence along the property line with agate for each property owner. Please include this as
well as a detail within the road plan set on the next submittal.
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.
(Rev. 1) Comment has been addressed.
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 11 is constructed.
5. Please provide the marks on the plan indicating the start and end point of the horizontal curves.
(Rev. 1) There is a horizontal curve of lllft on Wickham Place at Stat 13 +60 that may be
problematic given the design speed. I recommend correcting the horizontal alignment in the
area to avoid a compound curve using the lllft radius. This is not a requirement unless
VDOT needs this to be changed.
6. The stop sign at the end of Chesi Circle should be moved to the south of the "Do Not Enter" sign.
(Rev. 1) Comment has been addressed.
7. Please add speed limit signs to each street.
(Rev. 1) Comment has been addressed.
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).
(Rev. 1) Comment has been addressed.
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]
(Rev. 1) Comment has been addressed.
10. The north side of Wickham Way and west side of Wickham Place require a sidewalk unless
waived by the Planning Commission.
(Rev. 1) Comment has been addressed.
11. The 634 contour disappears at the front of the site. Please correct.
(Rev. 1) Comment has been addressed.
12. The concrete ribbon is described as 0.5ft wide in the alley detail but 2ft width in the concrete
ribbon detail. Please clarify.
(Rev. 1) Comment has been addressed.
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.
(Rev. 1) The planting plan is acceptable except that the middle tree on Lot 103 should be
removed.
14. Is a Splendens Scarlet Oak an acceptable street tree to VDOT? It does not appear to be on their
acceptable species list.
(Rev. 1) Comment has been addressed.
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)
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.
(Rev. 1) It has been determined by the County Engineer that an additional Stormwater
Management Facility maintenance agreement is not needed.
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.
(Rev. 1) Comment has been addressed.
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.
(Rev. 1) Comment has been addressed.
c. A sediment forebay is needed for the new discharge point.
(Rev. 1) Comment has been addressed.
d. The trail must double as a maintenance path for access to the sediment forebay and outlet
of Phase II development.
(Rev. 1) A graded access path to the forebay must be provided.
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.
(Rev. 1) Comment has been addressed.
4. In the biofilter cross - section, please remove the reference to the Luckstone Mix and replace with
"state- approved mix."
(Rev. 1) Comment has been addressed.
5. Please provide a forebay for the biofilter. The forebay can simply be a stone ring around the
inflow point.
(Rev. 1) Comment has been addressed.
6. Please correct the biofilter detail so that the 3" of pea gravel is exclusive of the biofilter mix.
(Rev. 1) Comment has been addressed.
7. The maintenance path to the biofilter must be gravel. The 3" of topsoil must be removed.
(Rev. 1) Comment has been withdrawn.
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.
(Rev. 1) To request an estimate for the SWM bond, please provide a Bond Estimate Request to
the County Engineer after the plan has been approved.
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.
(Rev. 1) Basin 1 should remain in place until the site is stabilized since the drainage area to it is
still over 3 acres. Basin 1 should remain in place in Phase 2. If Bain I is to get smaller, please
show the design of this facility and refer to it in step 12 of the construction sequence. Please try
to design Basin 1 for both phase I and phase 2 if possible to avoid disturbing this area 3 times.
Stockpiles adjacent to basins 1 and 3 will likely be necessary so that when the vast majority of
the site is stabilized, these facilities can be removed andfilled in with the adjacent soil. Please
also show the riser of sediment basin 3 on Phase L Please show safety fence around sediment
basin 3 on both phases.
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 II of the ESC plan.
(Rev. 1) Please extend the entrance so that its length is 70ft behind the diversion and note that
the wash off area will occur behind this diversion.
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.
(Rev. 1) Comment has been addressed.
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.
(Rev. 1) Comment has been addressed.
5. 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.
(Rev. l) Comment has not been fully addressed. No adequacy calculations have been
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
1 58645.944
1 38033.856
1 16.048
1 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.
(Rev. l) Comment has not been fully addressed. No adequacy calculations have been
received for the channel from the outlet of the biofilter to the stream. The 2-year
discharge must be contained within the existing banks without eroding.
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.
(Rev. 1) The county engineer did not make this evaluation.
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.
(Rev. 1) Comment has been addressed.
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.
(Rev. 1) The table provided needs to be updated because it does not match the
proposed underground detention system.
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.
(Rev. 1) The calculations and summary table for sediment basin 1 are acceptable. However, the
dewatering orifice for new sediment basin 3 appears to be larger than necessary. Please
provide the dewatering calculations for this orifice.
8. Please provide the calculations for Sediment Trap 3.
(Rev. 1) There appears to be a few problems with the calculations for revised sediment trap 1:
• the width of the dam is designated 16ft, rather than 2.5ft
• the wet storage slope is designated as 2:1, rather than 1:1
• the wet storage provided is smaller than the wet storage required
9. Please provide outlet protection on the temporary culvert in phase 2 of the ESC plan.
(Rev. 1) Comment has been addressed.
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.
(Rev. 1) Comment has been addressed.
11. Please revise the dates of construction within the ESC narrative.
(Rev. 1) Comment has been addressed.
12. The ESC portion of the WPO bond will be computed once all comments have been addressed.
(Rev. 1) To request an estimate for the ESC bond, please provide a Bond Estimate Request to
the County Engineer after the plan has been approved.
File: E2_esc swm rp_PBC_Wickham Pond Phase II Block Ldoc