HomeMy WebLinkAboutWPO200900041 Review Comments Stormwater Management Plan 2010-07-28ALg�,��
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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: Downtown Crozet Stormwater Management Project [WPO- 2009 - 00041]
Plan preparer: Kimley -Horn and Associates
Owner or rep.: Albemarle County Department of General Services, Gregor Patsch
Date received: (Rev. 1) 28 June 2010
31 July 2009
Date of Comment: (Rev. 1) 28 July 2010
21 September 2009
Engineer: (Rev. 1) Phil Custer
Current Development Engineering
The Erosion & Sediment Control and Stormwater Management plans for the Downtown Crozet
Stormwater Management Plan, received on 28 June 2010, have been reviewed. Current Development
Engineering has the following comments.
A. General review comments:
riease snow limits of the stream buffer of Powell's Creek on all plan sheets. Stream buffer
disturbance is authorized per Section 17- 320(B) of the Water Protection Ordinance.
(Rev. 1) Comment has been addressed.
Please provide the date and map number for the FEMA floodplain in the plan. This floodplain
should be shown on all sheets. This project falls under Section 18- 30.3.05.1.1.7 of the Zoning
Ordinance so the work and minimal fill will not require a Special Use Permit.
(Rev. 1) Comment has been addressed.
Please provide calculations of Powell's Creek to determine what the new limits of the 100 -
year floodplain will be with the proposed grading modifications on this parcel. This analysis
should be performed following the calculations procedures and standards specified by FEMA.
(Rev. 1) Since the issuance of this letter, this comment has been revised to read.
After further review, detailed calculations will not be required. However, the plan will
need to clearly delineate where the new 100 -year floodplain is estimated to be. Applying
the flood elevations from the latest FEMA study to the new topography proposed on this
plan would be acceptable.
A new graphical floodplain should be shown on the set by using the current floodplain
elevations (656 through 641) and the proposed topography of plan.
4. Approval of the plan by the Albemarle County Service Authority and/or the Rivanna Water
and Sewer Authority will be required before a grading permit is issued. The applicant should
work with these entities independently and forward their approval once it is received.
(Rev. 1) Approvals from these agencies have not yet been received by this office.
5. These plans need to include the County's General Construction Notes for ESC and
Stormwater Management. These notes can be found in the latest edition of the County Design
Manual, available online.
(Rev. I) Comment has been addressed.
Please submit documentation of all offsite easements necessary to perform the work and all
permanent easements resulting from the project.
(Rev. 1) Offsite easements have yet to be received by this office.
It appears that the 36" RCP proposed in TMP 56A2 -1 -7 lacks sufficient cover at Sta. 0 +45.
Please adjust the pipe profile or provide adequate cover.
(Rev. 1) Comment has been addressed.
B. Erosion and Sediment Control Plan:
An erosion and sediment control narrative is required. The narrative can be an expansion of
the Sequence of EC Measures already included in the plan, or a separate document. Please
include the total disturbed acreage of the project. [VESCH, DM]
(Rev. 1) Comment has not been addressed. An ESC narrative has not been received. A
narrative is required for each separate application. In the narrative, please note how many
acres are disturbed with this project.
Please clearly show the limits of construction for this project. It is unclear if the pipe between
56A2 -1 -7 and 56 -11 is being installed with this project. If it is to be installed, please provide
necessary computations, including associated drainage area maps. This pipe must be sized for
full development of the upstream drainage area.
(Rev. 1) Comment has not been addressed. No limit of construction line has been provided.
No computations for the drainage pipe have been provided. If these calculations have
been reviewed and approved by another agency within the county, please submit the
package with the approval stamp to include in this project's file.
Additionally, I recommend moving the endwall of the 36" pipe 5 -10ft closer to the stream
or farther uphill so that the endwall is not located above the sanitary sewer lateral.
10. Provide protection (silt fence) around the area of construction of the head wall /pipe outlet at
Jarmans Gap Rd., to protect the spoil from the trench.
(Rev. 1) Please provide silt fence as shown below to protect the stream from the bare earth
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11. Silt fence, or another E &SC measure, is needed to protect they downstream embankment of
the construction entrance onto Jarman's Gap.
(Rev. 1) Comment has been addressed.
12. Construction entrance onto Crozet Avenue must drain to a sediment trap. [VESCH 3.02, DM]
(Rev. 1) Please provide calculations and details (bottom area, wet area, weir length, etc) for
the sediment trap to make sure it meets all the design requirements of the VESCH. The
drainage area for this trap (once the diversion is extended and the cleanwater diversion is
eliminated) looks to be around 3 acres.
Please also remove the silt fence from the streambottom and western embankment of
Powell's creek.
13. Provide Safety Fence between Crozet Avenue and the project. [VESCH 3.01]
(Rev. 1) The existing fence will suffice.
14. Please provide dust control symbols throughout the plan. [DM]
(Rev. I) Comment has been addressed.
15. Please provide a soil map of the work area. This map can be a detail or a layer on an existing
sheet. [DM VESCH]
(Rev. 1) The ma is missinfl two soil types within the work area.
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16. Please provide a soil stockpile and staging area on the plan that are protected by erosion and
sediment control measures. [DM]
(Rev. 1) Comment has been addressed.
17. Remove the proposed diversion dike along the west side of Crozet Avenue. This is acting as a
"clean-water diversion" which is not a State approved Erosion & Sediment Control measure.
(Rev. 1) A "clean -water diversion" still remains in the set. Please remove and increase the
size of the trap, if necessary, or replace with a designed ditch that is either lined with sod or
riprap. Please eliminate the reference to the clean water diversion in the construction
sequence.
18. Please provide ESC measures for the offsite work on TMP 56A2 -1 -7. It appears silt fence
(installed parallel to contour lines) and CIP is required. [DM]
(Rev. 1) It appears as though the construction of the adjacent travelway is no longer
included within this project's scope. Therefore, CIP is no longer needed. The silt fence is
shown perpendicular to the contour lines and, in some cases, uphill of the disturbed area.
Please add silt fence above the endwall and 140ft toward Crozet Avenue on the south side of
the pipe construction.
19. Protection (silt fence) is needed on the west side of the swale /intermittent stream near the pond
forebay. [DM]
(Rev. 1) Comment has been addressed.
20. For better sediment capture, design the last cell of the wetlands as a sediment trap and replace
the silt fence along the eastern bank of Powell's Creek with a diversion dike to direct flow to
the trap. Extend the diversion dike along the creek up to the 656 contour. Protect the
downstream side of the diversion dike with silt fence if deemed necessary by the inspector.
(Rev. 1) Please provide calculations and details (bottom area, wet area, weir length, etc) for
the sediment trap to make sure it meets all the design requirements of the VESCH. Please
also show the drainage area of the trap. To me, the drainage area for this trap looks to be
around 3 acres.
Also, the diversion has not been extended to the 656 contour. The silt fence as proposed
will not work properly since it is placed perpendicular to contour lines. At the culvert
stream crossing, provide a ROW diversion to keep water out of the stream and directing it to
the trap.
21. The construction sequence states that excess cut will be used for the fill areas of the 16ft
access road. However, plans do not indicate where this access road requires fill. In the
narrative, please provide a note stating that all cut material not used on site will be transported
to a site with an approved erosion and sediment control plan. This should be coordinated with
the County Erosion and Sediment Control inspector. [VESCH, DM]
(Rev. 1) The notation that excess cut material is to be deposited on a site with an approved
ESC plan must also be noted in the ESC narrative when it is submitted.
22. A designed stream crossing is needed for the site to access the western portion of the
construction area. Please refer to VESCH Standard 3.24 when designing this crossing. Will
an access road be needed on the west side of the streamwork? [VESCH 3.24]
(Rev. 1) The culvert is too small for the stream crossing. Please size the crossing per
VESCH 3.24 or provide a temporary bridger per VESCH 3.24. Also, this crossing must be
included in the construction sequence before any other work proposed on the west side of
the intermittent stream
23. Please provide a USC symbol at all proposed utility crossings of the existing intermittent
stream. [VESCH 3.25]
(Rev. 1) Two utility stream crossings are required.
24. Please provide channel calculations at regular intervals along the newly constructed channel.
The new channel must be sized for full development, non - eroding for the 2 -year storm, and
able to carry the 10 -year storm within its banks. The calculations should clearly show n-
values, permissible velocity, and calculated velocity for each segment of a channel cross -
section. [DM]
(Rev. 1) This information has yet to be provided. If these calculations have been reviewed
and approved by another agency within the county, please submit the package with the
approval stamp to include in this project's file.
25. Work within the live stream must be minimized as much as possible. [VESCH MS 12] Except
for the work performed for the two utility stream crossings and the vehicular stream crossing,
the stream in the vicinity of the stormwater facility should be left in tact for as long as
possible. On sheet C2 -5, please hatch the western corner of the constructed wetland and the
northeastern corner of the forebay with a note stating that these areas are to be left undisturbed
until authorization from the County ESC is given to divert the stream into the forebay.
More detail regarding the work within the stream channel is needed. When the regrading of
the stream and banks commences, the work must be performed in the dry. To do this,
engineering review recommends damming the stream and pumping around the work area
progressing up or down the stream. If this option is chosen, please describe this work in the
construction sequence including a sentence stating that the contractor is to dam no more of the
stream can be upgraded in the course of a day; the stream must be clear of obstructions when
the contractor leaves the site each day. A diversion channel around the work area does not
seem feasible giving the constraints around the channel.
(Rev. 1) The description of the construction sequence regarding the forebay construction is
acceptable. However, the second paragraph of the comment seems to be neglected. In the
step that discusses the shaping of the channel sideslopes in the construction sequence,
please state that the contractor must not perform any stream work in wet conditions. Please
also state that the contractor must return streamflow to the channel at the end of each work
day clear of obstructions.
26. The construction of the sanitary sewer line is one of the last steps listed in the construction
sequence. It appears that this work item should be one of the first activities to keep work away
from the existing 10" VCP.
(Rev. 1) The construction sequence on C24 conflicts with the sequence on C2 -5. Please be
cognizant of comment #22 when revising the timing of construction of the sanitary sewer
line.
27. (Rev. 1) Please provide dewatering symbols (DW) in the forebay and wetland cells.
C. Stormwater Management Review Comments
1. The plan must state clearly what the goals of this regional facility are so that current
development may review the plan to those standards. For instance, if this project is intended
to be a supplement to existing SWM requirements, our review would be limited to making
sure freeboard and safety standards are met. But, if this facility is to provide full water quality
treatment and detention for the entire watershed, many issues will need to be resolved.
Additional comments will be required once the ambitions of this project are determined.
(Rev. 1) The design engineer has not responded to this comment. It is my understanding
that Water Resources Staff will be bringing this issue to the Board of Supervisors for
discussion in the coming months.
2. The current edition of the Virginia Stormwater Management handbook specifies that a
properly designed constructed wetland provides a removal rate of 30%. Since Crozet is
located within a water supply protection area, the required removal rates generated by new
development is higher than in all other development areas in Albemarle County, per existing
county policy which uses a modified version of the state's formula for phosphorous removal
requirements.
Please provide a county modified simple spreadsheet to determine the required removal rate
for this facility. When determining the existing and full buildout imperviousness for the
facility drainage area, current development strongly recommends providing a breakdown of
square footage of imperviousness allotted to each parcel to simplify future reviews and so that
fees can be calculated fairly.
(Rev. 1) The design engineer has not responded to this comment. It is my understanding
that Water Resources Staff will be bringing this issue to the Board of Supervisors for
discussion in the coming months.
3. Please provide volume calculations for each cell of the constructed wetlands facility. The
Virginia Stormwater Management Handbook states that 80% of the volume within a
constructed wetland should be held in depths of 0 -18 inches. Currently, the volume held in
these depths is only 25% of the total for the facility. However, because the forebay has been
oversized, the facility is likely to possess removal efficiencies on par with a Type I or Type II
retention basin. The adequacy of this facility will be confirmed or questioned once the total
post - development impervious area for the facility is determined and the goals of the facility are
finalized.
(Rev. 1) The design engineer has not responded to this comment. Stormwater quality credit
for upstream impervious area may not be a goal of this facility.
4. Please provide pre and post development drainage area maps for the facility. Each drainage
area map must include all items listed in the engineering review checklist found in the latest
edition of the design manual, available online. The post - development drainage area map must
account for all future public or private projects that may direct water to this facility.
(Rev. 1) The design engineer has not responded to this comment. It is my understanding
that Water Resources Staff will be bringing this issue to the Board of Supervisors for
discussion in the coming months.
5. If detention is one of the goals of this facility, please provide a routing of the 2, 10, and 100 -
year storms. If the drainage area is over 50 acres, please route the facility using two
hydrologic methods. (Will this facility attempt to detain to a forested or pasture rate of
development ?)
(Rev. 1) The design engineer has not responded to this comment. It is my understanding
that Water Resources Staff will be bringing this issue to the Board of Supervisors for
discussion in the coming months.
6. A soil study is needed to confirm that water will not infiltrate and dry out the constructed
wetlands. The soil identification from the County's GIS application shows that this area is
considerably pervious. A clay liner may be required.
(Rev. 1) I have reviewed the soil study. Comment has been addressed.
7. The planting mix in cells 2 and 4 appear to need just the high marsh specification because the
maximum depth is only 4 inches.
(Rev. 1) The planting plan has been changed to show no proposed landscape in the wetland
area. The Low Marsh (6"48" deep) hatching is shown above the water surface elevation
on the cell embankments. Please revise.
8. Please provide vehicular access along the west side of the facility adjacent to the stream for
maintenance purposes. Currently, this area appears too narrow in some sections to allow for
vehicular access.
(Rev. 1) The design engineer has not responded to this comment.
9. No trees, shrubs, or woody vegetation can be planted between the facility and the stream to
allow for vehicular access and embankment stability. [VSMH 3.01]
(Rev. 1) The design engineer has not responded to this comment.
File: CDE2 escswm PBC WPO- 2009 -00041 Downtown Crozet Stmwtr.doc