HomeMy WebLinkAboutWPO200900041 Review Comments Stormwater Management Plan 2009-09-21ALg�,��
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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: 31 July 2009
Date of Comment: 21 September 2009
Engineer: Current Development Engineering
The Erosion & Sediment Control and Stormwater Management plans for the Downtown Crozet
Stormwater Management Plan, received on 31 July 2009, have been reviewed. Current Development
Engineering has the following comments.
A. General review comments:
1. Please show 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.
2. 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.
3. 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.
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.
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.
6. Please submit documentation of all offsite easements necessary to perform the work and all
permanent easements resulting from the project.
7. 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.
B. Erosion and Sediment Control Plan:
8. 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]
9. 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.
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.
11. Silt fence, or another E &SC measure, is needed to protect they downstream embankment of
the construction entrance onto Jarman's Gap.
12. Construction entrance onto Crozet Avenue must drain to a sediment trap. [VESCH 3.02, DM]
13. Provide Safety Fence between Crozet Avenue and the project. [VESCH 3.01]
14. Please provide dust control symbols throughout the plan. [DM]
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]
16. Please provide a soil stockpile and staging area on the plan that are protected by erosion and
sediment control measures. [DM]
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.
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]
19. Protection (silt fence) is needed on the west side of the Swale /intermittent stream near the pond
forebay. [DM]
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.
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]
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]
23. Please provide a USC symbol at all proposed utility crossings of the existing intermittent
stream. [VESCH 3.25]
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]
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.
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.
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.
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.
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.
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.
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 ?)
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.
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.
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.
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]
File: CDE1 escswm ADP PBC WPO- 2009 -00041 Downtown Crozet Stmwtr.doc