HomeMy WebLinkAboutWPO200900041 Review Comments Erosion Control Plan 2011-02-28ALg�,��
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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. 2) 31 January 2011
(Rev. 1) 28 June 2010
31 July 2009
Date of Comment:
(Rev. 2) 28 February 2011
(Rev. 1) 28 July 2010
21 September 2009
Engineer:
(Rev. 1) Phil Custer
Current Development Engineering
The third submittal of the Erosion & Sediment Control and Stormwater Management plans for the
Downtown Crozet Stormwater Management Plan, received on 31 January 2011, has been reviewed.
Current Development Engineering has the following comments.
A. General review comments:
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.
(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.
(Rev. 2) Comment has been addressed.
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.
(Rev. 2) Comment remains unchanged.
5. These plans need to include th y'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. 1) Comment has been addressed.
6. 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.
(Rev. 2) Comment remains unchanged.
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.
(Rev. 1) Comment has been addressed.
B. Erosion and Sediment Control Plan:
tin erosion ana seaiment 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.
(Rev. 2) Comment has been addressed.
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 -1 oft closer to the stream or
farther uphill so that the endwall is not located above the sanitary sewer lateral.
(Rev. 2) On sheets C2 -3 and C2 -5, silt fence has been shown outside the limits of
construction. Please correct.
After looking through the Downtown Crozet book provided by Kimley Horn, the only
information I could find on the 36" drainage system from "Inlet Ex" to Endwall 12 is a
nearly illegible hydraulic grade line calculation table. This table uses a Q of the 52cfs for
the end of pipe, which may be an underestimate of 25% by my calculations. What was the
time of concentration used for this drainage system? How much of drainage area A2 is
routed to this 36" drainage system?
Alternatively, the applicant may provide a letter from another county agency that has
approved the calculations for this pipe system.
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
after the pipe is backfilled:
The comment was confusing the way it was written.
(Rev. 2) Comment has been addressed.
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.
(Rev. 2) Comment has been addressed.
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. 1) 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 map is missing two soil types within the work area.
(Rev. 2) Comment has been addressed.
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.
(Rev. 2) Comment has been addressed.
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.
(Rev. 2) The silt fence should be extended from the endwall 140ft east.
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.
(Rev. 2) Every aspect of this comment has been addressed except for the use of a ROW
diversion (mountable diversion) where the temporary stream crossing is located.
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.
(Rev. 2 Comment has been addressed.
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.
(Rev. 2) In the meeting held at the count offices on February IS, the County Engineer
stated the culvert crossing as proposed was acceptable.
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.
(Rev. 2) Comment has been addressed.
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.
(Rev. 2) At this time, it's important to note that adequate channel calculations are not
required considering just the development proposed in this plan; State Law states that
"stream restoration and relocation projects that incorporate natural channel design
concepts are not man -made channels and shall be exempt from any flow rate capacity and
velocity requirements for natural or man -made channels as defined in any regulations..."
Also, this project does not increase the peak flow rate, velocity, or volume for the watershed.
The only reason Current Development Engineering feels this channel should be analyzed
per MS -19 is so that this project can comfortably state to the Board of Supervisors when it
is considered as a regional facility that the stream upgrade addresses adequate channel
concerns for all upstream properties. If the manager of this project wishes, the following
adequate channel comments can be ignored and approval can still be granted.
The following comments are provided.
a. All adequate channel analyses must be performed using the 24 -hour storm.
b. The time of concentration should not increase by 12 minutes over 600ft given the
resulting velocities listed in the applicant's calculation table. Assuming a
conservative velocity of 4fps, the difference between the time of concentration of Sta.
1 +60 and Sta. 7 +60 should differ by 2.5 minutes. How was the time of concentration
increment calculated?
c. The Flowmaster program has provided an average velocity for the entire cross -
section. Please provide the velocities for each segment across the cross - section. It is
expected that a deeper section would possess a more erosive velocity than the outer
edges of the cross - section.
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.
(Rev. 2) Comment has been addressed.
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. l) 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.
(Rev. 2) Comment has been addressed.
27. (Rev. I) Please provide dewatering symbols (DW) in the forebay and wetland cells.
(Rev. 2) Comment has been addressed.
C. Stormwater Management Review Comments
1. _ _ie pl,-__ ____ - _gate clearly what th, foals 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.
(Rev. 2) Current Development staff has not reviewed this application for meeting any SWM
quality or quantity requirements for development within the upstream watershed because
the scope of the facility has yet to be 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.
(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.
(Rev. 2) Current Development staff has not reviewed this project for meeting any water
quality requirements of the upstream watershed.
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.
(Rev. 2) Current Development staff has not reviewed this project for meeting any water
quality requirements of the upstream watershed.
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.
(Rev. 2) Current Development staff has not reviewed this project for detention.
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.
(Rev. 2) Current Development staff has not reviewed this project for detention
requirements.
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. I) 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 " -18" deep) hatching is shown above the water surface elevation on
the cell embankments. Please revise.
(Rev. 2) The General Services Department, who is responsible for the perpetual
maintenance and viability of this facility, is reviewing this application as well. Current
Development staff will accept the collective review of the SWM plantings provided by the
staff of the General Services division responsible for this project. Modification to the plan to
address this comment is no longer necessary.
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.
(Rev. 2) The General Services Department, who is responsible for the perpetual
maintenance and viability of this facility, has determined that the ]Oft wide maintenance
path to outlet of the facility is not necessary for their maintenance program and state this
area can be accessed by ATV or by foot. Modification to the plan to address this comment
is no longer necessary.
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. l) The design engineer has not responded to this comment.
(Rev. 2) The General Services Department, who is responsible for the perpetual
maintenance and viability of this facility, has determined that the concern about
embankment stability that comes with woody vegetation is not significant enough in this
situation to warrant prohibiting revegetation in these areas. Modification to the plan to
address this comment is no longer necessary.
File: CDE3 escswm PBC WPO- 2009 -00041 Downtown Crozet Stmwtr.doc