HomeMy WebLinkAboutWPO200900014 Review Comments Stormwater Management Plan 2009-08-06� OF AL
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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: Biscuit Run Connector Road [WP0200900014]
Plan preparer: Scott Collins, Collins Engineering [scott @collins- engineering.com]
Owner or rep.: Forest Lodge, LLC
Plan received date: 31 March 2009
(Rev. 1) 22 June 2009
Date of comments: 14 May 2009
(Rev. 1) 6 August 2009
Reviewer: Amy Pflaum, Current Development Engineering
(Rev. 1) Phil Custer
The Water Protection Ordinance Application for the Biscuit Run Connector Road, submitted on 22 June
2009, has been reviewed. The previous plan set reviewed by engineering was for solely the construction of
the Connector Road. A plan submitted around the same time for Phases A and D was withdrawn when it
was decided that it would be best for review and construction if the plans were merged.
General Plan Comments
1. There is a notable elevation difference in the proposed grading of the connector road (stations
65 +00 to 80 +00) between these plans and the approved ZMA. The Director of Planning must
determine that the proposed grading is consistent with the application plan before a grading permit
can be issued. [18- 8.5.5.4.b] Alternatively, a site plan or subdivision plat must be submitted and
approved before a grading permit can be granted.
(Rev. 1) The plan has been given to the Planning Department to determine whether the plan is
consistent with the application plan regarding the proposed grading. Comments from the
planning department will be sent with this engineering comment letter.
To limit the disturbance in this area (ST -2) as much as reasonably possible, the vertical profile
of road AA could be adjusted so that the fill in the gully above ST -2 is not as tall as currently
proposed.
When the road plan is submitted, please also provide a preliminary vertical alignment for the
extension of Road AA to Avon Extended to verify that the elevation at the intersection of AA
and V is not limiting the design of the future connection.
2. Please submit a request for a critical slopes waiver per Section 18 -4.2.5 of the County Code.
(Rev. 1) Comment has not been addressed.
3. Please submit copies of required state and federal permits for work within and disturbance to
wetlands and waterways.
(Rev. 1) Comment has not been addressed.
Erosion Control Plan
4. The Erosion and Sediment Control Notes and Details sheet contains several redundancies. For
example, there are three different listings of "Erosion & Sediment Control Measures" each
containing a definition of a Construction Entrance. Please review all text on this page for
Albemarle County Community Development
Engineering Review Comments
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consolidation options.
(Rev. 1) Comment has been addressed.
5. Please label the Albemarle County Erosion and Sediment Control Notes as such.
(Rev. 1) Comment has been addressed.
6. Please include the Conservation and Preservation areas as Critical Areas on the E &S Narrative.
(Rev. 1) Comment has been addressed.
7. The Erosion and Sediment Control Narrative should include a discussion of the measures proposed
to meet the 80% sediment removal rate required by Proffer 5B. It should also discuss the
provisions of revegetation outlined in Proffer 5C.
(Rev. 1) The ESC narrative now acknowledges both of the ESC proffers. Please note that
proffer 5C states that the 9 -month timeline begins at the date the grading permit is given for the
entire limits of disturbance for each plan set and not for each phase within the plan. Once the
time limit is reached, all work shown in the ESC set must be terminated and permanently
seeded. Disturbance to all seeded areas will not be allowed until the surrounding final site
plans and subdivision lot plats are approved; a second mass grading plan will not be allowed.
If the intention of the applicant is to phase the mass grading plan as described in the narrative
(A, B, and C), it may be best to break this set up into at least three applications.
Please see the memo from the County Engineer regarding the applicant's proposals for ESC
measures "above- and - beyond" state minimum standards. (Proffer 5B) The county engineer's
commentary indicates that an adjustment to the ESC concept will need to be addressed first.
8. Some erosion and sediment control measures are too far from the road, please move measures
closer to the road to eliminate the likelihood of disturbance outside the limits.
(Rev. 1) This comment is no longer applicable.
9. Bio- retention Basin #3 is too far from the road and its conversion to a stormwater management
facility will be extremely delayed due to future development disturbing its drainage area.
(Rev. 1) This comment is no longer applicable.
10. It appears that the limits of disturbance and tree protection fencing area shown inside the stream
buffer on Sheet ESC -2. If the sediment basin can not be moved outside of the stream buffer, a
Request for Development Within a Stream Buffer must be submitted to and approved by the
Program Authority.
(Rev. 1) The area referred to in this comment appears to have been corrected. However, in a
few places throughout the plans, the preservation areas do not seem to be represented as it
existed in the approved rezoning plan. Please check the border and dimensions of all
preservation areas to assure that preservation areas are accurately shown. Please also
shade /hatch the preservation areas so that it is more prevalent when looking at the plan.
The sanitary sewer trunk line cannot disturb any preservation area. On this plan, a significant
portion of the preservation area is shown as conservation where a sewer trunk line is proposed.
When the sewer line plan is resubmitted, the sewer line should be pulled out of the preservation
area. If the applicant wishes to keep the sewer line where it is currently proposed, he should
contact the Planning Department to determine whether the preservation areas can be revised
through a variation.
11. The "clean water diversion" along the northern boundary of SB -area 9 will deposit concentrated
flow into the conservation area. Also, a "clean water diversion" is not an erosion and sediment
control measure recognized by the VSESCH, and can only be approved if a variance is granted by
the Program Authority.
(Rev. 1) Comment has not been addressed. The variation has been considered and engineering
review does not believe the use of a cleanwater diversion here is appropriate.
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Engineering Review Comments
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12. There are no E &S measures protecting the area within the limits of disturbance to the southwest of
S139. The limits of disturbance and tree protection are shown within the stream buffer.
(Rev. 1) This comment has been addressed.
13. It is unclear how the silt fence will "prevent excess disturbance" in the conservation area. This silt
fence is shown installed contrary to the requirements of the VESCH. Tree protection or safety
fence may be more appropriate.
(Rev. 1) Chain link fencing with concrete bases may not be able to stand on many of the steep
slopes within the property. Alternatives to the chain link with concrete bases include driving the
posts for the chain link fence into the ground or orange safety fence. If two different fence
types are provided, please clearly indicate the variety on the plan and legend.
Also, please note that any construction outside of the preservation area that damages or kills a
tree within the preservation area will be considered a zoning violation.
14. A portion of the diversion dike to SB8 is outside the limits of disturbance.
(Rev. 1) This diversion should be extended to the diversion to SB -4 and it should be on the
downhill side of the access path to the facility for as long as possible.
15. Please expand the limits of disturbance on Sheet ESC -3 to include the natural drainage swale that
will carry "bypass flow" to S138 in anticipation that stabilization or repairs may be required within
this swale.
(Rev. 1) Comment is no longer applicable.
16. The longer baffle in S138 appears to bisect, rather than redirect, the flow received from the
diversion dike.
(Rev. 1) Baffles in many sediment basins need to be reevaluated and extended to meet state
guidelines.
17. Limits of disturbance and tree protection fencing surrounding S137 are shown within the
Preservation Area.
(Rev. 1) This comment is no longer applicable because the ESC plan has been adjusted.
18. The diversion dike directing flow to S135 needs to be extended to the length of the drainage area
boundary.
(Rev. 1) This comment is no longer applicable.
19. The stockpile area next to SB 1 should be within the limits of the drainage area of SB 1.
(Rev. 1) Comment has been addressed. All stockpiles appear to drain to sediment basins or
traps.
20. MS -19 is not necessarily just and "end -point analysis ". Cross - sections of the streams and swales
within the property should also be studied for adequacy. It is recommended that a cross - section be
taken at each sediment basin/trap outlet.
(Rev. 1) Comment has not been addressed. No adequate channel analysis has been provided
for basin outlets between the pipe and existing streams on site. In several instances, channels
do not even appear to exist. Constructing adequate channels at basin outlets through the
preservation areas to reach existing streams will likely be unavoidable. Please contact the
Planning Department to determine whether the preservation areas can be modified without
Board approval.
21. In order to meet the requirements of MS -19 with respect to the existing intermittent streams and
swales within the property, the applicant will conduct current condition analyses of the channels
by performing a field visit to document (with notes and photographs) the current condition of the
full reach of the streams. If degradation of the streams should occur as a result of the on -going
construction or development of the road, the applicant is responsible for implementing corrective
measures to the satisfaction of the County.
(Rev. 1) Adequate channel concerns have not been addressed. Please make the following
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Engineering Review Comments
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adjustment /corrections to the analysis before resubmitting.
a. More cross sections should be provided along each stream and channel except for
Biscuit Run. One representative cross - section every 200ft should suffice.
b. All computations should be performed using the 24 -hour storm(SCS), not the Rational
Method.
c. Except for the roadside ditches and new swales created with this plan, all channels will
be considered natural channels and only require analysis of the 2-year storm (velocity
must be non - erosive and the flow contained within the channel banks).
d. For each channel cross - section segment (most of your cross - sections have at least four
segments) please provide a separate n- value, permissible velocity (Table 5 -22), and
computed velocity. It appears that the program used for this computation assumes a
uniform n -value across the whole channel cross - section and computes an average
velocity for the entire section. In reality, the water flowing within the banks of the
channel flows much quicker than the water that flows above the banks.
e. Pre - development flows are not required in adequate channel calculations. It should be
noted that in several locations the applicant shows that the post- development flows are
increasing with development which would mean the plan is not in compliance with 17-
314.
f. A drainage area map showing the hydrological data to each point of analysis should be
provided. [DM/
g. The farm road crossing downstream on SB -6 should be converted to an adequate
channel or calculations showing that this area is adequate should be provided.
h. The MS -19 analysis on the east end of the site along Route 20 must be re- evaluated.
The culverts running underneath Route 20 have not been analyzed. It appears that
runoff from this development will impact the stream located on the east side of Route
20. This stream should be evaluated at regular intervals including the drainage
structure that runs underneath Route 20 at the southeast corner of the property.
22. An E &S bond amount will be computed by the County once the plans have been approved.
(Rev. 1) Comment remains unchanged.
After the plans have been modified and a more extensive review was performed, the following comments
have been added.
23. (Rev. 1) Terminate the connector road so that a diversion can be placed on existing grade at the
base of the fill, rather than in fill, that can direct runoff to SB -l7. This may require a
modification to the inlet closest to SB -17.
24. (Rev. 1) The grading around SB -14 and ST -4 does not appear to work. Please re- examine the
grading in this area.
25. (Rev. 1) All diversions should be placed on the downhill side of all access roads for as long as
possible. When the diversion must cross an access road, please provide a ROW diversion.
26. (Rev. 1) Sediment Basin 4 appears to be difficult to construct correctly. Please modify this
facility to a more reasonable shape. It appears that either a portion of the road network should
not be constructed or the preservation area should be modified.
27. (Rev. 1) Please provide a note that no construction access will be allowed into or out of the site
from the existing farm road /Old Route 20.
28. (Rev. 1) Any diversion with a drainage area greater than 5 acres should be called out as "DV',
as opposed to "DD'; and geometrically designed.
29. (Rev. 1) Sediment basin designs should not rely on diversions to be constructed for the sole
reason of reducing drainage areas to certain facilities. Please remove the following diversions
from the plan and redesign the affected facilities, if necessary:
Albemarle County Community Development
Engineering Review Comments
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- the diversion to basin 2 that directs existing runoff from basin 3
- the diversion to basin I that directs runoff from trap 2; trap 2 should be made
into a sediment basin (considering the modifications required in Comment
#1); once proposed grade is reached, the storm drainage system is in place,
and the slope has been stabilized, the basin should be removed and the area
replanted to mimic the existing landscape
- the diversion to basin 16 that prevents runoff from entering SB 17 in the post
development scenario.
- the diversions to the culvert in Section C; the traps should be sized for the
overland flow without diversion; the natural drainage patterns should take
most of the runoff to the culvert without diversions
- the diversion to basin 3 that directs runoff from ST -I
30. (Rev. 1) A diversion is needed on the north side of the disturbance along road V directing
runoff to the trap 2 and away from Avon Rd. Extended.
31. (Rev. 1) Outlet protection for all sediment basins and culverts should meet the standards of
VESCH 3.18 and not VDOT EC -1. Also, outlet protection should be placed on existing grade
when possible and not halfway up embankments.
32. (Rev. 1) Please provide an appropriately sized culvert at the construction entrance.
33. (Rev. 1) Waterfront the wash rack must drain to a sediment trap or sediment basin. This
current plan does not protect the construction entrance until the drainage structures are in
place. The applicant can either provide a much longer entrance with the wash rack at a
location that drains to the trap or the trap should be located closer to route 20. Either way, the
construction entrance should also be shifted to the north side of the road cross section and the
trap should be pulled out of the buffer as much as possible (see Planning Department
comments). The trap can be removed once the drainage system is installed and wash water is
then directed to SB -3.
34. (Rev. 1) The maximum drainage area to Culvert Inlet Protection is 3 acres. Any drainage area
greater than I acre must use Sediment Trap Culvert Inlet Protection.
35. (Rev. 1) Please update all "Volume Provided" cells in the sediment basin and trap spreadsheets
to the actual values provided.
36. (Rev. 1) Please provide dewatering orifice calculations for each basin. All orifices appear to be
oversized.
37. (Rev. 1) If possible, all emergency spillways should be located in existing ground. If spillways
are provided on basin embankments, please provide adequate rip -rap lining until a verified
adequate channel is reached.
38. (Rev. 1) Please specify a non - grassed, low maintenance ground cover on all slopes steeper than
3:1. The proposed landscaping should be similar to those plants found in Table 3.37C of the
VESCH.
39. (Rev. 1) A few anti- vortex /trashracks are called out incorrectly.
40. (Rev. 1) Inadequate freeboard appears to exist on Basins 1, 3, 5, 11, and 13.
41. (Rev. 1) The maximum height for a sediment trap is 5ft (from toe of slope to top of the
embankment).
42. (Rev. 1) This WPO plan must be approved at the same time as the road plans.
43. (Rev. 1) A grading permit will not be issued until the Planning Department has confirmed that
Proffer 10 has been entirely addressed.
Stormwater Management Plan
1. (Rev. 1) The applicant has two options going forward with the SWMplan. Either remove all
references to future development other than stating that future site and subdivision plans will
Albemarle County Community Development
Engineering Review Comments
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need to prove compliance with water quality and quantity requirements or size and provide
computations for full development now. With the latter option, the applicant must provide on
sheet SWM - -3 impervious area credits within each street block (not the rezoning definition of a
block), exclusive of the impervious area for sidewalks and roadways that will be installed with
this plan, that future site and subdivision plans will be held to.
Compliance with Proffer 5D will be evaluated with the next review after the decision regarding
the stormwater plan is made.
2. (Rev. 1) A removal rate computation (Albemarle County's Modified Simple Spreadsheet) for
each facility is required. One overall analysis will not be accepted.
3. (Rev. 1) Detention compliance calculations should be provided for each facility rather than at
the end of a stream.
4. (Rev. 1) When routing the facilities please use the critical storm. The output indicates that a
specific time duration was entered for the storm.
5. (Rev. 1) Do not include the permanent ponding volume of the sediment forebay in the biofilter
routings. It should be assumed that any volume below the spillway of the forebay will be full of
water at the start of the design storm.
6. (Rev. 1) The 4% of the impervious area biofilter bed sizing method assumes that a Ift ponding
depth is provided.
7. (Rev.]) The rock weirs should not be used as the principal spillway for these facilities. There
is too much uncertainty in the construction and routing of these spillways to have faith that
detention will be met for each watershed. Please specify a weir wall or riser structure within
these facilities.
8. (Rev. 1) Stormwater facility access roads cannot be shown over the primary spillways.
9. (Rev. 1) Access roads to several facilities are shown over slopes steeper than 20% and in some
areas slopes steeper than 10% are not graphically proposed to be graveled.
10. (Rev. 1) Access roads should be shown to all sediment forebays and spillways in each facility.
11. (Rev. 1) Please remove "IOft min. "from the bottom of the typical biofilter detail.
12. (Rev. 1) Where possible, spillways should be located in existing grade and not on top of the
embankment.
13. (Rev. 1) The proposed planting plan within each biofilter does not appear to meet county
requirements.
14. (Rev. 1) A few of the pre - development time of concentrations appear to be too quick. For
instance, the velocity of the water within the intermittent stream on sheet SWM -1 is shown as
8.Ofps, but in the MS -19 calculations the velocity was closer to 2.6fps. Please re- evaluate all
time of concentrations.
15. (Rev. 1) Be consistent in the rational coefficient attributed for the existing condition. In the
pre- development analysis, existing surfaces are shown as having a coefficient of 0.3 but in the
post- development condition that changes to 0.2.
16. (Rev. 1) Please provide post- development rational method computation for each drainage area.
Please use 0.35 for all grassed areas and 0.9 for all roadway and sidewalk. (It may be easier if
the SCS method was used for SWM since it is required for adequate channel computations.)
17. (Rev. 1) Any drainage area greater than 50 acres requires checking the calculation with a
second hydraulic method (for SWM, not MS -19).
18. (Rev. 1) Engineering review recommends that the applicant speak to the Planning Department
regarding the conversion of some of these larger facilities to Retention ponds. The large
drainage areas and high discharges into many of these facilities could be a long term
maintenance issue.
19. (Rev. 1) Please provide a storm waterfacility maintenance agreement and fee for TMP 90 -6D.
20. (Rev.]) A SWM bond amount will be computed by the County once the plans have been
Albemarle County Community Development
Engineering Review Comments
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approved.
Once these comments have been addressed, please submit 4 copies of the revised plans and reports to
Current Development Engineering.
Current Development Engineering is available from 2:30 -4 PM on Thursdays to discuss these review
comments. Please contact Phil Custer at 434 - 296 -5832 ext. 3072 or email pcuster @albemarle.org to
schedule an appointment.
E2 esc swm PBC Biscuit Run Connector and Phases A and D.doc