HomeMy WebLinkAboutSDP201200013 Review Comments Final Site Plan and Comps. 2012-03-30COUNTY OF ALBEMARLE
Department of Community Development
401 McIntire Road, Room 227
Charlottesville, Virginia 22902-4596
Phone (434) 296-5832 Fax (434) 972-4126
Project: Dunlora Forest WP0201200028, SDP201200013
Plan preparer: Dominion Engineering [fax 979-16811
Owner or rep.: ROCK CREEK PROPERTIES LLC [fax 245-0895]
Plan received date: 7 March 2012
Date of comments: 30 March 2012
Reviewer: Max Greene
The site development plans (WP0201200028) submitted 7 March 2012 have received
Engineering Review and do not appear to meet Albemarle County minimum checklist
items for approval. Please adequately address the following comments for final approval:
A) Site Development Plan (SDP201200013)
1) Please update the date and source of the topographic information: All topography
should be at least visually field verified by the designer within the last year [18-
32.6.6, 14-302, Policy for date]
2) Sheet title for 12 of 28 will need to be corrected. This sheet shows a Public Road.
3) The Right-of-way for Private Road `B" needs to include all pavement for access
and maintenance.
4) All easements will be a minimum 20' wide.
B) Road and drainage plans (SDP201200013)
1) VDOT approval is required. Comments will be forwarded when received. [DM905]
2) The turnarounds will be designed per VDOT alternative turnarounds design. [14-410.C]
3) Drop inlets should have sump areas to help allow storm water to enter the structures
before overflowing down to the next structure.
4) Curb inlet structure 26A appears to be jutting past the curb and completely exposed on
the back slope.
5) Drainage for Private Road `B" does not appear to meet the minimum requirements .
Please add and inlet to the end of the parking row and show spot elevations for proper
drainage.
6) Please show the curb to curb width on private road "A" detail on sheet 8 of 28.
C) Stormwater Management (WP0201200028)
1) Detention analysis will be at the outlet of the structure for the site drainage area. Don't
include off-site areas.
2) Please turn/move emergency overflow outlet to coincide with the primary outlet to reduce
buffer area disturbance. It appears to be very close to the footbridge for the walking trail.
3) The gabion baskets used between the forebay, cell 1, and cell 2 are not a good measure to
Engineering Review Comments
Page 2 of 3
maintain a constant water level, because the design of a wetland is so sensitive to depths,
a more rigid outlet structure like a concrete weir is necessary
4) In swm 1, the 3" orifice is located at the bottom of cell 3. It should be located 1.5' from
the normal bottom elevation. The VSMH details also shows a micropool in front of the
outlet structure
5) The maximum slope on a pipe system is 16%, concrete anchors should be used on the
barrel from SWM 1. The outlet into SWM 2 might be highly erosive and warrant greater
detail with regard to OP
6) The sanitary sewer line through the stormwater facility will need to be moved.
Maintenance and repair work could break sanitary sewer line.
7) The water quality calculations for SWM -1 seem to be incorrect. The calc seems to refer
to a Type H pond when the 4 x Wqv calculation is shown. 14557 cf of water is needed in
the first 18" of each of the cells and the forebay, and I just don't see it in the calculations.
The application should include in plan view a way to show the high and low marshes and
water elevation of each cell to help with the review.
8) A low flow path is recommended in SWM -2.
D) C. Erosion Control Plan (WPO201200028)
1) Narrative Project description states total area disturbed is 16.34 ac. However, disturbed
areas add up to 18.05 on plan. Please verify total proposed area of disturbance.
2) Temporary sediment basin calculations are not included in submittal package. No review
was possible at this time. Additional comments may be forthcoming upon receipt of all
required calculations.
3) Drainage areas appear variable once grading operation begins. Please add a note to the
plan and narrative "In the event drainage areas change during construction, sediment
control structures will be reconstructed to adequately protect the new or anticipated
drainage area per VESCU.
E) MS-19/Routing Comments
1. Pre -Development and Post -Development drainage areas must not use offsite areas. The
pre -development target rates must be for the watershed roughly equivalent to the post -
development drainage area for SWM -2. The eastern tributary of DA -A and all of DA -13
must be removed from all computations for pre -development and post -development
runoff rates (though it's important to keep this tributary for the analysis of the
downstream channels). The calculation must be simplified to compare a pre -development
watershed for the western tributary of DA -A to the release rates from SWM -2.
2. The weighted curve number calculation for DA -X does not added up to 17.56 acres, but
15.57 acres. It seems that there is impervious area missing from this computation.
3. The 1. 5, 2, 10, and 100 year rainfall depths are 3.6", 3.7, 5.6", and 9.1", respectively.
The applicant's calculations seem to use older numbers.
4. The post -development curve for DA -X is much too low. For a townhouse development
and mostly C soils, Table 4-6a of the VSMH shows a CN of 90. It seems the error in the
applicants calculation is that the incorrect lawn and wooded values were used. For C
soils, use a CN of 74 and 70 for lawns and woods, respectively. This will have a dramatic
effect on the detention calculations.
Engineering Review Comments
Page 3 of 3
5. The adequate channel submittal package provided by the applicant did not meet our
general expectations as outlined on page 7 of 35 of the Design Manual. Each cross-
section must be broken down into individual segments that have an individual n -value,
permissible velocity, and computed velocity. The calculations package assumes uniform
n -value and permissible velocity for each cross-section and the program finds one average
velocity. Higher velocities are hidden in the deeper portions of the channel by the flatter
segments in the applicant's analysis. Also, photographs are needed for each cross-section
so the n -value and permissible velocities for each segment of each cross-section can be
superficially checked.
Regardless, the applicant's analysis concludes that erosive velocities are present in the
downstream channel after development and states that "over -detention" is desired to
alleviate the situation. The over -detention solution is offered by the state and requires the
release of the 1.5, 2, and 10 -year storms a factored pre -development forested rate. This
alternative is acceptable to county engineering but the analysis should only include onsite
drainage areas (X and the western half of A) in the computation. Right now, it's difficult
to determine what the appropriate targeted release rates should be because of the issues
with the CN and drainage areas previously commented on. Because the site is composed
of about 90% C soils, the pre -development forested CN is 69, not 51. Please provide a
clear calculation for the 1.5, 2, and 10 -year allowable release rate.
Please also provide an analysis of the channel from the confluence of the two channels
from DA -A and to the confluence with the channel from DA -B.
6. The curve numbers for DA -Y and DA -Z are too low. DA -Y is composed of a
development of 1/3 acre lots in B soils, which has a CN of 72, and a mostly wooded area
of C soils, which has a CN of 70. Given these watershed characteristics, a CN of 63 is
not possibly. A similar error was made for DA -Z.
Once these comments have been addressed, please submit 3 copies of the revised plans,
calculations, and narratives to Current Development Engineering along with the required review
fee and transmittal form.
Current Development Engineering is available from 2:30-4 PM on Thursdays to discuss these
review comments. Please contact Max Greene at 434-296-5832 ext. 3283 or email
mgreene@albemarle.org to schedule an appointment.
[17-204.f] An application for an erosion and sediment control plan that requires modifications, terms, or conditions to be
included in order for it to be approved shall be deemed to be withdrawn if the owner fails to submit a revised plan addressing the
omitted modifications, terms or conditions within six (6) months after the owner is informed of the omitted information as
provided under paragraph (B).
Filc: CDDEI_fsp,csc,swm_MRG_Dunlora Forest.doc