HomeMy WebLinkAboutWPO200800059 Review Comments Stormwater Management Plan 2009-02-27ALg�,��
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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: WPO- 2008 - 00059, Airport Runway Extension SWM Plan
Plan preparer: Delta Airport Consultants, Inc.
Owner or rep.: Barbara Hutchinson, Executive Director
Date received: 29 January 2009
Date of Comment: 27 February 2009
Engineer: Phil Custer
The SWM plan for the Airport Runway Extension Project, received on 29 January 2009, has been
reviewed. The plan cannot be approved as submitted and will require the following changes /corrections
prior to final approval.
A. General Review Comments
I. Safety Slabs should be specified as SL -1, not SL -7.
2. Outlet protection calculations do not appear to be given for all necessary temporary and
permanent outlets. A few of the outlets missing from the calculations package include D123,
D109, slope drain on sheet 15, slope drain on sheet 13, and D114. The dimensions of the
outlet protection should also be detailed in the drainage profiles.
3. The INV In is missing from D105 in the profile.
4. Many structures on Exhibit 1, "Phase 1 Design Drainage Map ", are not labeled with the same
title as they are in the set. Please make sure the exhibit is coordinated with the sheet set.
The grate inlet analyses for the DI -5 and DI -7 structures should use the inlet capacity charts in
the VDOT Drainage Manual rather than the weir equation calculation. (For DI -5, the DI -7
capacity chart can be used since the grate options are the same.)
The ditch design computations tables show EC -2 as proposed for some sections of the ditch
but sheets 13 -16 specify only EC -3. Please clarify.
It appears a few channels have been omitted from the calculations (from D112, from D114,
and from D121A). Additionally, there is no information in the plan set on the channel in the
calculation package labeled as "ERSA" (left and right).
The package is missing the culvert profiles for the two box culverts at the perimeter fence and
the two 36" culverts underneath Crickenberger Lane.
B. SWM Plan Review Comments:
The portion of this SWM plan dealing with water quality treatment is unclear and will require
greater clarification.
a. The applicant must submit copies of the County's Modified Simple Spreadsheet for each
SWM facility's drainage area. The spreadsheet used by the applicant to compute removal
rates does not yield the same results as the county's spreadsheet. For the west, east, and
north basins, I computed removal rates to be 44 %, 18 %, and 47 %, respectively.
b. In the SWM narrative, it appears that the applicant wishes to provide water quality
treatment with extended detention facilities. However, the removal rate of a standard
extended detention facility is only 35% (VSMH). A 50% removal rate can only be
achieved in an extended detention facility when a permanent pool wetland is provided
(Enhanced Extended Detention).
c. I could not find in the package the VSMH sizing calculation for the drawdown orifices in
the extended detention facilities. On the next submittal, please include these calculations.
d. All SWM treatment facilities require sediment forebays. Calculations are required for all
sediment forebays.
e. Additional comments may be required based on the changes and provided calculations.
2. With the provided information, it is difficult to definitively conclude that detention
requirements have been met, though it appears that the volume provided in the three facilities
are sufficient. Please see below for the a list of comments regarding the detention calculations:
a. The pre - development drainage area map should show the existing contours at the time that
the basin at the edge of the runway was designed. The predevelopment drainage area map
in the calculation package appears to be the existing conditions on site today.
b. It appears that the composite CN values used for each watershed are a bit low and may
underestimate the discharge to each facility. Please provide more detailed calculations for
each post and pre - development watershed. The soil types in each watershed have
significant impacts on CN values so it would be helpful to overlay the soils map on the
drainage area map exhibit. And, since the majority of the post - development drainage area
will include fill soil from offsite, it is necessary to identify the borrow site for this project
and identify the soil characteristics of it.
3. Please provide an embankment section and drainage structure /riser detail (similar to the ESC
details found on sheet 36) showing all critical information regarding the permanent setup of
each facility. The detail should show the embankment and note that it must be certified by a
geotechnical engineer if an impervious core is not provided specifically on the detail.
4. All basins appear to impound some volume of water permanently. The lowest orifice is above
the deepest contour in the provided routings.
5. Please use the latest values of the 24 -hour storm for Albemarle County found in the Design
Manual for all runoff calculations. (3.7 ", 5.6 ", and 9.1" for the 2, 10, and 100 -year storms)
6. The input for the routing of basin A shows a 48" manhole structure but there does not appear
to be one provided.
7. Please provide greater clarification regarding the elevation - discharge relationship of all of the
anti - vortex structures on the top of all risers. I do not understand the input values in the
routing provided by the applicant for each top of structure.
8. Please provide vehicle access to all sediment forebays, structures, and embankments. The
grade of the vehicular access can be no greater than 20% and must be surfaced (gravel,
pavement, geo -grid, or other) in all places where the grade is greater than 10 %.
9. From the forebay of basin C to the riser structure, please provide a low flow channel so the
slope from the forebay and the bottom of the basin does not erode.
10. On both the pre and post development maps, please provide the time of concentration and
average CN for all drainage areas.
11. A bond will be computed once all comments are addressed.
C. Erosion and Sediment Control Plan Update
1. We are still waiting for the analysis of the downstream channel. This issue must be resolved
before a grading permit can be issued.
Once the modifications have been made to address these comments, please submit one full copy of the plan
and calculations to Engineering for review. Please contact me at (434)296 -5832 ext. 3072 should you
have any questions.
File: El_swm_PBC _ wpo- 2008 -00059 Airport Runway Extension.doc