HomeMy WebLinkAboutWPO201000070 Review Comments Erosion Control Plan 2011-02-07� 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: National College (WPO- 2010 - 00070)
Re: Minimum Standard 19 (Adequate Channel) Calculations
Plan preparer: Mr. John Hash, PE, Timmons Group
Owner or rep.: Corolla Management Corporation
Plan received date: 28 December 2010
Date of comments: 7 February 2011
Reviewer: Phil Custer, Current Development Engineering
This memo is being written to provide comments regarding the adequate channel calculations for National
College. The second submittal of the plan had been reviewed with comments provided February 2nd.
However, the adequate channel information on Sheet C6.3 was overlooked. Please consider this comment
letter an addendum to John Diez's 2 °a review comment letter.
Please provide detailed calculations for the MS -19 analysis on channel adequacy. Please refer to
the Virginia Erosion and Sediment Control Handbook for proper guidelines.
(Rev. 1) The adequate channel calculations provided by the applicant are not sufficient. The
following comments are provided.
a. The channel sections provided by the applicant indicate constructed man -made ditches.
I could not find any instance along this natural channel that matched any of these
cross - sections. Analyses of irregular channel cross - sections with varying permissible
velocity, allowed velocity, and n -value for each segment across the cross - section are
necessary. These calculations can be done iteratively by hand or through a computer
program. If you do not have a program that will do this calculation, I can provide you
with a basic one created by the County Engineer, Glenn Brooks.
b. No analysis of the 24" culvert was provided. It appears that this culvert may cross
Route 29 independently, without mixing with the drainage system of the shoulder and
median ditches. On my site visit last week, I noticed that the 24" culvert did not connect
to the median structure. I was unable to confirm whether the structure south of the
entrance to JW Townsend was also separate from the 24" culvert.
c. There is no channel 120ft from the outlet of the detention system. It looks as though
there might have been a culvert placed in this location which was clogged by debris and
silted over. This is causing considerable problems on the downhill side of the culvert
where there is rapid erosion in the stream. After reviewing this section of the stream
especially, there does not appear to be an existing adequate channel downstream of
development. Some work appears to be needed on the adequate property. An
alternative could be providing much more detention onsite.
d. Please note that the 1% rule is not reached until 350ft downstream of the outlet of the
Route 29 culvert where the stream intersects Flat Branch. In an earlier conversation
with the applicant, County staff stated they would not require a detailed analysis east of
the Route 29 culvert.
e. Please show the stationing on a plan view drawing so the location of the cross - sections
Albemarle County Community Development
Engineering Review Comments
Page 2 of 2
can be more easily identified.
f. The profile centerline on C6.3 appears to be located outside of the stream.
2-7-20 1 1 MS -19 Mcmo national colleac.doc