HomeMy WebLinkAboutWPO201100043 Review Comments Erosion Control Plan 2011-10-18ALg�,��
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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- 2011 - 00043, Bramley Farm — E &S
Plan preparer:
Eugene Ryang (ERYANG @WATERSTREETSTUDIO.NET)
Owner or rep.:
Bran ley Farm, LLC
Date received:
7 July 2011
Rev. 1:
18 August 2011
Date of Comment:
14 July 2011
Rev. 1:
18 October 2011
Engineer:
Phil Custer
At the request of the previous engineer for this project, John Diez, I have reviewed the MS -19 analysis for
compliance with state and county standards.
5. MS -19 calculations and information must be submitted to illustrate that the channel downstream of
the sediment basin is adequate.
(Rev. 1) The adequate channel analysis provided by the applicant is not satisfactory. The
following modifications are necessary:
a. The analysis must use a hydrologic method utilizing the 24 -hour storm, not the rational
method. Minimum Standard 19 is very clear in this regard. "Properties and waterways
downstream from development sites shall be protected from sediment deposition,
erosion and damage due to increase in volume, velocity and peak flow rate of
stormwater runoff for a stated_ frequency storm of 24 -hour duration in accordance with
the following standards and criteria...'.
b. The volume reduction credit the applicant is asserting in this analysis is not valid. The
proposed infiltration and cistern practices intercept the rst portion of the rainfall and
do not capture the volume that the calculation assumes is being captured like a normal
detention basin where water is continuously being released from the site. In addition,
this calculation uses the modified rational equation which the previous comment stated
was not to be used for MS -19 analyses. Also, if the applicant wishes to use facilities,
whether volume or detention, in the MS -19 analysis, these facilities will need to be
approved, bonded, and inspected annually by the county. The owner will also need to
sign a maintenance agreement with the county. For these reasons, I recommend not
introducing these facilities into the MS -19 calculations.
c. Please cite an acceptable source for the use of check dams every 250ft as being
considered an "appreciable" obstruction. In the Virginia Erosion and Sediment
Control Handbook, check dams are designed in a way that spaces them based on the
slope of the channel so that the downstream toe of a check dam is equal in elevation to
the top of a downstream check dam. A check dam every 250ft will not have a
noticeable effect on the velocity of the majority of the channel.
d. Is this channel an intermittent stream?
e. New channels will need to be design and constructed through the sediment basin since
these areas will be destroyed during the construction and removal of the sediment
basin.
f. There is a detail in the bottom right corner of Sheet MS -1 that refers to the stone being
specified in the downstream channel. Please specify where the riprap in this detail is to
be utilized in Reach A -B or B -C.
g. The use of EC -3 matting in an existing, grassed channel is impractical. The matting
will kill a large percentage of the grass currently helping to stablize the channel and
the water will flow underneath the matting at nearly the same velocity as without.
h. Please provide photos of the downstream channel, especially at each analyzed cross -
section. [Design Manual]
CDE2_esc_PBC_11 -43 - Bramley Farm - E &S_MS -19 Analysis.doc