HomeMy WebLinkAboutWPO200600082 Review Comments Stormwater Management Plan 2009-11-13Philip Custer
From: Philip Custer
Sent: Thursday, November 12, 2009 7:42 PM
To: 'Roy Whitlock ';'gloeckner @embargmail.com'
Cc: Amy Pflaum; Andrew Lowe
Subject: SWM plan for Westminster Cantebury (WPO- 2006 - 00082)
Attachments: Impervious Area Comparisons.xlsx
Good evening,
At the meeting held on Friday 25 September 2009 attended by Roy Whitlock, Kurt Gloeckner, Amy Pflaum, Andy Lowe,
and myself, the status of the Stormwater Management Plan for the Westminster Cantebury Duplex replacement project
was discussed (WPO- 2006 - 00082). In short, the site was not constructed to plan and fell short of County Stormwater
requirements. At the time, it was not known how insufficient the constructed site was. A detailed review was
necessary. The purpose of this email is state Current Development's analysis of the existing situation and a
recommendation of a potential solution to the issue.
The first step in the analysis consisted of calculating the impervious area of the site both before and after the cottage
replacement project. (Please see the attached Excel file for the detailed calculations.) Before the replacement project,
the existing impervious area onsite (excluding Pantops Mountain Road) was 24,040sf. The approved site plan for the
replacement project showed approximately 42,480sf, an increase of 18,440sf. Next , I visited the site to determine how
much of each building drained to a biofilter facility (though most facilities on site will require reconstruction because
they were not built to plan). For the whole site, approximately 6,700sf of imperviousness drains directly to a biofilter.
The rest escapes the site without treatment down the eastern slope or into the drainage system along Pantops
Mountain Road.
After considering the situation of the current Stormwater Quality treatment deficit, it is my recommendation that the
applicant design a biofilter east of building 5 at the outlet of the stormwater pipe and culvert. The bed area of this
facility must be at least 900sf (the square footage of the rain gardens in the approved plan and roughly 2.5% of the
impervious area to the new biofilter). The biofilter should be cut into the hillside as much as possible to reduce the size
of an embankment above the residential development downstream. With the SWM plan amendment showing the
biofilter, we can ignore the issues with existing rain gardens and exclude them from the SWM plan so no future
inspections of these will be performed and the owner can modify the area as desired (though a minor site plan
amendment may be required).
Please contact me if you have any questions.
Thanks,
Phil
296 -5832 x3072