20161204 Ver 1_Additional Information Request_20161222

Strickland, Bev
From:Scott Burwell <[e-mail]>
Sent:Thursday, December 22, 2016 10:54 AM
To:Larsen, Cory
Cc:John Vilas; Adam Waldroup
Subject:Additional Information Request for Mauney Cove Convenience Center 20161204
Attachments:Mauney Cove Conv Geotech Report June 2016.pdf; Drainage Map Exhibit-ex-1.pdf
Follow Up Flag:Follow up
Flag Status:Flagged
Cory,

Hope you are doing well today. John Vilas forwarded me your email with questions/comments on Mauney Cove. Please
see below for my responses in red

1.Please provide a final drainage basin map delineating areas captured by the proposed bioretention cell. Ensure

that all new impervious surface area is treated by the BMP.
Please see the attached drainage area map. The pre development Impervious area is 16,279 SF. The post
development impervious area is 30,039 SF. The required treatment surface area is 13,760 SF. The actual
impervious surface area routed to the bioretention cell is 16,236 SF.

2.The bioretention cell is being proposed in the vicinity of Wetland-WA which is noted in the wetland

determination data to have surface saturation and water table present 3” below surface. Existing elevation
appears to be about 2586 ft. The bottom of the bioretention cell is located at 2582 ft and supplement form
states that the SHWT is at 2580 ft. A minimum of two feet of separation is required for the selected
BMP. Please revise the design or provide information supporting SHWT at elevation of 2580 such as a soils
report from a licensed soil scientist.
Please see the attached geotechnical report provided by Bunnell-Lammons Engineering, INC. Boring B-1
(located within the proposed bioretention cell location) indicates that the SHWT is at 2580’. The bottom of the
proposed bioretention cell is at 2582’, therefore providing 2’ of separation from the SHWT.

3.The supplement form indicates “Other” as the form of pretreatment provided. Please elaborate on how this will

be achieved.
The pretreatment requirement is fulfilled by routing the stormwater via sheet flow over a 6’x60’ section of sod
prior to entering the bioretention cell.

4.Please provide (or clarify where located) calculations that demonstrate stormwater discharges from the outfall

located adjacent to Mauney Cove Branch will be released at a non-erosive rate (defined as v ≤ 2 fps) for 10-year
storm flows in accordance with 401 stormwater requirements (attached).
The discharge velocity for the outfall located adjacent to Mauney Cove Branch was calculated at approximately
4 fps. The slope of the pipe cannot be adjusted flatter due to the constraints of the site relative to ground water
elevation and the vertical separation required with existing sewer line in the shoulder between the road and the
stream. The pipe discharges directly into the stream to allow stream flow to dissipate the velocity.

Feel free to give me a call if you have any questions or need to discuss further. I can print hard copies and mail the
attachments if you need them for your files, just let me know. Have a Merry Christmas and Happy Holidays!

Scott Burwell, PE
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Project Manager

55 Broad Street | Asheville, NC 28801
Phone: [phone] | Mobile: [phone] | Fax: [phone]
Email: [e-mail] | Website: www.mcgillengineers.com

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