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Home My WebLink About20170151_RE B-4960 culvert drainage area and flow rates_20161104 Strickland, Bev From:Wanucha, Dave Sent:Friday, November 04, 2016 4:45 PM To:'Smith, Charles' Cc:Thomson, Nicole J; Fisher, Laura Subject:RE: B-4960 culvert drainage area and flow rates Thanks for the information. Have a good weekend. Dave Wanucha Division of Water Resources Transportation Permitting Unit NC Department of Environmental Quality 336-776-9703 office 336-403-5655 mobile Dave.Wanucha@ncdenr.gov NC DEQ Winston Salem Regional Office 450 West Hanes Mill Road, Suite 300 Winston Salem, NC 27105 Email correspondence to and from this address is subject to the North Carolina Public Records Law and may be disclosed to third parties. From: Smith, Charles \[mailto:Charles.Smith2@aecom.com\] Sent: Friday, November 04, 2016 2:03 PM To: Wanucha, Dave <dave.wanucha@ncdenr.gov> Cc: Thomson, Nicole J <njthomson2@ncdot.gov>; Fisher, Laura <Laura.Fisher3@aecom.com> Subject: RE: B-4960 culvert drainage area and flow rates Dave, For a new crossing, I look at zoning, make an estimate of future impervious, and base the flow rates on that. In this case the 2006 land covers are adequate as explained below. The existing 66” culvert under I-40 will control hydraulically, so as long as the proposed pipe extension has as much or more capacity than the 66” pipe (which the 72” extension does) the configuration will work. In other words the existing 66” is more restrictive, so the proposed 72” extension pipe is adequate to receive flow from the 66” for any flow rate. NCDOT typically checks adequacy of the riprap on the banks downstream of box culverts based on the 10-year storm. I used the 50-year storm as a belt and suspenders approach, mostly because the existing scour hole is evidence of an 1 existing problem. The existing riprap which has been pushed away from the culvert outfall looks like Class B. Larger Class I riprap is proposed in the riprap basin to prevent displacement from happening in the future. So these are measures that have been proposed to ensure future stability. If increases in impervious area cause higher flow rates in the future, the existing 66” pipe will act as a throttle to reduce the impact of flow increases downstream. In other words, the upstream embankment of I-40 will act as a detention pond to some degree. This will not totally mitigate the effect of increased flow rates reaching the riprap basin, but it will reduce it by some percentage. I have not modeled the effects, but qualitatively this is the expected outcome. Finally, I ran a quick check in HY-8 to see what the riprap basin dimensions would look like using a 50-yr flow rate of over 287CFS assuming approximately 50% impervious, instead of the 50-yr flow rate (197CFS) assuming 2006 impervious cover. The Class I riprap remained adequate, and the dimensions only increased about 0.1’ in each direction. (The contractor cannot place riprap to that level of accuracy.) Charles R. Smith, PE Hydraulics Engineer, Transportation & Site Civil D +1-919-854-6240 Charles.Smith2@aecom.com AECOM 701 Corporate Center Drive, Suite 475 Raleigh, NC 27607, United States T +1-919-854-6200 aecom.com Built to deliver a better world LinkedIn Twitter Facebook Instagram From: Wanucha, Dave \[mailto:dave.wanucha@ncdenr.gov\] Sent: Friday, November 04, 2016 10:43 AM To: Smith, Charles Cc: Thomson, Nicole J; Fisher, Laura Subject: RE: B-4960 culvert drainage area and flow rates Charles, Thanks for the info. I confirmed the drainage area and percent impervious area using stream stats website. It appears that the land cover data are from 2006. When designing culvert size and riprap dissipator basin, how do you account for the potential error associated with dated land cover data and estimated flow stats? Dave W. Dave Wanucha Division of Water Resources Transportation Permitting Unit NC Department of Environmental Quality 336-776-9703 office 336-403-5655 mobile 2 Dave.Wanucha@ncdenr.gov NC DEQ Winston Salem Regional Office 450 West Hanes Mill Road, Suite 300 Winston Salem, NC 27105 Email correspondence to and from this address is subject to the North Carolina Public Records Law and may be disclosed to third parties. From: Smith, Charles \[mailto:Charles.Smith2@aecom.com\] Sent: Friday, November 04, 2016 9:17 AM To: Wanucha, Dave <dave.wanucha@ncdenr.gov> Cc: Thomson, Nicole J <njthomson2@ncdot.gov>; Fisher, Laura <Laura.Fisher3@aecom.com> Subject: B-4960 culvert drainage area and flow rates Dave, Please find attached the drainage area and flow rates for the Konica Drive extension culvert we discussed on the phone yesterday. Please feel free to contact me if you need anything further. Thank you. Charles R. Smith, PE Hydraulics Engineer, Transportation & Site Civil D +1-919-854-6240 Charles.Smith2@aecom.com AECOM 701 Corporate Center Drive, Suite 475 Raleigh, NC 27607, United States T +1-919-854-6200 aecom.com Built to deliver a better world LinkedIn Twitter Facebook Instagram 3