20110009 Ver 1_Staff Comments_20110202

11-0008 American Tobacco Trail
AML comments
February 2; 2011
1. The 2:1 side slopes that are used throughout this project are too steep: Please adjust the slopes
to 3:1 or flatter.
2. The DWQ does not allow the use of scour holes to meet diffuse flow requirements except in
unusual circumstances when flows are very small and space is very constrained. Scour holes
may be used to stabilize transitions from cut to fill slopes but not to diffuse flowfrom new
ditches that are directing flow through protected riparian buffers. As discussed in the meeting
between the consultants and the DWQ on December 15, 2010, the DWQ recommends the.
following approach to meeting diffuse flow requirements on this project:
a. Minimize the use of swales to the maximum extent practicable by super elevating the
greenway trail and allowing stormwater to sheet flow off the trail. The small drainage
areas on this project are unlikely to cause. significant amounts of runoff sheeting across
the trail.
b. Provide level spreaders that are designed in accordance with Chapter 8 of the- BMP
Manual. For every level spreader that is proposed, provide a Level Spreader
Supplement Form that is completely filled out with all required items attached. The
BMP Manual and Supplement Forms are available at
http://portal.ncdenr.or~/web/wq/ws/su/bmp-manual
3. For the scour holes that remain after implementing Item (2) above, please provide a detailed
justification for its need on the project (i.e., it is stabilizing acut-fill slope transition or flows are
very small and pace is very constrained).