SW3190201_NCDEQ Stormwater Review Comment Response_20190520

NCDEQ Stormwater Review Comment Response
O'Reilly Auto —Wingate, NC
Comments received via email from 519 — 5116.
Bohler Engineering responses in red.
• The calculations show that the peak elevation during the 1" storm is above the first weir,
resulting in a flow of 0.40 cfs through the outlet. The entire 1" storm must by treated by
infiltrating through the bioretention media.
Calculations revised to have first outlet above stage of 1" storm.
• The calculated storage in the bioretention cell should not take into account the porosity of the
media.
Storage within voids of media removed from calculations.
• The subcatchments were modeled using weighted curve numbers rather than the Discrete NRCS
Method in which pervious and impervious areas are routed separately. This artificially lowers
the calculated runoff volume.
Impervious and pervious areas separated in revised calculations.
• The routing appears to be calculating the flow through the bioretention media using the surface
area at elevation 563 rather than at the surface of the bioretention media. The calculation
shows the flow as 0.03 cfs, whereas it should be 0.007 acres * 1 in/hr * 43560 sf/ac * 1 ft/12 in
1 hr/3600 s = 0.007 cfs.
Calculations reviewed to confirm exfiltration is taken at elevation 561.5 and above.
• The grading sheet shows the bottom of the bioretention cell at El. 561, whereas the detail and
calculations show it as El. 561.5.
Grading sheet revised to show the planting surface elevation at 561.5.
All I need to see is that the water is infiltrating into the media fast enough that the water level
doesn't reach one foot or the elevation of the first outlet. The underdrain pipe looks to be sized
large enough to carry the flow through the media so the limiting factor will be infiltration into
the media. The easiest way to do that is to not even consider the underdrain and only model it
as 1 inch/hour discarded exfiltration at the planting surface.
Calculations revised so that the underdrain and IWS pipe is not modeled and the exfiltration
through the bioretention media at 561.5 is discarded. Stage in the 1" storm is less than 12"
above planting surface and below the top of the box, which is the only outlet.