Lynn, Please review this information and let me know if you have any questions or comments. Thanks, Clay Barnett, P.E. Town Engineer Town of Addison 16801 Westgrove Drive Addison, TX 75001-2818 Office: (972) 450-2857 From: Chris Robinson [mailto:crobinson@gra-ce.net] Sent: Friday, January 21, 2011 11:12 AM To: Clay Barnett Cc: Bruce Grantham Subject: Million Air - Orifice Calculation Revision Clay, As we discussed over the phone before today’s meeting, there is an issue concerning the orifice size. The HGL is being used to size the detention basin orifice because the outlet is under pressure. Currently the starting HGL for Line ‘A’ was assumed to be 0.8’ above the top pipe. While this is a conservative assumption for determining the hydraulics of the pipe system, it will result in an detention basin orifice size that is too large. It’s preferable to assume that the starting HGL for Line ‘A’ is at the top of pipe which would in turn lower the HGL for Line B at the detention basin and provide a more conservative orifice opening calculation. For the sake of time, the orifice calculation could be revised without rerunning the HGL calculations on the plans. The head (h1) outside the pond could be assumed to be 0.8’ lower than the HGL. A note could be added explaining how the ‘h’ value was determined. Chris Robinson, P.E. Grantham & Associates 6570 Naaman Forest Blvd. Suite #200, LB 2 Garland, TX 75044 972-864-2333 ext. 121 972-342-5351 cell 972-864-2334 fax