Revised responses below and new spreadsheet attached. From: Clay Barnett [mailto:cbarnett@addisontx.gov] Sent: Wednesday, January 12, 2011 2:26 PM To: Chris Robinson Subject: FW: Million Air Phase 2 Chris, I was going to look at this, but it is obvious that I am not going to get the opportunity this week. Could you have a look at it? Thanks, Clay Barnett, P.E. Town Engineer Town of Addison 16801 Westgrove Drive Addison, TX 75001-2818 Office: (972) 450-2857 From: Lynn Kadleck [mailto:lkadleck@verizon.net] Sent: Monday, January 10, 2011 6:51 PM To: Clay Barnett Cc: Bill Leggett Subject: Million Air Phase 2 Clay, I reviewed the attached table sent to me by Chris Robinson with Grantham. There are some discrepancies with the table and what we are allowed to do and that we designed accordingly. It was my understanding that we did not have to provide detention for any offsite areas and any areas already improved. The table includes area G, which is offsite (between the lease parcel and Taxiway V pavement). We included this area to offset areas Du and Ou since these two areas were not going through the detention. The total for G is 0.11 acre and for areas Du and Ou is 0.12 acre. In addition, the runoff coefficient for Area G existing was shown to be 0.5, which should 0.3 for unimproved conditions. This are will be changing since the access to the taxiway V will be revised to include most of the length adjacent to the lease parcel. This is offsite area and will be handled in the drainage calculations and system, but will not be detained. ***It has been decided to allow the offsite area next the taxiway to pass through the system undetained. The area must be still be included in the detention calculations. Areas Du and Ou must still be detained. Please see the revised detention calculation table attached.*** Area Di is within Amelia Earhart Drive which is offsite and is already improved. Areas J and Oi are already improved (they are paved).Therefore, these areas should not be subject to detention. Area C is a landscape area and should keep an unimproved runoff coefficient of 0.3. ***You are correct that areas Di, J, and Oi are not subject to detention. However, those areas have no effect on the size of the detention system. The values are cancelled out in the allowable discharge calculation because they are included in both the present conditions and future conditions, and they bypass the detention system.*** ***Using a coefficient of 0.3 for Area C is a judgment call. However, one could argue that a 0.9 is too low for the remaining areas because they are entirely covered with concrete or are roof tops. A coefficient of 0.9 was established for industrial zones with the assumption that there would be some pervious area included. I think it is reasonable to assume that a 0.9 is applicable to the entire area.*** My calculations provide for the actual unimproved area of 3.01 acres [3.73 acre lease parcel less 0.19 (C) less 0.38 (J) and less 0.18 (Oi)] (some differences due to roundoff), which makes the allowed release 6.7 cfs (see table on drainage area map), This is the release amount used in my calculation. The volume required by this calculation is as we shown on our plan which is 25,348 cubic feet. ***First, for clarity and accuracy, the detention calculations should be based on the various areas shown on the drainage area map, not a leased area. Second, the entire 3.01 acre area does not enter the detention system and therefore, does not equal the allowable discharge from the system. Please see the note on the spreadsheet for clarification.*** The submerged orifice calculation as shown on the profile sheet anticipates a release rate of 6.7 cfs which produces an opening of 12.4 inches. The head is the difference between the water surface in the detention tank less the hydraulic grade elevation in the outfall pipe which is the difference between 635.80 and 633.98 which equals 1.82 feet, not 4.63 feet as shown for the calculation by Chris. I have also attached the storm sewer profile sheet with the orifice equation shown with more detail. This equation/calculation was added to sheet 4, the DA Map. ***I agree with the method you used to calculate the orifice opening. However, an error was made in the calculation. Orifice Area=1.03 ft^2 which equals a 13.7” diameter opening. Please note that the calculation will have to be rerun with the correct allowable discharge value and new HGL*** I trust this explains and clarifies the issues regarding the detention. Please review and contact me with any questions or comments, or if there is a disagreement with my numbers. Regards, L. Lynn Kadleck, PE, RPLS Kadleck & Associates 555 Republic Drive, Suite 115 Plano, Texas 75074 972-881-0771 lkadleck@verizon.net