Clay, My response are below. You may have a different opinion on this comment (also shown below). The other issues he sighted are more straight forward. ***Using a coefficient of 0.3 for Area C is a judgment call. However, you could argue that a 0.9 is too low for the remaining areas because they are entirely covered with concrete or are roof top. 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.*** 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. ***Detention must be provided for all areas being improved with this project as stated in the two prior reviews. The exception was allowing the offsite areas to be included in the present conditions runoff calculation even though they do not currently drain to 36” pipe in Westgrove. This significantly reduced the size of the detention system. I believe this requirement was the reasoning behind reducing the amount of pavement being installed next to the taxiway. Area Du and Ou must be accounted for separately. They don’t offset each other. Plus, Area G passes through the system, Du and Ou do not.*** *** According to the Drainage Area Map, Area G includes a portion of the existing taxiway. Therefore, a coefficient of 0.5 seems more reasonable than 0.3 for the existing conditions. This works towards reducing the size of the detention structure.*** 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. They were included in the calculation table because they are part of the project site. 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, you could argue that a 0.9 is too low for the remaining areas because they are entirely covered with concrete or are roof top. 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 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. The note on the spreadsheet is correct. Allowable discharge (from the detention system) = Present conditions (or total existing runoff) – Total Undetained Q (or areas that are bypassing the detention system). The spreadsheet appears to be correct. As previously mentioned, Areas Di, J, and Oi can be removed from the Present Conditions and Future Conditions, but this will not change the overall calculations.*** 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.*** 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