Nancy and Clay, From a cost standpoint, the additional relief thru Mary Kay next to the multiple 66"s will run about $400K. Therefore, the recommended plan with the big Mary Kay detention runs about $7.4M. If we don't do the big Mary Kay detention and add the additional relief the recommended plan reduces to $3.7M, but we may end up with slightly higher downstream peak Q's and a 0.09' rise in 100-yr wsel in Dallas along Keller Springs Branch. A smaller relief could eliminate the downstream rise but would result in slightly more flooding in Addison. I recommend that we go talk to Dallas and pursue alternative without the big Mary Kay detention. Worst case, we could do some channel cleanup in Dallas and get rid of the rise or go with slightly smaller relief and a little more flooding in Addison. Also, I recommend that the master plan include a requirement that undeveloped properties (primarily Mark Kay and the site at Addison and KS Road) provide a minimum level of detention on their sites equal to that which currently stores on site for the 100-yr flood. Shall we call you all this afternoon to discuss? Thanks, Walter ________________________________ From: Chow, Andy Sent: Thursday, October 01, 2009 5:42 PM To: Cline, Nancy S.; Barnett, Clay Cc: Skipwith, Walter; Gaspard, Noelle Subject: Upper DNT SWMP - Results for Additional Alternatives Nancy and Clay, Per your request, we have the model results for additional alternatives - (1) No Action with Future Development and (2) Alternative No.1 without Detention. 1. No Action with Future Development o Discount ditch volumes in this model o Assume Tc for all future development site equal 5 minutes The model results show several locations have increased flood depths compared to existing conditions. The maximum increase of flood depth is 0.4' at Ratliff Lane. The peak outfall discharge is 1,452 cfs at Keller Springs Branch which is below the FEMA discharge of 1,570 cfs. In other words, if the vacant properties at (1) Keller Springs/Addison and (2) Keller Springs/Airport are developed without detention, 100-Yr flood depths will increase at 5 of the 11 critical locations, a maximum of 0.4' (Ratliff Lane). The existing flood depth at this location is 1.2'. 2. Alternative No.1 without Detention o Use the proposed storm drain improvements in Alternative 1A (1,600 LF of 2-9'x5' RCB) o Include all localized improvements (Chapter VI, Section D in the report.) o Increase storm drain capacity with an additional RCP in approximately 1,000 LF running parallel with the trunk thru the Mary Kay property. With an additional 66" RCP, the model results show flooding remains at 2 locations (Mary Kay Spill and Parkside Corporate Center) with 3.5' and 1.1' flood depths respectively. In these locations, only the Parkside Corporate Center seems critical (water in the building). The Mary Kay Spill location is at an open space which can be improved in conjunction with future expansion plan. Finished floor elevation at Parkside Corporate is needed to determine if the building will actually flood. However, the peak outfall discharge in Dallas increases to 1,671 cfs which is higher than the FEMA discharge. The impacts downstream at Keller Springs Branch are a maximum 0.09' rise in 100-Yr CWSEL. Based on an impact analysis of the Keller Springs Branch hydraulic model, the 100-Yr flood discharge can increase to 1,630 cfs and not cause any adverse impact on Keller Springs Branch. This translates to a 54" RCP instead of 66" RCP thru the Mary Kay property. However, flooding depths increase (1.7' versus 1.1') at the Parkside Corporate Center. The image below shows this location. For the Mary Kay Spill location, the flood depth remains the same (3.5'). This should be discussed with Dallas if the Town decides to go with this alternative. Please call and we can discuss these additional alternatives; then include them in the final report and presentation. If you have any questions, please let me know. K. Andy Chow, P.E., CFM Project Manager HALFF ASSOCIATES, INC. 1201 North Bowser Road Richardson, Texas 75081 Phone 214-346-6228 Fax 214-739-0095 www.halff.com CONFIDENTIALITY NOTICE: This e-mail message including attachments, if any, is intended for the person or entity to which is addressed and may contain confidential and/or privileged material. Any unauthorized review, use, disclosure or distribution is prohibited. If you are not the intended recipient, please contact the sender by reply e-mail and destroy all copies of the original message. Thank you.