Nancy, I think we need to compile comments from John Hill, Garver, Grantham and myself and sit down and meet with them. Have we gotten any comments from Garver or John Hill? Thanks, Clay Barnett, P.E. Town Engineer Town of Addison 16801 Westgrove Drive Addison, TX 75001-2818 Office: (972) 450-2857 From: Nancy Cline Sent: Tuesday, November 16, 2010 4:02 PM To: Clay Barnett Subject: FW: Bankston Flood Study comments Your thoughts? From: Bruce Grantham [mailto:bgrantham@gra-ce.net] Sent: Wednesday, November 10, 2010 5:48 PM To: Nancy Cline Cc: Chris Robinson Subject: Bankston Flood Study comments Nancy: I've looked over the Bankston Flood Study and have the following big picture comments which come from a cautious point of view given the history on this property. 1. This study does not take into account what happens in the field. That is, in a heavy rain event, the undersized culverts at Lindbergh cause a significant amount of ponding on the airport. 2. For the Arapaho Road extension storm drainage design, we assumed that the flow through the Lindbergh culverts would be determined by inlet control based on a headwater elevation that matched the adjacent Lindbergh pavement (which has never overtopped). This inlet control calculation resulted in a small reduction in the flow through the culverts compared to the flow that arrives at them based on our Rational Method calculations. This result is consistent with the ponding that occurs on the airport. 3. Given the flooding problems on the Bankston property and further downstream in Carrollton on this tributary, we assumed that the Lindbergh culverts would not be enlarged. 4. As it is imperative that any new Bankston development will not flood, the associated study warrants a higher level of analysis than the hydrologic calculations we performed for the downstream Arapaho Road system which is not the case with the Nave Engineering study. 5. The Rational Method does not account for the airport ponding. In addition, the flood study should also include a detailed analysis of the starting water surface elevation for the hydraulics. 6. Without reviewing the HE-RAS output in detail, it's unclear how the channel cross section can be significantly reduced as proposed (with retaining walls on the south and east property lines) without any increase in the water surface elevation or velocity. 7. It would be conservative for the Town to require a modeling approach that can be calibrated to observed conditions in the field, especially on the Bankston property and the airport. The proposed development can then be added to this model. I look forward to hearing your thoughts. Regards, Bruce -----Original Message----- From: Nancy Cline [mailto:ncline@addisontx.gov] Sent: Wednesday, November 10, 2010 9:22 AM To: Bruce Grantham Subject: Question Can I call you later to ask you a quick question about lake forest? Nancy ****************************************************************************************************************** This e-mail and any files or attachments transmitted with it contains Information that is confidential and privileged. This document may contain Protected Health Information (PHI) or other information that is intended only for the use of the individual(s) and entity(ies) to whom it is addressed. If you are the intended recipient, further disclosures are prohibited without proper authorization. If you are not the intended recipient, any disclosure, copying, printing, or use of this information is strictly prohibited and possibly a violation of federal or state law and regulations. If you have received this information in error, please delete it and notify Hamid Khaleghipour at 972-450-2868 immediately. Thank you. *******************************************************************************************************************