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ST9905-CS021114 MEMORANDUM FROM THE DEPARTMENT OF ENGINEERING To: From: Date: RE: Jim Witt, City Manager Clay Phillips, Deputy City Manager Kenneth M. Griffin, P.E., Dir. of Engineering/Public Works November 14, 2002 Bethel Road Project ST 99-05 Since 1998, the City of Coppell has been evaluating drainage associated with West Bethel Road. In 1998 the City of Coppell hosted a meeting between the then property owners of the two large tracts of land on the north side of Bethel Road to discuss the development of their property in regard to drainage in Bethel Road. Those tracts of land are now referred to as the "Champion" and "Duke- Weeks" tracts. The drainage basin for this portion of Bethel Road also includes approximately 240 acres of airport property discharging into Bethel Road at Royal Lane. In addition to that, the bulk postal facility and the Minyards tract also discharge into Bethel Road. In total, there was approximately 676 acres of land that discharged into Bethel Road near Coppell Road. Realizing that drainage was going to be the overriding design consideration for the construction of Bethel Road, a drainage study was commissioned by the City of Coppell called the "Southwest Coppell Storm Water Management Master Plan". It was a joint venture between the City of Coppell, NCH and Coppell Industrial NV. The study was prepared by Halff and Associates. The study pointed to the fact that detention would be required on the land north of Bethel Road and on the airport property. The detention on the airport property was crucial because of the short distance from Royal Lane to Freeport Parkway and the fact that there was no available land in that section for detention. In 2001, the City of Coppell entered into a design contract with TranSystems Corporation for the design of Bethel Road from the west city limits to Freeport Parkway. The design contract was structured so that there would be additional drainage studies to complement the previous drainage study. TranSystems' drainage study also pointed to the fact that detention would be required. By this time, the property north of Bethel Road had already been developed by "Champion" and "Duke-Weeks" and in both cases the City of Coppell required detention that would not allow the discharge of any additional water from the site than what would normally discharge in an undeveloped condition. Also, the water from the "Champion" and "Duke-Weeks" property was conveyed in an easterly direction and discharged into Grapevine Creek north of Loch Lane, not directly into Bethel Road. "CITY OF COPPELL ENGINEERING - EXCELLENCE BY DESIGN" Page 1 of 4 The challenge now is to design a drainage system that will convey a 100-year storm and at the same time not create any downstream erosion problems in Grapevine Creek. TranSystems has provided two drainage studies for the City of Coppcll to accommodate the drainage along Bethel Road. Both studies show that detention will be required on the airport property. The detention on thc airport property is needed to detain the increase in water runoff when thc 242-acres of airport property develops. The last design by TranSystems shows two detention ponds strategically located on the airport property generally along the west side of Royal Lane. The design of a drainage system to accommodate the developed runoff along Bethel Road is not an overwhelming engineering feat. Simply put, to convey the drainage along Bethel Road, it is only a matter of installing additional box culverts. Preliminary design estimates are that there would need to be seven 10'x5' box culverts along Bethel Road to accommodate the 100-year ultimate developed runoff. However~ the real challenge in this design is to balance the conveyance of water with the potential for downstream erosion in Grapevine Creek. As you may be aware, we have had erosion along a portion of Big Cedar and Country Estates and there is evidence of erosion in Hunterwood Park. To effectively collect all the ultimate developed stormwater in a drainage system in Bethel Road i.e., the construction of seven 10'x5' box culverts, and convey that water to Coppell Road and discharge it into Grapevine Creek just north of the Senior Citizens Center would not be the wisest approach to accommodate the drainage associated with Bethel Road. That approach would very effectively and rapidly convey a large amount of water to an earthen channel and in effect create the potential for additional downstream erosion problems. The goal in this drainage design is to detain the increased runoff difference between the undeveloped airport land as it exists today and the future developed airport land Detention on the airport property is a critical component of this design. To that end, I met with representatives of DFW Airport in July 2002 and presented them a copy of the revised draft drainage report prepared by TranSystems. That report detailed the necessity for detention on the airport property. I left a copy of the report for their review and asked them to respond to me concerning the likelihood of a detention basin on airport property. I was even willing to accept a letter from the airport stating that if at some point in the future they chose to develop then they would construct detention at that time. I have a legal opinion from Pete Smith that states that the City of Coppell cannot require the airport to abide by our subdivision rules and regulations in regard to the development of their property. Therefore, unless the airport decides to create detention, the City of Coppell cannot require detention, even on the section of land that lies within the City of Coppell. However, representatives of DFW Airport stated that they are bound by EPA and other environmental rules and regulations the same as any municipality. They also have to obtain all permits and approvals prior to development of their property. It was my hope based on the meeting, that I would receive correspondence from DFW Airport stating that they understood the drainage situation and they would be willing to construct detention on their property at a future date if they decided to develop the 240-acres that drain into the City of Coppell. After numerous telephone calls, I was told in October 2002 that there would be no letter forthcoming obligating DFW Airport for any future action. Page 2 of 4 "CITY OF COPPELL ENGINEERING - EXCELLENCE BY DESIGN" The situation we find ourselves in today is: · 1) The airport will not allow us to construct detention on their property; · 2) The airport will not commit to construct detention in the furore; and · 3) It is my professional opinion that it would not be wise to construct an efficient system to discharge all the future water into Grapevine Creek knowing that there are downstream erosion problems that exist today. I contacted our consultant, TranSystems, and asked them to evaluate the drainage system that was proposed with Bethel Road to detemfine what level of capacity we would have in the future if no detention was constructed on the airport property. In essence, what I was asking is: if we build a system today in anticipation of detention on the airport property in the future and the airport chose not to construct detention, but instead chose to discharge all of their water into our "undersized" drainage system what level of capacity would we have in our drainage system? The reply to that question is that we would have a drainage system capable of conveying about a 25-year fully developed storm. If the system only conveys approximately a 25-year storm, then any larger event that we have i.e., 50 or 100-year storm, creates the potential for street flooding along Bethel Road. The City of Coppell Subdivision Ordinance requires that drainage systems be designed for a 100- year fully developed storm event. Without a detention basin, the drainage system cannot be designed for a 100-year storm without discharging a substantial amount of water into an unimproved section of Grapevine Creek. At this point, I find myself at a standstill on moving ahead with this project. One the one side ifI build an efficient system to convey all the water in the future, I stand a very strong risk of creating erosion problems in Grapevine Creek. On the other side, ifI authorize the consultant to only design a system that will convey a 25-year storm I mn the risk of having street flooding along Bethel Road in those rare storm events that exceed a 25-year frequency. It is my recommendation that we design a drainage system capable of conveying a 25-year ultimate developed storm and work with the airport at some future date when they chose to develop the property in Coppell to try, to obtain some type of detention onsite. It's important to note that our system will convey the 100-year storm as long as the airport property remains undeveloped. Also, if the airport constructs detention in the furore, our system will still convey the 100-year storm. The only situation that reduces the capacity of our system would be if the airport fully developed their property and released all the water without detention. With an undersized system downstream, if and when the airport decides to develop, they will be in a predicament of how to convey their water because our system would not be adequate to allow the release of all of their water. So there is a possibility that they would be forced to have some type ofonsite detention just by virtue of the downstream system being undersized. To construct a system not in conformance with the Subdivision Rule and Regulations requires a variance from City Council. Therefore, I will take an agenda item on December 10, 2002 to request a variance. If you have any questions, please feel free to contact me. Page 3 of 4 "CITY OF COPPELL ENGINEERING - EXCELLENCE BY DESIGN" Summary of Key Dates: · February4,1999 "Southwest Coppell Storm Water Management Master Plan" by Halff & Associates. Sponsored by: The City of Coppell, NCH, & Coppell Industrial NV · March 22,1999 Legal opinion from Pete Smith that we cannot require DFW Airport to install detention. · April 13, 1999 Memo to Curtis Inglis, DFW Airport, providing summary of meeting between Halff & Assoc., and Tracy Thompson, Doug Bryan & Mike Pyles of DFW Airport and requesting clarification of drainage requirements on airport property. · May ll, 1999 Summary of reply from Curtis Inglis of DFW. Mr. Inglis spoke with Kevin Cox and Gary Keane and Mr. Inglis stated that development of airport property would be accomplished in responsible manner. · February 13, 2001 Entered into design contract with TranSystems Corporation for Bethel Road and associated drainage. · January 18, 2002 Received "Drainage Report Executive Summary" from TranSystems. Report outlined necessity for detention on DFW Airport property. · May30,2002 · July 18, 2002 · October 11, 2002 · October 14, 2002 · October25, 2002 Received "Revised Draft Drainage Report for Regional Detention Analysis / Bethel Road Improvements". Met with representatives of DFW Airport and provided copy of"Revised Draft Drainage Report for Regional Detention Analysis / Bethel Road Improvements" for their review. Was informed that representatives of DFW Airport would not commit to any detention now or in the future. Requested evaluation of drainage system in Bethel Road if n__qo detention was provided by DFW Airport when they developed and our system was constructed today as if there was no increase in runoff from DFW. Received letter from TranSystems that system would convey approximately a 25-year storm with ultimate development without detention. Page 4 of 4 "CITY OF COPPELL ENGINEERING - EXCELLENCE BY DESIGN"