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Freeport NIP(2.7)-CS021223JONES & CARTER, INC. �+ Consulting Engineers 3030 LBJ Freeway, Suite 910 972/488 -3880 Dallas,Texas 75234 Fax 972/488 -3 December 23, 2002 Ms. Teresa Turner Engineering Department City of Coppell 255 Parkway Coppell, Texas 75019 Re: SE Comer - Burns & Freeport Storm Drainage Dear Teresa: Per our meeting of December 3, 2002 and the subsequent correspondence sent to you December 6, 2002, we have completed a detailed analysis of the 36 -inch storm sewer (Line "SS2 ") and the existing storm system in Hammond Street and the effects of our 7.8f acre development. We have analyzed both of these systems for various 5 -year and 25 -year events using both the current design standards and the design standards at the time of their respective designs. This analysis has taken into account the fact of the existing conditions along the east and west sides of Hammond Street (i.e. the west drainage basins no longer drain into Hammond Street and the east basins are currently undeveloped). Based on this analysis, it appears that a 5 -year event, with a C factor of 0 .7 and an I factor of 5.88 (the original design conditions of the Hammond Street system) can be accommodated by both systems without any required on -site detention. Exhibits 1 and 2 show our analysis of a 25 -year event under current conditions, with and without the inclusive effects of minor head losses. Under both of these scenarios, we determined the maximum area of our site that could be accommodated by the Hammond Street system and the effect of the remaining area on the 36 -inch system (Line "SS2 "). Exhibits 1 and 2 show that under current 25 -year storm event criteria, the maximum developed acreage from our site that can be accommodated by the Hammond Street system is approximately 0.48 acres and 0.61 acres, respectively. However, in analyzing the remaining acreages of our tract under these same conditions on Line "SS2 ", our backwater analysis shows ponding at the lowest upstream curb inlet (see Lateral "A ") anywhere from 7.6 feet to 11.4 feet. Exhibits 3 and 4 show the analysis of 5 -year events under current standards (i.e. c = .9, I = 5.88) with and without minor losses. This analysis shows that, under current 5- year event criteria, this event can only be accommodated without the consideration of minor head losses while still having approximately 0.25 feet of water ponding in the gutter. It is our conclusion that the only scenario that works is a scenario in which both systems are analyzed at a c factor of 0.7 and a 5 -year intensity, I factor of 5.88. This scenario appears justified in considering the fact that the Hammond Street system was originally designed under these conditions. It also appears justified in that in the event that each system reaches capacity, the extreme event appears to sheet flow down Hammond Street into the existing ditch. At this point, our request is that the City respond to us in writing as to the validity of our analysis by the middle of January. Our developer has the site under contract and wishes to proceed, but does not want to proceed with any platting or construction plans until confirmation is received that they can utilize the site to fullest extent without the City requiring any on -site detention. In support of this request, I am also including copies of the drainage area maps for both systems and a copy of our preliminary grading plan for your use. Thank you for your consideration in these matters. Should you have any questions in regards to this letter, please feel free to call either myself or Thomas Stroh, P.E. at 972 - 488 -3880. Sincerely, Mark Donohue, P.E. cc: Mr. Jim Pritchard 1APrcjectsV &CDa11as\Pritchard Associates (D076) \002 Freeport & B=s9.etteACityStorm12- 23 -02.D0C ! #e !; !� f� && � LU 0 � c 2 w c � �■�■ #■ #■�, ■■ | , ■3,998 ■gym! §« #R R 2868■■ §§!�; |2■ e a a,P @#r a |e §g3R8 8688■■ 2262|;■■ § |I S!§§!! 6666! §§B # -| amcs00 k0a6dd6k k !— ;nR■„ ; ;R,� ■�■ §k§§§§ § 8888 §§|§|§§| B • !. 2222§§ §k K 0 aC ! ■! ■ ■mm m# « #amea § ■R■SRV st G ,,,s + ® !Q!!! 4 #!646!| ! � a �6 S!!55! !||!lRRR ® ! 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