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Coppell BC3-CS000503..."' Halff Associates, Inc. ENGINEERS · ARCHITECTS · SCIENTISTS · PLANNERS · SURVEYORS 8616 Northwest Plaza Drive Dallas, Texas 75225,4292 (214) 346~6200 Fax (214) 361~5573 MEMORANDUM TO: Mr. Kent Collins, P.E. DATE: May 3,2000 CC: Mick Granlund Brett Phillips AVO: FROM: Todd Abbott Emaih tabbott@halff, com PROJECT: 18428 Coppell Business Center III SUBJECT: Comment responses The following is a summary of the responses to your comments dated Apdl 18, 2000: 1. 2. o 9. 10. 11. Provide a paving plan in the civil set showing the pavement design and all applicable dimensions.. See Sheet SC-I for paving information. The drainage plan for this site needs to follow the original drainage area map for this site as much as possible, i If the nearby systems are not adequate to handle the runoff then on-site detention will be necessary. A small area was originally shown to drain to Wrangler Dr. The remaining area is shown to drain to the box culvert on the south side of the property. The relative areas to each system have been preserved from ~he original drainage area map. Please see the attached memorandum for a more detailed discussion of the drainage plan. Verify and provide information indicating that the box culvert has adequate capacity to carry the proposed storm discharge. Also, keep in mind that the box culvert is designed to handle a large portion of the properties to the ~south. That capacity needs to be reserved to the extent of the original design. Please see the attached memorandum for a more detailed discussion of the drainage plan. Provide profiles for all storm drains showing hydraulic information and all utility crossings. See Sheet STM-I. Limit the storm runoff directed to out any driveway to 2 cfs. See Sheet DA-I. The exact location of the box culvert in relation to existing pavement needs to be verified. If these connections interfere with the existing paving then permission will be required by the owner of the adjacent property. See Sheet STM-t. The contractor Is Instructed to verify the location and elevation of the box culvert. Iflthe paving is in conflict, it will be addressed at that time. Please consider providing a pdvate looped fire line along the south edge of the property. The proposed public hydrant leads fall under paving. This will not be allowed given the proximity to water lines in Crestside and Lakeshore. See Sheet WS-I. Provide 20'-wide utility easement for all acceptable public fire hydrants. See Sheet FP-'I. Provide a wastewater access device at the property line on the sanitary sewer service. See Sheet WS-I. The fire hydrant layout needs to be approved by the City Fire Marshal. He can be reached at (972) 304-3503. Texas Sprinkler is performing the fire hydrant layout and coordination. They have been notified of the approval requirements. Show the location of the stabilized construction entrance on the erosion control plan. Also consider providing additional silt prevention around the existing y-inlet to the south of the property. See Sheet EC-1. ,,,'" Halff Associates, Inc. ENGINEERS · ARCHITECTS · SCIENTISTS · PLANNERS · SURVEYORS / 8616 Northwest Plaza ~)rive Dallas, Texas 75225-4292 (214) 346-6200 Fax (214) 361-5573 MEMORANDUM TO: Mr. Kent Collins, P.E. DATE: CC: Brett Phillips /J ~ AVO: FROM: Pat Acker, P.E. /~ ~ Ernaih packer@halff, com SUBJECT: Site Drainage PROJECT: May 5, 2000 18428 Coppell Business Center III In response to your comments #2 and #3 on Apdl 18, 2000, the following is a detailed explanation of the design process .' ~nd rationale behind the storm sewer design for the above referenced site. The Northlake 635 Business Park drainage system was designed with lower rainfall intensities and runoff coefficients than the current drainage criteria required by the City of Coppell. Specifically, most of the sites within the business park were set ~up using a rainfall intensity of 7.8 in/hr, and a runoff coefficient of 0.90, in contrast to the 8.88 in/hr and 0.95 currently approved by the City of Coppell for this project. The result of this change in drainage criteria is that parts of the existing public storm Sewer system are undersized if analyzed using the current drainage criteria of 8.88 in/hr and 0.95. It is the City's policy to require all new on-site storm sewer to be designed to 8.88 in/hr and 0.95, while maintaining a release rate that does not adversely affect the original design parameters of the business park. In most cases, this requires on site detention of some sort. The particular drainage structure affecting this site is the 10' x 10' box culvert, which conveys off-site water along with the drainage from Lakeshore Dr. and Crestside Dr. into the open channel south of Lakeshore Dr. This box culvert receives off-site water from the west and south via open channel, which was calculated using an intensity of 11.6 in/hr and a runoff coeffiCient of 0.90. The remaining water received by the box culvert is via the storm sewer systems in Lakeshore Dr. and Crestside Dr. These systems were designed using the previously mentioned criteria of 7.8 in/hr and 0.90, and are not capable of carrying the current City drainage criteria as listed above. Because of this, all sites draining into the drainage systems are required to detain flows as necessary to maintain the original drainage criteria. It follows that the flows in the box culvert remain as designed in the original Northlake 635 Business Park plans. The site that is Coppell Business Center III is comprised of portions of drainage areas 13, 21,22, & 36. Please reference the attached Drainage Area Map. Only areas 13, 21, & 22 drain into the box culvert in question. Area 13 is shown to drain tO the Lakeshore Dr. storm sewer system. Area 21 is shown to drain directly to the 10' x 10' box culvert via "Y" inlet. Area 22 iS shown to drain to Crestside Dr. As per the design drawings for Coppoll Business Center III, the portions of these areas that are on the site in question are redistributed into 3 main collection points on the box culvert. Please reference Sheet DA-1 of the Coppell Business Center III Civil Package. This rerouting allows much of the water to bypass the systems in Lakeshore Dr. and Crestside Dr. and discharge directly into the box culvert. The box culvert was analyzed using the new connection points and current City drainage criteria of 8.88 in/hr and 0.95 fdr the Coppell Business Center III site. The result was a lowering of the hydraulic grade line for the box culvert due to the entering the box 'down stream" of the original connection points. This shows that the current design of the Coppell Business Center III with the current City drainage criteria has less impact on the box culvert than the original drainage plans for this site. I have included a mark up of the old box culvert design with the new flows and new connection points shown, along with the hydraulic ,,,"' Halff Associates, Inc. ENGINEERS · ARCHITECTS · SCIENTISTS · PLANNERS · SURVEYORS 8616 Northwest Plaza brive Dallas, Texas 75225-4292 (214) 346-6200 Fax (214) 36145573 calculations used. In summary, the design of the storm sewer of Coppell Business Center III has less impact on the existing drainage struc~res than the original layout of the Northlake 635 Business Park. This has been accomplished despite more conservative drainage criteria requirements placed on the site than those for which the site was originally designed. By discharging the majorit~f of the on-site water into the existing box culvert further downstream than originally planned, the hydraulic grade line for the box culvert has been lowered. I trust that this information is sufficient for your needs at this time. Should you have any questions, or require any additic~nal information, please contact myself (214) 346-6240 or Todd Abbott (214) 346-6248. / [] Attachments: Copies: File ~i~' ~?- ,?hi.s2 lo ii.s _o.02 134s.e ~,,.;~ ,'~ ',.~A.~'~.- i, ~., ( ,, ,., ,. ~i i.4Y Il f.S I.lO IO.S -iS' ¢.. /-J. 42 lO ?.1 2.2! lO.I ,~..;. .:. ~., . .,., ?., ,.,o. .....IS., · i,S2 IS ?.l O.°S Ss, .,~; Jr. SS: lO 11.8 S.SO Sil.4 iS.,' ,' ~2.,. Is i,.° o." ~,-.° :1i-.i,. ;. 2.ss Is i,.s s.so s~.o 12' ' 2'02 IS ?.l °.O° 14~! 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