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FS9905-CS 990616 9?22393820 06/16/99 WED 16:23 FAX 9722393820 DALLAS ~ 001 []--n Kimley-HOrn and Associates, Inc. VIA FACSIMILE Fax no. 972-304-3570 No. of pages.J, June 16 1999 Ken Griffin, P.E. Director of Engineering and Public Works City of Coppoll P.O. Box 478 Coppell, TX 75019-4409 Coppell, TX Re: CLOM'R for River Chase Elementary School (Elm Fork of the Trinity River) Dear Ken: The following is in response to the fax you sent: · Comment and Response 5 The report clarifies how the effective models were obtained, but it's confusing how the information was used: a) the effective model was not revised and b) a new model was created to perform the storage calculations (City requirement, i.e. FEMA doesn't need to review) Adding a p~ragraph in the report to clarify this will help FEMA during the review process; however, this doesn't affect any of the results in the report (you can choose to send this on to lvEMA "as is') Page 5 from our review copy is attached for your information. \~RANGF. RXVO M \PRO DXHYDRO\prOjC~IS\06312402%teviews~Rjv=r%finu12 doc 06/16/99 WED 16:24 FAX 9722393820 DALLAS ~ 002 and Associates, Inc. Mr Ken Caiffjn, Junc 6. 1999 Pg2 of 2 Comment and Response 6 In the report copy we have, the relationship between the finished floor elevation and water-surface elevation were discussed in general. What we were looking for is "what is the elevation of the finished floor of the building (s)" - i.e. specify numbers for water surface elevation and resulting finished floor elevation for the building(s) so that this information can be easily checked during the plan review process. Page 4 from our review copy is airached for your information If the above items are resolved to your satisfaction, it appears the report submiRal is complete. Note that neither of the above two items significantly affect the report for the FEMA submittal. You may choose to send the report to FEMA "as is2' If you have any questions, do not hesitate to call us at 972/770-1300. Very truly yours, KIMLEY-HORN AND ASSOCIATES, INC. Michelle Evans Reviewer SMS/faS Project Manager Ken Morrison, Morri.~n HydrOlOgy Engineering, Inc. via facsimile file \~.ANGER\VOL l \PROD\I IYDRO~rojectt,\O6S 12402\reviews~Rivcr~fi nal2.doc 06/16/99 WED 16:24 FAX 9722393820 DALLAS ~003 I! [ [ I I ! Morrlson Hydrololy Engineering, ln~ Page -4 City of Coppoll Requirements G~ne~l The City of Col:q~ll titrough a Fieodplain Management Ordinance CNo. 94639) has specific requirements tzlating to this projea, The lovest floor of all school construction must be elevated a mirthhum of t'wo feet above the Flood Insurance Study (FIS) b~e flood elevation or one foot above the design base flood elevation, whichcvcr is higher. This report has shown that it is proposed to fill the area according to the grading Shown on the Prol~ted Storage Computation drawing included in ag~ndix g. Hydrologic and Hydraulic Information A copy of tl~ City Wide Storm Water Mamigement Study for the City of Coppoll dated January 1991 was obtained (appendix 1). This document providsd the design flood elevations used in this study. Table 1 is a oompatison between all the flood elevations from sou~cet investigated in this study. The tract is affected by the ~o<xl elevations from tl~ ELm Pork of the Trinity River. However, this elevation is blocked fi~om the actual flow of the river. The ar~ is in an ineffective flow xe~m~ and thus has no impact on flood elevations us long as storage levels are not ~Iuced. This m~ that i~lling the tracts has no impact on the flood elevations. Stong Requirements Cit~ reqtti~vu,eets stipulate that ~ valley s~om~e t~hction shnll net exceed 0% for the 100-year design flood and 5% for the standard project flood. In order to determine storage wlun~ a HEC2 me4~l was completed for the existing and pwposed oondition. The only purpose of this model is to delermin~ storage voinme~ at the 100-yur Design flood aml the stinglard projec~ ~o~l. This model was net developed as a backwater model, so the roughness values, discharge valu~ and other such information is me~ninqoleSS and should be ignored. The elevations used were s~t by an X5 method so that the volume comparisons could be made. The proposed project meC. s or exceeds all the Floodplain Management Ordinance r,quirements. The proposed project does not increase ~oo~ levels, and storage values are maintained according to ordinance rex~rements. 06/16/99 WED 16:24 FAX 9722393820 DALLAS ~004 Morrisonitydrolj~yEnfine~ing, lnc Page 5 FEMA information General There are s~veml sources available to determine the FEMA ~o~i elava~ions for the site. They are as follows: 1. Request for Conditional letter of Map Revision on the Elm Fork of the Trinity River Sandy Lakt Road Bridge Hydraulic S~dy, Ha~ Associates Lnc., Dallas, Texas June 1994. 2, Floodplain HydeItalics Study Elm Fork Tnntty River for Conditional Letter of Map Revision Request 18. 9 Acre Sandy Lake Road Proper,Iv, Nathan D. Maier Inc,, Dallas, Texas July 1996. 3, Corps of Engineers HEC-KAS too<tel currently under x~wiew by FEMA FEMA Effective Information In order to decide model to use as the effective condition was made with FEMA's technical consultant Michael Baker Jr. Engineers. They agree that the most accurate model would be the Corps HEC-RAS model currently under review. However, this model has not been officially approved and can not be considered the effective model at this time. The next most current model available is the Floodplain Hydraulicssssssss Study Elm Fork Trinity River for Conditional Letter of Map Revision Request 18.9 Acre Sandy Lake Road Property, Nathan D. Maier Inc., Dallas, Texas July 1996. This model was provided from the City of Coppell. Thid report used the existing condition from this model as the effective FIS model. I~. ~ ~1 ~ ~ ~ ~ CiW of C~B. ~s ~ ~ ~ ~g ~n~fi~ Hydraulic Information The project axe= is modeled by ntl th~ studies inwsfignted m the same way. It is blocked from flow from the Elm Fork of the Trinity by high ~ This means Umt it is in an indfective flow ~ and filling of the site has no impact on the floodplain elevations, The site can be ftlled without alteration of the flood elevations or any other imVact ex~pt ~c~xl storage, and this has been addressed by offsetling storage compensalion, All ~:quited FEMA forms to support a ConditiOnal LeUer of Map Rcvision4~,~a.~.lilJ ar~ included in appendix 7. The project consisfs of ~il in the ~oo~plain with compensating excavation that results in no increase in floed levels and no decrease in lO0-year flood norage. Results The project site iS in an are~ that receives floodwaters from the Elm Fork of the Trinity River, but is isolated from the actual flow of the river. Because of this the axea can be filled without impacl to the flood elcvatlons of the river, This report has iclentified sZ0rage mitigation so that there is nO net decrease in flood storage.