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FS9903-CS 990401Morrison Hydrology Engineering, Inc. 210 Arnold Ave. ~Arlington, Texas 76010 Metro (817) 461-0321 Fax (817) 274-1.t58 E-Malh ronmor7777~aol, com April 1, 1999 Mr. Kenneth M. Griffin, P.E. Assistsnt City Managar City Engineer City of Coppell 25~ Parkway Blvd. P.O. Box 478 Coppell, Texas 75019 Off972-304-3679 Metto 972-471-2251 Fax 972-304 3673 Re: Response to P~iew Questions Regarding a CLOMR-F and CDC Submitted for a 10 and 4g Acre Tract South of Sandy Lake Road and West of MeArthur BIrd, The following comments and responses are offered in reply to the attached fncsimile from Kimley Horn and Associates. Comment 1 /1 site plan or exhibit was not included in the CLOblR-F submittal with proposed topography, and therefore cross-section information in the model was not verified When plans are submitted to the city, they should be checked to match the topography and conditions as indicated in the study. Rtply 1 Asitophtnhssnotbesnpreparedforthisprojeot. The usual procedure for FEMA submittals is to prepare a report before any site plan is completenl. FEMA does not require a site plan. A plan xvill be pre~nd for this project along with consmtotien doctwaents whan a request for a construction permit is requested. Comment 2 In general. the exhibits are missing legends far hatch marks, the font scale is tinreadable (n~d names, contour labels), and information is hard to find. F..rhibits do not need to he resubmitted for this CLOMR- F submiual to the city. Ina for the FEMA submittal the future LOMR-F submittal to the city, please revise these workmaps. This includes using a larger overall scale for the exhibits, larger font scales, identO~ing roads and site features mare clearly, and providing a legend per line or hatch marks. Please include the location of all exhibits. maps. and tables in the table of contents. The existing and proposed floodplain should he clearly identtfiecZ Morrison Hydrology Engineering, Ine. Page -2- Reponse 2 We used the Corps of Engineers Trinity River Topographic map as a base for this project. The read designations and contour labels are from ttmt document. For your cla~catkm we have atiached a larger scale map with rood names, contour labels, legends, and floodplain cross hatching clearly shown. In edditi~s~ we have referoncecl all e~,hibits, maps, and tables in the report table of contents. City Comments Comment 3 Do the hatched areas on the "FLOODPLAIN FILL PROJECT PROPOSED FLOODPLIIN AND FLOODWAY" exhibit represent the resulting ~oodplain after grading or the areas that will he graded? Response 3 The hatched areas shown on the Proposed Condition Map (appendix 6) represent the floodplain after fill Comment 4 Page 3 of the report states "A check was made to ensure that this elevation is higher than the design flood pins one foot and higher than the current Corps of Engineers study in this area~ " DIe reviewer is assuming the term "this elevation" is referring to the FEMA FIS elevation plus two feet, or in other words, the finished floor (FF) elevation~ If the FF elevations were computec~ where is the information? The reviewer is assuming this is based on Table lelevattons, pins the required one or two feet of fi'eeboaret Please provide this infarmatto~ reference table I in the report text, and state the conclusions of this comparison Response 4 We changed this rderenced portion of the report to read: This report has shown that it is proposed to fill the area shown on the floodplain maps to an elevatian that is one foot above the FIS base flood. It is further assumed that any gntcturas will have a finished floor one foot above finished grade. This report has verified that the fill as disansed above is higher than the design flood elevation plus one foot and also higher than the current Corps of Engineen study of the area. Comment 5 Please clari.~ in Table I that the Stormwater Management Informaton is based on ultimate flows and the FEMA FIS is based on existing flows. The FEMA F1S hyelrology is based on parameters for what year (i.e. 1996, 1970's, ecO? V/hat is the hydrology of the COE model based on (existing or ultimate)? Response 5 We modified the headings in Table 1 to make it more clear. The FEMA FIS is besed on flows detrtmined in the most current FEMA model. These are the same flows that have been used for evea'y project along the Trinity in Coppell and other cities. We have no idea what year was used as the basis for the FEMA hydrology. FEMA never uses ultimate condition hydrology as required by their regulations. We did n~ investigate the Ccq~ model as a Morrison Hydrolo~, Engineering~ Inc. Page-3- reference for ultimate conditions. Instead, we used the City of Coppell City Wide Storm Water Management Study ultimate discharges. Was the Nathan D. Maier CLOAtR then submitted as a LOMR to FEMA. In other words, was this project actually constructed or raodi~ed as specified in the CLOMR submittal to FEMA. Please clarify with a note by Table 1 that the FEMA FIS study refere,ced is the Nathan D, Maier sw, dy. Response 6 Neither the Nathan D. Maier nor the Halff CLOMR's were submitted as LOMR's. We assume that these projects were never completed in their entirity. Our report did not us~ proposed conditions from these reports since it has never bee~ approved by FEMA. Instead we used the "existing conditions" frorn these reports which had been approved. As stated on page 4 in our report, "The next most curnmt model available is the Hoodplain Hydraulics Study Elm Fork Trinity River for Conditional Letter of Map Revision Request 18.9 Acre Sandy Lake Road Property, Nathan D. Maie~ Inc., Dallas, Texas July 1996. This model was provided from the City of CoppelL This report used the existing condition from this model as the etfective FIS model." Comment 7 There appears to he a 75-~ to lO0-~ wide channel along Sandy Lake Road on the property. Informan'on Must he provided during the site planning process to ensure that · The fill does not interrupt historical flow flora upland areas into Denton Creek · Thereisadequate~apacityinthechanne~toc~nveysiteandup~andrun~~with~utir~pactingthesite · Flood elevations are determined for the creek andfinishedfioor elevations meet city criteria for this area Response 7 We strongly agree that during the site planning process the general civil engineer must ensexe that: · The fill do~ not interrupt historical flow ermn upland areas into Denton Creek · There is adequate ~tpacity in the channel to convey appropriate sit~ and upland runoff without impacting the site · Flood elevations de~,~amined for the crock and finished floor elevations me~-t city Oriteria for this area Comment 8 This channel must remain part of the mapped FEMA floodplain It is unclear based on exhibits and discussion tn the report whether this is the case. Currently, the floodplain for this channel is mapped up to MacArthur Boulevard P~sponse 8 A CLOMR-F does not result in a physical map revision, A letter is provided by FEMA approving fill on the site. Since a map revision completed by the Corps of Engineers is being reviewed by FEMA at this time. We intend to bring this fill modification to FEMA's attention so they can change the maps before Morrison Hydrology Eugt~e~rlRg, Page they are printed and distributed to the cities. However, this has no impact on FEMA approval of the subject study. Comment 9 Agree with statement that there is no conveyance in the southwest area of the site; however, please confirm once the site is regraded that the proposed pond area will function similarly to current conditions. Will the fill restrict floodwater from reaching the pond? How does the water flow into this area? Response 9 Since this area has no conveyance, as agreed by the reviewer, thee can be no hydraulic impact at the 100- year event. Hydraulic changas require conveyance according to hydraulic theory. Since the flood storage does not change there can be no impact to the hydrology. Hence, there can be no impact on hydraulics or hydrology. The general civil engineer who prepares the site grading plan must ensure that the fill will not restrict floodwater from reaching the pond. This same general civil engineer must det~anine how the stormwater flows through the site, The subject repcfi does not address these concerns. That will be left to the general civil engineers preparing the construction plans. Comment I 0 Methodology for hydrology has not been provided Please explain hew the discharge for the 48-acre tract was determined to be 5000 cubic feet per second (cJ~) for the l O0-year event and standard project flood. Please explain how the discharge for the lO-acre tract was determined to be lO00 c.]~ for the lO0- year event and standard project flood Response 10 We agree that methodology for the hydrology has not been provided. It was not the intmt of this report to remodel hydrolo~y. In order to compare flood volumes at the same elevation it was necessary to use the X5 procedure to set the model at the same elevation as the floodplaIn 100 year and 500 year (used for the Standard Project flood). Since the X5 method set the elevation an arbitrary discharge value must be used. The only re~wiction is that the discharge must be above the supercritical flow. The discharge values used could be any range of values since it is arbitrary. Comment 11 The report text specifies that the Cit)~Wide Storm Water Management Study was used to determine design flood elevations. The Nathan D. Maier study from July 1996 wets used as the (FEMA) existing conditions for the site. Please explain bow the water surface elevations were determined in the storage pond computations. It appears for the 48-acre tract that the ~uter surface elevation begins at the lO0-year or 500 year water surface elevation of the stormwater mat~gement study section 93620 (cross section 1) and ends at section 92300 (cross section 8). Please co~firnt Also, please show on the exhibit how cross sections 93620 and 92300 relate to cross sections 1 and 8. Morrison Hydrology EmZint~rin$, Inc. Response 11 As staled in comment number 10 the FEMA HS ~nd standard project flood elevations we,e set in the storage pond model. Since the storage model had more cross sections than the FEMA HS model it was necessary to use a s~'aight line averaging procedure to determine elevations between the FEMA seetkms. Cross Seaion I uses the 100 year and 500 year wate~ surface elevation of the stormwate~ manage~nant study section 93620 and cross section 8 uses the value for section 92300. The purlx~e of the storage compensation drawing vats to show that cross section 1 is located at main channel section 93620 and that ~ sectkin 8 is located at main channel section 92300. All the others were determined using a stntight line averaging method, Comment 12 Please explain how the water surface elevation was determined for cross sections 2 through 7 of the 48-acre tract roodsling run~ Response 12 Please refer to the above comments. The values for sections 2 through 7 were determined using a straight line averaging method. Comment 13 For the 10-acre tract modeling ru~ how were the lO0-year and SPF water surface elevations determined for cross sections 1 through 47 This information needs to be explained in the text of the report and calculations prov~ed Response 13 Please refer to the above comments. The values for sections 1 through 4 were determined using a s~raight line averaging me~od. The method was explained in more detail in the report. (see storage df~ii~inations). Comment 14 For the exhibits corresponding to the cross sections for the 48 acre and 10 acre tract · Please include north arrows on both exhibits · For the 10 acre tract exhibit, the scale is readable, however it is difficult to understand how this blown up area relates to the entire site and cross sections along Denton Creek Please provide more area on the exhibit (24"x36" instead of 81/2 "xl l · For the 48-acre tract exhlbtt~ please use a larger scale for the map and a larger font scale; the area of the site shown is adequate Response 14 · North arrows wife included on all drawings throughout the repo~ · Thedrawingsweremodi~edtoclenrlyshowthelocationofthel0acretract Morrison Hydrology~ Engineering Page --6- Comment 15 Please include the storage table located in Appendix 7 in the body of report as Table 2. Please state specific information and conclusions regarding this table and analysi~ in the body of the report, P, espo~15 T able 2 was included in the t~t portion of the report FEMA Requirements Comment 16 This submittal appears to meet the intent of the requirements far a Conditional Letter of Map Revision based on fill. R~lpon~ 16 Comment l 7 Before the stud~ is submitted to F~Za., please complete and us~ the at~t F~M/~ forra~. do~nload~d frora: www.f~Tna.~ov. If the engineer doe~ not have oc~',s~ to the Internet, these can be prorind b~ t~e cl~ upon reqv. e~t. R~pons~ 17 Virginia and Ms. Laura Bitthee, P.E., o~afirmnl that the fanas used in this Tho following ~mmeats potrain to the CiX: applicatioa: · It was our uacla'mmding that the gity would provi~ gopi~ of tho rqxtt to othe~ ~ties and agencies. Howorer, w~ will ~ glad to c~omply with any ~ity mluiremants in this regarcL ~ had seattwobouml~o~i~softh~re~orttoth~itvasinstruCced. · If further ~larifi~atioa of the storage ~alnlatioas ar~ ne~locl v~ will ~ glad to respond. It would probably help ff we gould me~'t with th~ re~ri~r to dla,~sa this rgl~t. K,~m1~y Horn and Assoc~tes ancl off~r~ ~s h~lp, and w~ are still availablo ifnml~t. Morrison Hydrology Engineering, Inc. Page -7- We have addressed all the questions and comments of the reviewer. Two copies of the revised report are included. One for submittal to FEMA and one for the city. Please let us know if we can be of assistance. Time is crucial. Sincerely, Ronald W. Morrison, P.E., C.F.M. Senior Hyclrologist