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999 <br /> iAFE OF COLORADO <br /> Colorado Water Conservation Board <br /> Department of Natural Resources 7IVED <br /> 721 Centennt,l 6uildin.; <br /> 1 J13 Sherman Street -SW LI W7 <br /> Denver,Colorado 80203 J\ 1 11tJYYU <br /> Yhune: (303) 1366-3441 <br /> FAX: (303)066-4474 Roy Roma <br /> orals dt Geology ""'r"" <br /> June 11; i997 "" 91 1=1S.tochhe.1d <br /> .as.*I)rtMOr,r1NK <br /> C- <br /> IvL. Jeffery Lipton Post-It"br tax transmittal memo 7671 +or pag.a . .� ;��,•r-wi n <br /> Director,Business Services To d From <br /> University of Colorado —� <br /> Campus Box 317 <br /> ca Co. <br /> Boulder,CO 80309-0317 Dept. Fhorn• _3 <br /> Fax� Fa:A <br /> Dear Mr. Lipton: &7 s'© <br /> A-s a follow-up to my October 16, 1996 letter to the University of Colorado, Boulder (see attached) and <br /> as a response to your letter dated June 10, 1997, I provide the following comments and recommendations <br /> re.-ardin.- the "Flood Control Dike around University of Colorado Property." The subject property is <br /> also known as the Flatiron Gateway Property Acquisition and Deeps Farm Pit. <br /> In my October 1996 lerter, there were a number of issues that required additional engineering submittals <br /> for the Colorado Water Conservation Board (CWCB) to complete its flood hazard review of the subject <br /> property which is located in the South Boulder Creek floudplain in Boulder County, Colorado. The <br /> ;. engineering technical suppors concerns are: <br /> • What 100- year flood water surface model is used for the determination of the base flood elevations <br /> and top of dike design for the existing and proposal levee system? <br /> • Does a 100- year flood hydros aph exist for design flood event for the levee system? <br /> • Has a geotechnical investigation been completed regarding the dike/levee stability as required by <br /> FEMA and the CWCR? <br /> Information and data in response to these concerns has been provided to the CWCB at technical review <br /> meetings on February 14, 1997 and April 3, 1997. <br /> The CWCB staff finds that the Flatiron's Company end Deer, Leonard Rice Consulting Water Engineers, <br /> Inc., is using the 1IEC-2 model which was developed by Greenhorns 8t O'Mara, Inc. This HEC-2 model <br /> is acknowledged by the CWCR and FF.MA as Ihr. designated 100- year flood profile for floodplain <br /> management purposes and the implementation of flood mitigation measures. Using this ]hydraulic <br /> information, the CWCB staff finds that the levee system will comply with the 3 foot frccboartd <br /> requirement. In addition, this Board has been advised that the Flatiron Company has an application <br /> submitted to the Mined Land Reclamation Board to add an additional foot of fill to the dike system, <br /> which will bring the system to 500-year flood protection. <br /> In response to the Board's concern regarding "the stability of the existing dike embankment," Leonard <br /> Rice has provided the Board with a geotechnicul investigation which addressee this concern. The report <br /> findings state that the permeability of the existing embankment is between .0009 in./hr. and .01 1 1 in./hr. <br /> When comparing this rate to the period of time that floodwa[ers will be in contact with the embankment, <br /> which is estimated to be 10 hours, a simple calculation will illustrate that there will be a very limited risk <br /> for a "piping" type embankment failure for the existing embankment. The floodwater contact time <br />