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<br />. <br /> <br />Colorado Water Conservation Board <br />Department of Natural Resources <br />721 Centennial Building <br />1 31 3 Sherman Street <br />Denver, Colorado 80203 <br />Phone, (303) 866-3441 <br />FAX: (03) 866-4474..". "p~;,~i:< <br /> <br />STATE Cj COLORADO <br />I) <br /> <br />-'.- <br /> <br />Roy Romer <br />Governor <br /> <br />June 11, 1997 <br /> <br />Ja.mes s. lochhei4d <br />Executive Director, DNR <br />DariesC. Lile, PE. <br />Director,CWCB <br /> <br />Mr. Jeffery Lipton <br />Director, Business Services <br />University of Colorado <br />Campus Box 317 <br />Boulder, CO 80309,0317 <br /> <br />Dear Mr. Lipton, <br /> <br />, <br /> <br />As a foIlow,up to my Oc_tober 16, 1996 letter to theUn,i,versity of Colorado, Boulder (see attached) and <br />as a response to your letter dated June 10, 1997, I provi& the following comments and recommendations <br />regarding the "Flood Control Dike,around University of Colorado Property." The subject property is <br />also known as the Flatiron Gateway,Property Acquisition and Deepe Farm Pit. <br /> <br />In my October 1996 letter, 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 floodplain in Boulder County, Colorado. The <br />engineering technical support concerns are: <br /> <br />. What 100, year flood watetsurface model is used for the determination of the base flood elevations <br />and top of dike design for thf' e,xisting and proposal levee system? <br />. Does a 100, year flood hydrograph exist for design flood event for the levee system? <br />. Has a geotechnical investigation been completed regarding the dikelIevee stability as required by <br />FEMA and the CWCB? - <br /> <br />Information and data in response to these concerns has been provided to the CWCB at technical review <br />meetings on February 14, 1997wd April 3, 1997. <br /> <br />The CWCB staff finds that the Flatiron's Company engineer, Leonard Rice Consulting Water Engineers, <br />Inc., is using the HEC,2 model which was developed by Greenhorne & O'Mara, Inc. This HEC-2 model <br />is acknowledged by the CWCB and FEMA as. the 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 fOGt freeboard <br />requiremeni. In addition, this Board has been advised that the Flatiron Company has an application <br />submitted to tile Mined Land Reclamation Board to add an additional foot of fiIl to the dike system, <br />which will bring the system to 500'year flood protection. <br /> <br />In response to the Board's concern regardij{g'~the~siabtlily oftheexisting dike ehibankment," Leonard <br />Rice has provided the Board with a geotechnical investigation which addresses this concern. The report <br />findings state that the permeability of the existing embankment is between .0009 inJhr. .and .0111 in./hr. <br />When comparing this rate to the period of time -that floodwaters will be ilt contact i.vith 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 /> <br />~ <br />