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„ Johnson, Kunkel & Associates, Inc. <br />CIVIL ENGINEERING • STRUCTURAL ENGINEERING • SURVEYING • MAPPING <br />ESTABLISHED 1972 <br />February 2, 2004 <br />Allen C. SorensorY/ <br />Division of Mineral and Geology <br />Department of Natural Resources <br />1313 Sherman St., Room 215 <br />Denver. CO 80203 <br />CrentivcM ~rrain~iSio~ <br />PP x <br />RE: Design Information for East Pit Inlet and Outlet Structures, Bunn Ranch Pit, Lafarge <br />eference No. 805-430, Technical evision TR-001, Permit No. M-19180-13_i~ <br />Dear Mr. Sorenson, <br />As we discussed on the phone toady, I am writing to clarity the design rationale for the above referenced <br />structures. <br />Given the complexity of river hydrology during flood stage events, an empirical approach was used. <br />Anecdotal reports, of the flood event precipitating the breach, describe the pond berm as submerged at <br />the peak of the event. The breach therefore conveyed the portion of the river flowing through this part of <br />the berm. It was therefore reasoned that a structure of equal or greater cross-sectional area would be <br />adequate to prevent further erosion during future overtopping events. The cross-sectional area of the <br />existing breach is 160 square feet. The new structures were designed with across-sectional area of 240 <br />square feet. <br />The FEMA 100 year B.F.E. for this area is 6191.4 feet (NGVD-29 datum). This equates to 6209.8 feet <br />when adjusted to the local datum used for the submitted drawings. The 100 year event will therefore just <br />overtop the berm. The river gradient in this area during the 100 year flood was also determined from the <br />FEMA map to be 0.004 feet per foot. Using this as the basis for a channel flow calculation through these <br />structures, results in a flow rate of 1,665 cfs and a velocity of 6.9 fps (see attachment for further details). <br />The rip-rap size of 8 inches, as specified on the drawing, is adequate for this velocity (per USBR sizing <br />chart). <br />Please let me know if you need any additional information. <br />Sincerely, <br />Stephen L. Miller, P.E. <br />Project Manager <br />cc: John Rozetle (via fax) <br />attachment <br />RECEIVED <br />FEB 0 4 2004 <br />GIYbt9fl of MineraVS '= !ic? ' . 4 <br />P.O. Box 409 • 1286 Chambers Ave. • Suite 200 • Eagle, Colorado 81631 • Phone: (970) 328-6368 • Fax: (970) 328-1035 <br />