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<br />71dtleApplegate,lnc. <br />Consultants for Land; Mineral and Water <br />February 1, 2000 <br />Mr. Tom Schreiner <br />Division of Minerals and Geology <br />Department of Natural Resources <br />1313 Sherman St., Room 215 <br />Denver, Colorado 80203 <br />999 <br />!~~CEIVED <br />Division of Minerab $l <br />~~ i~ 3 .7."~a <br />~~, <br />RE: Riverview Resources 112 Application, Aggregate Industries, File No. M-99-088 <br />Response to Hydrologic Review Continents <br />Dear Mr. Schreiner: <br />Following aze responses to your Hydrologic Review Comments regarding the Riverview <br />Resources Site in Weld County. The original comments were dated December 6, 1999. <br />A thorough floodplain impact analysis of the Riverview Mining and Reclamation Plans <br />was completed. The 1999 HEC-2 model of the Cache la Poudre River was used for this <br />analysis. Three scenarios were analyzed: existing conditions, worst case scenario during <br />mining, and reclaimed conditions. <br />The results of the model indicate that the maximum rise in the expected 100-year water <br />surface would be 0.13 feet, or less than 2 inches. The Phase 3 scenario that produced <br />these results included the processing plant in place in Cell 6, and stockpiles bordering the <br />floodway in Cell 4. It was modeled conservatively, not taking into account additional <br />conveyance area provided by the mining and reclamation of Ce112 downstream, and <br />blocking out the processing area completely without taking into account the individual <br />structures and stockpiles. The maximum rise of 0. l3 feet occurs upstream of the <br />processing area; downstream, the rise is closer to 0.04 feet, or less than half an inch. The <br />possible effect the mining plan will have on the floodplain is temporary, very small, and <br />considered insignificant within the accuracy limitations of the model. <br />Results of the reclaimed conditions model indicate that the 100-yeaz water surface <br />elevation would be lowered between 0.5 and 2.0 feet across the site. Accompanying <br />these lower water surface elevations is an increase in the river channel velocity just <br />downstream of the 83'~ Avenue bridge. This increase is a local effect only noted at one <br />cross section, and it seems to be caused by a decrease in flow area due to the lower water <br />surface elevation. Since there is already bank protection downstream of the bridge in the <br />form of concrete rubble, and this higher velocity, if it occurs, will only happen during a <br />significant flood event, this situation should not require mitigation. <br />11990 Grant St. • Suite 304 • Denver, CO 80233 5441 Boeing Drive. • Suite 200 • Loveland, CO 80536-8855 <br />(303) 452-6611 • Fax (303) 452-2759 (970) 461-9884 • Fax (970) 613-1177 <br />