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<br />Mr. Mark Klune <br />WRC File: 1664/2 <br />Page 2 <br /> <br />A railroad crossing approximately 80 feet upstream from the old <br />railway bed consists of a Pile Bent Bridge with 3 piers and a bottom <br />opening width of approximately 40 feet. <br />From the intersection of the arroyo with the Booth-Orchard Canai to <br />the Arkansas River, which lies some 2000 feet to the south, the <br />arroyo has been straightened, ditched and bermed. Some reaches of <br />the straightened ditch are sparsely covered with bushes and shrubs. <br />The longitudinal slope of this ditch is approximately 0.8%. <br />(2) The condition and cross-section of the Booth-Orchard Canal varies <br />greatly throughout its length along the property. In many areas no <br />discernible ditch is visible due to abandonment and erosion. Where <br />there is a distinguishable ditch, it is filled with trash and <br />debris. The ditch profile slope appears to be approximately 0.2% to <br />0.3%. In addition, review of the property maps provided by Valco <br />indicate that approximately 350 feet of the ditch along the northern <br />central part of the pit is on property not owned by Valco. <br />(3) The area tributary to the pit consists of range land in fair <br />condition. <br />Upon completion of the field investigation, WRC used the SCS TR-55 <br />hydrologic analysis to estimate the peak flows in the arroyo for the 5-, 10-, <br />25-, 50-, and 100-year storm events. Results of the analysis are attached to <br />this letter. Because the MLRB does not require that a certain frequency storm <br />be diverted around the mining operations, WRC decided to check the hydraulic <br />capacity of alternative ditches and based on that capacity conclude which <br />frequency storm could be reasonably diverted. Due to the existing condition, <br />and flat slope of the Booth-Orchard Canal, we recommended not considering <br />using the canal as an alternative method to carry the flow through or around <br />the pit area. <br />The capacity of the highway crossing structures (box culverts) was <br />determined using inlet control. This capacity is in the 1500-2000 cubic feet <br />per second (CFS) range or much greater than the 100 year peak flow of 1113 <br />cfs, i.e. the structures will cause no restriction in flows. <br />Based on the site inspection and concurrence by Mark Klune, Valco, Inc., <br />to eliminate the option of using the Booth-Orchard Canal, we investigated the <br />capacity of the straightened portion of the arroyo between the Booth-Orchard <br />Canal and the Arkansas River. Using field cross sections and longitudinal <br />slope, WRC computed the capacity of the ditch to be approximately 474 cubic <br />feet per second (CFS) in the shailowest part. Minimal berming of the east <br />side combined with removal of some of the brush would bring the capacity up to <br />600 cfs, equivalent to that of the 10-year frequency storm. <br />