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<br />BATTLE MOUNTAIN RESOURCES, SAN LUIS PROJECT <br />CMLRD ADEQUACY LETTER RESPONSES <br />Page 12 of 28 <br />19. What effect will the additional 80 cfs diverted to the north by the <br />east diversion ditch have on the drainageway receiving this flow? <br />Are any channel improvements proposed for this north drainage to <br />ensure stable conditions during the 100-yr, 24-hr storm runoff event? <br />RESPONSE: An analysis of the north drainage area was performed using the <br />computer program WASHED. In the analysis, the discharge from the east <br />diversion ditch (Drainage Area D), which is anticipated to be 80 cfs <br />' from the 100-yr, 24-hr storm, was routed through the drainage. A peak <br />discharge of 97 cfs was predicted. As shown on the enclosed <br />calculations the peak flow for the existing channel is routed before <br />the peak flow for the east diversion ditches resulting in only 17 <br />cfs of flaw in the existing channel at the time of passing of the <br />peak flow from the east diversion. <br />' The north drainage channel will be riprapped to stabilize the channel <br />during the 100-yr, 24-hr event. Based on the geometry of the existing <br />drainage, a riprap Dso of 0.71 ft is required to stabilize the <br />' channel. Existing channel characteristics and riprap sizing is <br />presented in Attachment 3. Riprap bedding material is described in <br />Item 43. <br />' 20. How was the SCS curve number of 70 derived? What soil and vegetation <br />conditions were assumed in the derivation? <br />1 RESPONSE: The SCS curve number of TO was chosen to be a conservative number <br />for runoff from the site to allow for conservative runoff control <br />designs. Based on a rangeland classification with an estimated 60% <br />cover and a hydrologic soil group classification of B (as discussed <br />' in the approved permit, Exhibits I and J), the curve number could <br />range between approximately 46 and 70 (Figures S-3 and S-4 from Peak <br />Flows in Colorado, U.S. Soil Conservation Service, 1984). 70 was <br />' chosen as the most conservative number. <br />21. A velocity of 4.5 fps will result in the south diversion ditch under <br />' design conditions. A velocity of 3.1 fps will result in the east <br />diversion ditch. Wi11 any attempt be made to improve channel bottom <br />conditions in these diversions and reduce possible sedimentation <br />downstream? The Division recommends the diversions be seeded after <br />construction for erosion control. <br />RESPONSE: The diversion ditches will be constructed primarily in the cemented <br />gravels of the Santa Fe formation. Locally, gravel particles within <br />the Santa Fe range from 0.75" to 3" in diameter. In addition, the <br />channel will be seeded after construction to aid in erosion control. <br />Natural armoring of the ditch bottom, coupled with the recommended <br />reseeding, is expected to maintain the stability of the diversion <br />ditch and control erosion. <br /> <br />