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III'I~II~I~I~ll~~l~~ Erosion Potential during Large Flood at Line Camp Pit Location Reclamation Permit Application No. M-2001-001 Prepared by Allen Sorenson Date: May 22, 2001 ~Z ~3 -5~R 3/~ The Dolores River through the mine reach is very straight, perhaps due to channelization to facilitate historic and ongoing agricultural uses. The straight river will flow at higher averaee velocity than a naturally winding river, but locally higher velocities that would be present at sharp outside river bends will not occur. Estimate of velocity through Line Camp reach during a lazge flood. Estimating tool is the velocity calculator in the Riptap Design System Version 2.0, West Consultants Inc., Cazlsbad, CA. Floodplain width at south end of excavation azea: 845 feet. Floodwater surface gradient:.00625. Flood discharge peak at Town of Dolores is 7000 cfs, 100 plus years of record; Dolores is several miles downstream and there aze several significant intervening tributaries. ' Manning's `n' :.030 for clean, straight natural stream; .035 for floodplain, pasture; .050 for floodplain, light brush and weeds; .100 for floodplain, dense trees. Flood Dischar a Mannin 's `n' Avera a Veloci 7000 cfs .035 4.79 ft/sec 7000 cfs .030 5.24 ft/sec 7000 cfs .050 3.87 fUsec 10,000 cfs .035 5.53 ft{sec 12,000 cfs .035 5.95 ft/sec 14,000 cfs .035 6.33 fUsec 14,000 cfs .050 5.10 ft/sec Conclusion All estimated velocities can be described as low to moderate, and would not be erosive over the short flood time period typical of the Dolores River. Vegetated banks could withstand these velocities quite well during a flood; cut banks denuded of vegetation can be expected to cut further and cave during the flood. However, the geomorphology of the river in its existing alignment is likely to cut away from the pits, if at all.