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Cotter Corporation Mr. George Van Slyke August 26, 1983 Page - 2 - is located. To estimate the extent of flood innundation and velocities, tfie U.S. Army Corps of Engineers Water Surface Profile Computer Model (HEC-2) was used. Both subcritical and supercritical runs were made to better estimate the flow regime through the mine site. Friction values were estimated for the channel bed and overbanks. For the channel bed, an "n" value of 0.040 was used, based on our experience and calculations using the bed material size and the Land and Carlson formula. Bed inateriai sizes were determined from sieve analyses of minus three-inch material and volumetric measurements of larger sizes. For the congested overbank area, an average "n" value of 0.035 was used. This is a conservative value for the overbank area because the congestion due to structures actually will cause a higher "n" value and lower velocities. Surveyed information was used to determine the channel and overbank profile and representative cross sections. Cross sections were taken at an average of 100-foot intervals in this reach. Results of the hydraulic analysis are shown on the 100-year Flood Plain Plan and Profile previously submitted. Scour Mechanism Ralston Creek at the Schwartzwalder Mine site is an alluvial creek con- taining bed and bank material in the gravel to boulder size range. Particles range from sand sizes to boulders about 15 inches in equi/alent diameter. Only 40 percent of the material is less than three inces in equivalent diameter. The left (north) bank of the channel is composed of the natural rock outcropping with some brush. The right (south) bank is composed of a thin layer of road base material overlying alluvium. A seven foot chain link fence, concrete wall, conveyor bent and other facilities in the south overbank will restrici high velocity flcws ~o the main channel. Flood flows will extend across the valley floor. Main channel velo~:ities will be high, ranging from 6.2 and 18.0 feet per second. There wi 11 be scour in and adjacent to the main channel; the depth of scour will tie controlled by the stream bed armoring process. This process occurs as flows remove the fines from the alluvium, leaving the cobble and boulder size fragments to serve as an armor layer. Velocities on the flood fringes are much lower. Scour depths in the alluvium at the flood fringes ~rre two tenths to six tenths of a foot before the armoring process restricts further scour. The flood waters will encroach on the ore loading pad and bins and ~ow flood fringe velocities may cause some removal of fines from the sorted