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Response to Exhibit C, Comment 5 <br />The course of the Uncompahgre River will be changed <br />during Phase II when a diversion dike is constructed to direct <br />the river flow into the Phase I pit. A sill will be constructed <br />at the upstream end of the streambed transition to prevent <br />headcutting and riprap will be placed at a 3:1 slope on the west <br />bank of the river to prevent its erosion. <br />The 1978 Floodplain Study of the Uncompahgre River <br />found, in the vicinity of the proposed gravel pit, the 100-year <br />floodplain to be 1700 feet wide. About 5000 feet upstream from <br />the site, the river spills out of a steep, confined channel and <br />spreads laterally across the valley floor. As it approaches the <br />mine site, the flow bifracates into east and west channels with <br />shallow, sheet flow occupying the space between the main <br />currents. The proposed gravel pit is within the main or east <br />channel. <br />The flow down the main channel can be estimated at its <br />bank-full volume with the remaining flood flow divided between <br />the west channel (west of the gravel pit) and sheet flow through <br />the remainder of the floodplain. Table 1 shows the bank-full <br />capacity of the main channel at the three cross-sections <br />delineated on Exhibit E of the revised Limited Impact <br />App i ~ <br />TABLE 1 <br />Cross-Section Are <br />(Ft ) <br />Bank-full Flow <br />Capacity Velocity <br />(cf~ (fps) <br />A (Downstream) 208.6 ' 1100.5 ', 5.3 <br />B (Mid-Pit) 72.0 366.9 ~ 5.1 <br />C (Upstream) 225.0 17 1.8 p j, 8.0 <br />~aw~..~ ,~r t.{ s o 1- <br />The 100-year flo~d flow for the Uncompahgre"River is <br />calculated to be 2250 cfs. Based on this, the bank-full flow at <br />the upstream end of the proposed pit is probably less than the <br />value shown in Table 1. A better estimate may be in the 1200 to <br />1500 cfs range. In any event, as stated in the response to <br />Exhibit C, Comment 3, the diversion dike is not intended to <br />withstand~lood flows It will fail before bank-full stage is <br />reached and permit the river /to follow its historic main channel. <br />3Summary of Basin and Streamflow Characteristics for Selected <br />Basins in Western Colorado and Adjacent States, U.S. Geological <br />Survey, Open File Report 84-137, 1984. <br />Page 3 <br />