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Doc Date:12/11/2001 III III'll'll"II'll sss segment of Tuttle Draw in the vicinity of the Nucla Mine has the lowest calculated • gradient (.0015 ft/ft) while the reach of Calamity nearest the Nucla East mining area (SW-N101 to SW-N107) is almost six times as steep (.0083 ft/ft). Gradients calculated for the monitored tributaries to each main channel (West Return Ditch and the Return Ditch above Site SW-N102) have gradients from 4 up to 16 times as great as the main channel reach. 5 ~ V ~redicted Peak Flow The annual mean and peak discharges were predicted for both Tuttle VVV and Calamity Draws~~~u sJJJi ng techniques outlined in the USCS Water-Resources Investigations Report 85-4086 titled Estimation of Natural Streamf low Characteristics in Western Colorado (Kircher et al. 1985). The techniques used to predict discharge characteristics of Tuttle and Calamity Draws involved the use of regression relationships determined from historical streamflow records in Western Colorado. Basin parameters and precipitation records were used to correlate discharge characteristics of four different regions of Western Colorado: 1) Mountain, 2) Rio Cra ndeq 3) Southwest and, 4) Northwest. Both Tuttle and Calamity Draws are situated in the southwestern region as outlined by Kircher et al. (1985). The regression model developed for this region is as follows: • 0 = aA(b1)P(b2)EB(b3)SB(b4) where, Q = discharge (cf s) 2 A = drainage area (mi ) P = mean annual precipitation minus 10 (in) EB =mean basin elevation minus 5000 per 1000 feet (ft) SB = mean basin slope (ft/ft) and, a, b1, 62, b3 and b4 are regression coefficients Using this regression model, the mean annual discharge and peak discharges for the 2-, 5-, 10-, 25-, 50-, 100- and 500-year recurrence intervals were predicted at two locations each in both Tuttle and Calamity Draws. For both Draws, discharge characteristics were calculated for the entire drainage (at each respective mouth), and at the most downstream • surface water monitoring site in each Draw (SW-N3, Tuttle Draw and SW-N103, Calamity Draw, see Exfiibit 7-1). Table 7-38 presents the regression equations and coefficients applied 7-110 Revised 04/11/88