2013-10-30_REVISION - M1977310 (3)

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JD -6 Mine — Drainage Design Plan calculate resistance to flow and mean velocity at the channel cross section. The roughness coefficient ( Manning's n) for each cross section was calculated based on the D84 particle size (i.e., the 84th - percentile particle size) and the relative roughness was adjusted based on changes in flow depth. Increased stage (higher water level) results in decreased roughness (lower Manning's n). Thorne and Zevenbergen (1985) identified the appropriate resistance equations for mountain streams by evaluating and testing formulas using relative submergence terms for estimating mean velocity in steep, cobble/boulder -bed channels. For small relative roughness values (relative submergence, R/d84 >1), Thorne and Zevenbergen recommended the Hey (1979) equation for estimating mean cross section velocity: V = 5.62 log a'R (gRS)1 12 (3.5d84) R —0.314 a'= 11.1Gm ) a x Where: V = Mean cross section velocity (ft/s) g = Acceleration due to gravity (ft/s) R = Hydraulic radius S = Energy slope (water- surface slope in uniform flow) d84 = Intermediate axis for the 84"' — percentile particle size (ft) Dmax = Maximum depth of section (ft) For large relative roughness values (relative submergence RM844 ), the Thorne and Zevenbergen method uses Bathurst's (1978) equation for estimating mean cross section velocity: V Gi.365d,,,) R 2.34 rWl7(;LE -0.08) (gRS)1 /2 = \Dl AE = 0.039 — 0.139 log ( R —) d84 Where: D = Mean flow depth (ft or m) W = Water surface width (ft or m) These formulas require that the following assumptions be considered: (1) that channel gradients generally exceed 1 percent (0.01 ft/ft), (2) channel beds are predominately cobble and boulder substrate, and (3) relative roughness is large. As implemented, the method also assumes uniform flow conditions in the channel. The public domain software WinXSPro, distributed by the U.S. Forest Service (Hardy, et al., 2005), was used for the channel stability analysis. The mean velocity and Froude number' calculated using the Thorne and Zevenbergen (1985) resistance equations were compared to the Maximum Permissible Velocity or the Froude number established for channel stability. The Urban Storm Drainage Criteria Manual (UDFCD 2001, 2008), recommends that structures be designed with a permissible velocity of 5 ft/sec or Froude number of 0.6 for soil types A and B and of 7 ft/sec or Froude number of 0.8 for soil types C and D. Engineered channels at the JD -6 Mine are constructed in soil types C and D (rock) and are thus designed to meet the v = 7 ft/sec or Fr = 0.8 criteria in erodible soils. The Froude Number is a dimensionless parameter that describes the ratio between the inertia and gravitational forces in a liquid, and determines whether a flow is subcritical (Fr<l), critical (Fr -1) or supercritical (Fr >l). 4148A.131028 Whetstone Associates