2013-10-31_REVISION - M1977284 (3)

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Mineral Joe Mine - Drainage Design Plan 6 1. Type and size of the materials that compose the bed and banks of the channel; and 2. Shape of the channel. Cowan (1956) developed a procedure for estimating the effects of these factors to determine the value of n for a channel. The value of n may be computed by: where : n = (nb +nl +n2 +n3 +n4)•m nb = a base value of n for a straight, uniform, smooth channel in natural materials nl = a correction factor for the effect of surface irregularities n2 = a value for variations in shape and size of the channel cross section, n3 = a value for obstructions n4 = a value for vegetation and flow conditions m= a correction factor for meandering of the channel Table 3. Manning's "n" Values Used in Cowan's Method for Channel Roughness Natural Borrow Diversion Short Channel Condition n Values Channels Ditches Ditches Natural Channels Material Involved nb Earth 0.02 0.02 Rock cut 0.025 0.022 0.022 0.022 Fine gravel 0.024 Coarse gravel 0.028 Degree of Irregularity n, Smooth 0 Minor 0.005 0.005 Moderate 0.01 0.01 0.01 0.01 Severe 0.02 Variations of Channel Cross Section n2 Gradual 0 Alternating occasionally 0.005 0.005 0.005 0.005 Alternating frequently 0.010 -0.015 0.01 Relative Effect of Obstructions n3 Negligible 0 Minor 0.010 -0.015 0.01 0.01 0.01 Appreciable 0.020 -0.030 0.02 Severe 0.040 -0.060 Vegetation n4 Low 0.005 -0.010 0.005 0.005 0.005 0.005 Medium 0.010 -0.025 High 0.025 -0.050 Very high 0.050 -0.100 Degree of Meandering m Minor 1 1 1 1 1 Appreciable 1.15 Severe 1.3 Calculated Manning's n value 0.067 0.052 0.047 0.05 4148B.131029 Whetstone Associates