Laserfiche WebLink
<br />2.05.4(2)(c) <br />of the major drainage showing 9/79, present, projected and final projected <br />contours. Drawing 2.05.4(2)(c)-3 represents the maximum and average slope <br />of the major drainage channel and its banks. Drawing 2.05.4(2)C-2a shows <br />a detailed cross-section of the typical channel bottom above the mine sur- <br />face disturbance and at the lower end of the proposed disturbance. The <br />upper end shows a channel bottom 8' wide with a relief of less than 6". <br />The lower end shows a 3' wide channel for the first 6" of relief and 17' <br />wide for a relief of 12". The reconstructed channel will have a minimum <br />width of 5' on the upper end and 10' on the lower end, with maximum widths <br />of 20'. The peak flow, 5.4 cfs, for a 10-year 24-hour storm at the upper <br />end was taken from areas © and ® of Table 2.05.3(4)(d)-5 in the permit <br />application. The peak flow for the lower end, 33.4 cfs, was calculated <br />using the method described in Table 2.05.3(4)(d)-5. The variables used in the <br />lower end peak flow were 75 acres with an average slope of 20% and reclaimed <br />to a CN of 74. Using the Manning Equation for open channel design as des- <br />cribed on Table 2.05.3(4)(d)-6, the water depth in the 5' channel with an <br />11% slope will be 2.5" with a velocity of 4.5 fps and the water depth in <br />the 10' channel with a slope of 8% will be 6" with a velocity of 5 fps. A <br />check half way up the reconstructed channel with a width of 7.5', a grade of <br />7% and a peak flow of 17 cfs gives a 6" water depth and a velocity of 4.3 <br />fps. Peak flows using a 100-year 24-hour storm (3", page 103 of the permit <br />application) are 8.6 cfs for the upper end and 89 cfs for the lower end. <br />The calculated flow depth and velocities are 3.5" at 5.1 fps for the upper <br />end and 9" at 7.4 fps for the lower end. All of the design velocities are <br />too fast for an unprotected dirt channel. Vegetation could handle velocities <br />of 5 to 6 fps, if well enough established. Due to the uncertainty of water <br />for establishing adequate vegetation in the channel, rip-rap is proposed. <br />REVISED August 21, 1984 128a <br />