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Attachment #4 <br />Riprap Sizing Calculations (2 pages) <br />Federal Highway Administration Method (4 pages) <br />Cotter JD -9 <br />Calculations were based on maximum runoff velocity for any given channel section_ This was <br />determined by using the minimum roughness coefficient and maximum slope found within the reach <br />being analyzed. Results of these analyses are found on the worksheets contained in Attachment 3. <br />Riprap was sized using the Federal Highway Administration HEC -11 methodology (Chapter 4: Design <br />Guidelines for Rock Riprap). Copies of the relevant portions of this document are included in this <br />Attachment #4 following these calculations. In general, this method provides a required D50 size for <br />riprap in a straight channel with known side - slopes, flow -depth, and discharge velocity. The equation is <br />metric and is as follows: D50 = 0.00594 M3 / d0-5 K"5 where V = channel velocity (meters/sec), d = <br />normal depth (meters), and K is a variable dependent on the riprap side -slope and angle of repose. <br />K = (1 —sin 20 /sin2�)0-5 , where A = riprap side -slope angle from horizontal (@ 3:1) , and = riprap angle of <br />repose (from horizontal, conservatively estimated at 1:1). <br />Therefore: K = 0.7746 (2:1 sides), K = 0.8944 (3:1 sides), K = 0.9900 (10:1 sides) <br />Proposed Sect. 20,3A. Road /Runoff Diversion Ditch, East side, 3:1 side slopes <br />Note: This section analyzed at steepest slope (17 %) and lowest n (0.035) to give highest velocity <br />V = 5.66 fps = 1.7252 mps (meters per sec) <br />d = 0.39ft = 0.1189m (meter) <br />D50 = 0.00594 (V)3 / do-5 W-5 <br />(from worksheets in attachment #3) <br />1350 = 0.00594 (1.7252)3 / (0.1189)0.5 (0.8944)1 -5 = 0.1016 meter =4.O inches <br />Therefore: 4" D50 specified is adequate <br />Proposed Sect. 30 -1, Road/Runoff Collection Ditch, West side, 3:1 side slopes <br />Note: This section analyzed at steepest slope (17 %) and lowest n (0.035) to give highest velocity <br />V = 5.56 fps = 1.6947 mps (meters per sec) <br />d = 0.38 ft = 0.1158 m (meter) <br />D. = 0.00594 (v)3 I d0.5 K1.5 <br />(from worksheets in attachment #3) <br />D50 = 0.00594 (1.6947)3 / (0.1158)'-'(0.8944) 1 5 = 0.1004 meter = 3.9 inches <br />Therefore: 4" D50 specified is adequate <br />