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Attachment #4 <br />Riprap Sizing Calculations <br />Federal Highway Administration Method <br />Riprap was sized using the Federal Highway Administration HEC -11 methodology (Chapter 4: Design <br />Guidelines for Rock Riprap). In general, this method provides a required D50 size for riprap in a straight <br />channel with known side - slopes, flow - depth, and discharge velocity. The equation is metric and is as <br />follows: D50 = 0.00594 M3 / d0-5 K'-5 where V = channel velocity (meters/sec), d = normal depth <br />(meters), and K is a variable dependent on the riprap side -slope and angle of repose. <br />K = (1 —siW9 /sine +)0'5 , where 8 = riprap side -slope angle from horizontal (@ 3:1) , and 4 = 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 />Ofifsite 20 channel above confluence with Pond 20 outlet (Section 20-3.5), 3:1 side slopes <br />V = 6.13 fps =1.87 mps (meters per sec) (from worksheets in this attachment #4) <br />d = 0.40ft = 0.1219m (meter) <br />D50 = 0.00594 (V)3 / do-5 W-5 <br />D50 = 0.00594 (1.87)3 / (0. 1219)0-5 (0.8944)15 = 0.1315 meter = 5.18 inches <br />Therefore: 6" D50 specified is adequate <br />