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APPENDIX C: DETERMINATION OF <br /> SCOUR VELOCITY <br /> An estimate of the scour velocity for overland <br /> flow systems was obtained from sedimentation <br /> theory. According to Metcalf and Eddy (1972), <br /> the horizontal velocity that will just produce scour <br /> in a settling basin can be determined from the fol- <br /> lowing relationship: <br /> Vs=Jgk 5 -1)gd 1/2 (Cl) <br /> f <br /> i <br /> where Vs= horizontal scour velocity (m s') <br /> s=specific gravity of particles <br /> d=diameter of particles (m) <br /> k= constant which depends on the type <br /> of material being scoured z 0.06 <br /> g=gravitational constant=9.81 m s' <br /> f=Darcy-Weisbach friction factor=0.025. <br /> Assuming a particle size of 0.1 mm and a specific <br /> gravity of 1.1 the scour velocity was calculated to be <br /> 0.0434 m s' or 2.6 m min'. This appears to be a <br /> reasonable estimate based on the performance of <br /> the CRREL overland flow site. Suspended solids <br /> removal began to decrease slightly at a detention <br /> time of 20 minutes,which is equivalent to a velocity <br /> of 1.5 m min'. <br /> The limiting overland flow rate that will produce <br /> a scour velocity (Vs) of 2.6 m min' can be determined <br /> from eq 13 as follows: <br /> T= 0.078L = L <br /> 1/3 s' <br /> Solving for q, <br /> 0.078 Vs =0-078 2.6 .0.20 <br /> 9= 5113 S1/3 5113 . <br /> Therefore,in order to avoid resuspension of <br /> solids and scour,the average overland flow rate <br /> should be limited to 0.20/Si/3 or less. <br /> 29 <br />