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=FNATVES FOR WATER OUAU7Y <br />itla for Vrrious Sediment Particles <br />Type of Panicle <br />Gravel <br />Coarse sand <br />�fedium sand <br />Fine sand <br />Silt <br />Clay <br />:h. <br />Settling <br />Velocity <br />100-200 ft/min <br />IS-20 R/min <br />2-10 ft/min <br />30-80 ft/h <br />1-2 ft/h <br />0.005-0.01 ft/day <br />s aod SetU(n` VeloclHa CNculaled Ustn� <br />3nd 20° C <br />Diameter (µm) <br />30 <br />40 <br />50 <br />. 60 <br />80 <br />t00 <br />Velociry (h/s) <br />0.0027 <br />0.0047 <br />0.0074 <br />0.011 <br />0.019 <br />0.029 <br />te Mase in Stormwater as Relrled <br />Average Settling <br />Velocity (R/h) <br />0.03 <br />0.33 <br />1.5 <br />7.0 <br />70.0 <br />_ntal Prolection Agency (Igg3), <br />7.5 SEDIMENTATION 235 <br />of particles in urban runoff with average settling velocity can be estimated <br />using Table 7.5. <br />7.5.3 Design Storm Stzfng <br />For some reservoirs, engineers have formulated empirical equations from <br />sediment removal (Brune, 1953). These are based on assumed particle sizes <br />(or a specific location and specific design flow. Design conditions are speci- <br />fied and sediment removal estimate for the design condition. <br />Stormwater particles may be discrete or flocculent. Classical equations for <br />xtding mechania can be used ro estimate discrete partide removal effi- <br />ciencies. For discrete nonflocculating particles without hindrance or help <br />from cach other, Stokes law can be used to define settling. There are many <br />simplifying assumptions using Stokcs law which cannot be duplicated in <br />stormwater ponds, such as discrete nonflocculant particles, equally dis- <br />tributed velociry vectors, plug flow, and complete removal once the particle <br />strikes the bottom. Nevertheless, the settling mechanisms fot discrete pani- <br />cles are important for some design and for an understanding of the removal <br />mechanisms. <br />htost particles that settle with velocities greater than that required by the <br />design should be effectively removed, where the design velociry is defined as <br />yo <br />io � — <br />tQ <br />µhere uo - design settling velocity, ft/h <br />ho - depth of setHing, ft <br />�a - pond detention time, h <br />and the aond detention time is <br />Vo <br />�d 6 3600Q <br />wherc ta � detention time, h <br />VP = volume of pond, ftl <br />Q= outHow rate based on a design storm, ft3/s <br />3b00 = seconds per hour <br />with a pond overflow rate of <br />OR = Q x 360� <br />A <br />Wherc OR = overflow rate, ft/h <br />A = surface area, ft� <br />(7.6) <br />(7.7) <br />(7.8) <br />