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<br />. <br /> <br />21 <br /> <br />. <br /> <br />2.3.2.4 Silt and Clay <br />The silt and clay classes are of considerable importance in the evaluation <br /> <br />of total sediment load when treating such problems as density currents, con- <br /> <br />solidation, channel stability, and the development of berms. Since silt sizes <br /> <br />lie below sieve range, size determinations must be made by a bottom withdraw1 <br /> <br />tube, microscopically, or indirectly by a technique involving the relative <br /> <br />motion between the sediment particles and a suspending fluid. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />2.3.3 Fall Velocity <br />The primary variable defining the interaction of sediment transport <br />with the bed, banks, or suspended in the fluid, is the fall velocity of the <br />sediment particles. The value of the fall velocity depends on several factors <br /> <br />such as the concentration, the presence of a wall, and the influence of tur- <br /> <br />bulent velocity fluctuations. Frequently, the standard fall velocity is <br /> <br />used. This is defined as the fall velocity of a particle in motionless <br />distilled water of infinite extent at 240 C. Fall velocity is an important <br />variable used to estimate resistance to flow and the rate of sediment transport. <br /> <br />It has been shown that the bed configuration, resistance to flow,.and rate of <br /> <br />bed material transport in a sand channel may change significantly when the <br />fall velocity of the bed material changes. <br />Fall velocity may be evaluated using various formulas and diagrams which <br /> <br />can be found in Sediment Transport Technology by Simons and Senturk (1977). <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />2.3.4 Specific Weight and Specific Gravity <br />Specific gravity is an important factor extensively used in the analysis <br /> <br /> <br />of hydraulic and sediment transport problems. The specific gravity is the <br /> <br /> <br />ratio of the weight of a solid or a fluid to the specific weight of water <br /> <br /> <br />at 40 C. The specific gravity of a sediment particle ranges from 2.3 for <br /> <br />. <br /> <br />. <br />