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<br />22 <br /> <br />. <br /> <br />coal to 7.6 for galena. Fortunately, water-borne sediments are mainly quartz <br /> <br />and felspathic minerals with a specific gravity of about 2.65. For this <br />reason, the specific gravity of water-borne sediments is often assumed to <br /> <br />be 2.65. However, this specific gravity must be assumed with caution. <br /> <br />Different kinds of specific weights and their determination are described <br />in Simons and Senturk (1977). <br /> <br />. <br /> <br />. <br /> <br />2.3.5 Size Distribution of Sediment <br />Sediment consists of many particles differing in size, shape, specific <br />gravity, and fall velocity. It is necessary to make statistical analyses of <br /> <br />these characteristics, as determined from an adequate number of samples of <br /> <br />sufficient size, in order to define fully the representative particle char- <br /> <br />acteristics of any sediment mixture as a whole. The most commonly used method <br /> <br />is size frequency distribution. Sieve analysis is the most commonly used <br /> <br />method for size frequency distribution. In general, the results are presented <br /> <br />as cumulative size frequency curves. The fraction or percentage by weight of <br /> <br />a sediment that is smaller or larger than a given size is plotted against <br /> <br />particle size. From the size frequency curve, it is possible to obtain <br /> <br />representative sizes of sediment for use in describing the hydraulic properties <br /> <br />of a sand mixture related to: (1) the resistance to flow due to grain rough- <br /> <br />ness, and (2) the threshold condition which defines the beginning of transport. <br />The sediment size frequency distribution is essentially a probabilistic <br />approach used to help describe the transported sediment and the sediment mix- <br /> <br />ture forming the bed of a river. Experimental analysis shows that a cumulative <br /> <br /> <br />size distribution curve is similar to the cumulative distribution function (CDF) <br /> <br /> <br />of the standardized normal distribution. The characteristics of the CDI' are <br /> <br /> <br />such that between the ordinates 0.8143 and 0.1586, the normal distribution <br /> <br /> <br />can be approximated by a straight line. In general, the straight line <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br />