Good, qualitative, visual estimates of mean size are possible with little 

 previous experience. With experience, such visual estimates become semi- 

 quantitative. Visual comparison vd.th a standard is a useful tool in recon- 

 naissance and in obtaining interim results pending a more complete laboratory 

 size analysis. 



a. Sieve Analysis . Sieves are graduated in size of opening according to 

 the U.S. standard series. These standard sieve openings vary by a factor of 

 1.19 from one opening to the next larger (by the fourth root of 2, or 0.25-phi 

 intervals); e.g., 0.25, 0.30, 0.35, 0.42, and 0.50 millimeter (2.00, 1.75, 

 1.50, 1.25, 1.00 phi). The range of sieve sizes used and the size interval 

 between sieves selected can be varied as required. Typical beach sand can be 

 analyzed adequately using sieves with openings ranging from 0.062 to 2.0 

 millimeters (4.0 to -1.0 phi), in size increments increasing by a factor of 

 1.41 (0.5-phi intervals). 



Sediment is usually sieved dry. However, for field analysis or for size 

 analysis of sediment with a high content of fine material, it may be useful to 

 wet-sieve the sediment. Such wet-sieve analyses are described by Lee, Yancy, 

 and Wilde (1970). 



Size analysis by sieves is relatively slow but provides a widely accepted 

 standard of reference. 



A sieve analysis is independent of sediment density. Sediment shape 

 variation can introduce error in that sieve analysis tends to measure the 

 smaller axis of individual grains; these axes do not fully characterize the 

 size or mass of elongated grains (Sengupta and Veenstra, 1968; Baba and Komar, 

 1981). 



g. Settling Tube Analysis . Recording the rate that sediment settles in a 

 fluid-filled tube provides a rapid measurement of sediment size with useful 

 accuracy (Gibbs, 1972). Size analyses using a settling tube are sensitive to 

 sediment density and to sediment shape. Settling velocity tends to be 

 controlled by the larger axes of individual grains (Sengupta and Veenstra, 

 1968; Mehta, Lee, and Christensen, 1980). With commonly occurring littoral 

 sands, the characteristic sediment size is related to the settling velocity of 

 grains in isolation or in bulk (Ch. 4, Sec. II,l,c). 



There are numerous types of settling tubes; the most common is the visual 

 accumulation tube (Colby and Christensen, 1956), of which there are also 

 several types. The type now used at CERC (the rapid sediment analyzer or RSA) 

 works in the following way: 



A 3- to 6-gram sample of sand is dropped through a tube filled with 

 distilled water at constant temperature. A pressure sensor near the bottom of 

 the tube senses the added weight of the sediment supported by the column of 

 water above the sensor. As the sediment falls past the sensor, the pressure 

 decreases. The record of pressure versus time is empirically calibrated to 

 give size distribution based on fall velocity. 



The advantage of settling tube analysis is its speed. With modern 

 settling tubes, average time for size analyses of bulk lots can be about one- 

 fifth the time required for sieve analyses. 



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