Square-Surface Sample is obtained by picking up and counting all the surface pebbles in
each predetermined size class within a small enclosed area of the bed. The area is taken to
be representative of the whole channel bed.
The pebble count method entails measurement of randomly selected particles in the field,
often under difficult conditions. Therefore, use of the Zeiss Particle-Size Analyzer should be
considered (Ritter and Helley 1968). For this method, a photograph of the stream bed is
made, preferably at low flow, with a 35 mm camera supported by a tripod about 2 m (6.6 ft)
above the stream bed, the height depending on the size of the bed material. A reference
scale, such as a steel tape or a surveyor's rod must appear in the photograph. The
photographs are printed on the thinnest paper available. An iris diaphragm, illuminated from
one side, is imaged by a lens onto the plane of a Plexiglass plate. By adjusting the iris
diaphragm the diameter of the sharply defined circular light spot appearing on the
photograph can be changed and its area made equal to that of the individual particles. As
the different diameters are registered, a puncher marks the counted particle on the
photograph. An efficient operation can count up to 1,000 particles in 30 minutes.
In the line sampling method of pebble count sampling, a line is laid out or placed either
across or along the stream. Particles are picked at random intervals along the line and
measured. The measured particles are classified as to size or weight and a percent finer
curve or table is prepared. Usually 100 particles are sufficient to give an accurate
classification of the size distribution of coarse materials.
Pipette Method of determining gradation of sizes finer than 0.062 mm is one of the most
widely accepted techniques utilizing the Oden theory and the dispersed system of
sedimentation. The upper size limit of sediment particles which settle in water according to
Stokes is about 1/16 mm or 0.062 mm. This corresponds to the lower size limit which can be
determined readily by sieves. This size is the division between sand and silt (Table 3.1) and
is an important division in many phases of sediment phenomena.
The fundamental principle of the pipette method is to determine the concentration of a
suspension in samples withdrawn from a predetermined depth as a function of settling time.
Particles having a settling velocity greater than that of the size at which separation is desired
will settle below the point of withdrawal after elapse of a certain time. The time and depth of
withdrawal are predetermined on the basis of Stokes law.
Satisfactory use of the pipette method requires careful and precise operation to obtain
maximum accuracy in each step of the procedure. Also, for routine analysis, special
apparatus can be set up for the analysis of a large number of samples. A complete
description of a laboratory setup and procedure for this method is given by Guy (1969).
Frequency Curves. The presentation of sediment size analysis is made in various formats.
A histogram is a graphical representation of the number, weight, or volume percentage of
items in given class intervals. An example of a histogram is shown in Figure 3.3a. The
abscissa scale represents the class intervals, usually in geometric progression, and the
ordinate scale represents either actual concentration or percent (by number, volume, or
weight) of the total sample contained in each class interval. If the class intervals are small,
the shape of the histogram will approach a continuous curve. The successive sizes
employed in the size analysis of sediment are usually in ratios of 2 or 2 .
3.8
Previous Page
