as percent finer than 0.062 mm with the analysis of the coarser material. If it is going to be
dry-sieved, the material retained on the sieve is oven dried for one hour after all visible water
has been evaporated. If the material is to be analyzed by wet-sieving or with the
accumulation tube, it is not dried.
Sieve Size Distribution in the sand and gravel range is generally determined by passing the
sample through a series of sieves of mesh size ranging from 32 mm to 0.062 mm. A
minimum of about 100 grams of sand is required for an accurate sieve analysis. More is
required if the sample contains particles of 1.0 mm or larger. Standard methods employed in
soil mechanics are suitable for determining the sieve sizes of sand and gravel sediment
samples.
Visual Accumulation (VA) Tube is used for determining the size distribution of the sand
fraction of sediment samples (0.062 mm < Ds < 2.0 mm). It is a fast, economical, and
accurate means of determining the fall velocity or fall diameter of the sediment. The
equipment for the visual accumulation tube analysis consists of: (1) a glass funnel about 250
mm (9.8 in.) long; (2) a rubber tube connecting the funnel and the main sedimentation tube,
with a special clamping mechanism serving as a "quick acting" valve; (3) glass sedimentation
tubes having different sized collectors; (4) a tapping mechanism that strikes against the glass
tube and helps keep the accumulation of sediment uniformly packed; (5) a special recorder
consisting of a cylinder carrying a chart that rotates at a constant rate and a carriage that can
be moved vertically by hand on which is mounted a recording pen and an optical instrument
for tracking the accumulation; and (6) the recorder chart which has a printed form
incorporating the fall diameter calibration.
In the visual accumulation tube method, the particles start falling from a common source and
become stratified according to settling velocities. At a given instant, the particles coming to
rest at the bottom of the tube are of one "sedimentation size" and are finer than particles that
have previously settled out and are coarser than those remaining in suspension.
It has been shown that particles of a sample in the visual tube settle with greater velocities
than the same particles falling individually because of the effect of mutual interaction of the
particles. The visual accumulation tube apparatus is calibrated to account for the effects of
this mutual interaction and the final results are given in terms of the standard fall diameter of
the particles.
Visual accumulation tube method may not be suitable for some streams that transport large
quantities of organic materials such as root fibers, leaf fragments, and algae. Also, extra
care is needed when a stream transports large quantities of heavy or light minerals such as
taconite or coal. The method is explained in detail by Guy (1977).
Pebble Count Method is used to obtain the size distribution of coarse bed materials (gravel
and cobbles) which are too large to be sieved. Very often the coarser material is underlain
by sands. Then, the underlying sands are analyzed by sieving. The two classes of bed
material are either combined into a single distribution or used separately. The large material
sizes are measured in situ by laying out a square grid. Within the grid, all the particle sizes
are measured and counted by size intervals. For large samples a random selection of
particles in the various classes is appropriate to develop frequency histograms of sediment
sizes.
3.7
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