Figure 3.2. Nominal diameter vs. fall velocity (Temperature = 24C) (Guy 1977,
U.S. Interagency 1957b).
3.2.4 Sediment Size Distribution
Four methods of obtaining sediment size distribution are described below: Sieve analysis,
visual accumulation (VA) tube analysis, pebble count method, and pipette analysis. Each
method for size distribution analysis is appropriate for only a particular range of particle sizes
(Table 3.2). All together the four methods provide a means of obtaining particle size
distributions for most bed material samples.
Table 3.2. Guide to Size Range for Different Types of Size Analysis.
Analysis
Quantity of
Size
Concentration
Sediment (g)
Range
(mg/l)
or Pebbles
Sieves
0.062-32
mm
-----
100-500
VA tube
0.062-2
mm
-----
0.05-15.0
Pipette
0.002-0.062 mm
2,000-5,000
1.0-5.0
Pebble Count
12-1000
mm
-----
100 pebbles
If the sediment sample to be analyzed (bed material or suspended sediment) has
considerable fine material (Ds < .062 mm) it must be separated prior to analysis. To separate
the coarser from the finer sediment, the sediment should be wet-sieved using distilled water
and a 250-mesh (0.062 mm) sieve. The material passing through the sieve can be analyzed
by pipette analysis if further breakdown of the fine sediment is desired, or dried and included
3.6
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