Sediment Properties - Sediment Transport in Irrigation Canals: A New Approach - page 50

Sediments normally have many particles with different sizes. The par-
ticle sizes result in a distribution, which is generally expressed as percent
by mass (weight) versus particle size. The most common method to deter-
mine the distribution of particle sizes (size frequency) is a sieve analysis,
which can be used for particles larger than 74 m. Next, the results are

presented as a cumulative size-frequency curve. The fraction or the per-
centage of the sediment (by mass) that is smaller than a given size is plotted
against the particle size (sometimes a fraction that is larger than a given
size is plotted). The cumulative size distribution of various sediment sizes
by mass can be approximated by a log-normal distribution, as shown in
Figure 3.1. A log-normal distribution might result in a straight line when
logarithmic probability paper is used.

µ

100

80

60

40

20

0

1 10 100

Diameter (mm)

Figure 3.1. Example of a
particle size distribution.

d 50 = 8.2 mm d 90 = 10 mm

From this cumulative size distribution the following sediment

characteristics can be defi ne d (van Rijn, 1993):
(a) Mean diameter ( d m or d ) is defined as:

P i d i
P i d m = d = (3.4)

where:
P i = fraction (percentage) with diameter d i (%)

d i = geometric mean of the size fraction limits = diameter for which

i % of the sample is finer than d i (mm)
(b) The median diameter ( d 50 ) is assumed to give the best representa-
tion of the sediment mixture and is described by the diameter ' d ' for
which 50% of the sample is finer than ' d '. Sometimes other values
than d 50 are used in the sediment transport predictors; examples are
d 16 , d 35 , d 65 , d 84
(c) The geometric mean diameter ( d g ) is defined as:

d 84 d 16

d g = (3.5)